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Messages - AbruptSLR

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1
The findings of the linked research indicate that with continued global warming more of the increased heat will be advected by the atmosphere and proportionately less will be advected by the ocean.  This clearly implies that the relatively rapid poleward advection of heat energy through the atmosphere from the tropical oceans, will bring about more rapid climate change than expected by consensus climate science.  Furthermore, the disproportional concentration of ocean heat in the Southern Ocean, serves to destabilize Antarctic marine glaciers faster than assumed by consensus climate science:

Title: "Study: Climate change reshaping how heat moves around globe"

https://phys.org/news/2019-01-climate-reshaping-globe.html

Extract: " This is the first study to examine current changes in heat transfer and to conclude that warming temperatures are driving increased heat transfer in the atmosphere, which is compensated by a reduced heat transfer in the ocean. Additionally, the researchers concluded that the excess oceanic heat is trapped in the Southern Ocean around the Antarctic."

Chengfei He et al. The transient response of atmospheric and oceanic heat transports to anthropogenic warming, Nature Climate Change (2019). DOI: 10.1038/s41558-018-0387-3

https://www.nature.com/articles/s41558-018-0387-3


2
This is just quick post to note that there a numerous paleo examples of abrupt (including multi-decadal) climate change; which projections from current Earth System Models (ESMs) cannot not match.  Thus climate science continues to investigate this issue such as the linked conference in Ireland later this year:

INQUA 2019 (July 25 to 31): Special Session Details

http://www.inqua2019.org/programme/special-sessions/


Title: "Abrupt changes in climate and ice sheets during glacial-interglacial cycles"

http://www.inqua2019.org/programme/special-sessions/#toggle-id-48

Ruza Ivanovic (Convenor)
Lauren Gregoire (Co-Convenor)
Laura Robinson (Co-Convenor)
Understanding how and why rapid environmental changes took place in the Quaternary remains a key challenge in the field of climate science. In particular, it remains difficult to reconcile the chain of events between recorded warming, cooling, iceberg calving, ice sheet melt and sea level rise. Many of these events have been linked with changing greenhouse gases, collapsing ice sheets and rapid reorganisations of ocean circulation. However, the fundamental questions remain: How can progressive climate trends trigger rapid changes? What are the internal instabilities and ice-ocean-atmosphere interactions that drove the sudden transitions? Are they stochastic responses in a variable Earth System or are the processes consistent across glacial-interglacial cycles? What was their environmental impact?
For this session, we invite contributions that seek to better constrain the chain of events surrounding abrupt changes in climate and ice sheets during glacial-interglacial cycles. We encourage submissions covering mechanistic-modelling, data acquisition and reconstructions of the events and their impact.
&

Title: "Abrupt climate changes: The view from lakes"

http://www.inqua2019.org/programme/special-sessions/#toggle-id-49

Jule Xiao (Convenor)
Jonathan Dean (Co-Convenor)
The Earth’s climate system has experienced a series of abrupt changes during the recent geological past. Abrupt climate changes occurring on centennial to multi-decadal scales could provide an analogue for what might happen with future global warming, thus have attracted increasing attention from paleoclimatologists. In this regard, lake records are of special importance due to the fact that they contain a diverse selection of proxies, respond sensitively to climate change and are highly resolved and geographically widespread. This session invites contributions of newly obtained high-resolution, multi-proxy data from lake sediments, to allow for the exchange of the latest results and ideas regarding changes in hydrology, ecology and climate recorded by lakes at different latitudes during the late Quaternary. This session will aim to identify the regional expression of different abrupt climate changes occurring in the past, thereby progressing our understanding of the mechanisms responsible for abrupt climate changes on different timescales and the possible environmental effects of future global warming in different regions.

3
The paleo findings cited in the linked article confirms that Arctic Sea Ice loss can lead to abrupt increases in Arctic Amplification:

Title: "Arctic sea ice loss in the past linked to abrupt climate events"

https://www.sciencedaily.com/releases/2019/02/190211164033.htm

Extract: "A new study on ice cores shows that reductions in sea ice in the Arctic in the period between 30-100,000 years ago led to major climate events. During this period, Greenland temperatures rose by as much as 16 degrees Celsius.
...
"The summer time sea ice in the Arctic has experienced a 40% decline in the last few decades, but we know that about two thirds of that reduction is caused by human-induced climate change. What we now need to determine is, what can be learnt from these past sea ice losses to enable us to understand what might happen next to our climate.""

See also:

Louise C. Sime, Peter O. Hopcroft, Rachael H. Rhodes (February 13, 2019), "Impact of abrupt sea ice loss on Greenland water isotopes during the last glacial period", PNAS,   https://doi.org/10.1073/pnas.1807261116

https://www.pnas.org/content/early/2019/02/12/1807261116

Abstract: "Greenland ice cores provide excellent evidence of past abrupt climate changes. However, there is no universally accepted theory of how and why these Dansgaard–Oeschger (DO) events occur. Several mechanisms have been proposed to explain DO events, including sea ice, ice shelf buildup, ice sheets, atmospheric circulation, and meltwater changes. DO event temperature reconstructions depend on the stable water isotope (δ18O) and nitrogen isotope measurements from Greenland ice cores: interpretation of these measurements holds the key to understanding the nature of DO events. Here, we demonstrate the primary importance of sea ice as a control on Greenland ice core δ18O: 95% of the variability in δ18O in southern Greenland is explained by DO event sea ice changes. Our suite of DO events, simulated using a general circulation model, accurately captures the amplitude of δ18O enrichment during the abrupt DO event onsets. Simulated geographical variability is broadly consistent with available ice core evidence. We find an hitherto unknown sensitivity of the δ18O paleothermometer to the magnitude of DO event temperature increase: the change in δ18O per Kelvin temperature increase reduces with DO event amplitude. We show that this effect is controlled by precipitation seasonality.

Significance

The Dansgaard–Oeschger events contained in Greenland ice cores constitute the archetypal record of abrupt climate change. An accurate understanding of these events hinges on interpretation of Greenland records of oxygen and nitrogen isotopes. We present here the important results from a suite of modeled Dansgaard–Oeschger events. These simulations show that the change in oxygen isotope per degree of warming becomes smaller during larger events. Abrupt reductions in sea ice also emerge as a strong control on ice core oxygen isotopes because of the influence on both the moisture source and the regional temperature increase. This work confirms the significance of sea ice for past abrupt warming events

4
The linked references indicate that at a global scale permafrost is degrading fasters than previously assumed by consensus climate science; which could accelerate the rate of climate change this century:

Title: "Some Arctic ground no longer freezing—even in winter"

https://www.nationalgeographic.com/environment/2018/08/news-arctic-permafrost-may-thaw-faster-than-expected/

Extract: "On January 16, 2019, a new global study published in Nature Communications confirmed that permafrost is thawing quickly across much of the world. Between 2007 and 2016, permafrost temperature increased by 0.29 ± 0.12 °C globally. The greatest warming was seen in parts of Siberia, up to 0.93 °C. Significant warming was also seen in Antarctica, and less in mountain regions. In much of the Arctic ground temperature increased because of rising average air temperatures, while increased snow thickness in some areas also contributed to warming the ground underneath."
&

Biskaborn et al. (2019), "Permafrost is warming at a global scale", Nature Communications 10, No. 264, https://doi.org/10.1038/s41467-018-08240-4

https://www.nature.com/articles/s41467-018-08240-4#Fig6

5
The linked reference provides paleo data (from the past 360,000 years) that the ENSO assumes a La Nina like pattern during glacial periods and assumes an El Nino like pattern during rapidly changing portions of interglacial periods.  As we are in the most rapidly changing interglacial period on record, this is not good news:

Zhang, S., Li, T., Chang, F. et al. Chin. J. (2017), "Correspondence between the ENSO-like state and glacial-interglacial condition during the past 360 kyr", Ocean. Limnol., 35: 1018. https://doi.org/10.1007/s00343-017-6082-9

https://link.springer.com/article/10.1007/s00343-017-6082-9#citeas

Abstract: "In the warming world, tropical Pacific sea surface temperature (SST) variation has received considerable attention because of its enormous influence on global climate change, particularly the El Niño-Southern Oscillation process. Here, we provide new high-resolution proxy records of the magnesium/calcium ratio and the oxygen isotope in foraminifera from a core on the Ontong-Java Plateau to reconstruct the SST and hydrological variation in the center of the Western Pacific Warm Pool (WPWP) over the last 360 000 years. In comparison with other Mg/Ca-derived SST and δ18O records, the results suggested that in a relatively stable condition, e.g., the last glacial maximum (LGM) and other glacial periods, the tropical Pacific would adopt a La Niña-like state, and the Walker and Hadley cycles would be synchronously enhanced. Conversely, El Niño-like conditions could have occurred in the tropical Pacific during fast changing periods, e.g., the termination and rapidly cooling stages of interglacial periods. In the light of the sensitivity of the Eastern Pacific Cold Tongue (EPCT) and the inertia of the WPWP, we hypothesize an inter-restricted relationship between the WPWP and EPCT, which could control the zonal gradient variation of SST and affect climate change."

6
The linked reference provides calibrated simulations of Pliocene Earth Systems, to indicate that with continued warming Arctic Amplification will likely occur faster during the Arctic winter than during the Arctic Summer:

Zheng, J., Zhang, Q., Li, Q., Zhang, Q., and Cai, M.: Contribution of sea ice albedo and insulation effects to Arctic amplification in the EC-Earth Pliocene simulation, Clim. Past, 15, 291-305, https://doi.org/10.5194/cp-15-291-2019, 2019.

https://www.clim-past.net/15/291/2019/

Abstract
In the present work, we simulate the Pliocene climate with the EC-Earth climate model as an equilibrium state for the current warming climate induced by rising CO2 in the atmosphere. The simulated Pliocene climate shows a strong Arctic amplification featuring pronounced warming sea surface temperature (SST) over the North Atlantic, in particular over the Greenland Sea and Baffin Bay, which is comparable to geological SST reconstructions from the Pliocene Research, Interpretation and Synoptic Mapping group (PRISM; Dowsett et al., 2016). To understand the underlying physical processes, the air–sea heat flux variation in response to Arctic sea ice change is quantitatively assessed by a climate feedback and response analysis method (CFRAM) and an approach similar to equilibrium feedback assessment. Given the fact that the maximum SST warming occurs in summer while the maximum surface air temperature warming happens during winter, our analyses show that a dominant ice-albedo effect is the main reason for summer SST warming, and a 1 % loss in sea ice concentration could lead to an approximate 1.8 W m−2 increase in shortwave solar radiation into open sea surface. During the winter months, the insulation effect induces enhanced turbulent heat flux out of the sea surface due to sea ice melting in previous summer months. This leads to more heat released from the ocean to the atmosphere, thus explaining why surface air temperature warming amplification is stronger in winter than in summer.


7
The linked SciAm article indicates that black carbon is currently contributing to high values of Arctic Amplification:

Title: "Scientists Track the Source of Soot That Speeds Arctic Melt"

https://www.scientificamerican.com/article/scientists-track-the-source-of-soot-that-speeds-arctic-melt/

Extract: "Research has found that black carbon emissions may be responsible for as much as half a degree Celsius of Arctic warming — that's about a quarter of the warming the Arctic has experienced over the last hundred years.

Recent research has warned that unless global greenhouse gas emissions begin falling substantially within the next decade or two, winter temperatures in the Arctic could skyrocket by more than 10 C."

8
The linked reference finds that methane emissions are increasing faster in the NH than previously assumed.  I suspect that this trend will eventually result in greater Arctic Amplification than previously assumed by consensus climate science:

Sudhanshu Pandey et al. (29 January 2019), "Influence of Atmospheric Transport on Estimates of Variability in the Global Methane Burden", Geophysical Research Letters, https://doi.org/10.1029/2018GL081092

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL081092

Abstract: "We quantify the impact of atmospheric transport and limited marine boundary layer sampling on changes in global and regional CH4 burdens estimate using tracer transport model simulations with annually repeating CH4 emissions and sinks, but varying atmospheric transport patterns. We find the 1σ error due to the transport‐sampling effect on annual global CH4 increases to be 1.11 ppb/yr and on zonal growth rates to be 3.8 ppb/yr, indicating the transport‐sampling effect becomes more critical at smaller spatiotemporal scales. We also find that the trends in inter‐hemispheric and inter‐polar difference of CH4 are significantly influenced by transport‐sampling. Contrary to a negligible trend in the inter‐hemispheric difference of measurements, we find, after adjusting for the transport‐sampling, a trend of 0.37 ± 0.06 ppb/yr. This is consistent with the emission trend from a 3D inversion of the measurements, suggesting a faster increase in emissions in the Northern Hemisphere than in the Southern Hemisphere."

9
The linked reference indicates that current IPCC carbon budgets only consider relatively fast Earth System responses; while slower Earth System responses like those associated with ENSO feedback mechanism are unstoppable by geoengineering, once triggered, and which will contribute to net global warming:

Xiao‐Tong Zheng et al. (05 February 2019), "Intensification of El Niño rainfall variability over the tropical Pacific in the slow oceanic response to global warming", Geophysical Research Letters, https://doi.org/10.1029/2018GL081414

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL081414

Abstract
Changes in rainfall variability of El Niño–Southern Oscillation (ENSO) are investigated under scenarios where the greenhouse gases (GHGs) increase and then stabilize. During the period of increasing greenhouse forcing, the ocean mixed layer warms rapidly. After the forcing stabilizes, the deeper ocean continues to warm the surface (the slow response). We show that ENSO rainfall variability over the tropical Pacific intensifies in both periods but the rate of increase per degree global mean surface temperature (GMST) warming is larger for the slow response because of greater relative warming in the base state as the mean upwelling changes from a damping to a driver of the surface warming. Our results have important implications for climate extremes under GMST stabilization that the Paris Agreement calls for. To stabilize GMST, the fast surface cooling offsets the slow warming from the prior GHG increase, while ENSO rainfall variability would continue to increase.

Plain Language Summary
The Paris Agreement calls for limiting global mean surface temperature (GMST) increase to well below 2 degrees at the end of the 21st century. This requires the greenhouse gas (GHG) concentration to peak and subsequently decline in next few decades. After the GHG concentration peak, the heat accumulated in the ocean surface layer continues to penetrate to the deeper ocean. This deeper ocean warming leads to a slow response of surface warming, further influencing the climate system. This study examines scenarios where GHGs increase and then stabilize to isolate the fast and slow responses of El Niño‐Southern Oscillation (ENSO) rainfall variability. We find intensification of ENSO rainfall variability both during the increase and after stabilization of GHG concentrations due to a persistent El Niño‐like mean warming pattern in the tropical Pacific. Furthermore, for unit GMST increase, the changes in the mean state temperature and ENSO rainfall variability in the eastern equatorial Pacific is larger during the slow response. These results imply that there is a need for GHG emission reduction in near future to avoid more extreme tropical rainfall during El Niño.

10
export from the Ross Sea sector in recent years; those who consider the impact of ice-climate feedback mechanisms realize that this observed increase in Antarctic sea ice, and associated freshening of the Southern Ocean, leads to increase upwelling of warm CDW and an associated acceleration of the destabilization of key Antarctic marine glaciers:

Ivana Cerovečki et al. (2019), "The effects of enhanced sea ice export from the Ross Sea on recent cooling and freshening of the Southeast Pacific", Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0205.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0205.1

Abstract: "The top 2000 m of the Southern Ocean has freshened and warmed over recent decades. However, the high-latitude (south of 50°S) southeast Pacific was observed to be cooler and fresher in the years 2008-2010 compared to 2005-2007 over a wide depth range including surface, mode, and intermediate waters. The causes and impacts of this event are analyzed using the ocean—sea-ice data-assimilating Southern Ocean State Estimate (SOSE) and observationally based products. In 2008-2010, a strong positive Southern Annular Mode coincided with a negative El Niño Southern Oscillation and a deep Amundsen Sea Low. Enhanced meridional winds drove strong sea ice export from the eastern Ross Sea, bringing large amounts of ice to the Amundsen Sea ice edge. In 2008, together with increased precipitation, this introduced a strong freshwater anomaly that was advected eastward by the Antarctic Circumpolar Current (ACC), mixing along the way. This anomaly entered the ocean interior not only as Antarctic Intermediate Water, but also as lighter Southeast Pacific Subantarctic Mode Water (SEPSAMW). A numerical particle release experiment carried out in SOSE , showed that the Ross Sea sector was the dominant source of particles reaching the SEPSAMW formation region. This suggests that large-scale climate fluctuations can induce strong interannual variability of volume and properties of SEPSAMW. These fluctuations act at different time scales: instantaneously via direct forcing, and also lagged over advective time scales of several years from upstream regions."

11
The linked reference adds information to Bamber's observation that the Arctic Ocean could become seasonally sea ice free circa 2040 (which would accelerate the ice-climate positive feedback mechanism):

J. A. Screen & C. Deser (05 February 2019), "Pacific Ocean Variability Influences the Time of Emergence of a Seasonally Ice‐Free Arctic Ocean", Geophysical Research Letters, https://doi.org/10.1029/2018GL081393

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL081393

Abstract

The Arctic Ocean is projected to become seasonally ice‐free before midcentury unless greenhouse gas emissions are rapidly reduced, but exactly when this could occur depends considerably on internal climate variability. Here we show that trajectories to an ice‐free Arctic are modulated by concomitant shifts in the Interdecadal Pacific Oscillation (IPO). Trajectories starting in the negative IPO phase become ice‐free 7 years sooner than those starting in the positive IPO phase. Trajectories starting in the negative IPO phase subsequently transition toward the positive IPO phase, on average, with an associated strengthening of the Aleutian Low, increased poleward energy transport, and faster sea‐ice loss. The observed IPO began to transition away from its negative phase in the past few years. If this shift continues, our results suggest increased likelihood of accelerated sea‐ice loss over the coming decades, and an increased risk of an ice‐free Arctic within the next 20–30 years.

Plain Language Summary
Manmade climate change is causing a rapid loss of Arctic sea ice. Summer Arctic sea ice is predicted to disappear almost completely by the middle of this century, unless emissions of greenhouse gases are rapidly reduced. The speed of sea‐ice loss is not constant over time, however. Natural climate variability can add to the manmade decline, leading to faster sea‐ice loss, or can subtract from the manmade decline, leading to slower sea‐ice loss. In this study, we looked at how natural climate variability affects the timing of an ice‐free Arctic. We found that a natural cycle called the Interdecadal Pacific Oscillation, or IPO for short, is particularly important. Arctic sea‐ice loss is faster when the IPO is moving from its cold to warm phase and slower when the IPO is moving from its warm to cold phase. This is because variations in the IPO cause changes in atmospheric wind patterns, which alter the amount of heat that is transported into the Arctic. Observations show that the IPO started to shift from its cold to warm phase in the past few years. If this shift continues, our results suggest that there is an increased chance of accelerated sea‐ice loss over the coming decades.

12
The linked reference provides paleo evidence that the eastern equatorial Pacific (EEP) can release previously sequestered carbon from the ocean into the atmosphere in a relatively short-timespan.  Thus, ESM projections should be updated to include this potentially significant source of carbon emissions into the atmosphere over the coming decades, with continued global warming:

Lowell D Stott et al. (2019), "Hydrothermal carbon release to the ocean and atmosphere from the eastern equatorial Pacific during the last glacial termination", Environmental Research Letters,

https://iopscience.iop.org/article/10.1088/1748-9326/aafe28/meta

Abstract: "Arguably among the most globally impactful climate changes in Earth's past million years are the glacial terminations that punctuated the Pleistocene epoch. With the acquisition and analysis of marine and continental records, including ice cores, it is now clear that the Earth's climate was responding profoundly to changes in greenhouse gases that accompanied those glacial terminations. But the ultimate forcing responsible for the greenhouse gas variability remains elusive. The oceans must play a central role in any hypothesis that attempt to explain the systematic variations in pCO2 because the Ocean is a giant carbon capacitor, regulating carbon entering and leaving the atmosphere. For a long time, geological processes that regulate fluxes of carbon to and from the oceans were thought to operate too slowly to account for any of the systematic variations in atmospheric pCO2 that accompanied glacial cycles during the Pleistocene. Here we investigate the role that Earth's hydrothermal systems had in affecting the flux of carbon to the ocean and ultimately, the atmosphere during the last glacial termination. We document late glacial and deglacial intervals of anomalously old 14C reservoir ages, large benthic-planktic foraminifera 14C age differences, and increased deposition of hydrothermal metals in marine sediments from the eastern equatorial Pacific (EEP) that indicate a significant release of hydrothermal fluids entered the ocean at the last glacial termination. The large 14C anomaly was accompanied by a ~4-fold increase in Zn/Ca in both benthic and planktic foraminifera that reflects an increase in dissolved [Zn] throughout the water column. Foraminiferal B/Ca and Li/Ca results from these sites document deglacial declines in [ ] throughout the water column; these were accompanied by carbonate dissolution at water depths that today lie well above the calcite lysocline. Taken together, these results are strong evidence for an increased flux of hydrothermally-derived carbon through the EEP upwelling system at the last glacial termination that would have exchanged with the atmosphere and affected both Δ14C and pCO2. These data do not quantify the amount of carbon released to the atmosphere through the EEP upwelling system but indicate that geologic forcing must be incorporated into models that attempt to simulate the cyclic nature of glacial/interglacial climate variability. Importantly, these results underscore the need to put better constraints on the flux of carbon from geologic reservoirs that affect the global carbon budget."

Caption for the attached image: "Figure 1. Nearly pure CO2 bubbles emanating from sediments that blanket an active hydrothermal system in the western tropical Pacific. Photos by Roy Price, courtesy of Jan Amend."

13
A few key related points from Jonathan Bamber:

Title: "The Carbon Brief Interview: Prof Jonathan Bamber"

https://www.carbonbrief.org/the-carbon-brief-interview-prof-jonathan-bamber

Extracts: "On Arctic sea ice loss: “We could potentially have an ice-free Arctic Ocean in summer by 2035-2040, which is potentially in my lifetime, not so far away. That really is incredible.”

On the west Antarctic ice sheet: “There were some papers…that suggest that we may even have passed that marine ice sheet instability threshold already for parts of west Antarctica.”

On Arctic permafrost: “When it melts, we believe most of that carbon will be emitted as methane, which is a much stronger greenhouse gas than CO2.”"

14
As I have been (/will be) traveling, the following are some random considerations as to why the WAIS may collapse sooner than even DeConto & Pollard estimate (see the first image, and I note that the second image indicates that SSP5-Baseline results in a faster rate of global warming than computed by DeConto & Pollard):

1. Finer mesh resolution typically result in faster ice mass loss projections from ice sheet models, and this is particularly true for models of the Thwaites Gateway, and I note that DeConto & Pollards mesh could be finer if they had access to more computational power.

2.  If I am correct that once the residual Thwaites Ice Tongue is cleared away (maybe within 5 to 20 years), that ice cliff calved icebergs will be able to float-out by traveling along the seafloor trench at the base of the Thwaites Ice Tongue, then an MICI mechanism should be able to develop for the Thwaites Glacier prior to the development of hydrofracturing in this area.

3. As climate change is currently increasing the frequency of cyclones in the Amundsen-Bellingshausen Sea Sector, increased storm surge activity will likely accelerate ice mass loss from this area.

4. The recently observed trend of accelerating ice flow velocities for both the Thwaites, and Pine Island, Glaciers (partially attributable to the loss of buttressing on the SW Tributary Glacier in 2018), results in increased friction-induced ice melting within the body of these glaciers; and such ice meltwater drips/rains down to create increased subglacial meltwater; which serves to accelerate the destabilization of such key marine glaciers.

5. The glacial beds for the WAIS marine glaciers are all more worn/scoured than was the case for all paleo-cases.  Thus, any ice mass models calibrated using paleo-data will consequently err on the side of least drama.

6. It is likely that glacial isostatic rebound will increase geothermal heat flux through the beds of key WAIS marine glaciers; which would serve to accelerate the destabilization of such glaciers.

7. Projections of increased snowfall in the coastal regions of the WAIS (with continued global warming), will increase the gravitational driving force associated with MICI and MISI ice mass loss in coming decades.

8. Projections of increased El Nino activity (with continued global warming) will increase the volume of warm CDW advected to the grounding lines of key WAIS marine glaciers, and will also increase the likelihood of hydrofracturing of key WAIS ice shelves in the coming decades.

9. DeConto & Pollard (& Hansen) used consensus values of ECS in their model projections; which indicates that their projections of ice mass loss err on the side of least drama.

10. The Eastern Thwaites Ice Shelf and the Pine Island Ice Shelf, both appear to be degrading faster than projected by DeConto & Pollard; which indicates that their ice mass loss projections err on the side of least drama.

11. The freshening of much of the surface waters of the Southern Ocean, due to both early ice mass loss from Antarctic ice shelves and increase precipitation onto the surface of the Southern Ocean, is accelerating the projected increased upwelling of warm CDW.

12. The early ice mass loss from the Greenland Ice Sheet is accelerating the bipolar seesaw mechanism, which is compounding the influence of the decadal slow down of the MOC.

13. At some point, sufficient ice mass loss from the WAIS will trigger increased seismic and volcanic activity, not evaluated by DeConto & Pollard.

14. Recent evidence indicates that the negative forcing associated with anthropogenic aerosols has been/is greater than previously assumed by consensus climate science; which implies that GMSTA will increase faster than projected as anthropogenic aerosol emissions are reduced and/or redistributed around the globe.

15
In light of the growth of the subglacial cavity at the base of the Thwaites Ice Tongue, I provide the first image to remind people that the Thwaites subglacial drainage system (of fresh meltwater) empties through this cavity into the ocean, which can contribute to a syphon action which advects more warm CDW into the cavity (which causes more ice melting).  The second image shows that when such fresh meltwater drains to the ocean at an ice cliff face the local vertical convection can work to destabilize that ice face (which may happen in this area of the seafloor trench at the base of the Thwaites Ice Tongue circa 2035 to 2040).

16
One cannot understand Hansen's ice-climate feedback mechanism, without understanding the bipolar seesaw mechanism:

Joel Pedro, Markus Jochum, Christo Buizert, Feng He, Stephen Barker, and Sune Rasmussen (2018), "Beyond the bipolar seesaw: toward a process understanding of interhemispheric coupling", Geophysical Research Abstracts, Vol. 20, EGU2018-2551, EGU General Assembly 2018

https://meetingorganizer.copernicus.org/EGU2018/EGU2018-2551.pdf

Abstract: "The thermal bipolar ocean seesaw hypothesis was advanced by Stocker and Johnsen (2003) as the ‘simplest possible thermodynamic model’ to explain the time relationship between Dansgaard Oeschger (DO) and Antarctic Isotope Maxima (AIM) events. Here, we combine palaeoclimate observations, theory and general circulation model experiments to advance from the conceptual model toward a process understanding of interhemispheric coupling and the forcing of AIM events. We present four main results:  (1) Changes in Atlantic heat transport invoked by the thermal seesaw are partially compensated by opposing changes in heat transport by the global atmosphere and Pacific Ocean. This compensation is an integral part of interhemispheric coupling, with a major influence on the global pattern of climate anomalies.  (2) A change in cross-equatorial heat advection is commonly assumed to explain Atlantic Ocean temperature anomalies in the thermal seesaw.  We suggest that wind driven deepening of the South Atlantic thermocline contributes, in addition to the change in advection, to explain the speed and spatial pattern of the temperature changes in the South Atlantic and the storage of heat at depth.  (3) We support the role of a heat reservoir in interhemispheric coupling but argue that its location is the global interior ocean north of the Antarctic Circumpolar Current (ACC), not the commonly assumed Southern Ocean.  (4) Energy budget analysis indicates that the process driving Antarctic warming during AIM events is an increase in poleward atmospheric heat and moisture transport following sea-ice retreat and surface warming over the Southern Ocean. Sea-ice retreat is itself driven by eddy-heat fluxes from the global ocean heat reservoir across the ACC, amplified by sea-ice–albedo feedbacks. Our results underline the coupled role of the ocean and atmosphere in signal propagation linking DO and AIM events."

17
The linked reference provides a limited-scale, real-world example of Hansen's ice-climate feedback.

Alessandro Silvano et al. (18 Apr 2018), "Freshening by glacial meltwater enhances melting of ice shelves and reduces formation of Antarctic Bottom Water", Science Advances, Vol. 4, no. 4, eaap9467, DOI: 10.1126/sciadv.aap9467

http://advances.sciencemag.org/content/4/4/eaap9467

Extract: "Strong heat loss and brine release during sea ice formation in coastal polynyas act to cool and salinify waters on the Antarctic continental shelf. Polynya activity thus both limits the ocean heat flux to the Antarctic Ice Sheet and promotes formation of Dense Shelf Water (DSW), the precursor to Antarctic Bottom Water. However, despite the presence of strong polynyas, DSW is not formed on the Sabrina Coast in East Antarctica and in the Amundsen Sea in West Antarctica. Using a simple ocean model driven by observed forcing, we show that freshwater input from basal melt of ice shelves partially offsets the salt flux by sea ice formation in polynyas found in both regions, preventing full-depth convection and formation of DSW. In the absence of deep convection, warm water that reaches the continental shelf in the bottom layer does not lose much heat to the atmosphere and is thus available to drive the rapid basal melt observed at the Totten Ice Shelf on the Sabrina Coast and at the Dotson and Getz ice shelves in the Amundsen Sea. Our results suggest that increased glacial meltwater input in a warming climate will both reduce Antarctic Bottom Water formation and trigger increased mass loss from the Antarctic Ice Sheet, with consequences for the global overturning circulation and sea level rise."

See also:

Title: "One of the most worrisome predictions about climate change may be coming true"

https://bangordailynews.com/2018/04/23/environment/one-of-the-most-worrisome-predictions-about-climate-change-may-be-coming-true/

Extract: "Two years ago, former NASA climate scientist James Hansen and a number of colleagues laid out a dire scenario in which gigantic pulses of fresh water from melting glaciers could upend the circulation of the oceans, leading to a world of fast-rising seas and even superstorms.

Hansen’s scenario was based on a computer simulation, not hard data from the real world, and met with skepticism from a number of other climate scientists. But now, a new oceanographic study appears to have confirmed one aspect of this picture — in its early stages, at least.

The new research, based on ocean measurements off the coast of East Antarctica, shows that melting Antarctic glaciers are indeed freshening the ocean around them. And this, in turn, is blocking a process in which cold and salty ocean water sinks below the sea surface in winter, forming “the densest water on the Earth,” in the words of study lead author Alessandro Silvano, a researcher with the University of Tasmania in Hobart, Australia.

Hansen said that “this study provides a nice small-scale example of processes that we talk about in our paper.”

18
While some readers may find the topic of the  linked reference to be of secondary importance to the theme of this thread, I liked the research in part because I think that stratospheric ozone depletion is an under appreciated topic;

Bahlmann, E., Keppler, F., Wittmer, J., Greule, M., Schöler, H. F., Seifert, R., and Zetzsch, C.: Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes, Atmos. Chem. Phys., 19, 1703-1719, https://doi.org/10.5194/acp-19-1703-2019, 2019.

https://www.atmos-chem-phys.net/19/1703/2019/

Abstract
Chloromethane (CH3Cl) is the most important natural input of reactive chlorine to the stratosphere, contributing about 16 % to stratospheric ozone depletion. Due to the phase-out of anthropogenic emissions of chlorofluorocarbons, CH3Cl will largely control future levels of stratospheric chlorine.
The tropical rainforest is commonly assumed to be the strongest single CH3Cl source, contributing over half of the global annual emissions of about 4000 to 5000 Gg (1 Gg = 109 g). This source shows a characteristic carbon isotope fingerprint, making isotopic investigations a promising tool for improving its atmospheric budget. Applying carbon isotopes to better constrain the atmospheric budget of CH3Cl requires sound information on the kinetic isotope effects for the main sink processes: the reaction with OH and Cl in the troposphere. We conducted photochemical CH3Cl degradation experiments in a 3500 dm3 smog chamber to determine the carbon isotope effect (ε=k13C/k12C−1ε=k13C/k12C-1) for the reaction of CH3Cl with OH and Cl. For the reaction of CH3Cl with OH, we determined an ε value of (−11.2±0.8-11.2±0.8 ) ‰ (n=3) and for the reaction with Cl we found an ε value of (−10.2±0.5-10.2±0.5) ‰ (n=1), which is 5 to 6 times smaller than previously reported. Our smaller isotope effects are strongly supported by the lack of any significant seasonal covariation in previously reported tropospheric δ13C(CH3Cl) values with the OH-driven seasonal cycle in tropospheric mixing ratios.

Applying these new values for the carbon isotope effect to the global CH3Cl budget using a simple two hemispheric box model, we derive a tropical rainforest CH3Cl source of (670±200) Gg a−1, which is considerably smaller than previous estimates. A revision of previous bottom-up estimates, using above-ground biomass instead of rainforest area, strongly supports this lower estimate. Finally, our results suggest a large unknown CH3Cl source of (1530±200) Gg a−1.

19
While E3SM (2017) (with an ECS of 5.2C) has been included within CMIP6 (initial runs of which can be accessed after March 2019 at the following linked website); currently no version of E3SM includes subroutines for modeling MICI (Marine Ice Cliff Instability) failure modes.  As Pollard and DeConto freely cooperate with any sincere climate researcher to make their MICI subroutines transparent; it can only be that DOE management officials decline to incorporate a freely available and well calibrated MICI subroutine into the current version of E3SM.

https://esgf-node.llnl.gov/projects/e3sm/

Furthermore, consensus climate science would be well advised to take the output from CMIP6 (and for E3SM 2017 separately) and use it as input to state of the art AI software, to generate output that can be used to improve all future ESM models especially including a future version of E3SM that incorporates a MICI subroutine.

20
This post is just a reminder that following our current BAU pathway, many regions of the Earth will experience the emergence of aridification by 2030, and about 25% of the Earth will be subject to desertification by 2050 (see the attached image) assuming ECS is about 3.1C:

Chang-Eui Park et al (2018), "Keeping global warming within 1.5 °C constrains emergence of aridification",  Nature Climate Change, doi:10.1038/s41558-017-0034-4
http://www.nature.com/articles/s41558-017-0034-4
&
http://www.nature.com/articles/s41558-017-0034-4.epdf?referrer_access_token=uomV724E6L7Azgr_quk6KdRgN0jAjWel9jnR3ZoTv0Md-_o5A1rhm4JzTytmIvTkmxaaT7siHGll-VC_gYGc86rzaCGyHAOR4rvUdRgQU-l25yhfdvQ3YA9Sav1Hp-9FF5MB-iTHjnQ37ZcRAyET_8yswPHc50VudILwTqUqzbfytP20P8axT5rTSHLyrhghYS-UOsW-GxJvHJaa_8Fq3dvxpyfvvNDVuVY3ll6olzw%3D&tracking_referrer=www.newsweek.com

Abstract: "Aridity—the ratio of atmospheric water supply (precipitation; P) to demand (potential evapotranspiration; PET)—is projected to decrease (that is, areas will become drier) as a consequence of anthropogenic climate change, exacerbating land degradation and desertification. However, the timing of significant aridification relative to natural variability—defined here as the time of emergence for aridification (ToEA)—is unknown, despite its importance in designing and implementing mitigation policies. Here we estimate ToEA from projections of 27 global climate models (GCMs) under representative concentration pathways (RCPs) RCP4.5 and RCP8.5, and in doing so, identify where emergence occurs before global mean warming reaches 1.5 °C and 2 °C above the pre-industrial level. On the basis of the ensemble median ToEA for each grid cell, aridification emerges over 32% (RCP4.5) and 24% (RCP8.5) of the total land surface before the ensemble median of global mean temperature change reaches 2 °C in each scenario. Moreover, ToEA is avoided in about two-thirds of the above regions if the maximum global warming level is limited to 1.5 °C. Early action for accomplishing the 1.5 °C temperature goal can therefore markedly reduce the likelihood that large regions will face substantial aridification and related impacts."

Edit: If it is not clear, the RCP8.5 pattern of drying contributes to higher values of ECS by various feedback mechanisms including an induced slowing of the MOC.

21
Here is an opinion piece (& related linked references) that supports the underlying tenets of this thread:

Title: "Are We Headed Toward the Worst-Case Climate Change Scenario?"

https://psmag.com/environment/are-we-headed-toward-the-worst-case-climate-change-scenario

Extract: "A series of recent studies and reports suggest that, without immediate and drastic action, the worst-case climate scenario will become the rule rather than the exception."

See also:

Title: "Accelerating changes in ice mass within Greenland, and the ice sheet’s sensitivity to atmospheric forcing"

https://www.pnas.org/content/116/6/1934
&

Title: "Melting Ice Sheets Could Worsen Extreme Weather"

https://www.scientificamerican.com/article/melting-ice-sheets-could-worsen-extreme-weather/


22
As it is my belief that potential future increases in ECS (this century) will be related to increases in El Nino event frequency, and to increasing Polar Amplification (both of which should be accelerated by ice-climate interactions), I provide the following three linked references which discuss modeling issues related to ENSO events and Polar Amplification, which are not fully addresses by current consensus model projections; but which do not themselves consider possible future ice-climate feedback mechanism:

Lorenzo M. Polvani & Katinka Bellomo (2019), "The Key Role of Ozone-Depleting Substances in Weakening the Walker Circulation in the Second Half of the Twentieth Century", Journal of Climate, https://doi.org/10.1175/JCLI-D-17-0906.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-17-0906.1

Abstract: "It is widely appreciated that ozone-depleting substances (ODS), which have led to the formation of the Antarctic ozone hole, are also powerful greenhouse gases. In this study, we explore the consequence of the surface warming caused by ODS in the second half of the twentieth century over the Indo-Pacific Ocean, using the Whole Atmosphere Chemistry Climate Model (version 4). By contrasting two ensembles of chemistry–climate model integrations (with and without ODS forcing) over the period 1955–2005, we show that the additional greenhouse effect of ODS is crucial to producing a statistically significant weakening of the Walker circulation in our model over that period. When ODS concentrations are held fixed at 1955 levels, the forcing of the other well-mixed greenhouse gases alone leads to a strengthening—rather than weakening—of the Walker circulation because their warming effect is not sufficiently strong. Without increasing ODS, a surface warming delay in the eastern tropical Pacific Ocean leads to an increase in the sea surface temperature gradient between the eastern and western Pacific, with an associated strengthening of the Walker circulation. When increasing ODS are added, the considerably larger total radiative forcing produces a much faster warming in the eastern Pacific, causing the sign of the trend to reverse and the Walker circulation to weaken. Our modeling result suggests that ODS may have been key players in the observed weakening of the Walker circulation over the second half of the twentieth century."
&

J. Ono et al. (2019), "Mechanisms for and Predictability of a Drastic Reduction in the Arctic Sea Ice: APPOSITE Data with Climate Model MIROC', Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0195.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0195.1

Abstract: "The mechanisms for and predictability of a drastic reduction in the Arctic sea ice extent (SIE) are investigated using the Model for Interdisciplinary Research on Climate (MIROC) version 5.2. Here, a control (CTRL) with forcing fixed at year 2000 levels and perfect-model ensemble prediction (PRED) experiments are conducted. In CTRL, three (model years 51, 56, and 57) drastic SIE reductions occur during a 200-yr-long integration. In year 56, the sea ice moves offshore in association with a positive phase of the summer Arctic dipole anomaly (ADA) index and melts due to heat input through the increased open water area, and the SIE drastically decreases. This provides the preconditioning for the lowest SIE in year 57 when the Arctic Ocean interior is in a warm state and the spring sea ice volume has a large negative anomaly due to drastic ice reduction in the previous year. Although the ADA is one of the key mechanisms behind sea ice reduction, it does not always cause a drastic reduction. Our analysis suggests that wind direction favoring offshore ice motion is a more important factor for drastic ice reduction events. In years experiencing drastic ice reduction events, the September SIE can be skillfully predicted in PRED started from July, but not from April. This is because the forecast errors for the July sea level pressure and those for the sea ice concentration and sea ice thickness along the ice edge are large in PRED started from April."
&

Xiaofan Li et al. (2019), "Contributions of Atmosphere–Ocean Interaction and Low-Frequency Variation to Intensity of Strong El Niño Events since 1979, Journal of Science, https://doi.org/10.1175/JCLI-D-18-0209.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0209.1

Abstract: "Evolutions of oceanic and atmospheric anomalies in the equatorial Pacific during four strong El Niños (1982/83, 1991/92, 1997/98, and 2015/16) since 1979 are compared. The contributions of the atmosphere–ocean coupling to El Niño–associated sea surface temperature anomalies (SSTA) are identified and their association with low-level winds as well as different time-scale variations is examined. Although overall SSTA in the central and eastern equatorial Pacific is strongest and comparable in the 1997/98 and 2015/16 El Niños, the associated subsurface ocean temperature as well as deep convection and surface wind stress anomalies in the central and eastern equatorial Pacific are weaker during 2015/16 than that during 1997/98. That may be associated with a variation of the wind–SST and wind–thermocline interactions. Both the wind–SST and wind–thermocline interactions play a less important role during 2015/16 than during 1997/98. Such differences are associated with the differences of the low-level westerly wind as well as the contribution of different time-scale variations in different events. Similar to the interannual time-scale variation, the intraseasonal–interseasonal time-scale component always has positive contributions to the intensity of all four strong El Niños. Interestingly, the role of the interdecadal-trend time-scale component varies with event. The contribution is negligible during the 1982/83 El Niño, negative during the 1997/98 El Niño, and positive during the 1991/92 and 2015/16 El Niños. Thus, in addition to the atmosphere–ocean coupling at intraseasonal to interannual time scales, interdecadal and longer time-scale variations may play an important and sometimes crucial role in determining the intensity of El Niño."


23
Perhaps the largest obstruction blocking societies willingness/ability to face the true risks of abrupt climate change (likely to be triggered by ice-climate feedback mechanisms not adequately addressed by consensus climate science reports), can be characterized by what Sir Francis Bacon described as the 'four idols' of the mind (see the first linked article).  <snip>

Hello ASIForum members, especially AbruptSLR!! I've registered on the forum and logged in for the express purpose of leaving this reply. Note, however, I've been following multiple threads, daily, here for nearly a decade. Also, I apologize in advance that this comment will stray slightly off-topic.

Thank you, ASLR, for that comment about Sir Francis Bacon and his "4 Idols" theory. His observations mesh closely with my own over the past 50+ years. However, I refer to the "obstructions" simply as "beliefs." Too many people, the world over, proclaim to believe many things. Often, those beliefs are contradictory to others claimed to be held and, more significantly, most people act antithetically to the beliefs they claim to hold. I'm sure most, if not all, of the beliefs people hold can be categorized into one or more of Bacon's "4 Idols," and they are the most critical factors of all impeding "progress" of any kind, not least of which being the continuing existence of the human species.

I thank you, ASLR, ad infinitum, for your continued work and comments in this forum, though I do miss your erudition in other threads but understand and accept your reasoning behind that decision. Please, keep up the good work, your words and supplied links generally "make my day."

ColinOH,

Thank you for your thoughtful words, and while talking about Bacon, I note that in an address entitled "The philosophy of Francis Bacon" delivered at Cambridge on the occasion of the Bacon tercentenary, 5 October, 1926", Cambridge: University Press, p. 67, the philosopher C.D. Broad stated:

"May we venture to hope that when Bacon's next centenary is celebrated the great work which he set going will be completed; and that Inductive Reasoning, which has long been the glory of Science, will have ceased to be the scandal of Philosophy?"

This makes it clear that the Broad was hoping that by 2026 that human effort (on empiricism and the scientific method) will bring science and philosophy sufficiently close together that philosophy will be able to employ inductive reasoning to provide usefully guidance regarding our currently unbalanced socio-economic systems (see the following link to a Wikipedia article about Sir Francis Bacon).

Title: "Francis Bacon"

https://en.wikipedia.org/wiki/Francis_Bacon

Bacon saw the scientific method as a philosophy, and I hope that by 2026 consensus climate scientists will broaden their limited definition of science in the Anthropocene to consider 'fat-tailed' risks driven by mankind's preconceived mental constructs/idols (desires & aversions).

Best regards,
ASLR

24
The linked 2017 reference was issued by the UK's 'The Royal Society', which worked in collaboration with the US's NAS, to provide climate updates to AR5.  The extracts below indicate that in general AR5 underestimated climate risks, and as I have cited in this thread, new climate findings published since 2017 indicate that 'The Royal Society' (2017) report also underestimates true climate risks:

Title: "Climate updates What have we learnt since the IPCC 5th Assessment Report?", November 2017 DES5123 ISBN: 978-1-78252-306-2

https://royalsociety.org/~/media/policy/Publications/2017/27-11-2017-Climate-change-updates-report.pdf

Extract: "With the next assessment report (AR6) not due until 2022, it is timely to consider how evidence presented since the publication of AR5 affects the assessments made then.

One important advance is that it is now known that as the climate warms it becomes less effective at emitting heat to space, mainly as a result of regional variations in surface warming. This means that climate sensitivity derived from historical data (which typically fails to fully represent regional areas that may be warmer or cooler than the average) gives an underestimate of the value for high carbon dioxide atmospheres. It is also now clear that the very slow changes in patterns of ocean surface warming are inadequately represented in time varying global climate models resulting in an underestimate of climate sensitivity.

Another approach, in which global climate models that have been assessed on the basis of their ability to reproduce observed changes in cloud cover and properties, such as ice content and reflectivity, shows that the best performers generally have higher sensitivities.

It has been demonstrated that incomplete geographical sampling of temperature can impact estimates of sensitivity. For example, the use of data with less coverage over the Arctic, where warming has been larger, has biased some climate sensitivity estimates to be too low.

… projections of future emissions will need to focus on both emissions of methane and the rate at which chemical reactions destroy it.

The ‘pause’ apparent in the data used in AR5 can be attributed to two main factors: observational biases and the variability caused by natural processes. There is some evidence that changes in atmospheric aerosols (small particles in the atmosphere) caused by human activities may have been an additional factor.

Concern about the likely long-term sea level rise is heightened by evidence that sea level was 6 – 9 m higher than today during the last interglacial period (125,000 years ago) when new climate reconstructions confirm that polar temperatures were comparable to those expected in 2100.

In summary, gradual climate change could trigger abrupt changes – with large regional and potentially global impacts – associated with thresholds in the Earth system. The possibility of crossing any of these thresholds increases with each increment of warming."

25
Perhaps the largest obstruction blocking societies willingness/ability to face the true risks of abrupt climate change (likely to be triggered by ice-climate feedback mechanisms not adequately addressed by consensus climate science reports), can be characterized by what Sir Francis Bacon described as the 'four idols' of the mind (see the first linked article).  Consensus science (as well as other populist movements) get(s) bogged down by various preconceptions of the human mind, and per the extract below philosopher Dale Jamieson indicates that individual mindfulness (and the scientific integrity that goes with it) is fundamental to over-coming such misguided preconceptions that inhibit society from effectively facing the true risks of abrupt climate change.  In this regards, consensus climate science is used by society to create, and then promote, preconceptions/dogmas in order to absolve leaders (& societies as a whole) from the effort to remain mindful of the world (& Earth Systems) around them with compassion for that world (e.g.: empathy for sustainability).  Such scientific mindfulness would enable scientist to consider/address issues beyond the preconceived specialist silos that they typically work within, so as to better address the many 'fat-tailed' risks of abrupt climate change:

Title: "The 17th-century philosopher whose scientific ideas could tackle climate change today"

https://www.weforum.org/agenda/2019/02/francis-bacon-the-17th-century-philosopher-whose-scientific-ideas-could-tackle-climate-change-today

Extract: "In his key work Novum Organum, Bacon identified “four idols” of the mind – false notions, or “empty ideas” – that don’t just “occupy men’s minds so that truth can hardly get in, but also when a truth is allowed in they will push back against it”. A true science, he said, should “solemnly and firmly resolve to deny and reject them all, cleansing our intellect by freeing it from them”.

Bacon was referring to our understanding of the world around us. But his point applies to our morality too. As the philosopher Dale Jamieson has argued, our natural moral understanding is too limited to grasp the moral consequences and responsibility that comes with a problem like climate change, in which diffuse groups of people cause a diffuse set of harms to another diffuse set of people, over a diffuse range of time and space.
Since the “idols of the tribe” are natural and innate, they are tricky to shift. As Jamieson argued, one way to combat them is for individuals to mindfully cultivate green virtues, such as rejecting materialism, humility about your own importance, and a broad empathy with your ecosystem."

For one example of more holistic thinking on climate risks, see also:

Title: "Climate change, water and the spread of diseases: Connecting the dots differently"

https://phys.org/news/2018-09-climate-diseases-dots-differently.html

26
As a follow-on to my Reply #582, I note that the first image shows a major calving event for the Thwaites Ice Tongue in September 2012, which coincided with the collapse of a portion of the subglacial cavity at the base of the Thwaites Ice Tongue as indicated by the abrupt drop in ice surface elevation in the region shown in the second image. This ice flow velocities of the Thwaites Ice Tongue temporarily surged following this September 2012 calving event.

Also, as a follow-on to my Reply #584, the third image shows the specific trenches in the ASE seafloor which advect warm CDW to the grounding of key marine glaciers as shown in the fourth image; which also shows that circa 2040 the CDW will raise-up towards the sea surface due to ice-climate mechanisms (which will accelerate the retreat of the grounding lines for the key Antarctic marine glaciers).

27
The linked reference discusses ice mass loss from the Amundsen Sea Embayment, ASE, with decadal patterns primarily in the ENSO cycle.  The first associate image shows the location of the study (which focused on the Dotson Ice Shelf) w.r.t. the rest of the ASE features (like Thwaites and PIG).  The second image shows the normalized ocean water temperature forcing vs ice mass loss with time for different ice features in the ASE. 

Adrian Jenkins  et al. (2018), "West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability", Nature Geoscience, volume 11, pages733–738, DOI: https://doi.org/10.1038/s41561-018-0207-4

https://www.nature.com/articles/s41561-018-0207-4

Abstract: "Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or an ongoing, possibly unstable, response to a past climate anomaly. Lengthening satellite records appear to be incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000 to 2016 near the Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice-shelf thinning and retreat of the grounding line, which re-advanced during a post-2011 cool phase. These observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The nonlinear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high."

Caption for the first image: "Fig. 1 Locations of Amundsen Sea observations used in this study. a, Map of Antarctica indicating grounded ice sheet (darker shading), floating ice shelves (lighter shading) and study area (blue box). b, Enlargement of study area showing regional bathymetry and catchments (darker shading) of Kohler, Thwaites and Pine Island glaciers (KG, TG, PIG). c, Enlargement of area near the moving Dotson Ice Front (blue box in panel b) showing locations and the years of summer (Dec–Mar) vertical profiles of seawater properties."

Caption for the second image: "Fig. 5 Multi-decadal history of ocean forcing and outlet glacier response in the eastern Amundsen Sea. Time series of glacier outflow changes (righthand axis, with one standard deviation error bars) and ocean forcing (red, warm conditions; blue, cool conditions) as documented here (darker shading) and as inferred (lighter shading) from central tropical Pacific sea surface temperatures (left-hand axis, both normalized). Shaded boxes (outlined and colour-coded by glacier) indicate the range of estimated times for the initiation of the most recent phase of rapid thinning at the grounding lines, while boxes without outlines are inferred times of initial and final detachment of Pine Island Glacier from a submarine ridge."

The third attached image shows alternate graphical representations for short-term climate sensitivity; where in Panel C one could image the red curve indicating ENSO induced fluctuations on climate sensitivity both before and after the potential abrupt collapse of the WAIS.

See also:

Ghil, Michael, Andreas Groth, Dmitri Kondrashov, and Andrew W. Robertson. 2018. “Extratropical sub-seasonal–to–seasonal oscillations and multiple regimes: The dynamical systems view.” The Gap between Weather and Climate Forecasting: Sub-Seasonal to Seasonal Prediction, edited by Andrew W. Robertson and Frederic Vitart, 1st ed., 119-142. Elsevier; DOI: 10.1016/B978-0-12-811714-9.00006-1

https://dept.atmos.ucla.edu/tcd/publications/gap-between-weather-and-climate-forecasting-sub-seasonal-seasonal-prediction

Abstract: "This chapter considers the sub-seasonal–to–seasonal (S2S) prediction problem as intrinsically more difficult than either short-range weather prediction or interannual–to–multidecadal climate prediction. The difficulty arises from the comparable importance of atmospheric initial states and of parameter values in determining the atmospheric evolution on the S2S time scale. The chapter relies on the theoretical framework of dynamical systems and the practical tools this framework helps provide to low-order modeling and prediction of S2S variability. The emphasis is on mid-latitude variability and the complementarity of the nonlinear-waves vs. multiple-regime points of view in understanding this variability. Empirical model reduction and the forecast skill of the models thus produced in real-time prediction are reviewed."

Also, I provide the fourth image for those who like to better appreciate how the warm circumpolar deep water, CDW, flows thru deep channels in the ASE seafloor, particularly during El Nino events.

Finally, I note that the CDW flow pattern shown in the four image helps to explain how the Manhattan-size subglacial cavity formed (and will likely continue growing) at the base of the Thwaites Ice Tongue.

28
As Shared Humanity has encouraged me to provide additional explanatory discussion for many/most of my posts, I return to the topic of the Manhattan-sized subglacial cavity at the base of the Thwaites Ice Tongue in Reply #552, where I provided the first image from Milillo et al. (2019).  Among other things, Panel C of this image shows a 2011.5 grounding line, a 2017.91 grounding line and the color shading shows the change in surface elevation between 2011.5 and 2017.51.

P. Milillo et al. (30 Jan 2019), "Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica", Science Advances, Vol. 5, no. 1, eaau3433, DOI: 10.1126/sciadv.aau3433

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

The extension of the grounding line from 2011.5 to 2017.91 occurs along the subglacial trench shown in the second image, and the change in surface elevation creates crevasses that eventually produce floating icebergs as shown in the third image.  The fourth image shows a Sentinel image for this area for January 7 2019; which shows plenty of crevasse in the ice immediately downstream of the 2017.91 grounding line; which in turn implies that icebergs could float out from this subglacial trench area as soon as the downstream mélange of icebergs float away within a decade or so.  If this is the case, then Panel A and B of the first image clearly show that any ice cliffs near the T2 or T4 areas of the subglacial trench would have sufficiently high ice faces above sea level to induce local ice cliff failures.  If so the local grounding line could rapidly down the negative bed slope into the Byrd Subglacial Basin anytime after about 2035.  Such failure mechanisms are not considered by consensus climate scientists.

29
I skimmed thru the Edwards paper, but the first thing that struck me that the diasgreement with pollard/diconto is primarily driven by disagreement on plausibility of high pliocene sealevel data. I shall have to read more carefully when i have some time.

sidd

I am generally impressed by the level of effort (and the quality of the work) that Pollard et al. have gone to in order to calibrate their MICI model of Antarctica, including the two linked 2018 references.  Thus, if Pollard & DeConto declined to be co-authors on the Edward et al. reference; I think that they did so for a very good reason.

D. Pollard et al. (11 September 2018), "Estimating Modern Elevations of Pliocene Shorelines Using a Coupled Ice Sheet‐Earth‐Sea Level Model", JGR Earth Surface, https://doi.org/10.1029/2018JF004745

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JF004745

Abstract: "A coupled ice sheet‐Earth‐sea level model is used to estimate the modern elevations of shoreline features that were formed at high sea level stands during the warm mid Pliocene ~3 million years ago. Knowledge of global mean sea level during this period is important as an indicator of possible future ice sheet retreat and sea level rise. However, local shoreline elevations can deviate from the eustatic mean by various geologic processes over the last 3 million years, including glacial isostatic adjustment of the solid Earth and gravitational field due to both Pliocene ice‐cover changes and more recent glacial cycles. Our coupled model includes glacial isostatic adjustment processes and simulates Antarctic ice sheet, global sea level, and solid Earth variations in the warmest mid‐Pliocene and over the last 40,000 years. Global maps of estimated modern elevations of Pliocene shoreline markers are produced for a standard radial profile of Earth viscosity and lithospheric thickness. Results are compared to an earlier study with an uncoupled Earth‐sea level model and a different methodology (Raymo et al., Nature Geoscience, 2011, https://doi.org:/10.1038/ngeo1118). As in that study, Pliocene shoreline elevations diverge significantly from the eustatic value in widespread regions, especially in the vicinity of present and former ice sheets. In some other regions, elevations are close to eustatic. The results emphasize that care should be taken in interpreting elevations of paleo‐shoreline markers."
&

David Pollard, Robert M. DeConto, Richard B. Alley (13 March 2018), "A continuum model of ice mélange and its role during retreat of the Antarctic Ice Sheet", Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2018-28

https://www.geosci-model-dev-discuss.net/gmd-2018-28/gmd-2018-28.pdf

Abstract: "Rapidly retreating thick ice fronts can generate large amounts of mélange (floating ice debris), which may affect episodes of rapid retreat of Antarctic marine ice. In modern Greenland fjords, mélange provides substantial back pressure on calving ice faces, which slows ice-front velocities and calving rates. On the much larger scales of West Antarctica, it is unknown if mélange could clog seaways and provide enough back pressure to act as a negative feedback slowing retreat.  Here we describe a new mélange model, using a continuum mechanical formulation that is computationally feasible for long-term continental Antarctic applications. It is tested in an idealized rectangular channel and calibrated very basically using observed modern conditions in Jakobshavn fjord, West Greenland. The model is then applied to drastic retreat of Antarctic ice in response to warm mid-Pliocene climate. With mélange parameter values that yield reasonable modern Jakobshavn results, Antarctic marine ice still retreats drastically in the Pliocene simulations, with little slowdown despite the huge amounts of mélange generated. This holds both for the rapid early collapse of West Antarctica, and later retreat into major East Antarctic basins. If parameter values are changed to make the mélange much more resistive to flow, far outside the range for reasonable Jakobshavn results, West Antarctica still collapses and retreat is slowed or prevented only in a few East Antarctic basins."

Edit: The attached image comes from Pollard, DeConto & Alley (2018), the second linked reference cited above, which indicates a relatively rapid rate of MICI collapse once mid-Pliocene conditions have been reached in modern times:

30
The UK's Met Office has projected that most likely, sometime in the next five years, GMSTA will at least temporarily exceed the IPCC's aspirational 1.5C target:

Title: "Met Office: World has 10% chance of ‘overshooting’ 1.5C within five years"

https://www.carbonbrief.org/met-office-world-has-10-per-cent-chance-overshooting-1-point-5-c-within-five-years

Extract: "The results find that, over a five-year period from November 2018 to October 2023, the global average temperature rise is likely to be between 1.03C and 1.57C. This is shown on the chart below, where blue shading represents the range of expected temperature rise for this period, when compared to temperatures in the “pre-industrial period” (1850-1990)."

Caption: "Expected rise in global temperature from November 2018 to October 2023 (blue), relative to “pre-industrial” temperatures (1850-1900). Actual temperature rise from 1960 to October 2018 is shown in black, the results of previous Met Office decadal projections are shown in red (hindcast) and results from the “Coupled Model Intercomparison Project” (CMIP5) are shown in green. There is a gap between the black line and blue shading because the observational data finishes in October 2018 and the projections start in November 2018. Shading shows range of confidence. Source: Met Office."

See also:

https://www.metoffice.gov.uk/news/releases/2019/forecast-suggests-earths-warmest-period


31
Per the linked article, the current projected weak El Nino event (see image), will likely keep the 2019 GMSTA above the 1.1C value for 2018:

James Hansen, Makiko Sato, Reto Ruedy, Gavin A. Schmidt and Ken Lo (06 February 2019), "Global Temperature in 2018 and Beyond"

http://www.columbia.edu/~jeh1/mailings/2019/20190206_Temperature2018.pdf

Abstract.  Global surface temperature in 2018 was the 4th highest in the period of instrumental measurements in the Goddard Institute for Space Studies (GISS) analysis.  The 2018 global temperature was +1.1°C (~2°F) warmer than in the 1880-1920 base period; we take that base period as an estimate of ‘pre-industrial’ temperature.  The four warmest years in the GISS record all occur in the past four years, and the 10 warmest years are all in the 21st century.  We also discuss the prospects for near-term global temperature change.

32
For your edification, I provide the following two relevant references:

Shengan Zhan et al. (2019), "A global assessment of terrestrial evapotranspiration increase due to surface water area change", Earth's Future, https://doi.org/10.1029/2018EF001066

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018EF001066

Abstract
Surface water, which is changing constantly, is a crucial component in the global water cycle as it greatly affects the water flux between the land and the atmosphere through evaporation. However, the influences of changing surface water area on the global water budget have largely been neglected. Here, we estimate an extra water flux of 30.38 ± 15.51 km3/yr omitted in global evaporation calculation caused by a net increase of global surface water area between periods 1984‐1999 and 2000‐2015. Our estimate is at a similar magnitude to the recent average annual change in global evapotranspiration assuming a stationary surface water area. It is also comparable to the estimated trends in various components of the hydrological cycle such as precipitation, discharge, groundwater depletion, and glacier melting. Our findings suggest that the omission of surface water area changes may cause considerable biases in global evaporation estimation, so an improved understanding of water area dynamics and its atmospheric coupling is crucial to reduce the uncertainty in the estimation of future global water budgets.

Plain Language Summary
Past studies have shown that global evapotranspiration has been increasing between the 1980s and 2000 and has been decreasing since 2000. These studies were done assuming surface water body areas (i.e. lakes and rivers) are constant throughout their study periods. However, surface water bodies on earth are changing constantly. Over the past 30 years, more than 90000 km3 of permanent water has disappeared while over 180000 km3 has emerged elsewhere. The conversion between land and water introduces a significant change of evapotranspiration from the earth's surface which has been neglected by past studies. Here, we quantify this change in evapotranspiration caused by such land‐water conversion to reduce the uncertainties in the estimation of global evapotranspiration trend. We find an increase in evapotranspiration caused by land‐water conversion of 30.38 {plus minus} 15.51 km3/yr between 1984‐1999 and 2000‐2015. The magnitude of this change is comparable to that of annual global evapotranspiration change assuming stationary surface water areas. Thus, surface water dynamics can lead to considerable changes in global evapotranspiration and should not be neglected in future global water budget studies.


&

Schröder, L., Horwath, M., Dietrich, R., Helm, V., van den Broeke, M. R., and Ligtenberg, S. R. M.: Four decades of Antarctic surface elevation changes from multi-mission satellite altimetry, The Cryosphere, 13, 427-449, https://doi.org/10.5194/tc-13-427-2019, 2019.

https://www.the-cryosphere.net/13/427/2019/

Abstract
We developed a multi-mission satellite altimetry analysis over the Antarctic Ice Sheet which comprises Seasat, Geosat, ERS-1, ERS-2, Envisat, ICESat and CryoSat-2. After a consistent reprocessing and a stepwise calibration of the inter-mission offsets, we obtained monthly grids of multi-mission surface elevation change (SEC) with respect to the reference epoch 09/2010 (in the format of month/year) from 1978 to 2017. A validation with independent elevation changes from in situ and airborne observations as well as a comparison with a firn model proves that the different missions and observation modes have been successfully combined to a seamless multi-mission time series. For coastal East Antarctica, even Seasat and Geosat provide reliable information and, hence, allow for the analysis of four decades of elevation changes. The spatial and temporal resolution of our result allows for the identification of when and where significant changes in elevation occurred. These time series add detailed information to the evolution of surface elevation in such key regions as Pine Island Glacier, Totten Glacier, Dronning Maud Land or Lake Vostok. After applying a density mask, we calculated time series of mass changes and found that the Antarctic Ice Sheet north of 81.5∘ S was losing mass at an average rate of −85±16-85±16  Gt yr−1 between 1992 and 2017, which accelerated to −137±25-137±25  Gt yr−1 after 2010.

33
The linked article (and following associated references), cites recent model work about ice sheet stability that gives more conservative estimates of abrupt ice mass lost from ice sheets this century, than that predicted by Pollard & DeConto (2016).  However, for me one critical consideration about these findings is the both Pollard & DeConto declined to serve as co-authors for this work because they believe that the projected SLR are too conservative.

Title: "Studies shed new light on Antarctica’s future contribution to sea level rise"

https://www.carbonbrief.org/studies-shed-new-light-on-antarcticas-future-contribution-to-sea-level-rise

Extract: "The findings of the new study show that “the jury’s definitely still out on MICI”, says Edwards. There is a real lack of published studies that incorporate the process, she says, adding: “We really need much higher resolution models to try including it, which test different representations.”

However, it certainly “does not mean that MICI is irrelevant and must be forgotten”, says Dr Cyrille Mosbeux, a postdoctoral scholar at the Scripps Institution of Oceanography who was not involved in either study. He tells Carbon Brief: “We still need to take account for as many processes as possible” in sea level rise estimates.

As “very little is known about MICI and it is hard to predict its effect in the future”, the new findings emphasise the need for improved models and continued observations around Antarctica, he adds.

Considering the theory of MICI was proposed less than eight years ago, it is still very early in terms of refining the estimates of what impact it could have. However, as scientific disagreements go, this is definitely the more cordial kind. Both DeConto and Pollard were originally co-authors on the new paper. They later recused themselves because they felt the results coming from Edwards’s statistical model were not consistent with what they were seeing from their own physics-based glacier model."

See also:

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

&
Seroussi, H. (2019) Fate and future role of polar ice sheets, Nature


34
Current (e.g. CMIP5) ESM projections have a difficult time estimating Polar Amplification, because of the numerous regional climate feedbacks; and the linked reference discusses more consensus work to include more accurate regional climate feedbacks into future (e.g. CMIP6).  While such hard consensus work is commendable; it generally ignores ice-climate feedback mechanisms such as that cited in Reply #574.  Hopefully, CMIP7 will make a bigger effort to model the numerous ice-climate feedback mechanisms.

Hugues Goosse et al. (2018), "Quantifying climate feedbacks in polar regions", Nature Communications, volume 9, Article number: 1919, doi: https://doi.org/10.1038/s41467-018-04173-0

https://www.nature.com/articles/s41467-018-04173-0

Abstract: "The concept of feedback is key in assessing whether a perturbation to a system is amplified or damped by mechanisms internal to the system. In polar regions, climate dynamics are controlled by both radiative and non-radiative interactions between the atmosphere, ocean, sea ice, ice sheets and land surfaces. Precisely quantifying polar feedbacks is required for a process-oriented evaluation of climate models, a clear understanding of the processes responsible for polar climate changes, and a reduction in uncertainty associated with model projections. This quantification can be performed using a simple and consistent approach that is valid for a wide range of feedbacks, offering the opportunity for more systematic feedback analyses and a better understanding of polar climate changes."

35
Abrupt, Any slowdown on the MOC would also reduce the ocean carbon sink. The North Atlantic down welling area sends vast amounts of organic matter and carbonates into the deep ocean where they remain until upwelling brings DIC ( dissolved inorganic carbon ) back into atmospheric contact , a cycle lasting hundreds of years to more than a thousand years .A slowdown with down welling in the Southern end of the MOC and would also result in a reduction in the ocean carbon sink.

Bruce,

All good points; which imply that not only would a slowdown of the MOC be a strong positive feedback mechanism (leading to a higher effective ECS); but would also lead to a stratified ocean (Canfield Ocean), with poor oxygen distribution and increased hydrogen sulfide content.

See:

Title "Canfield Ocean"
https://en.wikipedia.org/wiki/Canfield_ocean

Best,
ASLR

Edit: To point out the obvious implications of the two previously posted images (see Reply #570); the first image shows that the launch of an armada of icebergs from the Amundsen Sea Embayment, ASE, would rapidly float westward and would thus rapidly freshen the coastal surface waters over all AABW sources shown in the second image.  This freshening would slow AABW production at all sources essentially simultaneously; which would slow the MOC much quicker than considered by consensus climate science.

Edit2: For those who want a better numerical quantification of the impact on the MOC of the mechanism that I cite in my first Edit, I provide the third attached image.

36
The linked reference (& associated article) raises yet another potential longer-term positive climate feedback (not considered by consensus climate models) from a potential abrupt sea level rise in the coming decades; which is that as sea level rises and floods coastal areas, the carbon buried in the newly flooded coastal lands can become reactivated and emitted into the atmosphere (see the attached image) for thousands of years after the abrupt sea level rise:

Shelby L. Lyons et al. Palaeocene–Eocene Thermal Maximum prolonged by fossil carbon oxidation, Nature Geoscience (2018). DOI: 10.1038/s41561-018-0277-3

https://www.nature.com/articles/s41561-018-0277-3

Abstract: "A hallmark of the rapid and massive release of carbon during the Palaeocene–Eocene Thermal Maximum is the global negative carbon isotope excursion. The delayed recovery of the carbon isotope excursion, however, indicates that CO2 inputs continued well after the initial rapid onset, although there is no consensus about the source of this secondary carbon. Here we suggest this secondary input might have derived partly from the oxidation of remobilized sedimentary fossil carbon. We measured the biomarker indicators of thermal maturation in shelf records from the US Mid-Atlantic coast, constructed biomarker mixing models to constrain the amount of fossil carbon in US Mid-Atlantic and Tanzania coastal records, estimated the fossil carbon accumulation rate in coastal sediments and determined the range of global CO2 release from fossil carbon reservoirs. This work provides evidence for an order of magnitude increase in fossil carbon delivery to the oceans that began ~10–20 kyr after the event onset and demonstrates that the oxidation of remobilized fossil carbon released between 102 and 104 PgC as CO2 during the body of the Palaeocene–Eocene Thermal Maximum. The estimated mass is sufficient to have sustained the elevated atmospheric CO2 levels required by the prolonged global carbon isotope excursion. Even after considering uncertainties in the sedimentation rates, these results indicate that the enhanced erosion, mobilization and oxidation of ancient sedimentary carbon contributed to the delayed recovery of the climate system for many thousands of years."

See also:

Title: "Ancient climate change triggered warming that lasted thousands of years"

https://phys.org/news/2019-01-ancient-climate-triggered-thousands-years.html

Extract: ""We found evidence for a feedback that occurs with rapid warming that can release even more carbon dioxide into the atmosphere," said Shelby Lyons, a doctoral student in geosciences at Penn State. "This feedback may have extended the PETM climate event for tens or hundreds of thousands of years. We hypothesize this is also something that could occur in the future."

Increased erosion during the PETM, approximately 56 million years ago, freed large amounts of fossil carbon stored in rocks and released enough carbon dioxide, a greenhouse gas, into the atmosphere to impact temperatures long term, researchers said.

"One lesson we can learn from this research is that carbon is not stored very well on land when the climate gets wet and hot," Freeman said. "Today, we're pushing the system out of equilibrium and it's not going to snap back, even when we start reducing carbon dioxide emissions.""

Edit: See also:

https://www.sciencedaily.com/releases/2019/01/190122104515.htm



37
The linked paper provides useful analysis of the global GHG emissions impact of the USA's current fracking industry trends.  This finding indicate not only higher US but also higher non-US GHG emissions, and also finding that the additional methane emissions will dominate the impact of the additional CO₂ emissions:

The Greenhouse Gas Impacts of Increased US Oil and Gas Production
Feb 5, 2019 | Daniel Raimi


http://www.rff.org/research/publications/greenhouse-gas-impacts-increased-us-oil-and-gas-production?utm_source=newsletter&utm_medium=email&utm_campaign=newsletter_axiosgenerate&stream=top#_ga=2.176012642.1516993.1549398049-1865345678.1536676068

Key Findings
•   Comparing a high and low oil and gas production scenario, US emissions are 2% to 10% higher in 2030.
•   Non-US CO2 emissions are 450 million metric tons higher, similar to the 2016 fossil fuel emissions of Brazil.
•   The greenhouse gas impact of increased US oil production may be more substantial than increased gas production.
•   Differences in US GHG emissions under different scenarios are driven by methane, rather than carbon dioxide emissions.

Abstract: "Increased oil and natural gas production in the United States has decreased domestic natural gas prices and global oil prices, with major economic and environmental consequences. The resulting greenhouse gas (GHG) impacts have received substantial attention, with most focus on natural gas and relatively little on oil. In this paper, I provide a more comprehensive estimate of how increased production affects these emissions through changes in the US energy mix, associated methane emissions, and—crucially—global oil prices. Under a high oil and gas production scenario, US GHG emissions in 2030 are roughly 100 to 600 million metric tons of carbon dioxide equivalent (2 to 10 percent) higher than under a low production scenario with a range of assumptions about methane emissions. Under the high production scenario, lower global oil prices and increased consumption raise global carbon dioxide emissions in 2030 by roughly 450 to 900 million metric tons relative to a low production scenario, equivalent to 8 to 16 percent of projected US GHG emissions in 2030. This global projection assumes that OPEC or other nations do not coordinate production cuts to offset US gains. It also does not include domestic and global welfare effects of increased US production, which may be positive because of welfare gains from lower energy prices."

38
As both the Meridional Overturning Circulation, MOC, and associated ocean circulation patterns in the Southern Ocean, play a major role in ice-climate feedback mechanisms (including Agulhaus current leakage, CO2 venting, etc), I thought that I would post the two attached images to provide unfamiliar readers with a better frame of context:

Edit:  The caption for the third attached image (showing the sources of AABW formation) is as follows:

Caption for the third image: "The amount of Antarctic bottom water generated governs the relative strength of global deep ocean circulation. The Weddell and Ross Seas are well known regions of Antarctic bottom water formation, but in recent years it has emerged that the waters off Adelie Land in the vicinity of East 140 degrees Longitude is also an important region for bottom water generation"

Edit2: The fourth image shows that path that icebergs from the ASE would follow; westward along the Antarctic Coastal Current to the Weddell Sea, where the Antarctic Peninsula would kick the icebergs northward into the ACC (Antarctic Circumpolar Current), where-after they would circulate eastward around Antarctica.

39
what is the mechanism for soil drying to lead to increased ECS?
jai,

In addition to Bruce's excellent coverage of the direct positive carbon cycle feedback associated with soil moisture drying, I also note that per the information given in Reply #561, as lot of the soil drying is projected to occur in tropic rainforests (like the Amazon); which when coupled with additional rainfall and vegetation growth in Arctic regions, results in a feedback that induces the MOC to slowdown.  Per Hansen et al (2016), a slowing of the MOC contributes to a higher ECS due to such factors as the increased warming of, and evaporation from, the tropical oceans and increased penetration of the AMOC into the Arctic Ocean Basin near Norway.

40
The linked website provides biweekly plots of the observed ice flow velocities for the Thwaites Glacier since early 2014.  In this regards, the attached image shows the velocities for the period of Dec 20, 2018 (most of which is off the chart's scale) vs the mean since 2014, which shows that there has been a recent surge in velocities, possibly associated with (among other things) the loss of ice shelf buttressing of the Southwest Tributary Glacier (which is associated with the eastern shear margin of Thwaites Glacier).

http://www.cpom.ucl.ac.uk/csopr/iv/index.html?glacier_number=4&image_date=160305_160317

Edit: The second image shows the ice velocities for Nov 26 2018.

41
The linked reference indicates that Permafrost carbon feedback (PCF) will be stronger than currently assumed by consensus climate change models:

Katey Walter Anthony, Thomas Schneider von Deimling, Ingmar Nitze, Steve Frolking, Abraham Emond, Ronald Daanen, Peter Anthony, Prajna Lindgren, Benjamin Jones, Guido Grosse. 21st-century modeled permafrost carbon emissions accelerated by abrupt thaw beneath lakes. Nature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-05738-9

http://www.nature.com/articles/s41467-018-05738-9

Extract: "These finding demonstrate the need to incorporate abrupt thaw processes in earth system models for more comprehensive projection of the PCF this century."

See also:

Title: "'Abrupt thaw' of permafrost beneath lakes could significantly affect climate change models"

https://www.sciencedaily.com/releases/2018/08/180816143035.htm

42
The linked reference provides additional evidence that with continued anthropogenic radiative forcing, global soil moisture (SM) drying will act as a positive feedback mechanism, indicating higher values of ECS than currently assumed by consensus climate science:

Xihui Gu et al (31 January 2019), "Attribution of global soil moisture drying to human activities: a quantitative viewpoint", Geophysical Research Letters, https://doi.org/10.1029/2018GL080768

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL080768

Abstract
Anthropogenic impacts on widespread global soil moisture (SM) drying in the root zone layer during 1948‐2005 were evaluated based on the Global Land Data Assimilation System version 2 (GLDAS‐2) and Global Climate Models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) using trend analysis and optimal fingerprint methods. Both methods show agreement that natural forcing alone cannot drive significant SM drying. There is a high probability (≥90%) that the anthropogenic climate change signal is detectable in global SM drying. Specifically, anthropogenic greenhouse gas forcing can lead to global SM drying by 2.1×10‐3 m3/m3, which is comparable to the drying trend seen in GLDAS‐2 (2.4×10‐3 m3/m3) over the past 58 years. Global SM drying is expected to continue in the future, given continuous greenhouse gas emissions.

Plain Language Summary
Satellite observations and model simulations indicated widespread soil moisture (SM) drying in the root zone layer. Global‐scale SM drying has also been corroborated by meteorological drought indices. SM drying can accentuate the intensity of heat waves under global warming. Recent record‐breaking heat waves were amplified by SM drying, such as the 2003 European heat waves and 2010 Russia heat waves. the contributions of human activities to global‐scale SM changes have not been comprehensively evaluated. There is a high probability (≧90%) that the anthropogenic climate change signal in global SM drying is detectable. Specifically, anthropogenic greenhouse gas forcing can lead to global SM drying by 2.1×10‐3 m3/m3, which is comparable to the drying trend seen in GLDAS‐2 (2.4×10‐3 m3/m3) over the past 58 years. Global SM drying is expected to continue in the future, given continuous greenhouse gas emissions.

43
The linked article and associated linked reference, indicate that the WAIS grounding lines had retreated further 10,000 years ago (10 kya) than in modern times (see the first image & the following caption for the first image), will not save us from a potential WAIS collapse in the coming decades, due to the difference in time-scale of glacio-isostatic rebound then vs now.  However, in this post I note that this difference in time-scales for the glacio-isostatic rebound then & now, is not the only reason such paleo-evidence will not protect modern society from a potential WAIS collapse in the coming decades, including the following considerations:

1. The rate of change of net radiative forcing had essentially stopped by the Holocene Climate Optimum; whereas today the rate of increase of radiative forcing is so fast that numerous decadal-scale positive feedback mechanisms do not have sufficient time to dissipate before other decadal-scale positive feedback mechanism are trigger, which contribute to increased risk of a coming cascade of decadal-scale tipping points.  If for no other reason this consideration alone is sufficient to push GMSTA well above that during the Holocene Climate Optimum, which in turn increases the risk that hydrofracturing may collapse key West Antarctic ice shelves in coming decades; which would effectively eliminate the mechanism that allowed the WAIS to grow 10 kya.

2. The Antarctic Ozone Hole (created by anthropogenic emissions), lead to an increase of the velocities of westerly winds over the Southern Ocean (since about 1970); which has increased ocean upwelling; which has caused increased basal ice melting of key West Antarctic ice shelves and accelerated grounding line retreat for key WAIS marine glaciers.  As the ozone hole has begun to heal itself, the concurrent increase in regional atmospheric GHG concentrations have kept the westerly wind velocities in a near optimal range for inducing upwelling of warm CDW.

3. The freshening of the surface seawater near Antarctica together with an associated increase in Antarctic sea ice extent, has accelerated the advection of upwelled warm CDW over the local continental shelves where the warm CDW (circumpolar deep water) has accelerated ice mass loss of key ice shelves & marine glaciers.

4. Advanced ESMs have projected the increase in frequency of El Nino events, which Hansen has noted is already observable in the ENSO record (see the second image).  Not only will this trend increase the advection/telecommunication of heat energy from the Tropical Pacific Ocean to coastal regions of West Antarctica, but also will contribute eventually to increased regional snowfall; which will increase the gravitational head driving a potential MICI collapse of the WASI.

5. Atmospheric GHG concentrations are currently much higher than during the Holocene Climate Optimum; which will likely lead to still further acceleration of radiative forcing as anthropogenic aerosol emissions are rapidly reduced (and the remaining emission sources shifted to less optimal locations) in coming decades.

6. Finally, I noted that anthropogenic GHG emissions have been increasing since circa 1750, which has activated several  positive oceanic feedback mechanisms; and also the AMOC is currently slowing for at least the next several decades, which is a key positive ice-climate mechanism:


Potsdam Institute for Climate Impact Research (PIK). "What saved the West Antarctic Ice Sheet 10,000 years ago will not save it today." ScienceDaily. ScienceDaily, 14 June 2018.

https://www.sciencedaily.com/releases/2018/06/180614101245.htm

Summary: "The retreat of the West Antarctic ice masses after the last Ice Age was reversed surprisingly about 10,000 years ago, scientists found. The reason for the rebound is that, relieved from the weight of the retreating ice, the Earth crust lifted. This made the ice re-advance towards the ocean. Unfortunately, this mechanism is much to slow to prevent dangerous sea-level rise caused by West Antarctica's ice-loss in the present and near future."

Caption for first image: "New data indicates that the retreat of the West Antarctic ice masses after the last Ice Age in some parts of the continent was reversed surprisingly about 10,000 years ago. The maximum ice sheet extension is shown in green, the minimum extent in red, and the modern grounding line after the rebound in orange.

Extract: "A number of factors influences the ice-sheet behavior under warming. In the studied region sea mountains turned out to be rather important for the ice dynamics. The peaks of these mountains underneath the floating ice shelves reach up from the bottom of the ocean. When the bottom rises they can become ice rises within the ice shelf. Since they're made of solid rock, they increase the stability of the ice sheet. The scientists call this a buttressing effect. Conditions for ice re-growth might be less favorable in other areas.

Yet it is the time-scale that is key in the end. "What happened roughly 10,000 years ago might not dictate where we're going in our carbon dioxide-enhanced world, in which the oceans are rapidly warming in the Polar regions," says Scherer. "If the ice sheet were to dramatically retreat now, triggered by anthropogenic warming, the uplift process won't help regrow the ice sheet until long after coastal cities have felt the effects of sea level rise.""

See also:

J. Kingslake, R. P. Scherer, T. Albrecht, J. Coenen, R. D. Powell, R. Reese, N. D. Stansell, S. Tulaczyk, M. G. Wearing, P. L. Whitehouse. Extensive retreat and re-advance of the West Antarctic Ice Sheet during the Holocene. Nature, 2018; DOI: 10.1038/s41586-018-0208-x

https://www.nature.com/articles/s41586-018-0208-x



44
James Hansen has projected that ice-climate feedback may soon contribute to the creation of Category 6 hurricanes:

Title: "This is how the world ends: will we soon see category 6 hurricanes?"

https://peakoil.com/enviroment/this-is-how-the-world-ends-will-we-soon-see-category-6-hurricanes

Extract: "Meteorologists and scientists never imagined that there would be a need for a category 6 storm, with winds that exceed 200 miles per hour on a sustained basis, sweeping away everything in its path. Until now, such a storm wasn’t possible, so there was no need for a new category above category 5.

Jeff Masters, one of the most respected meteorologists in America, has begun to wonder publicly about the potential for a category 6 hurricane. He launched a lively debate among his colleagues with a provocative post in July of 2016 on the Weather Underground – a thought-provoking piece that prompted the Weather Channel and others to weigh in with their thoughts and theories as well."

45
The linked reference finds that climate change induced changes in LCC (land cover change, such as the dieback of the Amazon rainforest and the northward advancement of vegetation), will serve to slow the AMOC (which will reinforce any coming ice-climate feedbacks):

Armstrong et al. (2019), "Investigating the feedbacks between CO2, vegetation and the AMOC in a coupled climate model", Climate Dynamics, pp 1–16, https://doi.org/10.1007/s00382-019-04634-2

https://link.springer.com/article/10.1007%2Fs00382-019-04634-2

Abstract: "The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system, however its sensitivity to the terrestrial biosphere has been largely overlooked. Here the HadCM3 coupled climate model is run for millennial timescales to investigate the feedbacks between vegetation and the AMOC at increasing CO2. The impact of agricultural conversion (termed land-use change; LUC) and the role of the simulated ‘background’ vegetation (termed land cover change; LCC) are investigated. LUC cools climate in regions of high crop fraction due to increased albedo. LCC is shown to evolve at higher CO2, with a northward migration of the tree line in the Northern Hemisphere and dieback of the Amazon. This generally acts to enhance the impact of climate change primarily due to albedo changes. Density in the Greenland-Iceland-Norwegian (GIN) Seas is crucial in driving the AMOC. Increasing CO2 decreases regional sea surface density, reducing convection and weakening the AMOC. The inclusion of LCC is shown to be responsible for a significant proportion of this weakening; reflecting the amplification effect it has on climate change. This acts to decrease the surface density in the GIN Seas. At elevated CO2 (1400 ppm) the inclusion of dynamic vegetation is shown to drive a reduction in AMOC strength from 6 to 20%. Despite the cooling effect of LUC, the impact on the AMOC is shown to be small reflecting minimal impact it has on GIN Sea density. These results indicate the importance of including dynamic vegetation in future AMOC studies using HadCM3, but LUC may be insignificant. In the context of other climate models however, the importance of vegetation is likely to be overshadowed by other systemic model biases."

46
The linked reference suggests that it may be better to report GMSTA projection from ESMs that do a better job of projection such multi-decadal parameters as the IPO and the AMV, over multi-decadal periods.  If the IPCC adopted such a practice they would likely give more weight to ESM projections that also have high ECS values:

Jules B. Kajtar et al. (2019), "Global mean surface temperature response to large‐scale patterns of variability in observations and CMIP5", Geophysical Research Letters, https://doi.org/10.1029/2018GL081462

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL081462

Abstract
Global mean surface temperature (GMST) fluctuates over decadal to multidecadal time‐scales. Patterns of internal variability are partly responsible, but the relationships can be conflated by anthropogenically‐forced signals. Here we adopt a physically‐based method of separating internal variability from forced responses to examine how trends in large‐scale patterns, specifically the Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Variability (AMV), influence GMST. After removing the forced responses, observed variability of GMST is close to the central estimates of Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations, but models tend to underestimate IPO variability at time‐scales >10 years, and AMV at time‐scales >20 years. Correlations between GMST trends and these patterns are also underrepresented, most strongly at 10‐ and 35‐year time‐scales, for IPO and AMV respectively. Strikingly, models that simulate stronger variability of IPO and AMV also exhibit stronger relationships between these patterns and GMST, predominately at the 10‐ and 35‐year time‐scales, respectively.

Plain Language Summary
Despite the smooth and steady increase of greenhouse gas concentrations, the rate of global warming has not been as stable over the past century. There are periods of stronger warming, or even slight cooling, in the global mean temperature record, which can persist for several years or longer. These changes have been linked to regional climate patterns, most notably within the Pacific and Atlantic Ocean climate systems. Climate models do not exhibit the same level of variations in these Pacific and Atlantic oscillations as compared to the observed record, and the connections between these oscillations and the global temperature are also diminished. However, there is a tendency for those models that show stronger Pacific and Atlantic oscillations to also exhibit stronger relationships between these patterns and global temperature changes.

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To date I have posted only a limited amount of information about the risks of abrupt ice-climate feedback impacts (such as proposed by James Hansen, 2016).  This is probably the case because I believe that it is not possible to find solutions to address such abrupt impacts if society continues to decline to face the reality of such potential impacts; and also because assessing the risks and magnitudes of such abrupt ice-climate feedback impacts is both challenging and complex.

Many people (including many consensus climate scientists) seem to assume that the only principal way for the Earth to sustain catastrophic climate impacts is for anthropogenic carbon emissions to continue near a BAU pathway to nominally 2100 (see the first attached image from the first linked reference, and note that for high risk pathways the entire risk PDF shifts to the left and develops a fat right-tail).  However, if the WAIS begins to sustain an abrupt MICI type of collapse beginning around 2040, then the abrupt risk PDF would likely look like the 'fat-tailed' PDF shown in the second image (from the second linked reference); even if we only continue on a BAU carbon emission pathway until about 2030.

Finally, I note that many people only think of the risks associated with abrupt sea level rise, when they think of the potential collapse of the WAIS; however, my prior posts about cascading feedback mechanism tipping points triggered by the perturbance associated with an abrupt MICI type of collapse of the WAIS, should be sufficient to dispel such thinking:

Yangyang Xu and Veerabhadran Ramanathan (September 26, 2017), "Well below 2 °C: Mitigation strategies for avoiding dangerous to catastrophic climate changes", PNAS 114 (39) 10315-10323; https://doi.org/10.1073/pnas.1618481114

https://www.pnas.org/content/114/39/10315

Caption for first image: "Probability density function of projected warming for 2050 (A) and 2100 (B) for the baseline-fast (thick red line) and baseline-default (thick black line) scenarios. The base year for the warming estimates is 1900. The red dashed line shows the projection forced by the baseline-fast CO2 emission, but a positive carbon cycle feedback due to the ocean and land carbon uptake reduction is included. The blue dashed line in B shows the projection in which the aerosol forcing uncertainty is considered as well."
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From: NAS (2013), "Abrupt Impacts of Climate Change: Anticipating Surprises - Chapter: 4 The Way Forward"

https://www.nap.edu/read/18373/chapter/6#151

Caption for second image: "FIGURE The graph on the left represents the skewed distribution of uncertainties with a “fat tail.” The mean likelihood of occurrence at the level of severity anticipated is represented by a dotted line. The area to the left of the mean represents the likelihood for impacts less severe (the “a little better” case), while the area to the right shows the greater likelihood for extreme impacts (spanning “a little worse” to “a lot worse” cases). The graph on the right compares the normal distribution (black line) to the “fat tail” distribution (blue line). For some changes, more research has shown that the distribution of possible outcomes includes less likelihood of the most severe outcomes."

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The linked reference contains the cited extract regarding potential climate change surprises which gives a very high confidence that future changes outside the consensus projections cannot be ruled out; and give a medium confidence that consensus climate model projections are more likely to underestimate rather than to overestimate long-term future climate change:

USGCRP, 2017: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, 470 pp.

Title: "Chapter 15: Potential Surprises: Compound Extremes and Tipping Elements"

https://science2017.globalchange.gov/chapter/15/

Extract: "While climate models incorporate important climate processes that can be well quantified, they do not include all of the processes that can contribute to feedbacks, compound extreme events, and abrupt and/or irreversible changes. For this reason, future changes outside the range projected by climate models cannot be ruled out (very high confidence). Moreover, the systematic tendency of climate models to underestimate temperature change during warm paleoclimates suggests that climate models are more likely to underestimate than to overestimate the amount of long-term future change (medium confidence)."

Also, I note that USGCRP (2017) Chapter 15: Potential Surprises provides the following supporting evidence for their medium confidence assertion that consensus climate models underestimate paleo reconstructions of climate sensitivity

Extract: "The second half of this key finding is based upon the tendency of global climate models to underestimate, relative to geological reconstructions, the magnitude of both long-term global mean warming and the amplification of warming at high latitudes in past warm climates (e.g., Salzmann et al. 2013; Goldner et al. 2014; Caballeo and Huber 2013; Lunt et al. 2012)."

Note USGCRP (2017) classifies Medium Confidence as: "Suggestive evidence (a few sources, limited consistency, models incomplete, methods emerging, etc.), competing schools of thought"

Furthermore, the guide to USGCRP (2017) classifies these "Potential Surprises" as 'potential low probability/high consequence "surprises" resulting from climate change' and as 'high-risk tails and bounding scenarios'; and acknowledge that 'knowledge gaps' exist that limit their ability to precisely define the probability/risks associated with these "surprises".

https://science2017.globalchange.gov/chapter/front-matter-guide/

Extract: "Complementing this use of risk-focused language and presentation around specific scientific findings in the report, Chapter 15: Potential Surprises provides an overview of potential low probability/high consequence “surprises” resulting from climate change. This includes its analyses of thresholds, also called tipping points, in the climate system and the compounding effects of multiple, interacting climate change impacts whose consequences may be much greater than the sum of the individual impacts. Chapter 15 also highlights critical knowledge gaps that determine the degree to which such high-risk tails and bounding scenarios can be precisely defined, including missing processes and feedbacks."

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While general in nature, the linked Pik-Postdam website discusses tipping cascades of significantly interlinked positive feedback mechanisms potentially leading to abrupt climate change:

Title: "DominoES project    Domino Effects in the Earth System: Can Antarctica tip climate policy?"

https://www.pik-potsdam.de/research/projects/activities/dominoes

Extract: "Tipping elements are components of the Earth system that could be pushed into qualitatively different states by small external perturbations, with profound environmental impacts possibly endangering the livelihoods of millions of people. There are indications for significant interlinkages between climate tipping elements and even the potential for tipping cascades or domino effects from the climate to the social sphere. We will assess these effects for a highly relevant tipping chain connecting climatic tipping elements like Antarctica and Greenland with potential social tipping processes in public opinion formation and climate policy changes, and their societal implications.

DominoES is a joint project by the Potsdam Institute for Climate Impact Research (PIK) and the Leibniz Institute for the Social Sciences (GESIS) funded by the Leibniz Association (2017 - 2020)."

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It is good idea to keep an eye on Recovery/Slessor/Baily glaciers/ice stream for their possible ice mass loss later this century:

Anja Diez et al. (30 March 2018), "Basal Settings Control Fast Ice Flow in the Recovery/Slessor/Bailey Region, East Antarctica', Geophysical Research Letters, https://doi.org/10.1002/2017GL076601

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL076601

Abstract: "The region of Recovery Glacier, Slessor Glacier, and Bailey Ice Stream, East Antarctica, has remained poorly explored, despite representing the largest potential contributor to future global sea level rise on a centennial to millennial time scale. Here we use new airborne radar data to improve knowledge about the bed topography and investigate controls of fast ice flow. Recovery Glacier is underlain by an 800 km long trough. Its fast flow is controlled by subglacial water in its upstream and topography in its downstream region. Fast flow of Slessor Glacier is controlled by the presence of subglacial water on a rough crystalline bed. Past ice flow of adjacent Recovery and Slessor Glaciers was likely connected via the newly discovered Recovery‐Slessor Gate. Changes in direction and speed of past fast flow likely occurred for upstream parts of Recovery Glacier and between Slessor Glacier and Bailey Ice Stream. Similar changes could also reoccur here in the future."

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