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Messages - AbruptSLR

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1
As a follow-on to my last post I note that most of the ice mass loss modeled by Beltran et al. (2019 online) occurs both due to upwell of warm CDW (which the first image indicates is currently accelerating) and due to redirected warm ocean currents beneath key ice shelves (which is currently progressing for the FRIS as indicated by the second two image) due to changes in the local sea ice extent and the associate changes in local wind patterns.  Finally, I provide the fourth image to remind readers that we are currently near a fork in the road that may lead to a Hothouse Earth.

2

Sorry, I don't follow this argument. The Southern Ocean is supposed to warm with 5 °C within a GST rise of only 1.5 °C. How many millenia is that supposed to take?

...
The Southern Ocean is cooling, not warming. Surface temperatures cool, and there is some warming on deeper levels. How much is unclear.

To help limit the spread of your misunderstandings my first attachment is a pdf of Beltan et al. (2019 online, where the lead author is mistakenly identified as B. Catherine), which makes it clear that the authors are referring to a 5 oC increase of the surface water temperature within the study area in the Antarctic coastal waters and is associated with various factors including stratification of regional waters (due to freshening), warming of the surface water from the atmosphere and local upwelling of warm CDW.  Furthermore the first attached image (Fig 5) shows that within 1000 years of the majority of local ice mass loss has occurred; while the second image (Fig 6) shows the two step paleo warming process (& I note that in the modern world we are currently past Step 1).  Also, I note that the model that Beltran et al. (2019 online) could simulate MISI behavior but not MICI behavior thus I include the third attached image from Pollard and DeConto showing that using one version of an MICI model projected similar ice mass loss from Antarctica by 2170 that took the Beltran et al (2019) model approximately 1000 years to achieve.

Extract from Beltran et al. (2019 online): "We propose that the exceptional surface ocean warming during the MIS31 super-interglacial occurred in two steps (Fig. 6). [Step 1] The unusual MIS31 orbital configuration resulted in an extended period (~2000 years) of warm and long-lasting summer seasons that caused mild warming of Southern Ocean surface waters and the reduction of sea ice development or survival.  Concomitantly, the prevailing Southern Hemisphere Westerly Winds and the easterly coastal winds migrated to the south (Fig. 6).  The poleward shifted winds warmed the coastal waters, thus bring heat southwards thereby initiating basal melting of ice shelves and the retreat of marine-based groundling lines.  Our numerical ice sheet model demonstrates that a 0.5 oC ocean warming at the ice margin for c. 200 years is sufficient to cause ice retreat and that most of it can occur in less than 2000 years.  [Step 2] The surface warming in the coastal regions (9.5 oC at ODP Site 1101 and 5 oC at IODP Site 1361) could only take place after complete loss of the WAIS (Fig. 6) and would likely be amplified by ocean stratification feedbacks."


Caption for second image: "Fig. 6. Schematic evolution of ice retreat during MIS31 in response to poleward wind shift and subsequent ocean warming.  Step 1 shows the increased advection of warmer deep waters (red arrow, see Pedro et al., 2016).  Step 2 represents the run over ice melting at the maximum of MIS31 warmth.

Edit: I note that to access the pdf you need to click on the link named 10.10@j.quascirev.2019.106069

3
The linked article provides clear warning that climate change and nutrient pollution are both driving oxygen from the oceans, which is threatening many species of fish.  What is not stated is that a some point the loss of fish species will contribute to the stratification of the oceans, as many specifies of fish contribute to the mixing of seawater in the upper levels of the ocean (note that the video imbedded in the article presented a simple/clear message about some of the risks of climate change):

Title: "Climate change: Oceans running out of oxygen as temperatures rise"

https://www.bbc.com/news/science-environment-50690995

Extract: "Climate change and nutrient pollution are driving the oxygen from our oceans, and threatening many species of fish.

That's the conclusion of the biggest study of its kind, undertaken by conservation group IUCN.

As more carbon dioxide is released enhancing the greenhouse effect, much of the heat is absorbed by the oceans. In turn, this warmer water can hold less oxygen. The scientists estimate that between 1960 and 2010, the amount of the gas dissolved in the oceans declined by 2%.

That may not seem like much as it is a global average, but in some tropical locations the loss can range up to 40%.

Even small changes can impact marine life in a significant way. So waters with less oxygen favour species such as jellyfish, but not so good for bigger, fast-swimming species like tuna."

4
Now that Hausfather et al. (2019) has indicated that earlier/simpler climate models have a good record as compared to the observer record, and as in the following linked Carbon Brief article, Hausfather indicates that the majority of simpler climate models in CMIP6 are indicating lower values of ECS than the more sophisticated CMIP6 model projections; I strongly suspect that all of the left-tail advocates are already lining-up their strategies to keep the AR6 range for ECS as close as possible to the current AR5 range for ECS.

Title: "CMIP6: the next generation of climate models explained"

https://www.carbonbrief.org/cmip6-the-next-generation-of-climate-models-explained

Extract: "SSP5-3.4OS is an overshoot scenario (OS) where emissions follow a worst-case SSP5-8.5 pathway until 2040, after which they decline extremely rapidly with lot of late-century use of negative emissions.

However, despite making the models more realistic, it is not yet clear whether these improvements are translating into more accurate estimates of ECS. For example, a number of climate scientists have expressed scepticism of the high-end values, arguing that they are inconsistent with evidence from palaeoclimate records and other lines of evidence.

It remains to be seen how the IPCC AR6 will reconcile the high ECS from some models with other sources of evidence, and if they will update the “likely” sensitivity range."


Unfortunately, much of the risks of higher ECS values are associated with the Antarctic/Southern Ocean; which generally have not been modelled well by the CMIP5 and earlier models due to limited availability of data.  For instance, in Hausfather's Carbon Brief article he states:

"Researchers are currently looking into what is driving these high ECS values. In a number of models the increase in ECS appears to be due to their improved representation of clouds and aerosols; for example, how models treat supercooled clouds (below freezing but still liquid) in the Southern oceans can make a big difference in resulting sensitivity."

Furthermore, the CMIP5 and earlier climate models has all underestimated the ice-climate feedback mechanisms already being activated in the Antarctic/Southern Ocean system, as not only demonstrated by observed ice mass loss and ice shelve loss, but also by such recent paleo findings as Beltran et al. (2019 online or 15 January 2020 in print).

5
I think that the linked op/ed by Naomi Oreskes et al. (2019) is worth reading in its entirety as I only extract a few short sections:

Title: "Scientists Have Been Underestimating the Pace of Climate Change"

https://blogs.scientificamerican.com/observations/scientists-have-been-underestimating-the-pace-of-climate-change/

Extract: "… it was reported recently that in the one place where it was carefully measured, the underwater melting that is driving disintegration of ice sheets and glaciers is occurring far faster than predicted by theory—as much as two orders of magnitude faster—throwing current model projections of sea level rise further in doubt.

Consistent underestimation is a form of bias—in the literal meaning of a systematic tendency to lean in one direction or another—which raises the question: what is causing this bias in scientific analyses of the climate system?

In our new book, Discerning Experts, we explored the workings of scientific assessments for policy, with particular attention to their internal dynamics, as we attempted to illuminate how the scientists working in assessments make the judgments they do. Among other things, we wanted to know how scientists respond to the pressures—sometimes subtle, sometimes overt—that arise when they know that their conclusions will be disseminated beyond the research community—in short, when they know that the world is watching. The view that scientific evidence should guide public policy presumes that the evidence is of high quality, and that scientists’ interpretations of it are broadly correct. But, until now, those assumptions have rarely been closely examined."

6

The linked (open access) reference focuses on the impacts of marine instability primarily of the Wilkes Basin (but also other basins as indicated by the attached images showing impacts on AABW formation due to different assumed marine glacier instability scenarios in different basins) on Southern Ocean dynamics.  This research supports the Hansen et al (2016) findings:

Phipps, S. J., Fogwill, C. J., and Turney, C. S. M.: Impacts of marine instability across the East Antarctic Ice Sheet on Southern Ocean dynamics, The Cryosphere Discuss., doi:10.5194/tc-2016-111, in review, 2016.

http://www.the-cryosphere-discuss.net/tc-2016-111/

Abstract. Recent observations and modelling studies have demonstrated the potential for rapid and substantial retreat of large sectors of the East Antarctic Ice Sheet (EAIS). This has major implications for ocean circulation and global sea level. Here we examine the effects of increasing meltwater from the Wilkes Basin, one of the major marine-based sectors of the EAIS, on Southern Ocean dynamics. Climate model simulations reveal that the meltwater flux rapidly stratifies surface waters, leading to a dramatic decrease in the rate of Antarctic Bottom Water formation. The surface ocean cools but, critically, the Southern Ocean warms by more than 1 ºC at depth. This warming is accompanied by a Southern Ocean-wide "domino effect", whereby the warming signal propagates westward with depth. Our results suggest that melting of one sector of the EAIS could result in accelerated warming across other sectors, including the Weddell Sea sector of the West Antarctic Ice Sheet. Thus localised melting of the EAIS could potentially destabilise the wider Antarctic Ice Sheet.


Caption for attached image: "Figure 3. Rate of AABW formation (Sv) in the control simulation (black), and in experiments WILKES (red), WEST (green) and EAST (blue). Thin lines indicate individual ensemble members; thick lines indicate the ensemble means. The values shown are 100-year running means. Vertical dashed lines indicate the years in which the freshwater hosing begins and ends."


7
I just remind readers that researchers including Hellmer et al. (2012) [see the attached image comparing E3SM projections for FRIS vs Hellmer et al. (2012)] and Darelius et al. (2016) find that changes in sea ice extent near the Weddell Sea are changing local wind patterns that are currently driving warm modified CDW beneath the FRIS which should result in accelerated ice mass loss from the FRIS in coming decades:


Hellmer, H., Kauker, F., Timmermann, R. et al. Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current. Nature 485, 225–228 (2012) doi:10.1038/nature11064

https://www.nature.com/articles/nature11064

Abstract: "The Antarctic ice sheet loses mass at its fringes bordering the Southern Ocean. At this boundary, warm circumpolar water can override the continental slope front, reaching the grounding line through submarine glacial troughs and causing high rates of melting at the deep ice-shelf bases. The interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice-mass loss. Here we show that a redirection of the coastal current into the Filchner Trough and underneath the Filchner–Ronne Ice Shelf during the second half of the twenty-first century would lead to increased movement of warm waters into the deep southern ice-shelf cavity. Water temperatures in the cavity would increase by more than 2 degrees Celsius and boost average basal melting from 0.2 metres, or 82 billion tonnes, per year to almost 4 metres, or 1,600 billion tonnes, per year. Our results, which are based on the output of a coupled ice–ocean model forced by a range of atmospheric outputs from the HadCM3 climate model, suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern Weddell Sea due to thinning of the formerly consolidated sea-ice cover. The projected ice loss at the base of the Filchner–Ronne Ice Shelf represents 80 per cent of the present Antarctic surface mass balance. Thus, the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of Antarctic ice streams and ice shelves, and global sea level rise."

&

Darelius, E., Fer, I. & Nicholls, K. Observed vulnerability of Filchner-Ronne Ice Shelf to wind-driven inflow of warm deep water. Nat Commun 7, 12300 (2016) doi:10.1038/ncomms12300

https://www.nature.com/articles/ncomms12300

Abstract: "The average rate of melting at the base of the large Filchner-Ronne Ice Shelf in the southern Weddell Sea is currently low, but projected to increase dramatically within the next century. In a model study, melt rates increase as changing ice conditions cause a redirection of a coastal current, bringing warm water of open ocean origin through the Filchner Depression and into the Filchner Ice Shelf cavity. Here we present observations from near Filchner Ice Shelf and from the Filchner Depression, which show that pulses of warm water already arrive as far south as the ice front. This southward heat transport follows the eastern flank of the Filchner Depression and is found to be directly linked to the strength of a wind-driven coastal current. Our observations emphasize the potential sensitivity of Filchner-Ronne Ice Shelf melt rates to changes in wind forcing."

8
...

Certainly, the model results by Beltran et al. (2019) suggest that the current upwelling of modified CDW and advection to groundling lines of key Antarctic marine glaciers may well trigger sufficient ice mass loss to cause extensive modifications to oceanic and hydrologic systems that could lead directly to the collapse of the WAIS.

...

I thought it would be good to remind readers that Milillo et al. (2019) stated that: "We interpret the results in terms of buoyancy/slope-driven seawater intrusion along preferential channels at tidal frequencies leading to more efficient melt in newly formed cavities. Such complexities in ice-ocean interaction are not currently represented in coupled ice sheet/ocean models."

P. Milillo, E. Rignot, P. Rizzoli, B. Scheuchl, J. Mouginot, J. Bueso-Bello and P. Prats-Iraola (Jan 2019), "Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica", Science Advances  30, Vol. 5, no. 1, eaau3433, DOI: 10.1126/sciadv.aau3433

https://advances.sciencemag.org/content/5/1/eaau3433

Abstract
The glaciers flowing into the Amundsen Sea Embayment, West Antarctica, have undergone acceleration and grounding line retreat over the past few decades that may yield an irreversible mass loss. Using a constellation of satellites, we detect the evolution of ice velocity, ice thinning, and grounding line retreat of Thwaites Glacier from 1992 to 2017. The results reveal a complex pattern of retreat and ice melt, with sectors retreating at 0.8 km/year and floating ice melting at 200 m/year, while others retreat at 0.3 km/year with ice melting 10 times slower. We interpret the results in terms of buoyancy/slope-driven seawater intrusion along preferential channels at tidal frequencies leading to more efficient melt in newly formed cavities. Such complexities in ice-ocean interaction are not currently represented in coupled ice sheet/ocean models.

9
...
We show that the Paris Agreement target temperature of 1.5°C is sufficient to drive runaway retreat of the WAIS.

vox,

Thank you for the links to Beltran et al. (2019); which complements and adds details for the MIS 31 super interglacial that I posted about in Reply #2005, and by Coletti et al. (2015), see attached image; which also documents an interhemispheric linkage between the Arctic and Antarctic during super interglacial periods. 

Coletti, A. J., DeConto, R. M., Brigham-Grette, J., and Melles, M.: A GCM comparison of Pleistocene super-interglacial periods in relation to Lake El'gygytgyn, NE Arctic Russia, Clim. Past, 11, 979-989, doi:10.5194/cp-11-979-2015, 2015.

http://www.clim-past.net/11/979/2015/cp-11-979-2015.pdf
http://www.clim-past.net/11/979/2015/cp-11-979-2015.html

Abstract: "Until now, the lack of time-continuous, terrestrial paleoenvironmental data from the Pleistocene Arctic has made model simulations of past interglacials difficult to assess. Here, we compare climate simulations of four warm interglacials at Marine Isotope Stages (MISs) 1 (9 ka), 5e (127 ka), 11c (409 ka) and 31 (1072 ka) with new proxy climate data recovered from Lake El'gygytgyn, NE Russia. Climate reconstructions of the mean temperature of the warmest month (MTWM) indicate conditions up to 0.4, 2.1, 0.5 and 3.1 °C warmer than today during MIS 1, 5e, 11c and 31, respectively. While the climate model captures much of the observed warming during each interglacial, largely in response to boreal summer (JJA) orbital forcing, the extraordinary warmth of MIS 11c compared to the other interglacials in the Lake El'gygytgyn temperature proxy reconstructions remains difficult to explain. To deconvolve the contribution of multiple influences on interglacial warming at Lake El'gygytgyn, we isolated the influence of vegetation, sea ice and circum-Arctic land ice feedbacks on the modeled climate of the Beringian interior. Simulations accounting for climate–vegetation–land-surface feedbacks during all four interglacials show expanding boreal forest cover with increasing summer insolation intensity. A deglaciated Greenland is shown to have a minimal effect on northeast Asian temperature during the warmth of stages 11c and 31 (Melles et al., 2012). A prescribed enhancement of oceanic heat transport into the Arctic Ocean does have some effect on Lake El'gygytgyn's regional climate, but the exceptional warmth of MIS l1c remains enigmatic compared to the modest orbital and greenhouse gas forcing during that interglacial."

Extract: "The timing of significant warming in the circum-Arctic can be linked to major deglaciation events in Antarctica, demonstrating possible interhemispheric linkages between the Arctic and Antarctic climate on glacial–interglacial timescales, which have yet to be explained."

Certainly, the model results by Beltran et al. (2019) suggest that the current upwelling of modified CDW and advection to groundling lines of key Antarctic marine glaciers may well trigger sufficient ice mass loss to cause extensive modifications to oceanic and hydrologic systems that could lead directly to the collapse of the WAIS.

Beltran, Catherine et al. (2019 online or 15 January 2020), "Southern Ocean temperature records and ice-sheet models demonstrate rapid Antarctic ice sheet retreat under low atmospheric CO2 during Marine Isotope Stage 31, Quaternary Science Reviews, Volume 228, 10606, https://doi.org/10.1016/j.quascirev.2019.106069

https://www.sciencedirect.com/science/article/abs/pii/S0277379119306122?via%3Dihub

Abstract: "Over the last 5 million years, the Earth’s climate has oscillated between warm (interglacial) and cold (glacial) states. Some particularly warm interglacial periods (i.e. ‘super-interglacials’) occurred under low atmospheric CO2 and may have featured extensive Antarctic ice sheet collapse. Here we focus on an extreme super-interglacial known as Marine Isotope Stage 31 (MIS31), between 1.085 and 1.055 million years ago and is the subject of intense discussion. We reconstructed the first Southern Ocean and Antarctic margin sea surface temperatures (SSTs) from organic biomarkers and used them to constrain numerical ice sheet-shelf simulations. Our SSTs indicate that the ocean was on average 5 °C (±1.2 °C) warmer in summer than today between 50 °S and the Antarctic ice margin. Our most conservative ice sheet simulation indicates a complete collapse of the West Antarctic Ice Sheet (WAIS) with additional deflation of the East Antarctic Ice Sheet. We suggest the WAIS retreated because of anomalously high Southern Hemisphere insolation coupled with the intrusion of Circumpolar Deep Water onto the continental shelf under poleward-intensified winds leading to a shorter sea ice season and ocean warming at the continental margin. In this scenario, the extreme warming we observed likely reflects the extensively modified oceanic and hydrologic system following ice sheet collapse. Our work highlights the sensitivity of the Antarctic ice sheets to minor oceanic perturbations that could also be at play for future changes."

Best,
ASLR

10
The linked article and associated linked reference confirms that GHG emissions during the peak of the PETM were occurring about 10 times slower than current anthropogenic GHG emissions:

Title: "Carbon emissions from volcanic rocks can create global warming"

https://www.sciencedaily.com/releases/2019/12/191205073024.htm

Extract: "Geologists at the University of Birmingham have created the first mechanistic model of carbon emissions changes during the Paleocene-Eocene Thermal Maximum (PETM) -- a short interval of maximum temperature lasting around 100,000 years some 55 million years ago.

During PETM initiation, release of 0.3-1.1 PgC yr-1 of carbon as greenhouse gases to the ocean-atmosphere system drove 4-5°C of global warming over less than 20,000 years -- a relatively short period of time."

See also:

Stephen M. Jones, Murray Hoggett, Sarah E. Greene & Tom Dunkley Jones. Large Igneous Province thermogenic greenhouse gas flux could have initiated Paleocene-Eocene Thermal Maximum climate change. Nature Communications, 2019 DOI: 10.1038/s41467-019-12957-1

https://www.nature.com/articles/s41467-019-12957-1.epdf?shared_access_token=eJinQbvdEZd9ahXxNcs1ytRgN0jAjWel9jnR3ZoTv0NKzrEQ0tfq0ywvPqEwgn3CRBhC0EOyZalzcBDc1uFjXMiJ4lPtxegf_Pj0b5pds-B9v1i7Z55UIOQQnGSZzaa0zS67-jh5N7-FXcA10MuDGQ%3D%3D



11
As a follow-on to my last post, the linked reference discusses the implications of both Type 1 (false positive) and Type 2 (false negative) errors in climate science and assessments (see the attached image).

Anderegg, W.L. et. al. (September 2014), "AWARENESS OF BOTH TYPE 1 AND 2 ERRORS IN CLIMATE SCIENCE AND ASSESSMENT", BAMS - American Meteorological Society, DOI:10.1175/BAMS-D-13-00115.1

https://journals.ametsoc.org/doi/full/10.1175/BAMS-D-13-00115.1
&
https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-13-00115.1

Abstract: "Treatment of error and uncertainty is an essential component of science and is crucial in policy-relevant disciplines, such as climate science. We posit here that awareness of both “false positive” and “false negative” errors is particularly critical in climate science and assessments, such as those of the Intergovernmental Panel on Climate Change. Scientific and assessment practices likely focus more attention to avoiding false positives, which could lead to higher prevalence of false-negative errors. We explore here the treatment of error avoidance in two prominent case studies regarding sea level rise and Himalayan glacier melt as presented in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. While different decision rules are necessarily appropriate for different circumstances, we highlight that false-negative errors also have consequences, including impaired communication of the risks of climate change. We present recommendations for better accounting for both types of errors in the scientific process and scientific assessments."

Anderegg et al (2014) reference extensively discusses the Type 1 (false positive) statement in AR4 about the Himalayan glacier melt contribution to sea level rise; the correction of which in AR5, in my opinion, has caused consensus climate scientists to exhaust themselves on chasing Type 1 errors, leaving them with less energy to chase Type 2 errors (i.e. case where consensus scientists assume that no relationship exists when in fact one due exist [i.e. MICI mechanisms]).  Therefore, in addition to the points raised by Le Bars about Edwards et al. (2019), I add the following partial list of Type 2 errors that consensus scientists' models (both CMIP5 and to some degree CMIP6) are most probably making with regard to characterization of the risk of abrupt ice mass loss this century:

1. Consensus scientists over emphasize that Greenland is contributing about twice as much to current SLR than Antarctica. This can lead to a Type 2 error, of ignoring the freshening of the Southern Ocean associate with both ice mass loss from Antarctic ice shelves and increasing precipitation into the Southern Ocean, neither of which contribute to SLR, but both of which contribute to the early activation of ice-climate feedback mechanisms around Antarctica, which will contribute to more significant MICI action than consensus scientists are currently prepared to acknowledge.

2. The initial conditions and boundary conditions of virtually all consensus climate models err on the side of least drama with regard to such matters as: a) slowing of the MOC, b) sensitivity of the ENSO to shift towards more frequent El Nino events (which indicates higher end values for ECS) & initial Ocean Heat Content; c) negative forcing from aerosols which mask higher end values of ECS; d) the key Antarctic ice shelves are currently in much worse condition than assumed by essentially all climate models that I am aware of; e) finer meshes virtually always lead to greater marine glacier sensitivity to climate forcing, and current consensus models are far to coarse to capture key ice behavior (such as that at the base of the Thwaites Ice Tongue); f) new satellite information confirms that low level tropical cloud cover is dissipating with more global warming which implies that fast response climate sensitivity is increasing with time.

3. Many nonlinearly increasing positive carbon feedback mechanisms (such as permafrost, soil and forest degradation) are currently being masked by the likely increase in carbon absorption by vegetation that is temporarily benefiting from more atmospheric CO₂ and warming temperatures.  This is what Hansen calls a Faustian bargain, where consensus scientists do not need to acknowledge the nonlinearly increasing positive carbon feedback mechanisms (which is a Type 2 error).  Furthermore, biological systems are highly susceptible to damage from abrupt climate change, but most consensus climate models downplay the degree of this sensitivity; therefore most consensus models underestimate the likely coming damage to both terrestrial and oceanic carbon sinks with continuing anthropogenic forcing (including pollution, and land use changes).

4.  Consensus climate models cannot hind cast the degree of Arctic Amplification observed in the paleo-record, and thus these consensus climate models are clearly committing Type 2 errors by ignoring positive feedback mechanisms that contribute to Arctic Amplification (& thus to Antarctic marine glacial instability via bipolar seesaw mechanisms).

I could go on, but I am mere listing arguments that I have already made earlier in this thread, so those who are interested can scroll back to see other Type 2 errors (such as the positive feedback associated with the project future redistribution of anthropogenic aerosol emissions, and projected changes in VOCs from forests).

12
Some interesting reflections by Dewi Le Bars on the implications of two recent papers for the projections by DeConto & Pollard and for ice-climate feedback modelling (as proposed by Hansen et al, amongst others):
https://sites.google.com/site/dewilebars/sea-level-monthly-review/february-2019

On Edwards et al 2019:
'The claim that MICI is "not necessary" to reproduce past sea level high stands is both not really true and not really useful. The uncertainty range about what could have been the contribution of Antarctica to sea level during the Pliocene is 5-20 m and during the Last Interglacial it is 3.6-7.4 m. DeConto and Pollard’s model without MICI can reproduce up to 6 m and 5.5 m respectively for these two period (see Edwards et al. E.D. Fig. 4). So yes it can reproduce the lower part of the ranges. But most of the Pliocene range cannot be reproduced with the no-MICI assumption. What the figure shows is that the model with MICI covers a much bigger par of the possible Antarctic contribution for these periods. And still, even including MICI, the model can only explain a maximum of 12 m contribution for the Pliocene. Which means additional mechanisms would be necessary to cover the whole range of possible Antarctic contribution for that period. The claim that MICI is “not necessary” is also not very useful practically because projections with MICI are used to make high-end sea level scenarios. The important information is then is it possible or not? If it was not possible then it would be good news and decision makers wouldn't need to take it into account. "Not necessary" only has an impact on low-end scenarios, for which MICI would already not be used anyways.'

On Golledge et al 2019:
'Current state of the art (CMIP5 type) climate models do not include ice sheet models so the coupled effects between ice sheets and climate are a blind spot. In these climate models the ice sheets are just white mountains that do not change over time. They might have a snow layer on top of them but no ice. So snow falls on them accumulate a little bit and when it melts it is put in the nearest ocean grid box. If too much accumulates then it is put directly in the ocean to avoid infinite accumulation. What is missing is a model to transform the snow to ice and then transport it back to the sides of the ice sheet or to the ocean under the force of gravity. This is what ice sheet models do. Golledge et al. use the PISM ice sheet model for Greenland and Antarctica and couple them offline to LOVECLIM, an intermediate complexity climate model. Intermediate complexity means lower resolution and simpler physics compared to CMIP5 type climate models. It is the type of models generally used for long paleoclimate simulations.

What they find is that allowing feedbacks between the ice sheets and the climate model leads to strengthen both Antarctic and Greenland mass loss, by 100% and 30% respectively. For Antarctica this is not a surprise, although the magnitude is much bigger than I expected. Freshwater from the melting of ice leads to increase the ocean stratification, because it is is very light. This reduces vertical ocean mixing and as a result the surface of the ocean cools down while the subsurface warms up. Antarctica mostly looses mass from ice shelves basal melt and calving which is strengthened by warmer subsurface ocean temperature. For Greenland, it comes as a surprise to me that the feedback would increase the mass loss, because Greenland mostly looses mass from surface melt and a cooler atmosphere temperature would tend to reduce surface melt. Unfortunately the paper does not explain the mechanisms at play there (or did I miss it?).

There are a few issues with the ice sheet models that reduce my confidence in the projections. For Greenland the model is not able to reproduce the recent fast mass loss acceleration. Therefore the authors artificially impose the mass loss on the model in two ways: (1) decrease the friction between the ice and the bed (basal traction) to have a faster flow between 2000 and 2015 and (2) reduce the snowpack refreezing between 2000 and 2025. Refreezing is important for the mass balance because on ice sheets more than half of the snow that melts in the summer refreezes locally. It never reaches the ocean. Michiel van den Broeke had a similar comments in Trouw (in Dutch). You can force the model to agree with observations but if the model does not have the proper dynamics to explain observations there is no reason it is doing a good job for the future. For Antarctica, the model starts with enormous mass accumulation (1000 Gt/year in 1900) and accumulates mass until the 1980th. This is clearly not possible, such an accumulation would have been seen by tide gauge measurements. In fact as I said in the last review it is expected that Antarctica was slowly loosing mass in the 20th century. Also, the internal variability of grounded ice is so large in the model (Fig. 1a-d) that I do not understand what is going on physically (please let me know if you do).

In conclusion, the paper’s goal is important and it is the first time that two high resolution ice sheet models are coupled to a climate model. This is a big step in the right direction. However, I am not convinced by the results because of the issues mentioned above concerning the ice sheet models. Nevertheless, it is very instructive as it shows the long way that is left for ice sheet models to reach the level at which we can trust their future projections.'

As many left-tail advocates on this thread like to imply that Edwards et al. (2019) and Golledge et al. (2019), allows consensus climate science to effectively ignore MICI-type of ice-climate mechanism, I encourage readers to both re-read Lennart's quoted post from last March, and to consider:

1. The first image shows an abrupt SLR event about 127,000 year ago (during the Eemian) that Edwards et al. (2019) cannot replicate; and thus this significantly raises the risk that MICI-type of mechanisms are very relevant to our current situation w.r.t. a potential near-term collapse of the WAIS.

2. The second image shows how the pdf for ECS shifts to the right as more feedback mechanisms are considered (note that observation-based estimates of ECS not only ignore all slow-response mechanisms but also many fast response mechanisms).  However, as consensus climate science chooses to define ECS based on a doubling of CO2 (or CO2 equivalent) it ignores many ice-climate feedback mechanisms this century including both MISI and MICI mechanisms, which if considered would screw the effective ECS pdf still further to the right.  Furthermore, as the IPCC report use confidence levels centered on median values rather than average values of ECS, as illustrated by the third image, when the IPCC ignores many ice-climate feedback mechanisms for effective ECS they ignore that likelihood that the average effective ECS has shifted further to the right, which raises the risk of Earth Systems shifting into a high climate state during the Anthropocene as illustrated by the fourth attached image.


For reference see:

Edwards, T. L. et al. (2019) Revisiting Antarctic ice loss due to marine ice-cliff instability, Nature, doi:10.1038/s41586-019-0901-4

https://www.nature.com/articles/s41586-019-0901-4

&

Golledge, N. R. et al. (2019) Global environmental consequences of twenty-first-century ice-sheet melt, Nature, doi:10.1038/s41586-019-0889-9

https://www.nature.com/articles/s41586-019-0889-9

13
So a natural super-interglacial has been turned by us into a super-super-interglacial, and maybe we're even tipping the planet into a Hothouse Earth state:
https://www.pnas.org/content/115/33/8252

Again, I should have formulated more precisely, as Steffen et al 2018 do in their Hothouse Earth paper, referring to Ganopolski et al 2016:
"the rapid trajectory of the climate system over the past half-century along with technological lock in and socioeconomic inertia in human systems commit the climate system to conditions beyond the envelope of past interglacial conditions. We, therefore, suggest that the Earth System may already have passed one “fork in the road” of potential pathways, a bifurcation (near A in Fig. 1) taking the Earth System out of the next glaciation cycle (11)."

Maybe we've already passed this fork in the road, or maybe not. In risky territory we certainly seem to be.

I note that consensus climate science tends to use large ensemble of model projections (like those coming from CMIP6) in order to better address the 'signal-to noise' paradox in climate science as discussed in the linked reference (& associated linked article & first attached image).  While this may be an effect strategy to address the 'signal-to-noise' problem for ensembles of models that do not contain inherent bias in their formulations, if our current Earth Systems are subject to a cascade of Earth System tipping points changing our current 'climate state' to a 'Hothouse' Earth, then ensembles like CMIP6 could well be erring on the side of least drama.  For instance, CMIP6 excludes a large number of potential ice-climate feedback mechanism, largely by indicating that the participants calibrated their models against paleo-events that considered Heinrich and Dansgaard-Oeschger events as noise (see the second image); however, typically these ensembles cannot accurately project the super-interglacial periods cited by Lennart; which in my opinion is associated with the likely situation that super-interglacial periods are typically triggered by the collapse of marine ice sheets (such as the WAIS) while most Heinrich and Dansgaard-Oeschger events are related to ice calving from land, or land-terminating, ice sheets.  Furthermore, ensembles are not good at modeling decadal-scale feedback mechanisms; thus a MICI-type of collapse of the WAIS would likely not be simulated by the CMIP6 ensemble even if the modelers including MICI-mechanisms, which they do not:


Scaife, A.A., Smith, D. A signal-to-noise paradox in climate science. npj Clim Atmos Sci 1, 28 (2018) doi:10.1038/s41612-018-0038-4

https://www.nature.com/articles/s41612-018-0038-4

Abstract: "We review the growing evidence for a widespread inconsistency between the low strength of predictable signals in climate models and the relatively high level of agreement they exhibit with observed variability of the atmospheric circulation. This discrepancy is particularly evident in the climate variability of the Atlantic sector, where ensemble predictions using climate models generally show higher correlation with observed variability than with their own simulations, and higher correlations with observations than would be expected from their small signal-to-noise ratios, hence a ‘signal-to-noise paradox’. This unusual behaviour has been documented in multiple climate prediction systems and in the response to a number of different sources of climate variability. However, we also note that the total variance in the models is often close in magnitude to the observed variance, and so it is not a simple matter of models containing too much variability. Instead, the proportion of Atlantic climate variance that is predictable in climate models appears to be too weak in amplitude by a factor of two, or perhaps more. In this review, we provide a range of examples from existing studies to build the case for a problem that is common across different climate models, common to several different sources of climate variability and common across a range of timescales. We also discuss the wider implications of this intriguing paradox."

Extract: "We have provided a wide range of evidence for a ‘signal-to-noise paradox’ in climate science. The paradox lies in the fact that climate models are better able to predict observed climate variability than would be expected from their low signal-to-noise ratio. However, in many cases, the total amount of variability found in ensemble member simulations closely matches that found in observations, and so it is not just a simple case of models being too ‘noisy’ or containing too much variability. We instead conclude that the amplitude of predictable signals in response to boundary conditions or external forcing may be much too weak, especially in the Atlantic sector. This helps to explain why so many climate modelling studies show clear relationships between model and observations only after anomalies are ‘standardised’. These anomalously weak signals in predictions hamper the use of seasonal and decadal predictions, inhibit the validity of probabilistic and ensemble approaches and prevent the accurate estimation of forced climate variability in the Atlantic sector."

Caption for the first image: "Figure 1. This graph shows how ensemble predictions are better at predicting the real-world North Atlantic Oscillation (in black) than simulated ones (blue). The horizontal axis indicates the number of individual simulations contributing to each ensemble prediction, while the vertical axis measures the correlation between the ensemble average prediction and the year to year variations in the North Atlantic Oscillation (real or simulated)."

&

Title: "Clarity from Chaos: How Climate Models Could Be Better than We Think"

http://physicsbuzz.physicscentral.com/2019/08/clarity-from-chaos-how-climate-models.html

Extract: "Since Lorenz’s research took the field by storm, climate scientists have found the need to relinquish the traditional scientific love of causality. Weather and climate patterns are technically deterministic—if a perfect computer simulation had perfectly precise measurements of the millions of factors affecting the atmosphere, it could hypothetically predict future behavior exactly.  However, the impossibility of this task means that in practice, even a small error in measurement early on, or a slight misjudgment of, say, the number of butterflies passing through can easily compound.

Last year, Adam Scaife and Doug Smith of the UK’s Met Office published a review article in Nature Climate and Atmospheric Science which highlighted what they consider a “paradox” in ensemble predictions. The crux of the matter is this: for many models, the ensemble predictions provide a poor measure of the likelihood of a single simulated outcome… this might just suggest that chaos rules and there is little predictability, except for the surprising fact that the ensemble produces much more accurate predictions of the single real-world outcome. In other words, these models are better at predicting the real world than they are at predicting themselves!"

14
The linked reference indicates that the rate of freshening of the AABW is accelerating rapidly, thus supporting Hansen's ice-climate feedback mechanism:

Viviane V. Menezes, Alison M. Macdonald and Courtney Schatzman (25 Jan 2017), "Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean", Science Advances, Vol. 3, no. 1, e1601426, DOI: 10.1126/sciadv.1601426

http://advances.sciencemag.org/content/3/1/e1601426

Extract: "Southern Ocean abyssal waters, in contact with the atmosphere at their formation sites around Antarctica, not only bring signals of a changing climate with them as they move around the globe but also contribute to that change through heat uptake and sea level rise. A repeat hydrographic line in the Indian sector of the Southern Ocean, occupied three times in the last two decades (1994, 2007, and, most recently, 2016), reveals that Antarctic Bottom Water (AABW) continues to become fresher (0.004 ± 0.001 kg/g decade−1), warmer (0.06° ± 0.01°C decade−1), and less dense (0.011 ± 0.002 kg/m3 decade−1). The most recent observations in the Australian-Antarctic Basin show a particularly striking acceleration in AABW freshening between 2007 and 2016 (0.008 ± 0.001 kg/g decade−1) compared to the 0.002 ± 0.001 kg/g decade−1 seen between 1994 and 2007. Freshening is, in part, responsible for an overall shift of the mean temperature-salinity curve toward lower densities. The marked freshening may be linked to an abrupt iceberg-glacier collision and calving event that occurred in 2010 on the George V/Adélie Land Coast, the main source region of bottom waters for the Australian-Antarctic Basin. Because AABW is a key component of the global overturning circulation, the persistent decrease in bottom water density and the associated increase in steric height that result from continued warming and freshening have important consequences beyond the Southern Indian Ocean."

See also the associated article entitled: "Antarctic Bottom Waters Freshening at Unexpected Rate".

https://scripps.ucsd.edu/news/antarctic-bottom-waters-freshening-unexpected-rate

Extract: "Shift could disturb ocean circulation and hasten sea level rise, researchers say.

The team found that the previously detected warming trend has continued, though at a somewhat slower pace. The biggest surprise, however, was its lack of saltiness: AABW in the  region off East Antarctica’s Adélie Land has grown fresher four times faster in the past decade than it did between 1994 and 2007."

15
...
The ECS (or slow feedbacks as James Hansen refers to them) relate to the changes in albedo from melting ice sheets.  When the large ice sheets that covered North America and Europe during the ice ages, there were much, much larger albedo changes than would occur if Antarctica or Greenland were to lose their ice sheets.
...

When comparing glacial to interglacial climate sensitivity over the past 360,000 years, one also needs to consider the nonlinear impact of the equatorial Pacific SST for amplifying climate sensitivity:

Lo, L., Chang, S., Wei, K. et al. Nonlinear climatic sensitivity to greenhouse gases over past 4 glacial/interglacial cycles. Sci Rep 7, 4626 (2017) doi:10.1038/s41598-017-04031-x

https://www.nature.com/articles/s41598-017-04031-x

Abstract: "The paleoclimatic sensitivity to atmospheric greenhouse gases (GHGs) has recently been suggested to be nonlinear, however a GHG threshold value associated with deglaciation remains uncertain. Here, we combine a new sea surface temperature record spanning the last 360,000 years from the southern Western Pacific Warm Pool with records from five previous studies in the equatorial Pacific to document the nonlinear relationship between climatic sensitivity and GHG levels over the past four glacial/interglacial cycles. The sensitivity of the responses to GHG concentrations rises dramatically by a factor of 2–4 at atmospheric CO2 levels of >220 ppm. Our results suggest that the equatorial Pacific acts as a nonlinear amplifier that allows global climate to transition from deglacial to full interglacial conditions once atmospheric CO2 levels reach threshold levels."

Caption for image: "Nonlinearity of SST sensitivity to climate in the southern WPWP. The Solomon ΔSST and greenhouse gas radiative forcing ΔRFGHG data are plotted at a 1-kyr interval. Two groups (blue and red circles) were divided at a pCO2 level of 220 ppm via cluster analysis (see Methods). Purple triangles are standard deviations of the mean for ΔSST data points at a segment of radiative forcing corresponding to 10 ppm pCO2. Solid and dashed lines for each group represent the regression line and 95% confidence interval, respectively. The determined slopes are given as lines. The gray vertical bar marks the significant difference threshold for the non-linear SST changes at a pCO2 level of 220 ± 10 ppm."

16
The linked reference indicates that well oxygenated lake waters are an important, but long overlooked, source of methane emissions into the atmosphere.  Hopefully, CMIP7 will consider these findings:

Marco Günthel et al, Contribution of oxic methane production to surface methane emission in lakes and its global importance, Nature Communications (2019). DOI: 10.1038/s41467-019-13320-0

https://www.nature.com/articles/s41467-019-13320-0

Abstract: "Recent discovery of oxic methane production in sea and lake waters, as well as wetlands, demands re-thinking of the global methane cycle and re-assessment of the contribution of oxic waters to atmospheric methane emission. Here we analysed system-wide sources and sinks of surface-water methane in a temperate lake. Using a mass balance analysis, we show that internal methane production in well-oxygenated surface water is an important source for surface-water methane during the stratified period. Combining our results and literature reports, oxic methane contribution to emission follows a predictive function of littoral sediment area and surface mixed layer volume. The contribution of oxic methane source(s) is predicted to increase with lake size, accounting for the majority (>50%) of surface methane emission for lakes with surface areas >1 km2."

17
Some may not be familiar with the fact that in previous ice ages there use to be numerous marine ice sheets (of which the WAIS is the last remaining example) including in the Arctic Sea such as indicated by the accompany image of the Barents Sea Marine Ice Sheet with the associated Byornoy trough and renna.  I believe that the Byonrnoy trough system has several parallels with the Thwaites Glacier situation including: (a) the Byonrnoy trough has a large amount of sediment deposited at the base of the renna (flow stream or channel in Norwegian) in a similar manner to the sediment in and around the Thwaites grounding line; which allows the glacial ice to form a periodic seal with the sediment which periodical opens to allow burst of basal water to currently flow out for apparently periods of months; and (b) the upstream channel system branches quickly after this sediment filled area.  It should be noted that the Byrd Subglacial Basin connects directly to both the Siple Coast Ice Streams and the Weddell Sea Ice Streams, and this make the Thwaites system less stable than the Barents Sea Marine Ice Sheet was.

18
I have periodically suggested that the Jakobshavn Glacier calving front could experience a significant retreat by about +/- 2029; which might then trigger additional ice mass loss in West Antarctica due to the bipolar seesaw.  The linked reference supports the timing of such a significant ice mass loss from Jakobshavn circa 2029:

Guo, X., Zhao, L., Gladstone, R. M., Sun, S., and Moore, J. C.: Simulated retreat of Jakobshavn Isbræ during the 21st century, The Cryosphere, 13, 3139–3153, https://doi.org/10.5194/tc-13-3139-2019, 2019.

https://www.the-cryosphere.net/13/3139/2019/



I probably should have posted the first image with my immediate previous post, but nevertheless, it presents the measured monthly variations of the thickness of the CDW flow on the ASE shelf.  As the maximum thickness of CDW flow occurs in September of each year, and as the regional sea level rise around the ASE due to the northern hemisphere summer melt of the Greenland Ice Sheet, GIS (see the second image); this implies that the most likely month for the Thwaites (and nearby glaciers) to surge is September (as is what is what likely happened during the Northern Summer of 2012 when there was a lot of ice mass loss from the GIS, followed directly by a surge of ice mass loss from the gateway feeding the Thwaites Ice Tongue).

19
I have periodically suggested that the Jakobshavn Glacier calving front could experience a significant retreat by about +/- 2029; which might then trigger additional ice mass loss in West Antarctica due to the bipolar seesaw.  The linked reference supports the timing of such a significant ice mass loss from Jakobshavn circa 2029:

Guo, X., Zhao, L., Gladstone, R. M., Sun, S., and Moore, J. C.: Simulated retreat of Jakobshavn Isbræ during the 21st century, The Cryosphere, 13, 3139–3153, https://doi.org/10.5194/tc-13-3139-2019, 2019.

https://www.the-cryosphere.net/13/3139/2019/

Abstract
The early 21st century retreat of Jakobshavn Isbræ into its overdeepened bedrock trough was accompanied by acceleration to unprecedented ice stream speeds. Such dramatic changes suggested the possibility of substantial mass loss over the rest of this century. Here we use a three-dimensional ice sheet model with parameterizations to represent the effects of ice mélange buttressing, crevasse-depth-based calving and submarine melting to adequately reproduce its recent evolution. We are the first study on Jakobshavn Isbræ that solves for three-dimensional ice flow coupled with representations of hydro-fracturing-induced calving and mélange buttressing. Additionally, the model can accurately replicate interannual variations in grounding line and terminus position, including seasonal fluctuations that emerged after arriving at the overdeepened basin and the disappearance of its floating ice shelf. Our simulated ice viscosity variability due to shear margin evolution is particularly important in reproducing the large observed interannual changes in terminus velocity. We use this model to project Jakobshavn's evolution over this century, forced by ocean temperatures from seven Earth system models and surface runoff derived from RACMO, all under the IPCC RCP4.5 climate scenario. In our simulations, Jakobshavn's grounding line continues to retreat ∼18.5 km by the end of this century, leading to a total mass loss of ∼2068 Gt (5.7 mm sea level rise equivalent). Despite the relative success of the model in simulating the recent behavior of the glacier, the model does not simulate winter calving events that have become relatively more important.

Caption for image: "Figure 8Upper and lower estimates of July front positions within this century with colors indicating the date (color bar) for (a) lower bound with scalings of (1,0.8 ) and the HadGEM-ES forcing (b) upper bound of mass loss projection with (α, γ) parameter scalings of (1.2,1) and the seven-model-ensemble climate forcing."

20
As a follow-on to my last post about SLR, I provide the following links to visualization aids to help readers better understand some of the impacts of the risks of abrupt multi-meter SLR (e.g. see the attached image)

Title: "Use these tools to help visualize the horror of rising sea levels"

https://www.theverge.com/2019/2/17/18223808/climate-change-sea-level-rising-data-visualization-environment

&

https://ss2.climatecentral.org/#12/40.7298/-74.0070?show=satellite&projections=0-K14_RCP85-SLR&level=5&unit=feet&pois=hide

&

https://ss2.climatecentral.org/#12/40.7298/-74.0070?show=satellite&projections=0-K14_RCP85-SLR&level=5&unit=feet&pois=hide

21
I have not seen much information provided on sea level rise in the Arctic Ocean, thus I provide the following links and attached associated image.  While, the average SLR in the Arctic Ocean is below the global average the attached image makes it clear that this is due to the negative SLR associated with ice mass loss from Greenland and coastal glaciers; while other portions of the Arctic Ocean have rates of SLR that are two to three times greater than the global average.  For example, the image indicates that the coast of northern Norway may be subjected to accelerated SLR and associated erosion:

Stine Kildegaard Rose et al. Arctic Ocean Sea Level Record from the Complete Radar Altimetry Era: 1991–2018, Remote Sensing (2019). DOI: 10.3390/rs11141672

https://www.mdpi.com/2072-4292/11/14/1672

Abstract: "In recent years, there has been a large focus on the Arctic due to the rapid changes of the region. Arctic sea level determination is challenging due to the seasonal to permanent sea-ice cover, lack of regional coverage of satellites, satellite instruments ability to measure ice, insufficient geophysical models, residual orbit errors, challenging retracking of satellite altimeter data. We present the European Space Agency (ESA) Climate Change Initiative (CCI) Technical University of Denmark (DTU)/Technischen Universität München (TUM) sea level anomaly (SLA) record based on radar satellite altimetry data in the Arctic Ocean from the European Remote Sensing satellite number 1 (ERS-1) (1991) to CryoSat-2 (2018). We use updated geophysical corrections and a combination of altimeter data: Reprocessing of Altimeter Product for ERS (REAPER) (ERS-1), ALES+ retracker (ERS-2, Envisat), combination of Radar Altimetry Database System (RADS) and DTUs in-house retracker LARS (CryoSat-2). Furthermore, this study focuses on the transition between conventional and Synthetic Aperture Radar (SAR) altimeter data to make a smooth time series regarding the measurement method. We find a sea level rise of 1.54 mm/year from September 1991 to September 2018 with a 95% confidence interval from 1.16 to 1.81 mm/year. ERS-1 data is troublesome and when ignoring this satellite the SLA trend becomes 2.22 mm/year with a 95% confidence interval within 1.67–2.54 mm/year. Evaluating the SLA trends in 5 year intervals show a clear steepening of the SLA trend around 2004. The sea level anomaly record is validated against tide gauges and show good results. Additionally, the time series is split and evaluated in space and time."

See also:

Title: "New study reports sea level rise in the Arctic"

https://phys.org/news/2019-07-sea-arctic.html

Extract: "Over the past 22 years, sea levels in the Arctic have risen an average of 2.2 millimeters per year. This is the conclusion of a Danish-German research team after evaluating 1.5 billion radar measurements from satellites using specially developed algorithms.

The enormous volumes of fresh water released in the Arctic not only raise the sea level, they also have the potential to change the system of global ocean currents—and thus, our climate."

22
The linked reference makes various approximate estimates of the influence of iceberg calving on the retreat of the Thwaites Glacier.  I would take these as a lower bound estimates as they do not consider the current fragile/fractured nature of the Thwaites Ice Tongue; nor the risk that the currently grounded ice at the base of the ice tongue could simply float away in coming years due to thinning of the ice in this area:

Hongju Yu et al. (04 December 2019), "Impact of iceberg calving on the retreat of Thwaites Glacier, West Antarctica over the next century with different calving laws and ocean thermal forcing", Geophysical Research Letters, https://doi.org/10.1029/2019GL084066

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL084066

Abstract
Thwaites Glacier, West Antarctica, has been a major contributor to global sea level rise over the past decades. Prior studies illustrated the critical role of ice shelf melt and iceberg calving based on cliff height in driving the retreat of Thwaites glacier. Here, we simulate its evolution with various calving laws and rates of frontal melt by the ocean in the absence of a buttressing ice shelf. Over the next century, we find that volume losses increase by 15‐160% with a von Mises calving law compared to the case where the initial ice shelf is kept and the ice front is fixed at its current position, 10‐20% with a buoyancy‐driven calving law, and 5‐50% with frontal melt caused by ocean thermal forcing. Bed topography exerts the ultimate control on the evolution of Thwaites. In all simulations, once Thwaites Glacier retreats past the western subglacial ridge, the retreat becomes rapidly unstoppable.

23
The linked reference on the in situ mechanical properties of shallow gas hydrates in the seafloor states: "… we were able to observe the hydrate undergoing a catastrophic brittle failure".  To me this highlights the risk that free natural gas accumulating beneath an essentially impermeable shallow gas hydrate cap may eventually develop sufficient pressure to cause a 'catastrophic brittle failure' of the hydrate cap; which would likely result in an abrupt release of the free natural gas beneath the failed cap that would cause a gas bubble that would like reach the atmosphere without much absorption of the methane by the seawater, even in relatively deep continental shelves.  This is yet another risk factor than needs to be considered as the MOC slows and delivers more heat to numerous continental shelves around the world (including in the Arctic Ocean):
 
Jun Yoneda et al. (04 December 2019), "In situ mechanical properties of shallow gas hydrate deposits in the deep seabed", Geophysical Research Letters, https://doi.org/10.1029/2019GL084668

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL084668

Abstract

Natural gas hydrates (or methane hydrates) could become a major energy source but could also exacerbate global warming, because as the climate warms, hydrate deposits deep under the oceans or in permafrost may release methane into the atmosphere. There are many shallow deposits of gas hydrates in fine‐grained muddy sediments on the seafloor. However, the mechanical properties of these sediments have not yet been investigated because of the engineering challenges in coring and testing at in situ temperatures and pressures. Here we present the first uniaxial and triaxial strength and stiffness measurements of pure massive natural gas hydrates and muddy sediments containing hydrate nodules obtained by pressure coring. As a result, we were able to observe the hydrate undergoing a catastrophic brittle failure. Its strength and deformation moduli were 3 and 300 MPa, respectively. Muddy sediments containing hydrate nodules had the same strength as that of hydrate‐free sediments.

24
Talking about the implications of any given climate model, or any suite of model projections, is very tricky as '... all models are wrong but some models are useful".  Thus when reading the information from the first linked reference (& associated article), please bear in mind the limitations of the cited analyses and also the message from my immediate prior post (Reply #2131) that a CERN-type of global modeling effort (comparable to E3SM on steroids) following physical laws is justified by the many climate uncertainties:

Zeke Hausfather et al. (04 December 2019), "Evaluating the performance of past climate model projections", Geophysical Research Letters, https://doi.org/10.1029/2019GL085378

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL085378

Abstract
Retrospectively comparing future model projections to observations provides a robust and independent test of model skill. Here we analyse the performance of climate models published between 1970 and 2007 in projecting future global mean surface temperature (GMST) changes. Models are compared to observations based on both the change in GMST over time and the change in GMST over the change in external forcing. The latter approach accounts for mismatches in model forcings, a potential source of error in model projections independent of the accuracy of model physics. We find that climate models published over the past five decades were skillful in predicting subsequent GMST changes, with most models examined showing warming consistent with observations, particularly when mismatches between model‐projected and observationally‐estimated forcings were taken into account.

Plain Language Summary

Climate models provide an important way to understand future changes in the Earth's climate. In this paper we undertake a thorough evaluation of the performance of various climate models published between the early 1970s and the late 2000s. Specifically, we look at how well models project global warming in the years after they were published by comparing them to observed temperature changes. Model projections rely on two things to accurately match observations: accurate modeling of climate physics, and accurate assumptions around future emissions of CO2 and other factors affecting the climate. The best physics‐based model will still be inaccurate if it is driven by future changes in emissions that differ from reality. To account for this, we look at how the relationship between temperature and atmospheric CO2 (and other climate drivers) differs between models and observations. We find that climate models published over the past five decades were generally quite accurate in predicting global warming in the years after publication, particularly when accounting for differences between modeled and actual changes in atmospheric CO2 and other climate drivers. This research should help resolve public confusion around the performance of past climate modeling efforts, and increases our confidence that models are accurately projecting global warming.

See also:

Title: "Early climate modelers got global warming right, new report finds"

https://news.berkeley.edu/2019/12/04/early-climate-modelers-got-global-warming-right-new-report-finds/

Extract: "Climate skeptics have long raised doubts about the accuracy of computer models that predict global warming, but it turns out that most of the early climate models were spot-on, according to a look-back by climate scientists at the University of California, Berkeley, Massachusetts Institute of Technology and NASA.

Of 17 climate models published between the early 1970s and the late 2000s, 14 were quite accurate in predicting the average global temperature in the years after publication, said Zeke Hausfather, a doctoral student in UC Berkeley’s Energy and Resources Group and lead author of a new paper analyzing the models.

“The real message is that the warming we have experienced is pretty much exactly what climate models predicted it would be as much as 30 years ago,” he said. “This really gives us more confidence that today’s models are getting things largely right as well.”

One of the iconic climate models, and one that first brought the issue of climate change to broad public attention, was published by James Hansen of NASA in 1988. However, his predictions for temperatures after 1988 were 50% higher than the actual global mean temperatures in those years.

That is in part because Hanson did not anticipate the Montreal Protocol, a treaty that went into effect in 1989 and which banned chlorofluorocarbons, which are potent greenhouse gases. His estimates of future methane emissions were also off, Hausfather said.

“If you account for these and look at the relationship in his model between temperature and radiative forcing, which is CO2 and other greenhouse gases, he gets it pretty much dead on,” he said. “So the physics of his model was right. The relationship between how much CO2 there is in the atmosphere and how much warming you get, was right. He just got the future emissions wrong.”"
&

Title: "Even 50-year-old climate models correctly predicted global warming"

https://www.sciencemag.org/news/2019/12/even-50-year-old-climate-models-correctly-predicted-global-warming

Extract: "Climate change doubters have a favorite target: climate models. They claim that computer simulations conducted decades ago didn’t accurately predict current warming, so the public should be wary of the predictive power of newer models. Now, the most sweeping evaluation of these older models—some half a century old—shows most of them were indeed accurate.

“How much warming we are having today is pretty much right on where models have predicted,” says the study’s lead author, Zeke Hausfather, a graduate student at the University of California, Berkeley."

&

J. Hansen et al. (28 Aug 1981), "Climate Impact of Increasing Atmospheric Carbon Dioxide", Science, Vol. 213, Issue 4511, pp. 957-966, DOI: 10.1126/science.213.4511.957

https://science.sciencemag.org/content/213/4511/957

Abstract
The global temperature rose by 0.2°C between the middle 1960's and 1980, yielding a warming of 0.4°C in the past century. This temperature increase is consistent with the calculated greenhouse effect due to measured increases of atmospheric carbon dioxide. Variations of volcanic aerosols and possibly solar luminosity appear to be primary causes of observed fluctuations about the mean trend of increasing temperature. It is shown that the anthropogenic carbon dioxide warming should emerge from the noise level of natural climate variability by the end of the century, and there is a high probability of warming in the 1980's. Potential effects on climate in the 21st century include the creation of drought-prone regions in North America and central Asia as part of a shifting of climatic zones, erosion of the West Antarctic ice sheet with a consequent worldwide rise in sea level, and opening of the fabled Northwest Passage.

Edit, see also:

http://www.realclimate.org/index.php/archives/2019/12/how-good-have-climate-models-been-at-truly-predicting-the-future/


25
The linked reference is essentially calling for an international effort (comparable to CERN) to exceed the E3SM program to use exascale computing to adequately model physical laws of the numerous Earth Systems:

Tim Palmer el al., "The scientific challenge of understanding and estimating climate change," PNAS (2019). www.pnas.org/cgi/doi/10.1073/pnas.1906691116

https://www.pnas.org/content/116/49/24390

Abstract
Given the slow unfolding of what may become catastrophic changes to Earth’s climate, many are understandably distraught by failures of public policy to rise to the magnitude of the challenge. Few in the science community would think to question the scientific response to the unfolding changes. However, is the science community continuing to do its part to the best of its ability? In the domains where we can have the greatest influence, is the scientific community articulating a vision commensurate with the challenges posed by climate change? We think not.

Extract: "The idea that the science of climate change is largely “settled,” common among policy makers and environmentalists but not among the climate science community, has congealed into the view that the outlines and dimension of anthropogenic climate change are understood and that incremental improvement to and application of the tools used to establish this outline are sufficient to provide society with the scientific basis for dealing with climate change.

As climate scientists, we are rightfully proud of, and eager to talk about, our contribution to settling important and long-standing scientific questions of great societal relevance. What we find more difficult to talk about is our deep dissatisfaction with the ability of our models to inform society about the pace of warming, how this warming plays out regionally, and what it implies for the likelihood of surprises. In our view, the political situation, whereby some influential people and institutions misrepresent doubt about anything to insinuate doubt about everything, certainly contributes to a reluctance to be too openly critical of our models. Unfortunately, circling the wagons leads to false impressions about the source of our confidence and about our ability to meet the scientific challenges posed by a world that we know is warming globally.

The development of this new generation of models should be sustained, multinational, and coordinated as a flagship application of high-performance computing and information technology. Only as a coordinated technology project will it be possible to meet the computational challenges of running the highest possible resolution models and accessing their full information content. How to structure such an initiative can be debated; indisputable is the necessity to endow it with the same sense of purpose that has made past grand scientific challenges—from weather forecasting to moon landings—so successful.

As our nonlinear world moves into uncharted territory, we should expect surprises. Some of these may take the form of natural hazards, the scale and nature of which are beyond our present comprehension. The sooner we depart from the present strategy, which overstates an ability to both extract useful information from and incrementally improve a class of models that are structurally ill suited to the challenge, the sooner we will be on the way to anticipating surprises, quantifying risks, and addressing the very real challenge that climate change poses for science. Unless we step up our game, something that begins with critical self-reflection, climate science risks failing to communicate and hence realize its relevance for societies grappling to respond to global warming."

See also:

Title: "A CERN for climate change"

https://phys.org/news/2019-12-cern-climate.html

Extract: "Asked whether he feared their critique of the present state of Earth system modelling might be exploited by those attempting to cast doubt on present understanding of global warming, Stevens replies: "It is important that scientists speak candidly. It shouldn't come as a surprise that we can understand some things (like the world is warming because of human activities) but not everything (like what this warming means for regional changes in weather, extremes, and the habitability of the planet). By not talking about the limits of our understanding we run the risk of failing to communicate the need for new scientific approaches, just when they are needed most."

When asked whether spending new money on such an international climate modelling initiative can be justified, Professor Palmer said: "By comparison with new particle colliders or space telescopes, the amount needed, maybe around $100 million per year, is very modest indeed. In addition, the benefit/cost ratio to society of having a much clearer picture of the dangers we are facing in the coming decades by our ongoing actions, seems extraordinarily large. To be honest, all is needed is the will to work together, across nations, on such a project. Then it will happen.""

Edit: I note that consensus climate science is not particularly 'settled' science.

26
The linked reference provides numerous warning about the local impacts of global warming in the Arctic (I note that the statements about Antarctica are more general in nature as the authors do not adequately differentiate between surface warming in East and West, Antarctica):

Eric Post et al. (04 Dec 2019), "The polar regions in a 2°C warmer world", Science Advances, Vol. 5, no. 12, eaaw9883, DOI: 10.1126/sciadv.aaw9883

https://advances.sciencemag.org/content/5/12/eaaw9883

Abstract
Over the past decade, the Arctic has warmed by 0.75°C, far outpacing the global average, while Antarctic temperatures have remained comparatively stable. As Earth approaches 2°C warming, the Arctic and Antarctic may reach 4°C and 2°C mean annual warming, and 7°C and 3°C winter warming, respectively. Expected consequences of increased Arctic warming include ongoing loss of land and sea ice, threats to wildlife and traditional human livelihoods, increased methane emissions, and extreme weather at lower latitudes. With low biodiversity, Antarctic ecosystems may be vulnerable to state shifts and species invasions. Land ice loss in both regions will contribute substantially to global sea level rise, with up to 3 m rise possible if certain thresholds are crossed. Mitigation efforts can slow or reduce warming, but without them northern high latitude warming may accelerate in the next two to four decades. International cooperation will be crucial to foreseeing and adapting to expected changes.

Extract: "In contrast to the GIS, major mass loss over the coming decades from surface runoff is not expected for Antarctica under RCP4.5 or greater emissions (62). However, ongoing mass loss was recently triggered when warmer ocean waters thinned ice shelves, reducing their buttressing effect, allowing for faster flow of nonfloating ice into the ocean [reviewed in (71)]. Sufficient warming to trigger GIS-type ice-shelf loss and tidewater-calving retreat could contribute substantially to sea level rise in the next ~100 years especially from WAIS, even if iceberg calving is limited to rates already exceeded locally in GIS, owing to the much wider WAIS calving front that could develop (72, 73). In addition, because WAIS could produce higher cliffs with less drag from fjord sides than in the GIS, and thus greater stress imbalances driving calving, even faster sea level rise is possible (71).

Within the WAIS, Thwaites Glacier has undergone notably rapid ice loss and appears particularly vulnerable to accelerated ice loss with increased ice-shelf basal melt. In a recent comparison of two simplified model scenarios representing “constant climate” and “warming climate,” Thwaites Glacier collapsed in 80% of constant climate experiments and in 100% of warming climate experiments (74). Collapse of Thwaites Glacier and other Antarctic sources could contribute more than 3 m to global sea level rise over a time span that is poorly characterized but could be less than a century following initiation if ice-shelf loss and cliff retreat become important (72, 75). Further warming could extend these processes into marine basins of EAIS, potentially adding an additional 12 m or more of sea level rise further in the future (72). Geoengineering solutions have been proposed (76), but grave difficulties remain.

Recent work (77, 78) suggests that past ice sheet fluctuations can be modeled without invoking ice-shelf loss and subsequent cliff failure, favoring models that give smaller or slower sea level rise than calculated by some studies (72), but essentially all ice that flows into the ocean ends in calving cliffs. Ice-shelf loss has been observed in several cases with subsequent flow acceleration (75), so models lacking cliff physics are omitting known processes that are critical to ice loss. Uncertainties are very large on many aspects of this topic, including poor knowledge of the threshold warming of ocean or atmosphere needed to trigger major ice-shelf loss for vulnerable drainages. Large, rapid sea level rise under strong warming thus remains possible but unproven."

27
The linked reference indicates that the Ross Ice Shelf, RIS, is currently being stabilized by 'nails' from adjoining EAIS outlet glaciers (like Byrd Glacier) that are pinning the ice shelf to the Transantarctic Mountains.  Thus destabilization of the RIS would accelerated due to the extraction of the 'nails' associated with continued global warming:

Terence Hughes, Zihong Zhao, Raymond Hintz & James Fastook (27 May 2017), "Instability of the Antarctic Ross Sea Embayment as climate warms", Reviews of Geophysics, DOI: 10.1002/2016RG000545

http://onlinelibrary.wiley.com/doi/10.1002/2016RG000545/abstract

Abstract: "Collapse of the Antarctic Ice Sheet since the Last Glacial Maximum 18,000 years ago is most pronounced in the Ross Sea Embayment, which is partly ice-free during Antarctic summers, thereby breaching the O-ring of ice shelves and sea ice surrounding Antarctica that stabilizes the ice sheet. The O-ring may have vanished during Early Holocene (5000 to 3000 B.C.), Roman (1 to 400 A.D.), and Medieval (900 to 1300 A.D.) warm periods and reappeared during the Little Ice Age (1300 to 1900 A.D.). We postulate further collapse in the embayment during the post-1900 warming may be forestalled because East Antarctic outlet glaciers “nail” the Ross Ice Shelf to the Transantarctic Mountains so it can resist the push from West Antarctic ice streams. Our hypothesis is examined for Byrd Glacier and a static ice shelf using three modeling experiments having plastic, viscous, and viscoplastic solutions as more data and improved modeling became available. Observed crevasse patterns were not reproduced. A new research study is needed to model a dynamic Ross Ice Shelf with all its feeder ice streams, outlet glaciers, and ice calving dynamics in three dimensions over time to fully test our hypothesis. The required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf. Calving must then successively pull the outlet glacier “nails” so collapse of the marine West Antarctic Ice Sheet proceeds to completion."


29
...
1967 - Hansen warns Congress - but no number. Instead debate was about  what would happen if CO2 ppm doubled to 550 ppm.


Indeed, I was vaguely thinking of Hansen's 1988 Congressional testimony (not 1967), when Hansen presented the attached image (with 1960 as a baseline; which is close to a pre-industrial baseline) from the first linked article (that incorrectly thought that Hansen had over estimated climate sensitivity, while in fact GMSTA with a pre-industrial baseline will be over 1.1C in 2019; which, as the second link indicates shows that Hansen's model was spot on):

Title: "What do we learn from James Hansen's 1988 prediction?"

https://skepticalscience.com/Hansen-1988-prediction.htm

&

Title: "Judgment on Hansen's '88 climate testimony: 'He was right' "

https://www.yaleclimateconnections.org/2018/06/judgment-on-hansens-88-climate-testimony-he-was-right/

Extract: "Hansen in that 1988 congressional testimony nailed it, adds Texas A&M scientist Andrew Dessler. “You could have reached an alternative conclusion” based on the science at that time, he says, pointing to the 1990 IPCC conclusion that the observed warming at that point was consistent with global warming evidence, but also with natural variability."

Finally, as Lennart points-out the 350 ppm value was actually first cited in the following 2008 reference:

Hansen, J., Mki. Sato, P. Kharecha, D. Beerling, R. Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D.L. Royer, and J.C. Zachos, 2008: Target atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231, doi:10.2174/1874282300802010217.

30
The attached image is from the linked NOAA reference, which indicates a global mean sea level rise of 2.5m for the central 90% probability range augmented by MICI-type of Antarctic SLR contribution for RCP 8.5 to 2100.

Title: "Global and Regional Sea Level Rise Scenarios for the United States", January 2017.

https://tidesandcurrents.noaa.gov/publications/techrpt83_Global_and_Regional_SLR_Scenarios_for_the_US_final.pdf

While this does not constitute proof of abrupt sea level rise; it does constitute an official warning from NOAA of this possibility.

31
Didn´t we prove that for CO2 too?

I'm pretty sure.  But a lot of posters on this forum seem to think that we're doomed and it's too late to do anything.

In the early 1980's Hansen warned policy makers that the atmospheric CO2 concentration should not exceed about 350 ppm.  However (ignoring that the AGGI uses a GMP100 for methane of 25 instead of AR5's value of 36), the attached Annual Greenhouse Gas Index, AGGI, shows that since 1990 the radiative forcing from GHG has increased by 43% to 2018.  So I believe there is reason to question the resolve of policy makers to take action appropriate for the current level of climate change risk.

32

The Hansen paper I linked to specifically differentiated between the fast feedbacks (Charney sensitivity) and slow feedback ECS due to loss of ice sheets.  It also included a discussion of timeframes for ice sheet response to forcings and the fact that we can still avoid those impacts by reducing ghg emissions and stabilizing, then reducing, ghg atmospheric concentrations.

You appear to continue to ignore the science on the timing of ice sheet feedbacks.  Even in extreme models with instantaneous 2 degree increases in ocean temperature, continued forcing at the unrealistic RCP8.5 scenario and inclusion of the highly speculative MICI model, the Larsen C shelf wouldn't disintegrate until the 2050s and the Admunsen shelves in front of the PIG and Thwaites portions of the WAIS wouldn't go until the 2100s.

As a reminder the first image indicates that when the radiative forcing imbalance is about 4 W2/m above pre-industrial that the combined fast and slow feedbacks will result in ECS of about 7C; while the second image (from Hansen et al. - 2016)shows that the radiative forcing imbalance could reach 4 W2/m by about 2060 if the WAIS were to begin collapsing circa 2040-2045. 

The third image shows that only a few meters of hf (height above floatation) is holding down the grounded ice at the base of the Thwaites Ice Tongue (see the fourth image).  Thus, if the few meters of hf were to be overcome say due to ice thinning and ice melting (from mCDW), and the Thwaites Ice Tongue were to collapse (note the pending area of spalling shown in the fourth image), then all of the buttressing ice from the base of the ice tongue to the seafloor trough leading straight into the Byrd Subglacial Basin, BSB, could float away; which could well trigger an MICI-type of failure of the glacial ice in the BSB before 2045.

33
James Hansen produced a very easy to read, plain language paper (in support of a legal brief for a lawsuit, not a peer-reviewed science study) in 2018 that summarizes climate change.  It has a very good explanation of equilibrium climate sensitivity and slow feedbacks.

http://www.columbia.edu/~jeh1/mailings/2018/20181206_Nutshell.pdf

...

Of course the equilibrium climate sensitivity that Hansen is referring to in that paper is associated only with fast feedbacks (see dotted curve in the attached image, from Hansen and Sato - 2012); while it is open for discussion in this thread how fast the ice-climate feedbacks (such as albedo changes, see the solid curve in the attached image) will occur, particularly if the WAIS were to collapse abruptly in the coming decades.

34
Per the linked Twitter feed and associated linked analysis, Andrew Dessler stated:

"The first term is the unforced pattern effect, which is due to internal variability over the 20th century. The second is the forced pattern effect, due to the fact that the surface warming pattern during transient warming is not the same as the pattern at equilibrium.  Together, they could bias ECS measured over the 20th century low by about 0.5 K."

"Potential problems measuring climate sensitivity from the historical record", by Andrew Dressler, submitted to the Journal of Climate in 2019.

https://twitter.com/AndrewDessler/status/1202004214773190657

Abstract
This study investigates potential biases between equilibrium climate sensitivity inferred from warming over the historical period (ECShist) and the climate system’s true ECS (ECStrue). This paper focuses on two factors that could contribute to differences between these quantities. First is the impact of internal variability over the historical period: our historical climate record is just one of an infinity of possible trajectories, and these different trajectories can generate ECShist values 0.3 K below to 0.5 K above (5-95% confidence interval) the average ECShist. Because this spread is due to unforced variability, I refer to this as the unforced pattern effect. This unforced pattern effect in the model analyzed here is traced to unforced variability in loss of sea ice, which affects the albedo feedback, and to unforced variability in warming of the troposphere, which affects the short-wave cloud feedback. There is also a forced pattern effect that causes ECShist to depart from ECStrue due to differences between today’s transient pattern of warming and the pattern of warming at 2xCO2 equilibrium. Changes in the pattern of warming lead to a strengthening low-cloud feedback as equilibrium is approached in regions where surface warming is delayed: the Southern Ocean, East Pacific, and North Atlantic near Greenland. This forced pattern effect causes ECShist to be on average 0.2 K lower than ECStrue (~8%). The net effect of these two pattern effects together can produce an estimate of ECShist as much as 0.5 K below ECStrue.

35
I note that while the global average CO2 emission per capita has been essentially flat for the past decade, global fossil fuel emissions keep rising due to both population growth and due to economic growth in China:

Title: "Analysis: Global fossil-fuel emissions up 0.6% in 2019 due to China"

https://www.carbonbrief.org/analysis-global-fossil-fuel-emissions-up-zero-point-six-per-cent-in-2019-due-to-china

Extract: "After increasing at the fastest rate for seven years in 2018, global CO2 emissions are set to rise much more slowly this year – but will, nevertheless, reach another record high."

36
As SO2 emissions both have a significant negative impact on human heath and as it is easy to reduce such emissions using existing technology than are associated GHG emissions (see the linked article about reducing SO2 emissions from shipping); it is reasonable to assume that the trend of increasing SO2 emissions will be curtailed sooner that will be GHG emissions.  While from a health point of view this is good news; however, from a climate change risk perspective it is bad news as SO2 emissions represent a negative radiative forcing:

Title: "Shipping sector gears itself for new emissions regulations"

https://www.dw.com/en/shipping-sector-gears-itself-for-new-emissions-regulations/a-50836212

Extract: "From January 1, 2020, ships will be required by the IMO to reduce SO2 emissions by more than 80%. The options for doing so vary, from installing sulphur-removing technology known as scrubbers to moving away from oil-based fuel entirely. What doesn't vary all that much is the cost — complying with the new rules will be expensive.

This forthcoming measure only deals with sulphur though. Measures against carbon pollution, altogether harder to conceive and enforce, are yet to come.

Compliance is expected to be high."
For reference I provide the attached plot of global SO2 emissions (that generate a negative feedback) thru 2018 from the linked website.  It will be interesting to see how this timeseries progresses in the future:

https://openclimatedata.github.io/global-emissions/

37
The linked reference demonstrates that small changes in ice thickness on the edge of Antarctic ice shelves (like RIS) can induce thinning over distances of more than 900km of the rest of the ice shelf (see the attached image), which also reduces the buttressing action on the adjoining marine glacier:

R. Reese et al (2017), "The far reach of ice-shelf thinning in Antarctica", Nature Climate Change, doi:10.1038/s41558-017-0020-x

http://www.nature.com/articles/s41558-017-0020-x

Abstract: "Floating ice shelves, which fringe most of Antarctica’s coastline, regulate ice flow into the Southern Ocean. Their thinning or disintegration can cause upstream acceleration of grounded ice and raise global sea levels. So far the effect has not been quantified in a comprehensive and spatially explicit manner. Here, using a finite-element model, we diagnose the immediate, continent-wide flux response to different spatial patterns of ice-shelf mass loss. We show that highly localized ice-shelf thinning can reach across the entire shelf and accelerate ice flow in regions far from the initial perturbation. As an example, this ‘tele-buttressing’ enhances outflow from Bindschadler Ice Stream in response to thinning near Ross Island more than 900 km away. We further find that the integrated flux response across all grounding lines is highly dependent on the location of imposed changes: the strongest response is caused not only near ice streams and ice rises, but also by thinning, for instance, well-within the Filchner–Ronne and Ross Ice Shelves. The most critical regions in all major ice shelves are often located in regions easily accessible to the intrusion of warm ocean waters, stressing Antarctica’s vulnerability to changes in its surrounding ocean."

See also:
https://www.pik-potsdam.de/news/press-releases/tiny-ice-losses-at-antarctica2019s-fringes-can-accelerate-ice-loss-far-away

Caption: "Ross Ice Shelf: changes in speed resulting from 1m thinning (red: area of thinning, blue shading: resulting change in ice flow speed, ocean in grey). Fig. 2b from Reese et al, 2017"

38
I provide the linked open access reference that indicates that no later than 2070 we can expect the Filchner-Ronne Ice Shelf to be subjected to an marked increase of warm under-base water flow from off-shelf sources that will accelerate basal melting and calving.

Hartmut H. Hellmer, Frank Kauker, Ralph Timmermann, and Tore Hattermann (2017), "The Fate of the Southern Weddell Sea Continental Shelf in a Warming Climate:, Journal of Climate, doi: 10.1175/JCLI-D-16-0420.1

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0420.1

Abstract: "Warm water of open ocean origin on the continental shelf of the Amundsen and Bellingshausen Seas causes the highest basal melt rates reported for Antarctic ice shelves with severe consequences for the ice shelf/ice sheet dynamics. Ice shelves fringing the broad continental shelf in the Weddell and Ross Seas melt at rates orders of magnitude smaller. However, simulations using coupled ice–ocean models forced with the atmospheric output of the HadCM3 SRES-A1B scenario run (CO2 concentration in the atmosphere reaches 700 ppmv by the year 2100 and stays at that level for an additional 100 years) show that the circulation in the southern Weddell Sea changes during the twenty-first century. Derivatives of Circumpolar Deep Water are directed southward underneath the Filchner–Ronne Ice Shelf, warming the cavity and dramatically increasing basal melting. To find out whether the open ocean will always continue to power the melting, the authors extend their simulations, applying twentieth-century atmospheric forcing, both alone and together with prescribed basal mass flux at the end of (or during) the SRES-A1B scenario run. The results identify a tipping point in the southern Weddell Sea: once warm water flushes the ice shelf cavity a positive meltwater feedback enhances the shelf circulation and the onshore transport of open ocean heat. The process is irreversible with a recurrence to twentieth-century atmospheric forcing and can only be halted through prescribing a return to twentieth-century basal melt rates. This finding might have strong implications for the stability of the Antarctic ice sheet."

39
The first attached image from a 2002 National Geographic map of the Antarctic shows both the sea ice movement and the typical wind flow patterns (circa 2000 to 2001).  The sea ice movement indicates how both the Weddell and the Ross Sea areas manufacture and export sea ice; while the wind pattern shows how some snowfall could be blown into the ocean.

The second attached image from a 2002 National Geographic map of the Antarctic shows how the frontal zone of the RIS (Ross Ice Shelf) in the indicated area is subject to accelerated calving due to local upwelling of warm deep water.

40

 If an MICI-type of failure pushes an armada of icebergs into the Southern Ocean some decade from now, maybe a massive application of this technology could put the icebergs to beneficial use while concurrently cooling potential increases in tropical ocean SST values:


If some readers believe that it is totally unrealistic to assume that there could be sufficient economic benefits to pay for moving an armada of Southern Ocean icebergs to the tropical oceans (not only for freshwater supply but possibly for: OTEC (ocean thermal energy conversion); cooling of waste heat, etc.); then consider that:

1. Per the first image a nudge from oceangoing tugboats could push an iceberg out of the ACC and into one of the three indicated cold currents leading north from the Southern Ocean into the Pacific, Atlantic and Indian Oceans.

2. Per the second image these three cold current feed into warm currents that would carry any such icebergs directly to the tropical ocean regions of all three oceans without towing, and would slowly melt along the way, thus both cooling and freshening the surface waters of the tropical oceans.

3. Per the third image (showing a representative thermocline profile for tropical ocean regions) cooling of the SSTA in these regions would both slow/stop increase surface evaporation associated with global warming and for at least decades would prevent the tropical oceans SST from increase by 5C; which is projected to lead to an equable climate.

Such a form of geoengineering would be much less expensive than other currently conceived forms of geoengineering.

As much of the plankton growth in the oceans is limited by iron availability (particularly in the Southern Ocean), and as Antarctic icebergs contain significant quantities of iron, and thus it is likely that an armada of icebergs in the Southern Ocean due to a potential collapse of the WAIS in coming decades could produce an excess of iceberg iron flux.  Thus, if vessels were used to deflect significant numbers of icebergs from such an armada into surface currents leading to the tropical oceans, not only would such iceberg cool the SSTA of these tropical oceans, but would also contribute to plankton blooms that would help to sequester atmospheric carbon on the seafloors of the tropical oceans as would the icebergs remain in the armada w.r.t. the seafloor of the Southern Ocean:

Hopwood, M.J., Carroll, D., Höfer, J. et al. Highly variable iron content modulates iceberg-ocean fertilisation and potential carbon export. Nat Commun 10, 5261 (2019) doi:10.1038/s41467-019-13231-0

https://www.nature.com/articles/s41467-019-13231-0?draft=marketing

Abstract
Marine phytoplankton growth at high latitudes is extensively limited by iron availability. Icebergs are a vector transporting the bioessential micronutrient iron into polar oceans. Therefore, increasing iceberg fluxes due to global warming have the potential to increase marine productivity and carbon export, creating a negative climate feedback. However, the magnitude of the iceberg iron flux, the subsequent fertilization effect and the resultant carbon export have not been quantified. Using a global analysis of iceberg samples, we reveal that iceberg iron concentrations vary over 6 orders of magnitude. Our results demonstrate that, whilst icebergs are the largest source of iron to the polar oceans, the heterogeneous iron distribution within ice moderates iron delivery to offshore waters and likely also affects the subsequent ocean iron enrichment. Future marine productivity may therefore be not only sensitive to increasing total iceberg fluxes, but also to changing iceberg properties, internal sediment distribution and melt dynamics.

See also:
Title: "We Need to Protect Antarctic ‘Blue Carbon’"

https://www.wired.com/story/opinion-we-need-to-protect-antarctic-blue-carbon/

Extract: "As marine ice is lost in coastal waters, marine algae are blooming in higher densities, taking CO2 from the atmosphere to do so. Some of this sinks to the seafloor and is buried, and some is eaten by animals who are buried upon death. This “blue carbon sink” has doubled in the last two decades. As the climate warms, ice in polar seas decreases, marine life is clawing back via carbon sequestration."


41

Given that we can measure the concentration of methane in the atmosphere and thus calculate the total net emissions (all sources minus all sinks), if two sources were underestimated that implies that another source (or multiple sources) were overestimated or that the sinks were underestimated.


The attached NOAA plot of the atmospheric methane concentrations at the South Pole from 2006 to Dec 2, 2019 indicate that the trend line of this methane concentration is accelerating; thus if some methane sources are not changing then other sources are currently accelerating, and may accelerate even more in the future due to global warming.

The linked article & reference provide further insights about the origins of the current trend of increasing methane emissions and associated atmospheric methane concentrations:

Title: "Atmospheric Methane Levels Are Going Up—And No One Knows Why"

https://www.wired.com/story/atmospheric-methane-levels-are-going-up-and-no-one-knows-why/

Extract: "Levels of heat-trapping methane are rising faster than climate experts anticipated, triggering intense debate about why it's happening

“Is atmospheric methane increasing as a consequence of climate change, not of our direct emissions? Are some thresholds being passed?”

“It is a wicked problem,” Kort adds, “but it’s not unsolvable.”

Any convincing explanation needs to answer three questions. What explains the long-term increase in methane levels over the past 40 years? Why was there a pause? And why was there such an abrupt surge after 2006? Only three elements of the global methane budget are large enough to be plausible culprits: microbial emissions (from livestock, agriculture, and wetlands); fossil fuel emissions; and the chemical process by which methane is scrubbed from the atmosphere."

See also:

Giuseppe Etiope and Stefan Schwietzke (2019), "Global geological methane emissions: an update of top-down and bottom-up estimates", Elementa Science of the Anthropocene, 7: 47, doi: https://doi.org/10.1525/elementa.383

https://www.elementascience.org/articles/10.1525/elementa.383/

Abstract

A wide body of literature suggests that geological gas emissions from Earth’s degassing are a major methane (CH4) source to the atmosphere. These emissions are from gas-oil seeps, mud volcanoes, microseepage and submarine seepage in sedimentary (petroleum-bearing) basins, and geothermal and volcanic manifestations. Global bottom-up emission estimates, ranging from 30 to 76 Tg CH4 yr–1, evolved in the last twenty years thanks to the increasing number of flux measurements, and improved knowledge of emission factors and area distribution (activity). Based on recent global grid maps and updated evaluations of mud volcano and microseepage emissions, the global geo-CH4 source is now (bottom-up) estimated to be 45 (27–63) Tg yr–1, i.e., ~8% of total CH4 sources. Top-down verifications, based on independent approaches (including ethane and isotopic observations) from different authors, are consistent with the range of the bottom-up estimate. However, a recent top-down study, based on radiocarbon analyses in polar ice cores, suggests that geological, fossil (14C-free) CH4 emissions about 11,600 years ago were much lower (<15 Tg yr–1, 95% CI) and that this source strength could also be valid today. Here, we show that (i) this geo-CH4 downward revision implies a fossil fuel industry CH4 upward revision of at least 24–35%. (ii) The 95% CI estimates of the recent radiocarbon analysis do not overlap with those of 5 out of 6 other bottom-up and top-down studies (no overlap for the 90% CI estimates). (iii) The contrasting lines of evidence require further discussion, and research opportunities exist to help explain this gap.

Caption for image: "Comparison between current day estimates of geological and other methane sources. Geological emissions are based on the bottom-up and top-down estimates discussed in this work (see Fig. 1 and text). Other natural and anthropogenic emissions refer to the average (and range) of bottom-up and top-down estimates reported by Saunois et al. (2016). Note that a downward revision of the geological source requires an upward revision of the same magnitude for the fossil fuel industry (Section 4). DOI: https://doi.org/10.1525/elementa.383.f2"

42
I note that in the linked research: "Severe testing is applied to observed global and regional surface and satellite temperatures and modelled surface temperatures to determine whether these interactions are independent, as in the traditional signal-to-noise model, or whether they interact, resulting in steplike warming."  The reference concludes that indeed steplike warming occurs due to "… a store-and-release mechanism from the ocean to the atmosphere…" like the classical Lorenzian attractor case of ENSO decadal cycles.  Such steplike behavior raises the issue of what I call "Ratcheting Quasi-static Equilibrium States" (see the attached image) that can accelerate non-linear Earth Systems response beyond the linear Earth Systems response assumed by AR5/CMIP5 researchers.  As the authors point-out such AR5/CMIP5 researcher likely missed this behavior because: "This may be due in part to science asking the wrong questions."; and they advise that such AR5/CMIP5 researchers should change how they view the output from their models.  For example, the reference shows global warming increasing much faster for a steplike response if ECS is 4.5 than for a the traditional AR5/CMIP5 interpretation; which means that ESLD researchers are exposing society to far more risk of the consequences of high ECS values than AR5/CMIP5 are leading us to believe:

Jones, R. N. and Ricketts, J. H.: Reconciling the signal and noise of atmospheric warming on decadal timescales, Earth Syst. Dynam. Discuss., doi:10.5194/esd-2016-35, in review, 2016.

http://www.earth-syst-dynam-discuss.net/esd-2016-35/
&
http://www.earth-syst-dynam-discuss.net/esd-2016-35/esd-2016-35.pdf

Abstract. Interactions between externally forced and internally generated climate variations on decadal timescales is a major determinant of changing climate risk. Severe testing is applied to observed global and regional surface and satellite temperatures and modelled surface temperatures to determine whether these interactions are independent, as in the traditional signal-to-noise model, or whether they interact, resulting in step-like warming. The multistep bivariate test is used to detect step changes in temperature data. The resulting data are then subject to six tests designed to distinguish between the two statistical hypotheses, hstep and htrend. Test 1: since the mid-20th century, most observed warming has taken place in four events: in 1979/80 and 1997/98 at the global scale, 1988/89 in the Northern Hemisphere and 1968–70 in the Southern Hemisphere. Temperature is more step-like than trend-like on a regional basis. Satellite temperature is more step-like than surface temperature. Warming from internal trends is less than 40 % of the total for four of five global records tested (1880–2013/14). Test 2: correlations between step-change frequency in observations and models (1880–2005) are 0.32 (CMIP3) and 0.34 (CMIP5). For the period 1950–2005, grouping selected events (1963/64, 1968–70, 1976/77, 1979/80, 1987/88 and 1996–98), the correlation increases to 0.78. Test 3: steps and shifts (steps minus internal trends) from a 107-member climate model ensemble (2006–2095) explain total warming and equilibrium climate sensitivity better than internal trends. Test 4: in three regions tested, the change between stationary and non-stationary temperatures is step-like and attributable to external forcing. Test 5: step-like changes are also present in tide gauge observations, rainfall, ocean heat content and related variables. Test 6: across a selection of tests, a simple stepladder model better represents the internal structures of warming than a simple trend, providing strong evidence that the climate system is exhibiting complex system behaviour on decadal timescales. This model indicates that in situ warming of the atmosphere does not occur; instead, a store-and-release mechanism from the ocean to the atmosphere is proposed. It is physically plausible and theoretically sound. The presence of step-like – rather than gradual – warming is important information for characterising and managing future climate risk.

Extract: "Climate conceptualised as a mechanistic system and described using classical statistical methods is substantially different from climate conceptualised as a complex system.
With record atmospheric and surface ocean temperatures in 2015/16 variously being described as a singular event, a reinvigoration of trend-like warming or a wholesale shift to a new climate regime, this issue is too important to be left unresolved."

43
The first linked reference studies ice-climate feedback calibrated to 'freshwater hosing' events in the North Atlantic over the past 720,000 years, in order to study state dependence of climatic instabilities within a CMIP class of climate model.  Such research can help to calibrate models (say CMIP7) for such 'freshwater hosing' events such as the possible collapse of the WAIS this century:

Ayako Abe-Ouchi, et. al. (2017), "State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling", Science Advances, Vol. 3, no. 2, e1600446, DOI: 10.1126/sciadv.1600446

http://advances.sciencemag.org/content/3/2/e1600446
&
http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/218067/1/sciadv.1600446.pdf

Abstract: "Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear.  Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets."

The Last Glacial Termination, LGT, occurred from 18,000 to 11,650 kya, and the following reference, reconstructs the dynamic response of the Antarctic ice sheets to warming in this period in order to better evaluate Hansen's ice-climate feedback mechanisms.  The abstract from the second linked reference concludes: "Given the anti-phase relationship between inter-hemispheric climate trends across the LGT our findings demonstrate that Southern Ocean-AIS feedbacks were controlled by global atmospheric teleconnections.  With increasing stratification of the Southern Ocean and intensification of mid-latitude westerly winds today, such teleconnections could amplify AIS mass loss and accelerate global sea-level rise."

Fogwill, et. al. (2017), "Antarctic ice sheet discharge driven by atmosphere-ocean feedbacks at the last Glacial Termination", Scientific Reports 7, Article number 39979, doi:10.1038/srep39979

https://www.nature.com/articles/srep39979

Finally (for this post), can you imagine how the timing of a rain-dominated Arctic will be affected by Hansen's ice-climate feedback mechanism driven by a WAIS collapse circa 2040-2060 (which almost all ESM projections currently ignore), and or pulses of methane emission from thermokarst lakes?  I also note that the third linked reference assumes that ECS is only around 3C.

Richard Bintanja and Olivier Andry (2017), “Towards a rain-dominated Arctic”, Geophysical Research Abstracts Vol. 19, EGU2017-4402

http://meetingorganizer.copernicus.org/EGU2017/EGU2017-4402.pdf

Abstract: “Current climate models project a strong increase in Arctic precipitation over the coming century, which has been attributed primarily to enhanced surface evaporation associated with sea-ice retreat. Since the Arctic is still quite cold, especially in winter, it is often (implicitly) assumed that the additional precipitation will fall mostly as snow. However, very little is known about future changes in rain/snow distribution in the Arctic, notwithstanding the importance for hydrology and biology. Here we use 37 state-of-the-art climate models in standardised twenty-first century (2006–2100) simulations to show that 70◦ – 90◦N average annual Arctic snowfall will actually decrease, despite the strong increase in precipitation, and that most of the additional precipitation in the future (2091– 2100) will fall as rain. In fact, rain is even projected to become the dominant form of precipitation in the Arctic region. This is because Arctic atmospheric warming causes a greater fraction of snowfall to melt before it reaches the surface, in particular over the North Atlantic and the Barents Sea. The reduction in Arctic snowfall is most pronounced during summer and autumn when temperatures are close to the melting point, but also winter rainfall is found to intensify considerably. Projected (seasonal) trends in rain/snowfall will heavily impact Arctic hydrology (e.g. river discharge, permafrost melt), climatology (e.g. snow, sea ice albedo and melt) and ecology (e.g. water and food availability).”

See also the fourth linked reference:

R. Bintanja et al. Towards a rain-dominated Arctic, Nature Climate Change (2017). DOI: 10.1038/nclimate3240

http://www.nature.com/nclimate/journal/v7/n4/full/nclimate3240.html

Extract: "Rain causes more (extensive) permafrost melt [Refs. 7,26], which most likely leads to enhanced emissions of terrestrial methane [Ref. 27] (a powerful greenhouse gas), more direct runoff (a smaller seasonal delay) and concurrent freshening of the Arctic Ocean [Ref 18]. Rainfall also diminishes snow cover extent and considerably lowers the surface albedo of seasonal snow, ice sheets and sea ice [Ref. 9] , reinforcing surface warming and amplifying the retreat of ice and snow; in fact, enhanced rainfall will most likely accelerate sea-ice retreat by lowering its albedo (compared with that of fresh snowfall) "

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The first two linked articles appear in the May 28 2014 online version of Nature, about new paleo-evidence about how quickly the AIS can contribute to rapid SLR (including during Meltwater Pulse 1A):

Trevor Williams, (2014), "Climate science: How Antarctic ice retreats", Nature, doi:10.1038/nature13345

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13345.html

Summary: "New records of iceberg-rafted debris from the Scotia Sea reveal episodic retreat of the Antarctic Ice Sheet since the peak of the last glacial period, in step with changes in climate and global sea level."

&

M. E. Weber, P. U. Clark, G. Kuhn, A. Timmermann, D. Sprenk, R. Gladstone, X. Zhang, G. Lohmann, L. Menviel, M. O. Chikamoto, T. Friedrich & C. Ohlwein, (2014), "Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation", Nature, (2014), doi:10.1038/nature13397

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13397.html

See also:

http://news.discovery.com/earth/global-warming/antarctic-iceberg-flotilla-caused-huge-sea-level-rise-140528.htm

Extract: "Antarctica's melting glaciers launched so many icebergs into the ocean 14,600 years ago that sea level rose 6.5 feet (2 meters) in just 100 years, a new study reports. The results are the first direct evidence for dramatic melting in Antarctica's past — the same as predictions for its future.

"The Antarctic Ice Sheet had been considered to be fairly stable and kind of boring in how it retreated," said study co-author Peter Clark, a climate scientist at Oregon State University. "This shows the ice sheet is much more dynamic and episodic, and contributes to rapid sea-level rise.""

Also, the following extract from the third linked article about the Weber et al (2014) paper, not only reinforces the importance of AIS SLR contribution to Meltwater Pulse 1A, but more importantly that the fresh melt water causes a stratification of ocean water with a cool surface and warmer deep waters that creates a positive feedback mechanism that accelerates the rate of grounding-line retreat of Antarctic marine glaciers, particularly like those in the ASE; which supports Hansen et al (2016)'s ice-climate feedback mechanism
 
http://www.abc.net.au/science/articles/2014/05/29/4014978.htm

Extract: "Feedback system

Recent studies have shown that a significant amount of warming occurs directly from the ocean transferring heat to the ice shelves from underneath and causing melt.
"Our models indicate that when you add the fresh water, you initiate a positive feedback through subsurface ocean warming," says Menviel.

Fresh water from the Antarctic ice sheet melts into the Southern Ocean causing stratification of ocean water into separate layers, resulting in cool water on the surface, and warmer water deeper down which further erodes the ice sheet.

"So what starts as a small melting can be amplified leading to more rapid melting than just through changes in atmospheric temperature," says Menviel."

Thus there is plenty of paleo-evidence of past armadas of icebergs being released by the AIS.

45
...
The consensus ECS is from 2.0 to 4.5 degrees K with most paleo evidence indicating a most likely ECS around 3 degrees K.  So with the adjustment, the one example given went from being outside of the consensus (at 1.9 K), to almost at the median value (3.2 K).

While the linked (open access) reference has many appropriate qualifying statements and disclaimers, it notes that the AR5 paleo estimates of ECS were linear approximations that change when non-linear issues are considered.  In particular they find for the specific ECS, S[CO2,LI], during the Pleistocence (ie the most recent 2 million years) that:

"During Pleistocene intermediate glaciated climates and interglacial periods, S[CO2,LI] is on average ~ 45 % larger than during Pleistocene full glacial conditions."

Therefore, researchers such as James Hansen who relied on paleo findings that during recent full glacial periods ECS was about 3.0C, did not know that during interglacial periods this value would be 45% larger, or 4.35C.

Köhler, P., de Boer, B., von der Heydt, A. S., Stap, L. B., and van de Wal, R. S. W. (2015), "On the state dependency of the equilibrium climate sensitivity during the last 5 million years", Clim. Past, 11, 1801-1823, doi:10.5194/cp-11-1801-2015.

http://www.clim-past.net/11/1801/2015/cp-11-1801-2015.html
http://www.clim-past.net/11/1801/2015/cp-11-1801-2015.pdf

46

Given that we can measure the concentration of methane in the atmosphere and thus calculate the total net emissions (all sources minus all sinks), if two sources were underestimated that implies that another source (or multiple sources) were overestimated or that the sinks were underestimated.


The attached NOAA plot of the atmospheric methane concentrations at the South Pole from 2006 to Dec 2, 2019 indicate that the trend line of this methane concentration is accelerating; thus if some methane sources are not changing then other sources are currently accelerating, and may accelerate even more in the future due to global warming.

47
The linked reference look at CMIP5 model projections and finds that: "… using a single model to approximate the internal climate variability produces distributions that are too narrow and do not fully represent the uncertainty in the climate system property estimates."

Thus, in my opinion when AR6 looks at the final CMIP6 projections they should report the combined internal climate variabilities for all the reported CMIP6 models; which will likely be much higher than for the CMIP5 model projections.  The deep uncertainty associated with the full range of internal climate variability with CMIP6 will likely represent a significant climate risk:

Libardoni, A.G., C.E. Forest, A.P. Sokolov and E. Monier (2019): Underestimating Internal Variability Leads to Narrow Estimates of Climate System Properties. Geophysical Research Letters, 46(16), 10000-10007, doi: 10.1029/2019GL082442.

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL082442

Abstract: Probabilistic estimates of climate system properties often rely on the comparison of model simulations to observed temperature records and an estimate of the internal climate variability. In this study, we investigate the sensitivity of probability distributions for climate system properties in the Massachusetts Institute of Technology Earth System Model to the internal variability estimate. In particular, we derive probability distributions using the internal variability extracted from 25 different Coupled Model Intercomparison Project Phase 5 models. We further test the sensitivity by pooling variability estimates from models with similar characteristics. We find the distributions to be highly sensitive when estimating the internal variability from a single model. When merging the variability estimates across multiple models, the distributions tend to converge to a wider distribution for all properties. This suggests that using a single model to approximate the internal climate variability produces distributions that are too narrow and do not fully represent the uncertainty in the climate system property estimates.

48
The first attached Global Carbon Project image essentially shows that the reductions in coal use have been balanced by an increase in natural gas (methane has up to a GWP 36-times that of CO2) consumption; while the reported increases in wind, solar and hydropower have largely been balanced by a decrease in nuclear power consumption.  Thus with regard to net CO2-equivalent emissions, we are just moving the deck chairs on the Titanic.

Edit: Current world population is about 7.75 billion people, and the second attached image shows the assumed population growth for the five SSP families of forcing scenarios.  As all scenarios assume a currently growing global population, this implies that GHG emission will continue to grow for some time due to population growth alone

49

 If an MICI-type of failure pushes an armada of icebergs into the Southern Ocean some decade from now, maybe a massive application of this technology could put the icebergs to beneficial use while concurrently cooling potential increases in tropical ocean SST values:


If some readers believe that it is totally unrealistic to assume that there could be sufficient economic benefits to pay for moving an armada of Southern Ocean icebergs to the tropical oceans (not only for freshwater supply but possibly for: OTEC (ocean thermal energy conversion); cooling of waste heat, etc.); then consider that:

1. Per the first image a nudge from oceangoing tugboats could push an iceberg out of the ACC and into one of the three indicated cold currents leading north from the Southern Ocean into the Pacific, Atlantic and Indian Oceans.

2. Per the second image these three cold current feed into warm currents that would carry any such icebergs directly to the tropical ocean regions of all three oceans without towing, and would slowly melt along the way, thus both cooling and freshening the surface waters of the tropical oceans.

3. Per the third image (showing a representative thermocline profile for tropical ocean regions) cooling of the SSTA in these regions would both slow/stop increase surface evaporation associated with global warming and for at least decades would prevent the tropical oceans SST from increase by 5C; which is projected to lead to an equable climate.

Such a form of geoengineering would be much less expensive than other currently conceived forms of geoengineering.

50
As some readers may tire of pessimistic science reports on climate change risks; I provide the linked article about a Middle Eastern businessman that plans to tow Antarctic icebergs to more tropical ocean water (such as the U.A.E.).  If an MICI-type of failure pushes an armada of icebergs into the Southern Ocean some decade from now, maybe a massive application of this technology could put the icebergs to beneficial use while concurrently cooling potential increases in tropical ocean SST values:

Title: "Why a Middle Eastern business thirsty for water can't just tow an iceberg from Antarctica"

https://www.abc.net.au/news/2019-08-14/why-a-middle-eastern-business-cant-just-tow-antarctica-iceberg/11318638

Key points:
•   A firm from the United Arab Emirates wants to tow an iceberg to provide fresh water
•   Experts and regulators have identified numerous assessments, requirements, and hurdles to the plan
•   The head of the firm remains "optimistic" he can secure relevant approvals

Edit, see also:

Title: "Icebergs could be towed from Antarctica to solve Cape Town drought, expert says"

https://news.sky.com/story/icebergs-could-be-towed-from-antarctica-to-solve-cape-town-drought-expert-says-11354852

Extract: "Plans have been unveiled to tow icebergs from Antarctica to South Africa to help solve Cape Town's crippling water shortage.

Mr Sloane, who led the re-floating of the Costa Concordia cruise liner in 2014, said his team could wrap passing icebergs in fabric skirts to protect them and reduce evaporation.

"We want to show that if there is no other source to solve the water crisis, we have another idea no one else has thought of yet," he said.

Large tankers would guide the blocks into the Benguela Current that flows along the west coast of southern Africa, before a milling machine would cut into the ice.

A single iceberg "could produce about 150 million litres per day for about a year," around 30% of Cape Town's needs, Mr Sloane said.

The director of the marine salvage firm Resolve Marine said he was planning to hold a conference later this month to try and sell the $130m (£95m) project to officials and investors."

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