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Messages - AbruptSLR

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1
This is a follow-on post to my Reply #939,  which reviewed selected abstracts from the 2018 WAIS Workshop.  This post provides selected quotes and annotations from somewhat more technical abstracts to better access the current stability of the PIG/Thwaites combined catchment basin, and its risk of transitioning from its current state of a slow MISI-type of collapse to a potentially fast MICI-type of collapse (in the coming decades):

1. Wilson et al. (2018) indicate that with regards to the PIG/Thwaites catchment basin that: " Low mantle viscosity shortens the GIA response time scale from thousands of years, to hundreds or even tens of years."  In this regards I note that: a) the GIA is too slow to pin either the PIG or Thwaites Glacier; b) the high GIA means that early GRACE (gravitational) assessments of ice mass loss from this basin are too low because they did not correct for the mass of magma moving into the basin, and c) the high GIA will result in strong seismic/volcanic activity if/when a MICI-type of collapse occurs.
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/WilsonT2018.pdf

Extract: "Deformation rates measured by GPS in the Amundsen Embayment region are some of the fastest rates ever recorded for glacial isostatic adjustment (GIA).  GIA modeling indicates a very low mantle viscosity, consistent with seismic observations in this region (Barletta et al., 2018).  Low mantle viscosity shortens the GIA response time scale from thousands of years, to hundreds or even tens of years."
&

2. Muro et al. (2018), indicate that the bed conditions for the Thwaites Glacier are highly variable.  While consensus sciences like to focus on the sticky bed conditions that might slow the rate of ice mass loss under a MISI-type of future scenario, under a MICI-type of scenario with ice cliff failures initiating near the base of the Thwaites Ice Tongue; such local sticky bed conditions may have little influence on the rate of ice mass loss, as the calved bergy bits would float away from the calving ice face.
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/MutoA2018.pdf

Extract: "Seismic measurements on Thwaites Glacier show a spatially variable bed, with implications for ice-sheet stability. … Modeling suggests that the grounding-line-retreat rate in response to oceanic warming is strongly influenced by such variations in bed character as well as by the topography, highlighting the need for more geophysical surveys to reveal the bed conditions for Thwaites Glacier and other important outlets."
&

3. Nakayama et al. (2018) discuss "Pathway of Circumpolar Deep Water into Pine Island and Thwaites ice shelf cavities and to their grounding lines".  They find that: "… submesoscales are formed due to instabilities associated with the positive potential vorticity patches located in the sub ice-shelf mixed layer, particularly near strong topographic features."  Consensus scientists may believe that the influence of such submesoscale vorticity patches is random; however, in my opinion the paleorecord of rapid ice shelf collapse and of grounding line retreat for the PIG and Thwaites Glacier indicate that they submesocale vorticity patches contribute to the formation of groves the promote basal ice melting in these ice shelves.
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/NakayamaY2018.pdf

Extract: "We calculate time-mean and time-evolving fields of velocity and investigate the mechanisms of how CDW is transported into the ice shelf cavities and to their grounding lines. We find a prominent submesoscale variability in the ice cavity, with scales of motion O(1-5km) and Rossby numbers O(1). Preliminary analysis shows that these submesoscales are formed due to instabilities associated with the positive potential vorticity patches located in the sub ice-shelf mixed layer, particularly near strong topographic features."
&

4. Parizek et al. (2018)'s work is entitled: "Ice Cliffs: A Region Primed for Enhanced Flow or Failure?", as some consensus scientists were hoping that high stresses upstream of an ice cliff face would accelerate ice thinning sufficiently to inhibit ice cliff failure modes; however, this research indicates that that is not the case.
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/Parizek_B_2018.pdf

Extract: "Therefore, our findings indicate that while higher stresses enhance flow thinning, they do not necessarily cause cliffs to go away."
&

5. Bassis et al (2018)'s work is entitled: "Anatomy of the Marine Ice Cliff Instability".  Some consensus scientists have discounted MICI projections on the basis that the physics of this mechanism is not sufficiently understood to be accepted.  In this regards, Bassis et al (2018) use fracture mechanics to better understand: a) the formation of localize rifts that can help destabilize ice shelves and b) detail the mechanics of how tall ice cliffs on grounded marine glacier fail.  Such research is key to getting MICI-types of projections accepted into future consensus science documents like CMIP7 and AR7.

https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/BassisJ2018.pdf

Extract: "We first tested the model by applying it to study the formation of localized rifts in shear zones of idealized ice shelves. These experiments show that wide rifts localize along the shear margins and portions of the ice shelf where the stress in the ice exceeds the yield strength. These rifts decrease the buttressing capacity of the ice shelves, but can also extend to become the detachment boundary of icebergs. Next, application of the model to idealized glaciers shows that for grounded glaciers, failure localizes near the terminus in “serac” type slumping events followed by buoyant calving of the submerged portion of the glacier. The combination of further failure and ductile flow cause the glacier to thin towards buoyancy resulting in a floating ice tongue consisting of yielded ice—similar to what is currently observed at Jakobshavn Glacier in Greenland."
&

6. McCormack et al. (2018)'s work is entitled: "The impact of bed elevation resolution on Thwaites Glacier ice dynamics".  Their findings indicate that ice model meshes need to be on the order of 500m in order to adequately represent the ice behavior for the threshold of the Thwaites Glacier.  As most consensus science models have not used this level of mesh resolution, the findings of ice mass loss projections presented in such consensus documents as AR5 cannot be relied upon to adequately characterize the risks of rapid ice mass loss initiation in the Thwaites Glacier threshold region:
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/McCormackF2018.pdf

Extract: "The modeled velocities converge for increasing bed elevation resolution and for most of the basin the differences between the 250 m and 500 m simulation velocities are within 5%, which is within the bounds of uncertainty associated with the velocity datasets used to initialize our model simulations. Our results indicate that a bed elevation of 500 m resolution is sufficient in simulating ice dynamics (velocities, basal shear stresses, strain rates) consistent with those using the higher resolution bed elevation data."

2
Hansen warned that if Canadian tar sands are developed, it is game over for climate change action.

Title: "Canada’s Tar Sands Province Elects a Combative New Leader Promising Oil & Pipeline Revival"

https://insideclimatenews.org/news/18042019/tar-sands-future-alberta-premier-conservative-jason-kenney-pipelines-carbon-tax-trudeau

Extract: "The home province of Canada's tar sands elected a combative, conservative leader this week who came out swinging on the side of the country's struggling oil industry. Jason Kenney promised to cancel Alberta's carbon tax, lift a cap on greenhouse gas emissions from the tar sands and create a "war room" to combat the oil industry's opponents."

3
As those who are serious will look at the abstracts from the 2018 WAIS Workshop (see linked agenda), to better access the current stability of the PIG/Thwaites combined catchment basin, and its risk of transitioning from its current state of a slow MISI-type of collapse to a potentially fast MICI-type of collapse (in the coming decades), in this post I will only provide annotated highlights of key abstracts to better characterize this very real risk:

1. Schroeder et al (2018) indicate that: "… thickness change of the Thwaites Eastern Ice Shelf between 1978 and 2009, revealing the loss of over half of its thickness over the past three decades."  As the Eastern Thwaites Ice Shelf continues to thin, its risk of abrupt collapse increases rapidly in coming decades:
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/SchroederD2018.pdf

2. Hoffman et al (2018) indicate that: "Remote-sensing observations and modeling suggest that marine ice-sheet instability may already be occurring for the Thwaites Glacier Basin, West Antarctica. … our results highlight that glacier speed and ice discharge respond quickly to transient forcing (i.e., climate variability) and changes in ice-front geometry, complicating predictions of ice discharge flux on decadal timescales."  This indicates that while the PIG/Thwaites catchment basin is currently in a MISI-state it could transition to a MICI-state on decadal timescales.
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/HoffmanA2018.pdf

3. Schwans et al (2018) indicate that: "Results demonstrate how the timing and pattern of GL retreat on TG depend on bed character, and highlight key areas of TG’s ice tongue that are critical to its stability on short timescales."  I have previously noted that the most likely location for ice cliff fails to begin in the PIG/Thwaites basin is near the base of the Thwaites Ice Tongue (& once initiated could relatively rapidly spread to adjoining areas after the Thwaites Eastern Ice Shelf collapses); and Schwans et al (2018) confirm the critical risk that ice cliff failure many begin in this location on short-timescales.
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/SchwansE2018_0.pdf

4. Lenaerts et al. (2018) indicate that: "Remote sensing data indicate that the Thwaites Glacier (TG) system has experienced rapidly enhanced solid ice discharge, grounding line retreat on a reverse-sloping bed, and ice shelf thinning since the 1990s. Contemporaneous observations of surface mass balance (SMB) indicate that catchment-wide accumulation has not changed; … Controversially, our analysis indicates that high snowfall events on the TG are not controlled by the Amundsen Sea Low, but are clearly linked to atmospheric blocking at mid-latitudes. This atmospheric pattern, aided by the unique location and geometry of TG, enables intrusion of marine air that originates at lower latitudes."  This indicates that near-term increases in snowfall that could increase the stability of the PIIS and Thwaites Eastern Ice Shelf, is not occurring; however, there is a likely potential that high snowfall rates could occur in the PIG/Thwaites basin in a few decades after the key ice shelves have collapsed and ice failures are occurring (and the increased gravitational driving for of such future snowfall would accelerate any ice cliff failures occurring at that future time).
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/LenaertsJ2018.pdf

5. Neff & Steig (2018) indicate that: "Ice and sediment core data suggest that initiation of ice-shelf retreat and ice loss in the ASE may have been in response to atmosphere-ocean forcing from the strong 1939-42 El Niño. … A deep ice core at Hercules Dome, near where East Antarctica meets West in the Transantarctic Mountains, would provide critical boundary conditions for the magnitude and rate of ice-sheet collapse during the last interglacial period (120-130 kyr)."  This implies that data from the strong 1939-42 El Nino event could be used to better calibrate the risk of rapid grounding line retreat at ice-ocean-atmosphere interfaces as shown in the attached first image for the Hercules Dome site.
https://www.waisworkshop.org/sites/waisworkshop.org/files/webform/2018/abstracts/NeffP2018.pdf

6. Chu et al. (2018) indicate that:  "These new observations reveal that subglacial water is being actively routed from Bentley trench into the Pine Island Catchment, mirroring ice flow directions. We find that variations in subglacial hydrology instead of small-scale basal roughness explain the location of the western margin of PIG. Through examinations of bed echo characteristics, our results also show that basal water draining from the Bentley Trench flows into an extensive area of distributed water in the upper Pine Island Catchment, but switches to more concentrated flow paths as the water approaches the grounding line. This water transition is similar to one discovered beneath Thwaites Glacier, suggesting t

4
The linked reference indicates a positive feedback mechanism between early spring rainfall over the Arctic Ocean and increasing reduction in Arctic sea ice albedo, initiated by changes in surface ablation caused by the early rain.  This positive feedback mechanism could contribute to the Arctic albedo flip projected by Hansen many years ago:

Dou, T., Xiao, C., Liu, J., Han, W., Du, Z., Mahoney, A. R., Jones, J., and Eicken, H.: A key factor initiating surface ablation of Arctic sea ice: earlier and increasing liquid precipitation, The Cryosphere, 13, 1233-1246, https://doi.org/10.5194/tc-13-1233-2019, 2019.

https://www.the-cryosphere.net/13/1233/2019/

Abstract
Snow plays an important role in the Arctic climate system, modulating heat transfer in terrestrial and marine environments and controlling feedbacks. Changes in snow depth over Arctic sea ice, particularly in spring, have a strong impact on the surface energy budget, influencing ocean heat loss, ice growth and surface ponding. Snow conditions are sensitive to the phase (solid or liquid) of deposited precipitation. However, variability and potential trends of rain-on-snow events over Arctic sea ice and their role in sea-ice losses are poorly understood. Time series of surface observations at Utqiaġvik, Alaska, reveal rapid reduction in snow depth linked to late-spring rain-on-snow events. Liquid precipitation is key in preconditioning and triggering snow ablation through reduction in surface albedo as well as latent heat release determined by rainfall amount, supported by field observations beginning in 2000 and model results. Rainfall was found to accelerate warming and ripening of the snowpack, with even small amounts (such as 0.3 mm recorded on 24 May 2017) triggering the transition from the warming phase into the ripening phase. Subsequently, direct heat input drives snowmelt, with water content of the snowpack increasing until meltwater output occurs, with an associated rapid decrease in snow depth. Rainfall during the ripening phase can further raise water content in the snow layer, prompting onset of the meltwater output phase in the snowpack. First spring rainfall in Utqiaġvik has been observed to shift to earlier dates since the 1970s, in particular after the mid-1990s. Early melt season rainfall and its fraction of total annual precipitation also exhibit an increasing trend. These changes of precipitation over sea ice may have profound impacts on ice melt through feedbacks involving earlier onset of surface melt.

5
Methane emissions from ice sheets are currently ignored by consensus climate science, but the linked reference (& associated linked article) demonstrate that the Greenland Ice Sheet releases significant quantities of methane, and the researchers suspect that the Antarctic Ice Sheet is potentially an even larger source of methane emissions:

Guillaume Lamarche-Gagnon et al. Greenland melt drives continuous export of methane from the ice-sheet bed, Nature (2018). DOI: 10.1038/s41586-018-0800-0

https://www.nature.com/articles/s41586-018-0800-0

Abstract: "Ice sheets are currently ignored in global methane budgets. Although ice sheets have been proposed to contain large reserves of methane that may contribute to a rise in atmospheric methane concentration if released during periods of rapid ice retreat, no data exist on the current methane footprint of ice sheets. Here we find that subglacially produced methane is rapidly driven to the ice margin by the efficient drainage system of a subglacial catchment of the Greenland ice sheet. We report the continuous export of methane-supersaturated waters (CH4(aq)) from the ice-sheet bed during the melt season. Pulses of high CH4(aq) concentration coincide with supraglacially forced subglacial flushing events, confirming a subglacial source and highlighting the influence of melt on methane export. Sustained methane fluxes over the melt season are indicative of subglacial methane reserves that exceed methane export, with an estimated 6.3 tonnes (discharge-weighted mean; range from 2.4 to 11 tonnes) of CH4(aq) transported laterally from the ice-sheet bed. Stable-isotope analyses reveal a microbial origin for methane, probably from a mixture of inorganic and ancient organic carbon buried beneath the ice. We show that subglacial hydrology is crucial for controlling methane fluxes from the ice sheet, with efficient drainage limiting the extent of methane oxidation to about 17 per cent of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive fluxes (4.4 to 28 millimoles of CH4 per square metre per day) rivalling that of major world rivers. Overall, our results indicate that ice sheets overlie extensive, biologically active methanogenic wetlands and that high ates of methane export to the atmosphere can occur via efficient subglacial drainage pathways. Our findings suggest that such environments have been previously underappreciated and should be considered in Earth’s methane budget."

See also:
Title: "Melting ice sheets release tons of methane into the atmosphere, study finds"

https://phys.org/news/2019-01-ice-sheets-tons-methane-atmosphere.html

Extract: "The Greenland Ice Sheet emits tons of methane according to a new study, showing that subglacial biological activity impacts the atmosphere far more than previously thought.

Professor Jemma Wadham, Director of Bristol's Cabot Institute for the Environment, who led the investigation, said: "A key finding is that much of the methane produced beneath the ice likely escapes the Greenland Ice Sheet in large, fast flowing rivers before it can be oxidized to CO2, a typical fate for methane gas which normally reduces its greenhouse warming potency."

Lead author, Guillaume Lamarche-Gagnon, from Bristol's School of Geographical Sciences, said: "What is also striking is the fact that we've found unequivocal evidence of a widespread subglacial microbial system. Whilst we knew that methane-producing microbes likely were important in subglacial environments, how important and widespread they truly were was debatable. Now we clearly see that active microorganisms, living under kilometres of ice, are not only surviving, but likely impacting other parts of the Earth system. This subglacial methane is essentially a biomarker for life in these isolated habitats."

With Antarctica holding the largest ice mass on the planet, researchers say their findings make a case for turning the spotlight to the south. Mr Lamarche-Gagnon added: "Several orders of magnitude more methane has been hypothesized to be capped beneath the Antarctic Ice Sheet than beneath Arctic ice-masses. Like we did in Greenland, it's time to put more robust numbers on the theory.""

6
When the fossil fuel industry is actively adding to oil & gas proven reserves and production, it is difficult for me to believe that any of the Paris Agreement goals will be met:

Title: "EIA's Annual Energy Outlook 2019 projects growing oil, natural gas, renewables production"

https://www.eia.gov/todayinenergy/detail.php?id=38112

The first image shows the EIA's January 2019 US Oil & Gas production projections through 2050.
The second image shows BP's estimates of years of remaining fossil fuels assuming consumption at 2015 rates and estimated reserves per 2016 (I note that this only shows conventional reserves).

7
Hansen warned consensus climate scientists not to over emphasize the carbon sink associated with the multidecadal spurt of climate change induced plant growth, such as the observed 'Arctic greening'.  Now, recent research (see linked articles) indicates that substantial parts of the Arctic are turning brown due to a combination of extreme weather, invasive insects and increased wildfires.  If (as is likely) this Arctic browning trend continues, not only will much of the sequestered carbon be returned to the atmosphere, but also the associated reductions in albedo will act as a positive feedback from more global warming:

Title: "Climate change made the Arctic greener. Now parts of it are turning brown."

https://www.sciencenews.org/article/climate-change-arctic-browning

Extract: "Warming trends bring more insects, extreme weather and wildfires that wipe out plants

For more than 35 years, satellites circling the Arctic have detected a “greening” trend in Earth’s northernmost landscapes. Scientists have attributed this verdant flush to more vigorous plant growth and a longer growing season, propelled by higher temperatures that come with climate change. But recently, satellites have been picking up a decline in tundra greenness in some parts of the Arctic. Those areas appear to be “browning.”"
&

Title: "Extreme Weather Is Turning the Arctic Brown, Signaling Ecosystem’s Inability to Adapt to Climate Change"

https://www.smithsonianmag.com/smart-news/extreme-weather-turning-arctic-brown-signaling-ecosystems-inability-adapt-climate-change-180970896/

Extract: "Vegetation affected by extreme warming absorbs up to 50 percent less carbon than healthy green heathland"


8
The linked article indicates that the oil & gas industry are succeeding to ensure that oil and gas supplies exceed demand for many years to come:

Title: "Companies are finding lots of oil again"

https://www.axios.com/oil-discovery-increase-2019-72761320-0cbe-4ae3-942a-a815e69f1fe6.html

Extract: "The oil industry is finding lots of hydrocarbons thus far in 2019, putting discoveries on pace to grow by 30% this year if they keep it up, the consultancy Rystad Energy said this week.

The push for substantial new discoveries shows no signs of slowing down, with another 35 high impact exploration wells expected to be drilled this year, both onshore and offshore," Rystad wrote.

Bigger-than-expected U.S. shale growth has also eased concerns.

"Forecasts of a supply gap persist, but they’re being pushed further out into the future," Bloomberg reported in late January, and IEA has warned against complacency."

9
If nothing else, the findings of the linked reference could be used to better calibrate state-of-the-art ESM climate change projections w.r.t. the risks of future 'extreme ocean anoxic event conditions':

D. Hülse, S. Arndt, A. Ridgwell (15 March 2019), "Mitigation of Extreme Ocean Anoxic Event Conditions by Organic Matter Sulfurization", Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003470

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018PA003470

Abstract
Past occurrences of widespread and severe anoxia in the ocean have frequently been associated with abundant geological evidence for free hydrogen sulfide (H2S) in the water column, so‐called euxinic conditions. Free H2S may react with, and modify, the chemical structure of organic matter settling through the water column and in marine sediments, with hypothesized implications for carbon sequestration. Here, taking the example of Ocean Anoxic Event 2, we explore the potential impact of organic matter sulfurization on marine carbon and oxygen cycling by means of Earth system modeling. Our model experiments demonstrate that rapid sulfurization (ksulf≥ = 105 M−1 year−1) of organic matter in the water column can drive a more than 30% enhancement of organic carbon preservation and burial in marine sediments and hence help accelerate climate cooling and Ocean Anoxic Event 2 recovery. As a consequence of organic matter sulfurization, we also find that H2S can be rapidly scavenged and the euxinic ocean volume reduced by up to 80%—helping reoxygenate the ocean as well as reducing toxic H2S emissions to the atmosphere, with potential implications for the kill mechanism at the end‐Permian. Finally, we find that the addition of organic matter sulfurization induces a series of additional feedbacks, including further atmospheric CO2 drawdown and ocean reoxygenation by the creation of a previously unrecognized net source of alkalinity to the ocean as H2S is scavenged and buried.

10
If nothing else, the findings of the linked reference could be used to better calibrate state-of-the-art ESM climate change projections w.r.t. 'the role of the Southern Ocean in abrupt transitions and hysteresis' in the MOC:

Sophia K.V. Hines, Andrew F. Thompson, Jess F. Adkins (15 March 2019), "The Role of the Southern Ocean in Abrupt Transitions and Hysteresis in Glacial Ocean Circulation", Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003415

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018PA003415

Abstract: "High‐latitude Northern Hemisphere climate during the last glacial period was characterized by a series of abrupt climate changes, known as Dansgaard‐Oeschger events, which were recorded in Greenland ice cores as shifts in the oxygen isotopic composition of the ice. These shifts in inferred Northern Hemisphere high‐latitude temperature have been linked to changes in Atlantic meridional overturning strength. The response of ocean overturning circulation to forcing is nonlinear and a hierarchy of models have suggested that it may exist in multiple steady state configurations. Here, we use a time‐dependent coarse‐resolution isopycnal model with four density classes and two basins, linked by a Southern Ocean to explore overturning states and their stability to changes in external parameters. The model exhibits hysteresis in both the steady state stratification and overturning strength as a function of the magnitude of North Atlantic Deep Water formation. Hysteresis occurs as a result of two nonlinearities in the model—the surface buoyancy distribution in the Southern Ocean and the vertical diffusivity profile in the Atlantic and Indo‐Pacific basins. We construct a metric to assess circulation configuration in the model, motivated by observations from the Last Glacial Maximum, which show a different circulation structure from the modern. We find that circulation configuration is primarily determined by North Atlantic Deep Water density. The model results are used to suggest how ocean conditions may have influenced the pattern of Dansgaard‐Oeschger events across the last glacial cycle."

11
The Early Paleozoic Era ran from about 542 million, to about 251 million, years ago, and included a major extinction event at the end of the Ordovician period (485 to 443 million years ago).  The linked new research on biodiversity in these periods, offers some insights on our current period of a Sixth Extinction event.  The finding of the new research [Rasmussen et al. (2019)], the very large extinction event during the end of the Ordovician period was not primarily due to global cooling (as was previously assumed) but rather by climate change associated with increased volcanic activity.  As high levels of GHG emissions from strong volcanic activity can abruptly increase global warming, the Rasmussen et al. (2019) findings suggest that biological systems do not adapt well to abrupt increases in GMSTA; which has gloomy implications for our likely pathway this century:

Title: "New research about biodiversity reveals the importance of climate on today's abundance of life"

https://phys.org/news/2019-04-biodiversity-reveals-importance-climate-today.html

Extract: "Biodiversity Natural history museum paleontologists in Copenhagen and Helsinki have succeeded in mapping historical biodiversity in unprecedented detail. For the first time, it is possible to compare the impact of climate on global biodiversity in the distant past—a result that paints a gloomy picture for the preservation of present-day species richness. The study has just been published in the prestigious American journal, Proceedings of the National Academy of Sciences (PNAS).

Furthermore, we find that the very large extinction event at the end of the Ordovician period (485—443 million years ago), when upwards of 85 percent of all species disappeared, was not "a brief ice age—as previously believed—but rather a several million years long crisis interval with mass extinctions. It was most likely prompted by increased volcanic activity. It took nearly 40 million years to rectify the mess before biodiversity was on a par with levels prior to this period of volcanic caused death and destruction," says Christian Mac Ørum."
&

Christian M. Ø. Rasmussen et al. Cascading trend of Early Paleozoic marine radiations paused by Late Ordovician extinctions, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1821123116

https://www.pnas.org/content/116/15/7207

Significance

The first 120 million years of Phanerozoic life witnessed significant changes in biodiversity levels. Attempts to correlate these changes to potential short-term environmental drivers have been hampered by the crude temporal resolution of current biodiversity estimates. We present a biodiversity curve for the Early Paleozoic with high temporal precision. It shows that once equatorial sea-surface temperatures fell to present-day levels during the early Mid Ordovician, marine biodiversity accumulation accelerated dramatically. However, this acceleration ceased as increased volcanism commenced during the mid-Late Ordovician. Since biodiversity levels were not restored for at least ∼35 million years, this finding redefines the nature of the end Ordovician mass extinctions and further reframes the Silurian as a prolonged recovery interval.

Abstract

The greatest relative changes in marine biodiversity accumulation occurred during the Early Paleozoic. The precision of temporal constraints on these changes is crude, hampering our understanding of their timing, duration, and links to causal mechanisms. We match fossil occurrence data to their lithostratigraphical ranges in the Paleobiology Database and correlate this inferred taxon range to a constructed set of biostratigraphically defined high-resolution time slices. In addition, we apply capture–recapture modeling approaches to calculate a biodiversity curve that also considers taphonomy and sampling biases with four times better resolution of previous estimates. Our method reveals a stepwise biodiversity increase with distinct Cambrian and Ordovician radiation events that are clearly separated by a 50-million-year-long period of slow biodiversity accumulation. The Ordovician Radiation is confined to a 15-million-year phase after which the Late Ordovician extinctions lowered generic richness and further delayed a biodiversity rebound by at least 35 million years. Based on a first-differences approach on potential abiotic drivers controlling richness, we find an overall correlation with oxygen levels, with temperature also exhibiting a coordinated trend once equatorial sea surface temperatures fell to present-day levels during the Middle Ordovician Darriwilian Age. Contrary to the traditional view of the Late Ordovician extinctions, our study suggests a protracted crisis interval linked to intense volcanism during the middle Late Ordovician Katian Age. As richness levels did not return to prior levels during the Silurian—a time of continental amalgamation—we further argue that plate tectonics exerted an overarching control on biodiversity accumulation.

12
I think that it is always good to compare observed versus consensus science projections, and in this regards, the February 2018 Climate Lab Book article by Ed Hawkins compares observed GMSTA versus CMIP5 projections, with the attached image from the linked article showing comparisons for FAR, SAR, TAR, AR4 & AR5 thru 2035.

Title: "Comparing CMIP5 & observations"

https://www.climate-lab-book.ac.uk/comparing-cmip5-observations/

Extract about the first image: "In addition, the figure below updates Fig. 1.4 from IPCC AR5, which compares projections from previous IPCC Assessment Reports with subsequent observations. The HadCRUT4.4 observations from 2013-2015 are added as black squares. Note that previous reports made differing assumptions about future emissions. This figure has not yet been updated to include 2016-7 temperature data."

Note the original caption for AR5 Fig. 1.4 is: "Figure 1.4 |  Estimated changes in the observed globally and annually averaged surface temperature anomaly relative to 1961–1990 (in °C) since 1950 compared with the range of projections from the previous IPCC assessments. Values are harmonized to start from the same value in 1990. Observed global annual mean surface air temperature anomaly, relative to 1961–1990, is shown as squares and smoothed time series as solid lines (NASA (dark blue), NOAA (warm mustard), and the UK Hadley Centre (bright green) reanalyses). The coloured shading shows the projected range of global annual mean surface air temperature change from 1990 to 2035 for models used in FAR (Figure 6.11 in Bretherton et al., 1990), SAR (Figure 19 in the TS of IPCC, 1996), TAR (full range of TAR Figure 9.13(b) in Cubasch et al., 2001). TAR results are based on the simple climate model analyses presented and not on the individual full three-dimensional climate model simulations. For the AR4 results are presented as single model runs of the CMIP3 ensemble for the historical period from 1950 to 2000 (light grey lines) and for three scenarios (A2, A1B and B1) from 2001 to 2035. The bars at the right-hand side of the graph show the full range given for 2035 for each assessment report. For the three SRES scenarios the bars show the CMIP3 ensemble mean and the likely range given by –40% to +60% of the mean as assessed in Meehl et al. (2007). The publication years of the assessment reports are shown. See Appendix 1.A for details on the data and calculations used to create this figure."

To me this first image together with the second image (note one needs to add about 0.26C to GMSTA relative to 1961 to 1990 in order to compare to GMSTA values relative to the last 19th century) indicate several things including:

1.  The FAR estimates of GMSTA were reasonable, and then politics influenced the consensus science SAR & TAR estimates to make them err on the side of least drama.

2. The mean GMSTA projection using the SRES A1B radiative forcing scenario underestimate the observed GMSTA values thru 2018 (& projected by Gavin Schmidt thru 2019) shown in the second image.  This raises the prospect that consensus climate science thru AR5 may have underestimated climate sensitivity.

13
Apparently, illegal logging (primarily by Chinese companies) is contributing to a radical deforestation of Mozambique (see image).  This is just one example of how quickly anthrogenic damage can be done to the biosphere.

Title: "Mozambique reforms timber sector to counter illegal logging"

https://www.reuters.com/article/us-mozambique-forest-logging/mozambique-reforms-timber-sector-to-counter-illegal-logging-idUSKBN1KG1F8

14
Climate warming that is expected to reduce soil moisture, and therefore increase soil aeration, of Northern peatlands.  Thus, the finding of the linked reference that increased soil drying will decrease soil organic carbon (SOC) stocks in peatlands, implies that ECS will be higher, this century, than previously assumed by consensus climate science:

Anna M. Laine et al. Warming impacts on boreal fen CO 2 exchange under wet and dry conditions, Global Change Biology (2019). DOI: 10.1111/gcb.14617

https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.14617

Abstract: "Northern peatlands form a major soil carbon (C) stock. With climate change, peatland C mineralization is expected to increase, which in turn would accelerate climate change. A particularity of peatlands is the importance of soil aeration, which regulates peatland functioning and likely modulates the responses to warming climate. Our aim is to assess the impacts of warming on a southern boreal and a sub‐arctic sedge fen carbon dioxide (CO2) exchange under two plausible water table regimes: wet and moderately dry. We focused this study on minerotrophic treeless sedge fens, as they are common peatland types at boreal and (sub)arctic areas, which are expected to face the highest rates of climate warming. In addition, fens are expected to respond to environmental changes faster than the nutrient poor bogs. Our study confirmed that CO2 exchange is more strongly affected by drying than warming. Experimental water level draw‐down (WLD) significantly increased gross photosynthesis and ecosystem respiration. Warming alone had insignificant impacts on the CO2 exchange components, but when combined with WLD it further increased ecosystem respiration. In the southern fen, CO2 uptake decreased due to WLD, which was amplified by warming, while at northern fen it remained stable. As a conclusion, our results suggest that a very small difference in the WLD may be decisive, whether the C sink of a fen decreases, or whether the system is able to adapt within its regime and maintain its functions. Moreover, the water table has a role in determining how much the increased temperature impacts the CO2 exchange."

15
In contrast to overly-simplified consensus climate models, improved modeling of root dynamics in the more advanced E3SM; finds that climate change driven root dynamics will result in a net loss in plant induced carbon sequestration in the soil; which of course implies higher values of ECS than assumed by earlier consensus climate models:

B. A. Drewniak (03 January 2019), "Simulating Dynamic Roots in the Energy Exascale Earth System Land Model", JAMES,

https://doi.org/10.1029/2018MS001334

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018MS001334

Abstract: "Roots are important contributors to plant development, functioning to provide nutrients and water for plant growth. However, roots and their functions are often simplified in Earth system models, which limit the feedback of root foraging strategy on plant productivity, and their impacts on the carbon cycle. The goal of this study is to introduce a new method to resolve the vertical structure of roots over time. The method allows plasticity of rooting depth distribution under nonuniform profiles of water and nitrogen, which influences aboveground dynamics. The dynamic root model optimizes root distribution for both water and nitrogen uptake but gives priority to plant water demands. I implement this new method in the Energy Exascale Earth System model. The resulting root distribution maintains agreement with observations in most ecosystems and marginally improves the gross primary productivity estimated by the model, compared to satellite observations. Increases in gross primary productivity are simulated in desert and boreal ecosystems. However, the model does not capture deep roots in the dry tropics, and therefore, productivity losses are observed in parts of the Amazon and the African savannah. I discuss details of the model algorithm, along with some sensitivity studies that shed light on the model behavior in water‐limited ecosystems. The study shows that additional model processes, such as climate dependent root depth, root hydraulics, root form and function, and better nitrogen uptake, should be considered to improve the root water uptake in the Energy Exascale Earth System Land Model (ELM)."
&

Extract: "Globally, the net effect is a loss of productivity and carbon storage."


16
In contrast to some earlier (overly simplified) consensus research, the linked reference finds that "… interactions between convective aggregation and the spatial distribution of SST appear …" to increase ECS.  As ice-climate feedbacks tend to increase SST gradients; Coppin & Bony (2018)'s finding indicate that ice-climate feedbacks will increase ECS.

David Coppin and Sandrine Bony (28 November 2018), "On the Interplay Between Convective Aggregation, Surface Temperature Gradients, and Climate Sensitivity", JAMES, https://doi.org/10.1029/2018MS001406

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018MS001406

Abstract: "This study explores the extent to which convective aggregation interacts with sea surface temperature (SST) and affects climate sensitivity. For this purpose, radiative‐convective equilibrium simulations are run with a general circulation model coupled to an ocean mixed layer, and several types of perturbations are imposed to the ocean‐atmosphere system. Convective aggregation turns out to be much more sensitive to temperature in coupled experiments than in prescribed SST experiments. But changes in convective aggregation induced by a doubling of the CO2 concentration are always smaller than changes associated with the transition from a non‐aggregated to an aggregated state. If aggregation changes were acting alone, they would exert a strong negative feedback on global mean surface temperature. However, in a coupled framework, aggregation changes interact with the SST and generate SST gradients that strengthen the positive low‐cloud feedback associated with changes in SST pattern. This overcompensates the negative feedback due to aggregation changes and leads to a larger equilibrium climate sensitivity than in the absence of SST gradients. Although this effect might be model specific, interactions between convective aggregation and the spatial distribution of SST appear crucial to assess the impact of convective aggregation on climate sensitivity."

See also:

Cook, T. (2019), Improving estimates of long-term climate sensitivity, Eos, 100, https://doi.org/10.1029/2019EO116981. Published on 05 March 2019.

https://eos.org/research-spotlights/improving-estimates-of-long-term-climate-sensitivity

Extract: "In contrast to earlier studies, which found that aggregation decreases climate sensitivity when coupled with a simplified ocean, the team instead found that aggregation can increase it. Because these results indicate that aggregation strongly interacts with sea surface temperature gradients, this study casts significant doubt on the appropriateness of using experiments with fixed and uniform sea surface temperatures to understand the effects of convective aggregation on climate sensitivity."

17
I provide the following link about pre-2017 research on the rate of subsea permafrost degradation, and associated methane emissions, in the ESAS.  This research identifies numerous mechanisms that could accelerate methane emissions from the ESAS faster than assumed in AR5:

Natalia Shakhova, et al (2017), "Current rates and mechanisms of subsea permafrost degradation in the East Siberian Arctic Shelf", Nature Communications 8, Article number: 15872,
doi:10.1038/ncomms15872

http://www.nature.com/articles/ncomms15872

Abstract: "The rates of subsea permafrost degradation and occurrence of gas-migration pathways are key factors controlling the East Siberian Arctic Shelf (ESAS) methane (CH4) emissions, yet these factors still require assessment. It is thought that after inundation, permafrost-degradation rates would decrease over time and submerged thaw-lake taliks would freeze; therefore, no CH4 release would occur for millennia. Here we present results of the first comprehensive scientific re-drilling to show that subsea permafrost in the near-shore zone of the ESAS has a downward movement of the ice-bonded permafrost table of ∼14 cm year−1 over the past 31–32 years. Our data reveal polygonal thermokarst patterns on the seafloor and gas-migration associated with submerged taliks, ice scouring and pockmarks. Knowing the rate and mechanisms of subsea permafrost degradation is a prerequisite to meaningful predictions of near-future CH4 release in the Arctic."

See also:

http://envisionation.co.uk/index.php/nick-breeze/203-subsea-permafrost-on-east-siberian-arctic-shelf-now-in-accelerated-decline

18
The linked reference confirms that: "Climate change is an issue which elicits low engagement, even among concerned segments of the public."  Furthermore, it indicates the current communication of consensus science is insufficient to significantly motivate public engagement in Climate Action.  This low public engagement implies that pathways with higher radiative forcing are more likely to be followed than those with lower radiative forcing:

Morris, B.S., Chrysochou, P., Christensen, J.D. et al. (2019), "Stories vs. facts: triggering emotion and action-taking on climate change", Climatic Change, pp 1–18, https://doi.org/10.1007/s10584-019-02425-6

https://rd.springer.com/article/10.1007%2Fs10584-019-02425-6

Abstract: "Climate change is an issue which elicits low engagement, even among concerned segments of the public. While research suggests that the presentation of factual information (e.g., scientific consensus) can be persuasive to some audiences, there is also empirical evidence indicating that it may also increase resistance in others. In this research, we investigate whether climate change narratives structured as stories are better than informational narratives at promoting pro-environmental behavior in diverse audiences. We propose that narratives structured as stories facilitate experiential processing, heightening affective engagement and emotional arousal, which serve as an impetus for action-taking. Across three studies, we manipulate the structure of climate change communications to investigate how this influences narrative transportation, measures of autonomic reactivity indicative of emotional arousal, and pro-environmental behavior. We find that stories are more effective than informational narratives at promoting pro-environmental behavior (studies 1 and 3) and self-reported narrative transportation (study 2), particularly those with negatively valenced endings (study 3). The results of study 3 indicate that embedding information in story structure influences cardiac activity, and subsequently, pro-environmental behavior. These findings connect works from the fields of psychology, neuroscience, narratology, and climate change communication, advancing our understanding of how narrative structure influences engagement with climate change through emotional arousal, which likely incites pro-environmental behavior as the brain’s way of optimizing bodily budgets."

19
The first linked reference (Willeit et al 2019) and the associated Real Climate article, provides an example of how consensus science findings can inappropriately serve to encourage decision makers to continue on with BAU behavior (for at least several more years) for reasons including:

1. The research indicates that an ECS value of 3C is characteristic for the past 3 million year paleo record; without stating that ECS during glacial periods is nonlinearly lower than during interglacial periods (see Li Lo et al. 2017 and the first attached image).  Thus as we are in an interglacial period Willeit et al 2019 finding indicate that ECS is currently nonlinearly higher than 3C as supported by initial finding from CMIP6 projections.

2. The Willeit et al 2019 findings are the first time that models have been able to match the past 3 million year paleo record; which indicates that earlier guidance was non-conservative as the new findings show a strong sensitivity of the cryosphere to atmospheric CO₂ concentrations.  Furthermore, Willeit et al. 2019 do not use a dynamic model for the WAIS, and thus their findings would average out any multi-decadal change in the planetary energy imbalance noted by Hansen et al (2016).

Finally, I note that the second attached image from the Real Climate article shows that for the last 3 million years GMSTA has not been above 2C thus, if we collectively exceed the 2C value we may well be subject to abrupt climate change surprises as cautioned by the NAS in 2013.

M. Willeit et al. (03 Apr 2019), "Mid-Pleistocene transition in glacial cycles explained by declining CO2 and regolith removal", Science Advances, Vol. 5, no. 4, eaav7337, DOI: 10.1126/sciadv.aav7337

http://advances.sciencemag.org/content/5/4/eaav7337

Abstract
Variations in Earth’s orbit pace the glacial-interglacial cycles of the Quaternary, but the mechanisms that transform regional and seasonal variations in solar insolation into glacial-interglacial cycles are still elusive. Here, we present transient simulations of coevolution of climate, ice sheets, and carbon cycle over the past 3 million years. We show that a gradual lowering of atmospheric CO2 and regolith removal are essential to reproduce the evolution of climate variability over the Quaternary. The long-term CO2 decrease leads to the initiation of Northern Hemisphere glaciation and an increase in the amplitude of glacial-interglacial variations, while the combined effect of CO2 decline and regolith removal controls the timing of the transition from a 41,000- to 100,000-year world. Our results suggest that the current CO2 concentration is unprecedented over the past 3 million years and that global temperature never exceeded the preindustrial value by more than 2°C during the Quaternary.

See also:

Title: "First successful model simulation of the past 3 million years of climate change"

http://www.realclimate.org/index.php/archives/2019/04/first-successful-model-simulation-of-the-past-3-million-years-of-climate-change/#more-22376

Extract: "The simulations further suggest that global temperature never exceeded the preindustrial value by more than 2°C during the Quaternary. Ice sheet evolution is very sensitive to temperature, and the initiation of NH glaciation at around 3 million years ago would not have been possible in the model if global temperature would have been higher than 2°C relative to preindustrial during the early Quaternary."
&

Li Lo et al (2017), Nonlinear climatic sensitivity to greenhouse gases over past 4 glacial/interglacial cycles", Scientific Reports 7, No. 4626, DOI: https://doi.org/10.1038/s41598-017-04031-x

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

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

20
Since we all know how 2018 ended (emissions wise),


For those who do not know how 2018 ended (emissions wise), I provide the attached image indicating that CO2 emissions were very close to those in the RCP 8.5 scenario.

21
As a follow-on to my last post, the linked reference confirms that the low emissions scenarios evaluated under the Paris Agreement are not as safe as previously assumed by consensus climate science.  This supports concerns that a domino-effect cascade of positive feedback mechanisms could occur when GMSTA is between 1.5 and 2C:

Felipe Feijoo et al. (2019), "Climate and carbon budget implications of linked future changes in CO2 and non-CO2 forcing", Environmental Research Letters, Vol 14, No. 4

https://iopscience.iop.org/article/10.1088/1748-9326/ab08a9/meta

Abstract
The approximate proportional relationship between cumulative carbon emissions and instantaneous global temperature rise (the carbon budget approximation) has proven to be a useful concept to translate policy-relevant temperature objectives into CO2 emissions pathways. However, when non-CO2 forcing is changing along with CO2 forcing, errors in the approximation increases. Using the GCAM model to produce an ensemble of ~3000 scenarios, we show that linked changes in CO2 forcing, aerosol forcing, and non-CO2 greenhouse gas (GHG) forcing lead to an increase in total non-CO2 forcing over the 21st century across mitigation scenarios. This increase causes the relationship between instantaneous temperature and cumulative CO2 emissions to become more complex than the proportional approximation often assumed, particularly for low temperature objectives such as 1.5 °C. The same linked changes in emissions also contribute to a near-term increase in aerosol forcing that effectively places a limit on how low peak temperature could be constrained through GHG mitigation alone. In particular, we find that 23% of scenarios that include CCS (but only 1% of scenarios that do not include CCS) achieve a temperature objective of 1.5 °C without temperature overshoot.

22
The linked article (and associated linked report) indicates that since 1948 not only has Canada's mean surface warming been more than twice the global average, but also that the Arctic has warmed almost three times faster than the global average.  This indirectly implies that climate sensitivity is relatively high:

Title: "Canada warming at twice the global rate, climate report finds"

https://www.theguardian.com/world/2019/apr/02/canada-climate-change-warming-twice-as-fast-report

Extract: "While global temperatures have increased 0.8C since 1948, Canada has seen an increase of 1.7C – more than double the global average.

And in the Arctic, the warming is happening at a much faster rate of 2.3C, the report says."

See also:

Title: "Canada in a Changing Climate"
https://changingclimate.ca/

23
Perhaps some readers think that I am being overly harsh when I implied in my last post (Reply #828) that consensus climate scientist may well be hallucinating with regards to their collective expectations of how Mother Nature is most like going to respond to anthropogenic radiative forcing in the coming several decades (say from 2040 to 2060).  Therefore, I provide this follow-on post in order to briefly discuss why the concept of carbon budgets (which consensus climate scientists began formulating circa 2009 and cited explicitly in AR5 and then revised upward in SR15, see the first linked article), actually provides dangerous guidance to policymakers, as is lightly touched upon by the second linked article.

In addition to the discussion provided in the two linked articles, I raise the following points, that help to clarify why I believe that the carbon budget guidance provided relatively recently by SR15 is actually quite dangerous:

1. SR15 assumes that the values of TCR estimated by CMIP5 were too high and thus SR15 expanded the AR5 carbon budget to stay below 1.5C; while preliminary results from CMIP6 indicates that actually CMIP5 under estimated TCR and thus the carbon budget should have been decreased.  In the way of explaining the differences between CMIP5 and the preliminary CMIP6 projections, I note that the more advanced CMIP6 models: a) estimate that aerosol interactions in recent decades have been more negative than CMIP5 assumed (which means that TCR was higher than CMIP5 assumed); b) estimate that cloud interaction feedback mechanism are progressively becoming more positive (which means that TCR is progressively increasing in magnitude); and c) estimate that some ice-climate mechanisms are already being activated.

2. SR15 assumes that world governments will aggressively implement negative emissions technology (like BECCS); while recent studies of food supply clearly indicate that in coming decades there will be insufficient land for food production let alone for BECCS.

3. SR15 underestimates the global warming potential of future methane emissions, and totally ignore ice-climate feedback mechanisms (including those that are already occurring).

4. SR15 ignores the fact that anthropogenic radiative forcing has been occurring since at least 1750 and thus numerous slow-response positive feedback mechanisms are already being activated, and thus it may be advisable to use ECS, rather than TCR, when calculating the remaining carbon budget.

5.  If Hawkins et al. (2017) is correct that one needs to add about 0.675C (or at least 0.6C) to the consensus science GMSTA values; then not only are we closer to Mid-Pliocene conditions than most people assume, but also that ECS is higher than AR5 assumes.

6. If DeConto & Pollard (2018)'s estimates of how the WAIS will respond to Mid-Pliocene conditions is correct, then we are currently at risk of triggering an irreversible abrupt collapse of the WAIS; which is not considered by any consensus science carbon budget.

If reader wants more reasons that consensus climate science estimates of our remaining carbon budget (including that from SR15), may be dangerous; just scroll be through this thread:

Title: "Analysis: Why the IPCC 1.5C report expanded the carbon budget"

https://www.carbonbrief.org/analysis-why-the-ipcc-1-5c-report-expanded-the-carbon-budget

Extract: "The newly published Intergovernmental Panel on Climate Change’s (IPCC) special report on 1.5C (SR15) significantly expands the budget for a 66% chance of avoiding 1.5C to the equivalent of 10 years of current emissions. This compares to the IPCC’s fifth assessment report (AR5), which put it at around three years.

Based on estimates made in the IPCC’s fifth assessment report (AR5), there would be around 120 gigatonnes of CO2 (GtCO2) remaining from the beginning of 2018 – or around three years of current emissions – for a 66% chance of avoiding 1.5C warming. For a 50/50 chance of exceeding 1.5C, the remaining budget was a modestly larger 268GtCO2 – or around seven years of current emissions.

The IPCC’s new SR15 significantly revises these numbers. It raises the budget for a 66% of avoiding 1.5C to 420GtCO2 – or 10 years of current emissions. Similarly, the budget for a 50/50 chance of exceeding 1.5C is increased to 580GtCO2 – 14 years of current emissions.

Even the revised 1.5C carbon budget is unlikely to be the end of the debate, however, given a number of large remaining uncertainties. These include:
•   The precise meaning of the 1.5C target.
•   Disagreement about what “surface temperature” actually refers to.
•   The definition of the “pre-industrial” period.
•   What observational temperature datasets should be used.
•   What happens to non-CO2 factors that influencing the climate.
•   Whether Earth-system feedbacks like melting permafrost are taken into account.

Finally, the emission scenarios considered in the new SR15 also tend to emit far more than the budget would allow, but make up for it with the large-scale use of negative emissions in the future. The large carbon budget uncertainty and reliance on negative emissions – basically, sucking CO2 from the atmosphere and permanently storing it – suggest that the idea of a carbon budget may be of limited use for strict mitigation targets such as 1.5C."
&

Title: "How the “Carbon Budget” Is Causing Problems"

https://www.scientificamerican.com/article/how-the-carbon-budget-is-causing-problems/

Extract: "Confusion over how much CO2 can be emitted could undermine global climate action

Uncertainties about the value make it difficult to develop policies based around a single budget, they suggest. And the debate about the exact number may actually provide a kind of political flexibility for world leaders, enabling them to endlessly argue that there’s still time for action, even while time is actually running out.

“Instead of oversimplifying, the scientific community should seek to discuss and emphasize the persistent uncertainties,” he wrote—making it clear, instead, that any given estimate includes a wide range of assumptions about all kinds of factors, and that these are all part and parcel of any given budget."

24
Ice-climate feedback mechanisms are numerous, interacting, complex and already happening, as demonstrated by the linked open access reference, that demonstrates how freshening of local Antarctic surface water (due significantly from the basal ice melting of local Antarctic ice shelves, largely driven by changes in wind patterns due largely to the formation of the Antarctic ozone hole and subsequent increase in local GHG concentrations), reduces the full-depth convection and formation of Dense Shelf Water (DSW).  This in turn slows the formation of Antarctic Bottom Water, AABW; which then slows the MOC as well as acceleration the retreat of the grounding lines of key Antarctic marine glaciers.  These ice-climate feedback mechanisms are self-reinforcing:

Alessandro Silvano et al. (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.full

Abstract
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: "Study Reveals Dangerous Antarctic Feedback Loop"

https://www.ecowatch.com/antarctic-climate-change-study-2562971817.html

Extract: "... Silvano's research backs up a study co-authored by early climate change alarm-sounder Dr. James Hansen in 2016, which used computer models to predict that melting ice in Antarctica and Greenland would warm the oceans below the surface and increase the rate of melting, increasing storms and leading to sea level rise of "several meters over a timescale of 50 to 150 years," according to the abstract.

"Our study shows for the first time actual evidence of this mechanism. Our study shows that it is already happening," Silvano told The Washington Post."



25
As a follow-on to my last post & again without much back-up, the attached image shows the extent of the Arctic Coastal Plain (particularly in Alaska but also in Siberia & Canada) that are subject to inundation if the WAIS were to collapse abruptly.  Furthermore, I note that such inundated Arctic Coastal Plains contain extensive reserves of methane (natural gas) hydrates that could be destabilized by heat transferred from the relatively warm, inundating ocean water.

26
Just as a general comment (without a lot of back-up), in my last post I noted that the Thwaites and the Pine Island Drainage Basins could link-up into one single drainage basin, and I note that if the worst case ice-cliff failure scenarios were to occur for these two combined drainage basins, then per the fingerprint effect (see the attached image of the fingerprint effect is the entire WAIS were to collapse), the sea level at the Ross Ice Shelf (RIS) might drop abruptly which might cause flexural fissures near the grounding line of the RIS; which might contribute to a potential abrupt collapse of the RIS.

27
Now that the field phase of the International Thwaites Glacier Collaboration is complete, I can recommend periodically monitoring the linked websites in the coming weeks & months for updates as the field data is analyzed from the eight associated projects (see the attached image):

Title: "The International Thwaites Glacier Collaboration"

https://thwaitesglacier.org/
&
https://thwaitesglacier.org/projects

Extract: "UK and US scientists are collaborating to investigate one of the most unstable glaciers in Antarctica, the Thwaites Glacier, roughly the same size as Florida or Britain."

See also:

For the project Twitter site, see:

https://twitter.com/GlacierThwaites?ref_src=twsrc%5Etfw%7Ctwcamp%5Eembeddedtimeline%7Ctwterm%5Eprofile%3AGlacierThwaites&ref_url=https%3A%2F%2Fthwaitesglacier.org%2F

Also, I note that both Jeremy Bassis & Doug Benn are involved in the DOMINOS project & thus this hierarchical approach to computer modeling will certainly include ice-cliff failure mechanisms.

Title: "Disintegration of Marine Ice-sheets Using Novel Optimised Simulations (DOMINOS)"

https://thwaitesglacier.org/projects/dominos

Extract: "Currently, it is difficult to model calving and its complex interactions with atmospheric and oceanic conditions. The DOMINOS team will use a novel ice-dynamic model suite coupled with an ocean forcing model suite. This model suite includes a discrete element model capable of simulating coupled fracture and ice-flow processes, a 3D full Stokes continuum model, and the continental scale ice-dynamics model BISICLES. Ice dynamics models will be coupled to an ocean forcing model suite including simple plume models, intermediate complexity 2-layer ocean models and fully 3D regional ocean models. This hierarchical approach will use high-fidelity process models to inform and constrain the sequence of lower-order models needed to extrapolate improved understanding to larger scales and has the potential to radically reduce uncertainty of rates of marine ice sheet collapse and associated sea level rise."
&

Furthermore, I note that the Thwaites Drainage Basin can merge together with the Pine Island Drainage Basin as the Thwaites Glacier Eastern Shear Margin migrates outward (& I note that this outward migration has been accelerated by the recent reduction in ice shelf buttressing on the Southwest Tributary Glacier.

Title: "Thwaites Interdisciplinary Margin Evolution - The Role of Shear Margin Dynamics in the Future Evolution of Thwaites Drainage Basin (TIME)"

https://thwaitesglacier.org/projects/time

Extract: "If the Thwaites Glacier Eastern Shear Margin migrates outwards it could join the Pine Island Glacier, connecting the two glaciers which are already making large contributions to sea-level rise."

See also:
Title: "Melting at Thwaites grounding zone and its control on sea level (MELT)"

https://thwaitesglacier.org/projects/melt

Extract: "MELT is an ice-based project to understand how warm waters are affecting the Thwaites Glacier at the grounding line – the point where the glacier goes afloat to become ice shelf. This will allow the glacier’s potential sea-level contribution to be more accurately predicted."
&

Title: "Thwaites Offshore Research (THOR)"

https://thwaitesglacier.org/projects/thor

Extract: "Thwaites Offshore Research (THOR) is a ship-based and ice-based project that will examine the sedimentary records both offshore from the glacier and beneath the ice shelf, together with glacial landforms on the sea bed, to reconstruct past changes in ocean conditions and the glaciers response to these changes."
&

Title: "Geophysical Habitat of Subglacial Thwaites (GHOST)"

https://thwaitesglacier.org/projects/ghost

Extract: "GHOST is an ice-based project which will examine the bed beneath the Thwaites Glacier, to assess whether conditions are likely to allow rapid retreat, or if the retreat may slow or stop due to a ridge 70 km inland."
&

Title: "Geological History Constraints on the Magnitude of Grounding-Line Retreat in the Thwaites Glacier System (GHC)"

https://thwaitesglacier.org/projects/ghc

Extract: "GHC will gather information about past ice sheet behavior and relative sea level change in the Thwaites Glacier system. Determining the timing and magnitude of past episodes of thinning and retreat and subsequent re-advance is important to provide a context for the current and future behavior of Thwaites Glacier and its influence on global sea level."
&

Title: "Thwaites-Amundsen Regional Survey and Network Integrating Atmosphere-Ice-Ocean Processes (TARSAN)"

https://thwaitesglacier.org/projects/tarsan

Extract: "TARSAN is a ship-based project studying how atmospheric and oceanic processes are influencing the behavior of the Thwaites and Dotson Ice Shelves – neighboring ice shelves which are behaving differently."
&

Title: "Processes, drivers, Prediction: modeling the History and Evolution of Thwaites (PROPHET)"

https://thwaitesglacier.org/projects/prophet

Extract: "PROPHET is a computer modelling based project which aims to improve the representation of several key processes (calving, ice damage and basal conditions), which are not currently characterized well in large scale ice-flow models."
&

Title: "The Future of Thwaites Glacier and its Contribution to Sea-level Rise Science Coordination Office Proposal (SCO)"

https://thwaitesglacier.org/projects/sco

Extract: "The ITGC Science Coordination Office coordinates the eight funded research projects to deliver the best possible science for the funding agencies and the public of both the US and UK."


28
The late Cretaceous period ended about 66 million years ago, and thus is relatively close to the often discuss PETM period, and is thus relevant to the possible future conditions that Earth may be headed towards.  The linked reference offers new paleo-information that can used to better calibrate advanced ESM projects, particularly those associated with potential sea ice losses in the Arctic Ocean:

I. Niezgodzk et al. (20 March 2019), "Was the Arctic Ocean ice free during the latest Cretaceous? The role of CO2 and gateway configurations", Global and Planetary Change, https://doi.org/10.1016/j.gloplacha.2019.03.011

https://www.sciencedirect.com/science/article/pii/S0921818118306775

Abstract
The Arctic region is thought to play a key role in unraveling Mesozoic climate evolution. However, Late Cretaceous climate reconstructions in the high latitudes suffer from contradicting paleoclimatic interpretations. Toward the end of the Cretaceous hot-house, atmospheric CO2 concentration declined potentially enabling the formation of sea-ice in the Arctic Ocean. We use a coupled atmosphere-ocean climate model to investigate possible effects of different atmospheric CO2 levels and gateway configurations between the North proto-Atlantic Basin and the Arctic Ocean on the formation of Arctic sea-ice in the latest Cretaceous. Sensitivity tests were run with two atmospheric CO2 levels (840 and 1120 ppm, representing 3× and 4× pre-industrial concentrations, respectively) with six paleogeographic configurations. In the experiment with 840 ppm CO2, seasonal Arctic sea-ice is observed in each gateway configuration in December–June, while for 1120 ppm sea-ice in the central Arctic is either limited or absent, depending on gateway configuration. This suggests the existence of a CO2 threshold, estimated between 3× and 4× pre-industrial (PI) CO2 levels. For higher atmospheric CO2 levels sea-ice formation can only occur by the combined effect of cold winds blowing over the Arctic from continental North America during boreal winter and seawater freshening. The latter can be caused by either very limited or an absence of gateway connections between the Arctic and the open ocean. Such a configuration likely developed in the latest Cretaceous, i.e. close to the Cretaceous/Paleogene boundary interval.

29
When one considers the implications of the combined influences of both a MICI-type abrupt sea level rise (beginning circa 2040), with the associated greater storm activity (ala Hansen's 'Storms of My Grandchildren), on infragravity (IG) waves (see the linked reference); one realizes that the impacts on coastal regions will be significantly greater than current assumed by planners.

As indicated in the linked article oceanic IG waves are primarily produced in shallow (or shoaling) water from the gravitational energy released from the shorter period wind-generated waves as the wind waves enter the shallow water. IG waves have long periods (on the order of 20 seconds) and thus they can damage coastlines more than wind-driven waves (think hurricanes, typhoons & cyclones) as indicated in the linked reference.  Furthermore, reflected IG waves (say off-of a beach) can form soliton waves that do not dissipate with distance traveled, as is currently the case where IG soliton wave travel from the south coast of Alaska all the wave to Western Antarctica; where their long-period energy can reach underneath ice shelves and cause them to flex.

Finally, I note that there are many relatively flat coastal plains around the world that would be inundated by abrupt sea level rise (and/or inundated by storm related rainfall); which would create a large number of new shallow/shoaling water zones that would create many more IG waves than currently occur today; which would be accelerate the collapse of more Antarctic ice shelves (which would be a positive feedback on both abrupt SLR & on ice-climate feedback mechanisms) and would cause more coastal damage around the world.

Xavier Bertin et al. (February 2018), "Infragravity waves: From driving mechanisms to impacts", Earth-Science Reviews, Volume 177, Pages 774-799, https://doi.org/10.1016/j.earscirev.2018.01.002

https://www.sciencedirect.com/science/article/pii/S0012825217303239

Abstract
Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth).


30

...
Moreover values were not used for the full arctic (60-90N).

While I agree that accuracy is important, and appreciate Zeke's work on this, it really is obscene to be carrying on this debate as though we're still serious about the possibility of keeping warming well below 2C given what other recent studies have said, and the early results for ECS from CMIP6.

...

wdmn,

I generally do not have time to address most of the assumptions that work their way into various posts in this thread, but I thought that I would clarify my position on some of the matters that you raise.

First, the Arctic Circle is from 66° 33' 39" N. to 90oN, so the plot you referenced is more relevant than Zeke Hausfather's; and I was merely pointing out that different posts were comparing apples to oranges.

Second, in Zeke Hausfather's following linked article on the SSP scenarios, he notes that SSP2 is most comparable to the relatively new RCP 7.0, and he claims that these scenarios are more likely to be more representative of future anthropogenic forcing than either SSP5 baseline or RCP 8.5.  However, before the end of April 2019 global population will be at 7.7 billion people, which is above all of the SSPs estimated global populations in 2019 as shown in the first image from Hausfather's article, and I note that in 2100 RCP 8.5 assumed a world population of about 12 billion while SSP5-baseline assumes a world population of about 7.38 billion people (indicating that SSP5-baseline was rigged to match the radiative forcing of RCP 8.5, significantly by ignoring global population projections).  Thus SSP5-baseline seriously underestimates global population, which means to me that even if the people ignored by SSP5-baseline were to only use sustainable energy (see the second attached image for the year 2018) then we would still be following SSP5-baseline and not SSP2.

Title: "Explainer: How ‘Shared Socioeconomic Pathways’ explore future climate change"

https://www.carbonbrief.org/explainer-how-shared-socioeconomic-pathways-explore-future-climate-change

Extract: "While RCP8.5 lives on in the form of the SSP5 baseline, it is now just one of many possible no-new-policy futures. The fact that only one of the SSPs, SSP5, can reach the level of emissions found in RCP8.5 suggests that it may not now be best suited for use as the sole baseline scenario in future research.

If any SSP can be said to be characteristic of current conditions it is SSP2, where social, economic and technological trends do not shift markedly from historical patterns. Greenhouse gas concentrations in the SSP2 baseline roughly correspond to the new RCP7.0, which shows lower emissions and nearly 1C less warming than RCP8.5 – though still 3.8-4.2C of warming above pre-industrial levels."

The second image comes from the IEA report on global energy use thru 2018 at:

https://www.iea.org/geco/

Finally, I remind readers that I think that we will only follow SSP5-baseline through circa 2060 when I expect global socio-economic collapse will drop anthropogenic GHG emissions down to something like SSP1.

Best,
ASLR

31
At least last year, increasing sustainable energy production has just allowed the world to use more energy; thus still maintaining high CO₂ emissions:

Title: "Global carbon dioxide emissions reached record high in 2018"

https://www.axios.com/global-co2-emissions-rose-again-2018-aceffb3b-0f5c-42e9-80b0-238ab397d6c8.html

Extract: "Global carbon dioxide emissions from energy generation climbed for the second straight year in 2018 and reached a record high as global energy demand surged, a new International Energy Agency (IEA) report shows."
&

See also, "Global Energy & CO2 Status Report
The latest trends in energy and emissions in 2018"

https://www.iea.org/geco/

Extract: "Global energy consumption in 2018 increased at nearly twice the average rate of growth since 2010, driven by a robust global economy and higher heating and cooling needs in some parts of the world. Demand for all fuels increased, led by natural gas, even as solar and wind posted double digit growth. Higher electricity demand was responsible for over half of the growth in energy needs. Energy efficiency saw lacklustre improvement. As a result of higher energy consumption, CO2 emissions rose 1.7% last year and hit a new record."


32
The linked reference finds that Fennoscandian cloud cover is changing now in response to anthropogenic global warming (in recent decades) in the same manner that it has changed in past periods of climate change.  Such findings may help to explain why so many CMIP6 programs are exhibiting relatively high values of TCR/ECS; because net cloud feedback may well have become markedly more positive in recent decades:

Young, Giles H. F.., Gagen, Mary H.., Loader, Neil J.., McCarroll, Danny., Grudd, Håkan., Jalkanen, Risto., Kirchhefer, Andreas. & Robertson, Iain. (2019). Cloud cover feedback moderates Fennoscandian summer temperature changes over the past 1000 years. Geophysical Research Letters, https://cronfa.swan.ac.uk/Record/cronfa48822, doi:10.1029/2018GL081046

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL081046

Abstract: "Northern Fennoscandia has experienced little summer warming over recent decades, in stark contrast to the hemispheric trend, which is strongly linked to greenhouse gas emissions. A likely explanation is the feedback between cloud cover and temperature. We establish the long‐ and short‐term relationships between summer cloud cover and temperature over Northern Fennoscandia, by analyzing meteorological and proxy climate data. We identify opposing feedbacks operating at different timescales. At short timescales, dominated by internal variability, the cloud cover‐temperature feedback is negative; summers with increased cloud cover are cooler and sunny summers are warmer. However, over longer timescales, at which forced climate changes operate, this feedback is positive, rising temperatures causing increased regional cloud cover and vice versa. This has occurred both during warm (Medieval Climate Anomaly and at present) and cool (Little Ice Age) periods. This two‐way feedback relationship therefore moderates Northern Fennoscandian temperatures during both warm and cool hemispheric periods."

Plain Language Summary: "Temperatures have increased globally over recent decades, strongly linked to increases in greenhouse gases. However, over Northern Fennoscandia summer temperatures have increased little over this period, although this region should be strongly affected by global warming. We suggest that changes in summer cloud cover, driven by global temperature changes, are responsible for this moderation of temperatures. This is happening now and during past episodes of climate change. We produce a new reconstruction of summer cloud cover for this region and compare it to existing temperature reconstruction to establish the relationship between temperature and cloud cover. We find that over short timescales, increased cloud cover leads to cooler temperatures and vice versa. However, over longer timescales (decades to centuries), we find that increased global temperature leads to increased northern cloud cover, which reduces local temperatures (the medieval period and at present). The opposite being true in globally cool periods, such as the Little Ice Age. These finding are important as they help to explain the feedback relationship between cloud cover and temperature, which is one of the major uncertainties in modeling future climate. Our data also confirm models of climate that suggest a poleward movement of storm tracks during recent warming."

33
The linked UNEP reference has a lot of nice graphics about the changing Arctic (too many to choose just a few to post in this reply); however, most of the climate change comments focus on the RCP 4.5 pathway; which to my way of thinking is overly optimistic but as indicated by the extract still results in "… winter temperatures over the Arctic Ocean would increase 3 to 5°C by mid-century and 5 to 9°C by late century (relative to 1986–2005 levels) …":

Title: "Global Linkages A graphic look at the changing Arctic"

https://wedocs.unep.org/bitstream/handle/20.500.11822/27687/Arctic_Graphics.pdf?sequence=1&isAllowed=y

Extract: "Continuing global emissions at rates of a medium-emission scenario (RCP4.5) projects global warming of 2.4 ± 0.5°C above pre-industrial levels by 2100 (Collins et al., 2013 (AR5)). At this rate of emissions, winter temperatures over the Arctic Ocean would increase 3 to 5°C by mid-century and 5 to 9°C by late century (relative to 1986–2005 levels) (AMAP, 2017a)."

Edit, per Hausfather, the UNEP document, cited above, got the AMAP (2017) quote slightly in error and it should be as cited in the extract below:

Title: "Factcheck: Is 3-5C of Arctic warming now ‘locked in’?" by Zeke Hausfather

https://www.carbonbrief.org/factcheck-is-three-five-celsius-of-arctic-warming-now-locked-in

Extract: "The reference for these numbers is the 2017 Arctic Monitoring and Assessment Programme (AMAP) report. The 2017 AMAP report states:

“Over the Arctic Ocean, which is ice-free in early winter in some models and covered by thin sea ice during late winter, the warming is 3–5C by mid-century and 5–9C by late century under RCP4.5.”"

34
The linked open access reference finds a 'widespread decrease of firn air content in western Greenland'.  This is not good new, as the air content of the firn decreases (to zero) the surface meltwater runoff will increase substrantially:

Vandecrux, B., MacFerrin, M., Machguth, H., Colgan, W. T., van As, D., Heilig, A., Stevens, C. M., Charalampidis, C., Fausto, R. S., Morris, E. M., Mosley-Thompson, E., Koenig, L., Montgomery, L. N., Miège, C., Simonsen, S. B., Ingeman-Nielsen, T., and Box, J. E.: Firn data compilation reveals widespread decrease of firn air content in western Greenland, The Cryosphere, 13, 845-859, https://doi.org/10.5194/tc-13-845-2019, 2019.

https://www.the-cryosphere.net/13/845/2019/

35
The linked reference indicates that future Arctic Sea Ice loss under BAU forcing will be significantly influenced by Ocean Heat Transport (OHT) from the North Atlantic through the Barents Sea and into the Arctic Ocean Basin and that:  Internal OHT variability is associated with both upstream ocean circulation changes, including AMOC, and large-scale atmospheric circulation anomalies reminiscent of the Arctic Oscillation."  Furthermore, the reference finds that: "The future long-term increase in Atlantic heat transport is carried by warmer water as the current itself is found to weaken."  This last sentence indicates that as the AMOC slows down the ocean water in the associated North Atlantic current gets warmer and it is the increased warmth of the ocean water which is critical to Arctic Sea Ice loss.  Finally, I note that ice-climate feedback associated with both future GIS and future AIS ice mass loss will work to accelerated the current slow down of the AMOC which will result in additional warming of the ocean water in the associated North Atlantic current; which will then accelerate future Arctic Sea Ice loss:

Marius Årthun et al. (13 March 2019), "The role of Atlantic heat transport in future Arctic winter sea ice loss", Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0750.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0750.1

Abstract: "During recent decades Arctic sea ice variability and retreat during winter have largely been a result of variable ocean heat transport (OHT). Here we use the Community Earth System Model (CESM) large ensemble simulation to disentangle internally and externally forced winter Arctic sea ice variability, and to assess to what extent future winter sea ice variability and trends are driven by Atlantic heat transport. We find that OHT into the Barents Sea has been, and is at present, a major source of internal Arctic winter sea ice variability and predictability. In a warming world (RCP8.5), OHT remains a good predictor of winter sea ice variability, although the relation weakens as the sea ice retreats beyond the Barents Sea. Warm Atlantic water gradually spreads downstream from the Barents Sea and further into the Arctic Ocean, leading to a reduced sea ice cover and substantial changes in sea ice thickness. The future long-term increase in Atlantic heat transport is carried by warmer water as the current itself is found to weaken. The externally forced weakening of the Atlantic inflow to the Barents Sea is in contrast to a strengthening of the Nordic Seas circulation, and is thus not directly related to a slowdown of the Atlantic meridional overturning circulation (AMOC). The weakened Barents Sea inflow rather results from regional atmospheric circulation trends acting to change the relative strength of Atlantic water pathways into the Arctic. Internal OHT variability is associated with both upstream ocean circulation changes, including AMOC, and large-scale atmospheric circulation anomalies reminiscent of the Arctic Oscillation."

36
Tall Ice-Cliffs Trigger Big Calving Events—and Fast Sea-Level Rise 
https://phys.org/news/2019-03-tall-ice-cliffs-trigger-big-calving.html
...

Thanks for the links.  Richard Alley talked about this mechanism verbally in some of his videos dating back to before 2013, and I am glad that he has finally co-authored a paper describing it more rigorously.

Also, for those who did not follow the link to the source reference, I provide the following:

Byron R. Parizek et al. Ice-cliff failure via retrogressive slumping, Geology (2019). DOI: 10.1130/G45880.1

https://pubs.geoscienceworld.org/gsa/geology/article/569567/Icecliff-failure-via-retrogressive-slumping

Abstract: "Retrogressive slumping could accelerate sea-level rise if ice-sheet retreat generates ice cliffs much taller than observed today. The tallest ice cliffs, which extend roughly 100 m above sea level, calve only after ice-flow processes thin the ice to near flotation. Above some ice-cliff height limit, the stress state in ice will satisfy the material-failure criterion, resulting in faster brittle failure. New terrestrial radar data from Helheim Glacier, Greenland, suggest that taller subaerial cliffs are prone to failure by slumping, unloading submarine ice to allow buoyancy-driven full-thickness calving. Full-Stokes diagnostic modeling shows that the threshold cliff height for slumping is likely slightly above 100 m in many cases, and roughly twice that (145–285 m) in mechanically competent ice under well-drained or low-melt conditions."

Edit, the following caption is for Figure 1 from the reference with the figure subdivided into three panels (A, B & C) shown in the attached three images:

Caption: "Figure 1. Helheim Glacier (East Greenland) ice-cliff geometry schematic during calving event. Glacier-front geometries along the last 1 km of the A-A′ profile shown in Figure 3A are from terrestrial radar interferometer data and assumed hydrostatic equilibrium for ice mélange before slumping begins (A), during slump (B), and after calving event concludes (C). Black arrows indicate relative glaciostatic and hydrostatic stress imbalance along ice front assuming negligible backpressure from mélange (A,C) and after mass loss due to slumping (B). Bed elevation is based on mass-conservation gridding (Morlighem et al., 2014) where available, and is unknown elsewhere, as denoted by question marks. Time (UTC, 12 August 2014) for each panel is noted in upper right corner."

37
I agree with Trenberth's 2011 position that the null hypothesis should no longer be that human activity has no role in climate change, and that the null hypothesis should be changed to that human activity is assumed to be involved in a particular climate change observation unless proved otherwise.  Thus when consensus climate scientists make Type II errors they will be following the Precautionary Principle:

Kevin E. Trenberth (03 November 2011), "Attribution of climate variations and trends to human influences and natural variability", WIREs Climate Change, https://doi.org/10.1002/wcc.142

https://onlinelibrary.wiley.com/doi/abs/10.1002/wcc.142

Abstract: "Past attribution studies of climate change have assumed a null hypothesis of no role of human activities. The challenge, then, is to prove that there is an anthropogenic component. I argue that because global warming is “unequivocal” and ‘very likely’ caused by human activities, the reverse should now be the case. The task, then, could be to prove there is no anthropogenic component to a particular observed change in climate, although a more useful task is to determine what it is. In Bayesian statistics, this change might be thought of as adding a ‘prior’. The benefit of doubt and uncertainties about observations and models are then switched. Moreover, the science community is much too conservative on this issue and too many authors make what are called ‘Type II errors’ whereby they erroneously accept the null hypothesis. Global warming is contributing to a changing incidence of extreme weather because the environment in which all storms form has changed from human activities. WIREs Clim Change 2011, 2:925–930. doi: 10.1002/wcc.142"

38
The first linked Barron's article, explains that the information/guidance that consensus climate scientists have managed to convey to business, makes those businesses conclude that 'it still makes financial sense to burn the globe".  As the article indicates this leads some people to conclude that we are headed to a 'climate Minsky moment', comparable to the 2008 financial collapses where a few 'outlier' warnings (Minsky in the case of the market and Hansen in the case of climate) were discounted by the consensus which lead to a collapse (in 2008 for the economy and still unfolding for the climate).  It seems clear to me that consensus climate science has conveyed a message to the markets that climate change impacts will unfold so slowly that it makes good financial sense for business people to continue to game the market.  Clearly, climate scientists need to stop erring on the side of least drama, and instead they need to convey clearer information about the magnitude and the time-scale of our current climate risks (which are unfolding in real time in front of our eyes):


Title: "It Still Makes Financial Sense to Burn the Globe"

https://www.barrons.com/articles/climate-change-is-still-too-cheap-esg-investing-51553104880

Extract: "It still makes good financial sense to burn the globe, although we know that we will have to pay a steep price for it."

See also:

Title: "Minsky moment"

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

Extract: "The term was coined by Paul McCulley of PIMCO in 1998, to describe the 1998 Russian financial crisis, and was named after economist Dr. Hyman Minsky, who noted that bankers, traders, and other financiers periodically played the role of arsonists, setting the entire economy ablaze. Minsky opposed the deregulation that characterized the 1980s."
&

Title: "Mark Carney warns of climate change threat to financial system"

https://www.theguardian.com/business/2018/apr/06/mark-carney-warns-climate-change-threat-financial-system

Extract: "The governor of the Bank of England has warned of the “catastrophic impact” climate change could have for the financial system unless firms do more to disclose their vulnerabilities."

39
The linked websites indicate that the IPCC is not ignoring ocean and cryosphere interactions for future climate change projections, and that a promising Special Report on this topic will be issued in September 2019.  Unfortunately, I strongly suspect that this special report will not address MICI risks, and will likely discount the speed and intensity of many of the coming ice-climate feedback mechanisms:

Title: "The Ocean and Cryosphere in a Changing Climate"

https://www.ipcc.ch/report/srocc/

Extract: "During its 45th Session (Guadalajara, Mexico, 28 – 31 March 2017), the IPCC Panel approved the outline of the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). The report will be finalized in September 2019."

See also:

Title: "Decision IPCC/XLV-2. Sixth Assessment Report (AR6) Products, Outline of the Special Report on climate change and oceans and the cryosphere"

https://www.ipcc.ch/site/assets/uploads/2018/04/Decision_Outline_SR_Oceans.pdf

Extract: "Chapter 1: Framing and Context of the Report (~15 pages)
• Integrated storyline of the report, chapter narrative, chapter sequence and their linkages (including coverage of extremes and abrupt change and irreversible changes)
• Definition of ocean and cryosphere and their components
• Observing capacities, progress and limitations (e.g., time series and spatial coverage)
• Assessment methodologies, including indigenous and community knowledge, risk, including cascading risks, and applications of detection and attribution
• Role of ocean and cryosphere in the climate system, including characteristics, ocean heat content in Earth’s energy budget, key feedbacks and time scales
• Implications of climate-related ocean and cryosphere change for resources, natural systems (e.g., change and loss of habitat, extinctions), human systems (e.g., psychological, social, political, cultural and economic aspects), and vulnerability assessments, adaptation limits, and residual risks
• Solutions, including policy options and governance, and linkages of this report to relevant institutional and policy contexts (e.g., UNFCCC, Paris Agreement and SDGs, Sendai Framework)
• Treatment of vulnerabilities and marginalized areas and people (e.g., gender) in this report
• Scenarios and time frames considered in this report
• Treatment of uncertainty

Chapter 3: Polar Regions (~50 pages)
• Changes in atmospheric and ocean circulation that influence polar regions, including climate feedbacks and teleconnections and paleo perspectives
• Greenland and Antarctic ice sheets and ice shelves, Arctic and Antarctic glaciers, mass change, physics of dynamical instability and accelerated ice discharge; consequences for ocean circulation and biogeochemistry, and sea level
• Changing snow cover, freshwater ice and thawing permafrost (terrestrial and subsea); carbon flux and climate feedbacks; impacts on infrastructure and ecosystems; community- based adaptation
• Changing sea ice; effects on ocean and atmospheric circulation and climate, including teleconnections; implications for ecosystems, coastal communities, transportation and industry
• Changing polar ocean (physical, dynamical and biogeochemical properties), implications for acidification, carbon uptake and release; impacts on ecosystems and their services (e.g., fisheries); adaptation options (e.g., ecosystem-based management and habitat protection) and limits to adaptation
• Access to resources and ecological, institutional, social, economic, livelihood and cultural consequences of polar change, including issues of international cooperation
• Responses to enhance resilience

Chapter 6: Extremes, Abrupt Changes and Managing Risks (~20 pages)
• Risks of abrupt change in ocean circulation and cryosphere and potential consequences
• Extreme ENSO events and other modes of variability and their implications
• Marine heat waves and implications
• Changes in tracks, intensity, and frequency of tropical and extra-tropical storms and associated wave height
• Cascading risks (e.g., storm surge and sea level rise), irreversibility, and tipping points
• Monitoring systems for extremes, early warning and forecasting systems in the context of climate change
• Governance and policy options, risk management, including disaster risk reduction and enhancing resilience"

40
While water supply for human use will rapidly become a problem (with continued global warming), Africa will likely be hit particularly hard due to its projected rapid population growth (see the attached image).  Certainly ice-climate feedbacks will make this existing problem worse, with continued global warming:

Title: "World Water Development Report 2019 - Leaving No One Behind"

https://en.unesco.org/water-security/wwap/wwdr/2019
&
https://unesdoc.unesco.org/ark:/48223/pf0000367276

Extract: "Billions still lack safe water and sanitation facilities, and people are being left behind for reasons related to their gender, ethnicity, culture and/or socioeconomic status, among others. Exclusion, discrimination, entrenched power asymmetries, poverty and material inequalities are among the main obstacles to fulfilling the human rights to water and sanitation and achieving the water-related goals of the 2030 Agenda for Sustainable Development.

The wealthy generally receive high levels of service at very low price, while the poor often pay a much higher price for services of similar or lesser quality."

41
Just to support James Hansen's recommendation to limit atmospheric CO₂ concentrations to 350ppm, the linked article indicate that it is likely that we have already triggered a Sixth Mass Extinction, which is not only morally wrong, and which threatens our collective food supplies in the coming decades; but also will likely act as a positive feedback mechanism for accelerating more warming:

Title: "The sixth mass extinction, explained"

https://theweek.com/articles/823904/sixth-mass-extinction-explained

Extract: ""We are sleepwalking toward the edge of a cliff," said Mike Barrett, executive director at WWF.

Potentially enormous. The loss of species can have catastrophic effects on the food chain on which humanity depends. Ocean reefs, which sustain more than 25 percent of marine life, have declined by 50 percent already — and could be lost altogether by 2050. This is almost certainly contributing to the decline of global marine life, down — on average — by 50 percent since 1970, according to the WWF. Insects pollinate crops humans eat. "This is far more than just being about losing the wonders of nature, desperately sad though that is," the WWF's Barrett said. "This is actually now jeopardizing the future of people. Nature is not ‘nice to have' — it is our life-support system.""
&

Title: "Earth’s Sixth Mass Extinction Has Begun, New Study Confirms"

https://www.iflscience.com/plants-and-animals/earth-s-sixth-mass-extinction-has-begun-new-study-confirms/

Extract: "We are currently witnessing the start of a mass extinction event the likes of which have not been seen on Earth for at least 65 million years. This is the alarming finding of a new study published in the journal Science Advances."

42
I have made a point that I do not think that the WAIS will begin a MICI type of collapse until about 2040 (note that SSP5-Baseline projects a GMSTA of about 2C, relative to 1986-2005, by 2040); however, that do not mean that I do not think that we will activate numerous nonlinear positive self-reinforcing feedback mechanisms between now and then, which not only increases the probability of a MICI type of WAIS collapse this century, but also increases the probably that we cross a tipping point for abrupt climate change well before 2040 as cited in the two linked sources and illustrated by the attached image of abrupt regional feedback mechanisms.

Title: "Climate report understates threat"

https://thebulletin.org/2018/10/climate-report-understates-threat/

...

I would like to note that most of the abrupt regional feedback mechanisms identified in quote above came from CMIP5; however, if CMIP6 has several ESMs with ECS greater than 5C, then CMIP6 will most likely identify many more abrupt feedback mechanisms including several related to ice-climate feedback mechanisms (omitted in CMIP5) even though CMIP6 only considers MISI types of ice sheet collapse mechanisms and omits consideration of MICI type collapse mechanisms.

Furthermore, I note CMIP5 many very well have underestimated the negative feedback from both natural and anthropogenic aerosols, which means that if rainforests (especially the Amazon) degrades rapidly and/or anthropogenic aerosol emissions are rapidly decreased in the coming decades, we are likely to cross many more tipping point thresholds earlier than identified in CMIP5.

43
While the laws of nature will be followed no matter what, it is the judicial system that determines what is legal fact with regard to human actions (e.g. see the linked article about goverments suing regarding professionally irresponsible behavior and or fraud by the fossil fuel industry); and consensus climate science evaluations play a major role with regard to what judges and juries determine to be legally liable facts about anthropogenic radiative forcing.

Consensus science has helped to establish a legal expectation that staying well below a GMSTA of 2C (relative to pre-industrial) is acceptable; however, James Hansen has stated that establishing an acceptable limit of 350ppm of atmospheric CO₂ concentrations would be advisable.  If consensus science would follow the Precautionary Principle, then it would be easier for youth groups to be successful in their lawsuits; and Greta Thunberg's youth movement might then file multiple lawsuits against governments for failing to adequately safeguard their collective futures (including lawsuits to block irresponsible implementation of geoengineering):

Title: "DC moves closer to climate lawsuit against Exxon"

https://thehill.com/policy/energy-environment/434590-dc-building-team-for-exxon-climate-challenge

Extract: "The D.C. government is beefing up its legal team ahead of an anticipated legal challenge against Exxon.

If D.C. moves forward with a lawsuit against Exxon, it will be joining a handful of other states and municipalities looking into how the oil and gas giant may have failed to publicize science it had linking emissions to global warming.

New York sued Exxon in October for allegedly engaging in "a longstanding fraudulent scheme.""

Edit: To be clear, if youth groups were successful in hundreds of lawsuits against hundreds of governments worldwide, then we would likely see these governments enact progressive carbon tax programs around the world.

Edit2, With regards to Hansen's 350ppm acceptable limit for atmospheric CO2 concentrations, the first attached image shows that we past that limit around 1987; and thus I assume that Hansen also meant that atmospheric CH4 concentrations should have an acceptable limit around 1685ppb where they were around 1987 as indicated by the second attached image.

44
The linked article indicates that the USA and Saudi Arabia recently blocked a U.N. resolution to make the UNEA (U.N. Environment Assembly) the governing body for regulating any future potential implementation of geoengineering.  Apparently, the USA and Saudi Arabia did not want a U.N. body to limit/regulate the impacts of geoengineering on the overall environment and on small countries:

Title: "U.S. Blocks U.N. Resolution on Geoengineering"

https://www.scientificamerican.com/article/u-s-blocks-u-n-resolution-on-geoengineering/

45
The first two (associated) linked sources indicate that due to global warming intense El Nino events will become more frequent (estimated to occur about every 10 years); while the third linked source indicates that there is a good chance that there will be an intense El Nino event in the 2019-2020 season; which would be only four years after the 2015-16 intense El Nino event.  This is a clear indication that the likely range for ECS is higher than AR5 indicates; and this adds to the probability that the WAIS may initiate a MICI type of collapse circa 2040:

Wenju Cai et al. (2018), "Increased variability of eastern Pacific El Nino under greenhouse warming", Nature 564, 201-206, https://doi.org/10.1038/s41586-018-0776-9

https://www.nature.com/articles/s41586-018-0776-9?WT.feed_name=subjects_ocean-sciences

Extract: "An increase in SST variance implies an increase in the number of 'strong' EP-El Nino events (corresponding to large SST anomalies) and associated extreme weather events."

See also:

Title: "El Nino events to become 'stronger' and more intense, study finds"

https://www.smh.com.au/environment/climate-change/el-nino-events-to-become-stronger-and-more-intense-study-finds-20181212-p50lrv.html

"They are stronger and more frequent," Dr Cai said, adding the likelihood of intense El Nino events as measured by sea-surface temperatures will increase from about one every 15 years now to every 10 years on average during this century.

Big El Ninos of recent decades include 1982-83, 1997-98 and 2015-16."

Also, particularly see:

Title: "'Monster' El Nino a chance later this year, pointing to extended dry times"

https://www.smh.com.au/environment/climate-change/monster-el-nino-a-chance-later-this-year-pointing-to-extended-dry-times-20190315-p514hi.html

Extract: "The prospect of a big El Nino later this year was raised at an international conference of climate scientists in Chile earlier this month.

They considered parallel years, such as 2014 when a near-El Nino was reached before conditions revived a year later, creating one of the three most powerful such events in the past half century.

"There is more heat now below the surface waiting to be tapped than there was in early 2015," said Michael McPhaden, a senior scientist with US National Atmospheric and Oceanic Administration who attended the Chilean event.

"If westerly wind bursts of sufficient amplitude, duration and zonal extent develop along the equator in the next couple of months, 2019-20 could be very exciting," he said.

"While it's not a slam dunk that El Nino is going to persist, I think that the odds have certainly increased over one to two months ago," Phil Klotzbach, a research scientist at Colorado State University, said. " We've had a big build up of heat in the eastern and central Pacific."

Cai Wenju, a senior CSIRO scientist who has published widely on the El Nino Southern Oscillation climate pattern, said the chance of El Nino returning is high.

A return of westerlies by about June to halt the easterly tradewinds “could spark the fire and there’s a lot of fuel", Dr Cai said.

“If it’s similar to 2015, the impact this time will be big," he said."

46
The linked open access reference is relevant to projecting the future stability of Arctic sea ice and the underlying ocean stratification and warming of the inflowing waters:

H. Sadatzki, T. M. Dokken, S. M. P. Berben, F. Muschitiello, R. Stein, K. Fahl, L. Menviel, A. Timmermann, E. Jansen, Sea ice variability in the southern Norwegian Sea during glacial Dansgaard-Oeschger climate cycles. Sci. Adv. 5, eaau6174 (2019).

http://advances.sciencemag.org/content/5/3/eaau6174

Abstract
The last glacial period was marked by pronounced millennial-scale variability in ocean circulation and global climate. Shifts in sea ice cover within the Nordic Seas are believed to have amplified the glacial climate variability in northern high latitudes and contributed to abrupt, high-amplitude temperature changes over Greenland. We present unprecedented empirical evidence that resolves the nature, timing, and role of sea ice fluctuations for abrupt ocean and climate change 32 to 40 thousand years ago, using biomarker sea ice reconstructions from the southern Norwegian Sea. Our results document that initial sea ice reductions at the core site preceded the major reinvigoration of convective deep-water formation in the Nordic Seas and abrupt Greenland warming; sea ice expansions preceded the buildup of a deep oceanic heat reservoir. Our findings suggest that the sea ice variability shaped regime shifts between surface stratification and deep convection in the Nordic Seas during abrupt climate changes.

See also: "New PhD thesis solves mysteries around large abrupt climate changes"

https://www.bjerknes.uib.no/en/article/news/new-phd-thesis-solves-mysteries-around-large-abrupt-climate-changes


Extract: "Henrik Sadatzki defends on Friday 15.02.2019 his thesis for the PhD degree at the University of Bergen. The thesis is entitled: “Sea ice variability in the Nordic Seas over Dansgaard–Oeschger climate cycles during the last glacial – A biomarker approach”.

47
I have previously noted that earlier consensus climate science reports like AR5 have made liberal use of caveats to exclude several right-tailed feedback mechanisms from the confidence ranges that they report for various climate change parameters (see also my comments in at least Replies #707 & #751).  Furthermore, in at least Reply #719 I recommended that consensus science generate families of Maximum-credible Climate-risk Scenarios (MCSs) in order to better assess right-tail climate risks.

In this post I briefly note consensus science only formally cites two limit states (i.e. the aspirational 1.5C GMSTA target and the 'Well below' 2C GMSTA limit by 2100).  To my Civil Engineering way of thinking the 1.5C target can be associated with an 'Operations' limit state where the proper function of our socioeconomic systems begin to fail; while the 'Well below' 2C limit can be associated where significant human life safety becomes threatened.

However, also to my Civil Engineering way of thinking, modern civil society depends on numerous other limit states (in civil engineering designs there are several other limit states such as: 'Progressive Collapse', 'Fatigue' and 'Durability limit states), that it would be good for consensus science to formally specify acceptable limit states for this century such as: a) Sea Level Rise and the associated rate of Sea Level Rise; b) Tropical Pacific SSTA, and c) Cascades of tipping points leading to abrupt changes in climate state.  Also, I note that in limit state design engineers assign partial load factors of safety for each separate forcing according to its uncertainty and partial resistance factors of safety to reflex the fragility of each separate system component according to its uncertainty.  Furthermore, I note that all civil engineering designs to adapt to climate change use the incomplete forcing from consensus science reports like AR5/CMIP5 and use load and resistance safety factors that were empirically established for quasi-static climate change, and that little or no attempts have been made to develop such factors of safety appropriate to maintain civil society under conditions of rapid/dynamic climate change.

Finally, I provide the linked UC Berkeley article and associated images on how the real process of science works; which per the attached images includes community feedback and societal expectations of benefits from scientific work.  In this sense the real consensus climate science process is politicized by community resistance to change and by societal expectation that science bring benefits ('good science') rather than reporting increasing risks/dangers ('bad science'):

Title: "The real process of science"

https://undsci.berkeley.edu/article/howscienceworks_02

Extract: "At first this process might seem overwhelming. Even within the scope of a single investigation, science may involve many different people engaged in all sorts of different activities in different orders and at different points in time — it is simply much more dynamic, flexible, unpredictable, and rich than many textbooks represent it as."

48
We shouldn't forget that shrubs thrive (relative to trees as explained in the linked reference) in relatively cold regions, and there continued expansion into the tundra decreases albedo & thus increases Arctic Amplification:

Treml et al. (2019), "Differences in growth between shrubs and trees: How does the stature of woody plants influence their ability to thrive in cold regions?", Agricultural and Forest Meteorology, https://doi.org/10.1016/j.agrformet.2019.02.036

https://www.sciencedirect.com/science/article/pii/S0168192319300954

Abstract
Shrubs can be found far above or beyond cold tree limits. However, the mechanisms shrubs employ to thrive at sites not allowing the development of trees remain poorly understood. We hypothesize that shrubs are advantaged over trees thanks to: (i) their low stature reflected in a better thermal environment; (ii) differences in temperature thresholds of wood formation; and (iii) a shorter period of wood formation in the slender stems of shrubs with narrow cells compared to tree stems with large cells. We studied wood formation of Picea abies (trees) and Pinus mugo (shrubs) growing on the same site in the treeline ecotone of the Krkonoše Mts in the Czech Republic. We measured air temperature near tree (shrub) tops, stem temperature and soil temperature in the root zone. In addition, we determined the number of cells in individual phases of wood phenology. We then computed the duration of individual wood-phenology phases and temperature thresholds for the onset of wood formation. Our results show that in the growing season, shrubs experience higher amplitude of air and stem temperatures compared to trees. Mean growing season air and stem temperatures are similar between the two growth forms whereas mean soil temperatures are lower for shrubs because their dense canopies shade the ground. Temperature thresholds for wood formation are either similar (3 °C for soil temperature, onset of cell division) or greater by 1.2–2.6 K (onset of cell enlargement) for P. mugo shrubs than for P. abies trees, depending on the temperature metrics considered. Although we found ambiguous differences in the onset of wood formation, this was completed earlier in P. mugo than in P. abies, leading to a generally shorter growing period of shrubs (103 days) than trees (125 days). In conclusion, the main advantage of shrubs over trees resides in the earlier completion of wood formation and thus a shorter growing season. Trees with wide cells at stem base require more time for cell differentiation and maturation than shrubs with narrow cells. Other differences are either of lesser importance (the ambient thermal environment) or probably species-specific (temperature thresholds for wood formation).

Edit: I also not that the continuing expansion of digging creatures (like ground squirrels, etc.) into the tundra (with continued warming) accelerates permafrost degradation and thus also contributes to increasing Arctic Amplification.

49
The linked open access commentary provides a convenient summary of the nature and likely impacts of Arctic Amplification:

Twila A. Moon et al. (07 March 2019), "The expanding footprint of rapid Arctic change", Earth's Future, https://doi.org/10.1029/2018EF001088

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018EF001088

Abstract
Arctic land ice is melting, sea ice is decreasing, and permafrost is thawing. Changes in these Arctic elements are interconnected, and most interactions accelerate the rate of change. The changes affect infrastructure, economics, and cultures of people inside and outside of the Arctic, including in temperate and tropical regions, through sea level rise, worsening storm and hurricane impacts, and enhanced warming. Coastal communities worldwide are already experiencing more regular flooding, drinking water contamination, and coastal erosion. We describe and summarize the nature of change for Arctic permafrost, land ice, and sea ice, and its influences on lower latitudes, particularly the United States. We emphasize that impacts will worsen in the future unless individuals, businesses, communities, and policy makers proactively engage in mitigation and adaptation activities to reduce the effects of Arctic changes and safeguard people and society.

50
CMIP6 is already being conducted with at best consideration of MISI types of ice mass loss from both the GIS & AIS, and completely without consideration of MICI types of ice mass losses, therefore, a discussion of the implications of the growing number of consensus climate scientists who are pointing to the findings of Edwards et al. (2019) as justification for ignoring MICI types of ice mass loss in their model projections this century may be academic.  Nevertheless, here I point out that Edwards et al. (2019) used a statistical emulator of ice sheet mass loss over the past 1 million years to indicate that most likely MISI behavior could account for the paleo record; however, this finding is a far different matter than concluding that MICI types of behavior can be discounted in model projections for the rest of this century.

First, as SSP5-Baseline indicates that we may well be at Mid-Pliocene (3.3 Ma–3 Ma) conditions by (or before) 2040, the Edwards et al. (2019) emulation of the past 1Ma are likely not relevant; while Pollard, DeConto & Alley (2018)'s evaluation of MICI under Mid-Pliocene conditions are more relevant.

Second, we are approaching Mid-Pliocene conditions thousands of times faster than occurred during the Mid-Pliocene; which Pollard, DeConto & Alley (2018) took into account by abruptly introducing Mid-Pliocene conditions onto modern AIS conditions in their model.

Third, both Edwards et al (2019) and Pollard, DeConto & Alley (2018) use AIS boundary & starting conditions for the modern AIS that are less aggressive than what is currently observed in 2019 w.r.t. such factors as: a) the large subglacial cavity at the base of the Thwaites Ice Tongue; b) the loss of ice shelf buttressing of the Southwest Tributary Glacier in Pine Island Bay; and c) the amount of ice-climate feedback mechanisms that are already being activated (including MOC slowing, shifting of the ENSO towards more frequent El Nino events & increased advection of warm CDW towards the grounding lines of key AIS marine glaciers).

I understand that CMIP6 is already pushing the computational capacity of the participation ESMs; I think that it is wasteful of research funds to be investigating scenarios with little chance of occurring, and also I note that currently the cost of using Exascale computers is dropping by half about every 3 months.  Therefore, I believe that rather than likely committing a Type 2 error (a false negative, i.e. assuming that MICI ice mass loss will not occur this century when it actually might occur after 2040); it would be better for consensus climate scientists to work harder (possibly by using Exascale computing resources) on including MICI mechanisms in at least some of their scenarios so as to at least evaluate right-tail risks.

Finally, I provide the attached images related to use of the scientific method (without comment), as I believe that the proper use of this method would not result in MICI behavior being discounted in on-going ESM modeling efforts.

Edit: I forgot to mention that the emulator used by Edwards et al. (2019) did not include the impact of the Antarctic ozone holes (as this did not occur in paleo-times); which this ozone hole has accelerated ice mass loss from Antarctica and projections of the impact of continuing GHG emissions on the westerly winds over the Southern Ocean, indicate that as the ozone hole heals the regional impact of increasing GHG atmospheric concentrations will keep these regional westerly winds in an optimal zone for promoting ice mass loss for at least the next few decades.

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