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Author Topic: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios  (Read 85668 times)

Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #150 on: December 20, 2014, 01:01:49 PM »
And here the AGU-lecture by Jim White on Abrupt Climate Change, which puts Alley's presentation in a broader context:
https://virtualoptions.agu.org/media/C23D-01.+Nye+Lecture%2C+Presented+By+James+White/0_r289t1qf

Bottom line: "speed kills'.

folke_kelm

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #151 on: December 20, 2014, 04:59:22 PM »
no other comment than: That is a great lecture, thanks.

Laurent

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #152 on: December 20, 2014, 05:34:07 PM »
1°C in 5 years repeatedly in the past...jeessss !

Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #153 on: December 20, 2014, 06:15:28 PM »
1°C in 5 years repeatedly in the past...jeessss !

Yes, in Greenland, but still.

jai mitchell

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #154 on: December 24, 2014, 01:22:37 AM »
It should be noted that the sea level rises that he is discussing are predicated on continential ice sheets.  In addition, the warming that he is discussing happened only during the glacial maximum. (until the end of the younger dryas.)

so initial conditions are very different than today.
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #155 on: April 20, 2015, 03:14:44 PM »
The linked reference discusses 143 high latitude (in both the Arctic & Antarctic) ice-sheet grounding-zone wedges (GZWs), which are asymmetric sediment paleo deposits along the grounding line of an episodically pinned (generally for periods of decades to centuries) but otherwise retreating ice-sheet (such as is the case for both the PIG & Thwaites marine glaciers).  At one time researchers thought that such GZWs would serve to help stabilize otherwise retreating glaciers, but that is no longer thought to be the case.  Now I take them as evidence that marine glaciers (such as those in the WAIS) can retreat rapidly until they are temporarily pinned again.

C.L. Batchelor, and J.A. Dowdeswell (2015), "Ice-sheet grounding-zone wedges (GZWs) on high-latitude continental margins", Marine Geology, Volume 363, 1 May 2015, Pages 65–92, doi:10.1016/j.margeo.2015.02.001


http://www.sciencedirect.com/science/article/pii/S0025322715000304

Abstract: "Grounding-zone wedges (GZWs) are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line-source at the grounding zone of marine-terminating ice sheets during still-stands in ice-sheet retreat. GZWs form largely through the delivery of deforming subglacial sediments. The presence of GZWs in the geological record indicates an episodic style of ice retreat punctuated by still-stands in grounding-zone position. Moraine ridges and ice-proximal fans may also build up at the grounding zone during still-stands of the ice margin, but these require either considerable vertical accommodation space or sediment derived from point-sourced subglacial meltwater streams. By contrast, GZWs form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding-zone. An inventory of GZWs is compiled from available studies of bathymetric and acoustic data from high-latitude continental margins. The locations and dimensions of GZWs from the Arctic and Antarctic, alongside a synthesis of their key architectural and geomorphic characteristics, are presented. GZWs are only observed within cross-shelf troughs and major fjord systems, which are the former locations of ice streams and fast-flowing outlet glaciers. Typical high-latitude GZWs are less than 15 km in along-flow direction and 15 to 100 m thick. GZWs possess a transparent to chaotic acoustic character, which reflects the delivery of diamictic subglacial debris. Many GZWs contain seaward-dipping reflections, which indicate sediment progradation and wedge-growth through continued delivery of basal sediments. GZW formation is inferred to require high rates of sediment delivery to a fast-flowing ice margin that is relatively stable for probably decades to centuries. Although the long-term stability of the grounding zone is controlled by ice-sheet mass balance, the precise location of any still-stands is influenced strongly by the geometry of the continental shelf. The majority of high-latitude GZWs occur at vertical or lateral pinning points, which encourage grounding-zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #156 on: April 20, 2015, 06:45:55 PM »
While one must be very careful when extrapolating the relevance of paleo behavior to possible future modern response to global warming; if one were to do so then one of the most relevant examples would be the Pliocene transition from the Marine Isotope Stage M2 (from roughly 3.32 to 3.264Ma) to Marine Isotope Stage M1 (with the transition at about 3.264 Ma, see the first attached image [& associated caption below] and the linked reference which is open access).  While, the linked reference focuses on the glacial M2 stage rather than the interglacial M1 stage, this reference still provides a state-of-the-art model assessment of this critical time period.  Among many different insights that can be gained by reading the reference I offer one illustrated by the second attached image (with the third attached image providing the scales for the second image) that shows the difference in mean annual surface temperature (MAT) from the pre-industrial condition to the M2 condition with 280 ppmv CO₂ but without the glacial ice sheets indicated in the fourth attached image from Figure 2D (note that I provide the scale for this fourth image in the next post).  While this may seem to some to be much to do about nothing, I find it interesting that with the GIS, ASE & the Totten marine glaciers degraded to the elevations where they might possibly be by the end of 2100 (if we stay on a BAU pathway) then very roughly just due to elevation effects alone (i.e. with CO₂ at 280ppmv) the MAT in the WAIS, Southern GIS and Totten areas will be about 5C warmer than pre-industrial temperatures; which may need to be added on top of RCP 8.5 MAT projections (from pre-industrial) by 2100 that do not include the adjustment for ice sheet elevation drops.

Edit: Needless to say, an extra 5C MAT contributing to possible ice surface melting makes the Pollard et al 2015, hydrofracturing and cliff failure scenario much more plausible this century.

Aisling M. Dolan, Alan M. Haywood, Stephen J. Hunter, Julia C. Tindall, Harry J. Dowsett, Daniel J. Hill, Steven J. Pickering (2015), "Modelling the enigmatic Late Pliocene Glacial Event — Marine Isotope Stage M2", Global and Planetary Change, 128, 47–60,


http://www.sciencedirect.com/science/article/pii/S0921818115000399

Caption for first attached image, Figure 1: "Marine isotope stage M2 in relation to the long-term climate evolution of the Late Pliocene as shown in the benthic oxygen isotope record of Lisiecki and Raymo (2005). The PRISM warm interval (3.264 to 3.025Ma) is shown by the shaded grey bar.  Modern day benthic values are represented by the horizontal dashed line in the top panel. Obliquity (°), eccentricity, precession and the July insolation at 65°N (Wm−2) is also shown for reference (Laskar et al., 2004).

Caption for second attached image, Figure 3A&B: "Model predictions for (left) mean annual surface temperature (°C) and (right) mean annual precipitation (mm d−1) as an anomaly from the pre-industrial control experiment (Pre-IndCtrl) for each of the glacial sensitivity experiments (280 ppmv)."

Extract associated with the second attached image, Figure 3A&B: "In spite of the M2 orbit and no increase in CO2, the simulation using the PRISM3 distribution of ice sheets (PRISM-M2M2Orbit 280) remains warmer than pre-industrial (0.64 °C) but with relatively small overall polar amplification (Table 2). However, the warming seen in PRISM-M2M2Orbit 280 is primarily due to the warmer than pre-industrial temperatures simulated over ice sheets regions where ice has been removed in the PRISM3 reconstruction (Fig. 3a; Dowsett et al., 2010)."
« Last Edit: April 21, 2015, 01:03:51 AM by AbruptSLR »
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #157 on: April 20, 2015, 06:46:55 PM »
The attached image provides the elevation scale for the fourth image (Figure 2D) in the immediately prior post (Reply #156).
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #158 on: April 21, 2015, 11:12:03 PM »
The linked reference examines a period of abrupt climate change during MIS 3 (~35-55 ka); which could help to calibrate ESMs to project the implications of modern radiative forcing.

Gibson, K. A., R. C. Thunell, E. J. Tappa, L. C. Peterson, and M. McConnell (2015),"The influence of rapid, millennial scale climate change on nitrogen isotope dynamics of the Cariaco Basin during marine isotope stage 3", Paleoceanography, 30, 253–268. doi: 10.1002/2014PA002684.

http://onlinelibrary.wiley.com/doi/10.1002/2014PA002684/abstract

Abstract: "Understanding changes to the marine nitrogen cycle on millennial and shorter time scales can help determine the influence of rapid climate change on the fixed N pool and its sources and sinks. Rapid changes in denitrification have been observed in the eastern tropical North Pacific (ETNP) and Arabian Sea; however, millennial scale δ15N records in regions influenced by N2 fixation are sparse. We present a sedimentary δ15N record from the Cariaco Basin during marine isotope stage (MIS) 3 (~35–55 ka). The δ15N record displays a pattern of millennial scale variability that tracks the Greenland ice core Dansgaard-Oeschger cycles, with higher values observed during interstadial periods, lower values during stadial periods, and abrupt transitions in between. Conditions during interstadials are similar to those at present in the Cariaco Basin, with the sedimentary δ15N signal reflecting a combination of local processes and an imported regional signal. If interpreted to reflect regional processes, the interstadial δ15N values (average ~5.1‰) support the argument that N2 fixation did not increase in the tropical North Atlantic during the last glacial. The lower δ15N values during stadials, when lower sea level resulted in increased physical isolation of the basin, can be explained primarily by local processes. In spite of the importance of local processes, striking similarity is observed between the Cariaco record and millennial scale δ15N records from the ETNP and Arabian Sea. The apparent synchronicity of changes observed in all three regions suggests an atmospheric teleconnection between the three sites and high-latitude climate forcing during MIS 3."

See also:

http://www.reportingclimatescience.com/news-stories/article/rapid-climate-change-seen-in-ancient-records.html
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #159 on: April 30, 2015, 02:11:43 AM »
The linked reference discusses paleo findings (from the last ice age) about how long (about 200 years) it takes for Antarctic to notice abrupt warming (or cooling) in the Northern Hemisphere:

WAIS Divide Project Members (2015), "Precise interpolar phasing of abrupt climate change during the last ice age", Nature, Volume: 520, Pages: 661–665, doi:10.1038/nature14401


http://www.nature.com/nature/journal/v520/n7549/full/nature14401.html


Abstract: "The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics."

See also a discussion of this paper by Eric Steig at:

http://www.realclimate.org/
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AbruptSLR

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The linked reference provides additional paleo evidence that ocean-ice interaction plays a dominant role in ice mass loss in the WAIS both in the past and in the future:

Brenda L. Hall, George H. Denton, Stephanie L. Heath, Margaret S. Jackson & Tobias N. B. Koffman (2015), "Accumulation and marine forcing of ice dynamics in the western Ross Sea during the last deglaciation", Nature Geoscience, doi:10.1038/ngeo2478


http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2478.html


Abstract: "The grounding line of the ice sheet in the Ross Sea, Antarctica, retreated between the Last Glacial Maximum and the present. However, the timing of the retreat and the interplay of factors controlling ice stability in this region remain uncertain. Here we use 180 radiocarbon dates to reconstruct the chronology of moraine construction on the headlands adjacent to western McMurdo Sound. On the basis of these dates we then assess the timing of ice expansion and retreat in the Ross drainage system that is fed from both the East and West Antarctic ice sheets. We find that grounded ice in the western Ross Sea achieved its greatest thickness and extent during the last termination, between 12,800 and 18,700 years ago. Maximum ice thickness at our site coincides with a period of high accumulation as recorded by the West Antarctic Ice Sheet Divide ice core. Recession of the ice sheet from the headland moraines began about 12,800 years ago, despite continued high accumulation and the expansion of land-based glaciers at this time. We therefore suggest that the grounding-line retreat reflects an increased marine influence as sea levels rose and the ocean warmed. We suggest that future instability in the ice sheet grounding line may occur whenever the ocean forcing is stronger than forcing from accumulation."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Lennart van der Linde

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On the melting of the Laurentide Ice Sheet during the last deglaciation:
http://www.nature.com/ngeo/journal/v8/n7/full/ngeo2463.html

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #162 on: August 03, 2015, 05:18:01 AM »
The linked reference discusses Pollard, DeConto, Chang, Applegate and Haran (2015) findings on modeling both the last deglacial and future variations of the AIS,  Per the abstract: "One robust conclusion is that for the warmer future RCP scenarios, most reasonable parameter combinations produce retreat deep into the West Antarctic interior. Recently proposed mechanisms of hydrofracturing and ice-cliff failure accelerate future West Antarctic retreat, and later produce retreat into East Antarctic basins."

David Pollard, Robert DeConto, Won Chang, Patrick Applegate, and Murali Haran (2015), "Large-Ensemble modeling of last deglacial and future variations of the Antarctic Ice Sheet", Geophysical Research Abstracts, Vol. 17, EGU2015-5717, EGU General Assembly 2015


http://meetingorganizer.copernicus.org/EGU2015/EGU2015-5717.pdf

Abstract: "Recent observations of thinning and retreat of the Pine Island and Thwaites Glaciers identify the Amundsen Sea Embayment (ASE) sector of West Antarctica as particularly vulnerable to future climate change. To date, most future modeling of these glaciers has been calibrated using recent and modern observations.

As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the last deglacial retreat of Antarctica, making use of geologic data from ~20,000 years BP to present, focusing on the ASE but including other sectors of Antarctica.  Following several recent ice-sheet studies, we use Large-Ensemble statistical techniques, performing sets of ~500 to 1000 runs with varying model parameters.  The model is run for the last 40 kyrs on 10 to 20-km grids, both on continental domains and also on nested domains over West Antarctica. Various types of objective scores for each run are calculated using reconstructed past grounding lines, relative sea level records, measured uplift rates, and cosmogenic elevation-age data. Runs are extended into the future few millennia using RCP scenarios. The goal is to produce calibrated probabilistic ranges of model parameter values and quantified envelopes of future ice retreat.  Preliminary results are presented for Large Ensembles with (i) Latin HyperCube sampling in high-dimensional parameter space, using statistical emulators and Markov Chain Monte Carlo techniques, and (ii) dense "factorial" sampling with a smaller number of parameters. Different ways of combining the types of scores listed above are explored. One robust conclusion is that for the warmer future RCP scenarios, most reasonable parameter combinations produce retreat deep into the West Antarctic interior. Recently proposed mechanisms of hydrofracturing and ice-cliff failure accelerate future West Antarctic retreat, and later produce retreat into East Antarctic basins."
« Last Edit: August 03, 2015, 05:25:25 AM by AbruptSLR »
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #163 on: August 03, 2015, 05:22:50 AM »
The linked reference models the Antarctic during the Eemian and shows strong evidence for a severe ice-sheet retreat for the WAIS:

Johannes Sutter, Malte Thoma, Klaus Grosfeld, Paul Gierz, and Gerrit Lohmann (2015), "Learning from the past: Antarctic Eemian ice sheet dynamics as an analogy for future warming", Geophysical Research Abstracts, Vol. 17, EGU2015-13255-2, EGU General Assembly 2015


http://meetingorganizer.copernicus.org/EGU2015/EGU2015-13255-2.pdf


Abstract: "Facing considerable warming during this century the stability of the West Antarctic Ice Sheet is under increasing scrutiny. Recent observations suggest that the marine ice sheet instability of the WAIS has already started. We investigate the dynamic evolution of the Antarctic Ice Sheet during the last interglacial, forcing a state of the art 3D ice sheet model with Eemian boundary conditions. We elucidate the role of ocean warming and surface mass balance on the coupled ice sheet/shelf and grounding line dynamics. Special focus lies on an ice sheet modeling assessment of Antarctica’s potential contribution to global sea level rise during the Eemian. The transient model runs are forced by time slice experiments of a fully coupled atmosphere-ocean global circulation model, as well as different sets of sea level and bedrock reconstructions. The model result show strong evidences for a severe ice-sheet retreat in West Antarctica, leading to substantial contribution to global sea level from the Southern Hemisphere.  Additionally we compare future warming scenarios of West Antarctic Ice Sheet dynamics to our paleo ice sheet modeling studies."

« Last Edit: August 03, 2015, 12:31:40 PM by AbruptSLR »
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sidd

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #164 on: August 03, 2015, 07:49:09 AM »
the Pollard grid it too large, i think, but what shocks me is the sensitivity in Sutter to SLR forcing.

Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #165 on: August 03, 2015, 08:02:00 AM »
David Pollard, Robert DeConto, Won Chang, Patrick Applegate, and Murali Haran (2015), "Large-Ensemble modeling of last deglacial and future variations of the Antarctic Ice Sheet", Geophysical Research Abstracts, Vol. 17, EGU2015-5717, EGU General Assembly 2015

Good to see Pollard & DeConto teaming up with Applegate.
Maybe they should now team up with Hansen, Rignot et al as well...

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #166 on: August 03, 2015, 12:43:56 PM »
David Pollard, Robert DeConto, Won Chang, Patrick Applegate, and Murali Haran (2015), "Large-Ensemble modeling of last deglacial and future variations of the Antarctic Ice Sheet", Geophysical Research Abstracts, Vol. 17, EGU2015-5717, EGU General Assembly 2015


Good to see Pollard & DeConto teaming up with Applegate.
Maybe they should now team up with Hansen, Rignot et al as well...


See also:
Calibrating an ice sheet model using high-dimensional binary spatial data
Authors: Won Chang, Murali Haran, Patrick Applegate, David Pollard
(Submitted on 8 Jan 2015 (v1), last revised 2 Jul 2015 (this version, v4))

http://arxiv.org/abs/1501.01937
http://arxiv.org/pdf/1501.01937v4.pdf


Abstract: Rapid retreat of ice in the Amundsen Sea sector of West Antarctica may cause drastic sea level rise, posing significant risks to populations in low-lying coastal regions. Calibration of computer models representing the behavior of the West Antarctic Ice Sheet is key for informative projections of future sea level rise. However, both the relevant observations and the model output are high-dimensional binary spatial data; existing computer model calibration methods are unable to handle such data. Here we present a novel calibration method for computer models whose output is in the form of binary spatial data. To mitigate the computational and inferential challenges posed by our approach, we apply a generalized principal component based dimension reduction method. To demonstrate the utility of our method, we calibrate the PSU3D-ICE model by comparing the output from a 499-member perturbed-parameter ensemble with observations from the Amundsen Sea sector of the ice sheet. Our methods help rigorously characterize the parameter uncertainty even in the presence of systematic data-model discrepancies and dependence in the errors. Our method also helps inform environmental risk analyses by contributing to improved projections of sea level rise from the ice sheets.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #167 on: August 06, 2015, 07:26:37 PM »
The linked July 2015 paper (see also the attached image) includes discussion of paleo-evidence that abrupt collapse of the WAIS helps to trigger Arctic amplification.  As DeConto is one of the primary authors and he is also one of the main authors of the Pollard, DeConto and Alley 2015 on ASLR from Antarctic cliff failures and hydrofracturing; I think that we should all take these paleo findings very seriously:

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

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

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

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

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #168 on: August 27, 2015, 10:56:47 PM »
The linked reference provides paleo evidence that the formation of the ACC helped contributed to the lower atmospheric carbon dioxide levels since that time 30 million years ago:

Howie D. Scher, Joanne M. Whittaker, Simon E. Williams, Jennifer C. Latimer, Wendy E. C. Kordesch and Margaret L. Delaney (2015), "Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies", Nature, doi:10.1038/nature14598


http://www.nature.com/nature/journal/v523/n7562/full/nature14598.html


Abstract: "Earth’s mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere, and influences vertical ocean structure, deep-water production and the global distribution of nutrients and chemical tracers. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth’s present and past climate system, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent–ocean boundaries, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian–Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time."



Caption for first image: "The Antarctic Circumpolar Current blocks the Southern Hemisphere equivalent of the Gulf Stream from delivering heat to Antarctica"
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Greenbelt

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #169 on: September 01, 2015, 12:05:38 AM »
And here the AGU-lecture by Jim White on Abrupt Climate Change, which puts Alley's presentation in a broader context:
https://virtualoptions.agu.org/media/C23D-01.+Nye+Lecture%2C+Presented+By+James+White/0_r289t1qf

Bottom line: "speed kills'.


Here is a version of the talk that isn't behind a registration wall
http://climatestate.com/2015/04/06/abrupt-climate-change-past-present-and-future/

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #170 on: September 30, 2015, 10:33:49 PM »
The linked reference reviews paleo data about the importance of the Southern Ocean in regulating natural CO₂ atmospheric concentrations

Andrew J. Watson, Geoffrey K. Vallis & Maxim Nikurashin (2015), "Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO₂", Nature Geoscience, doi:10.1038/ngeo2538


http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2538.html


Abstract: "Atmospheric CO2 concentrations over glacial–interglacial cycles closely correspond to Antarctic temperature patterns. These are distinct from temperature variations in the mid to northern latitudes, so this suggests that the Southern Ocean is pivotal in controlling natural CO2 concentrations. Here we assess the sensitivity of atmospheric CO2 concentrations to glacial–interglacial changes in the ocean’s meridional overturning circulation using a circulation model for upwelling and eddy transport in the Southern Ocean coupled with a simple biogeochemical description. Under glacial conditions, a broader region of surface buoyancy loss results in upwelling farther to the north, relative to interglacials. The northern location of upwelling results in reduced CO2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60 ppm of atmospheric CO2. We therefore suggest that the direct effect of temperatures on Southern Ocean buoyancy forcing, and hence the residual overturning circulation, explains much of the strong correlation between Antarctic temperature variations and atmospheric CO2 concentrations over glacial–interglacial cycles."
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solartim27

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #171 on: September 30, 2015, 10:52:36 PM »
The northern location of upwelling results in reduced CO2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60 ppm of atmospheric CO2.
So I am reading that as a positive feedback without glaciers, correct?
FNORD

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #172 on: September 30, 2015, 11:19:10 PM »
The northern location of upwelling results in reduced CO2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60 ppm of atmospheric CO2.

So I am reading that as a positive feedback without glaciers, correct?


solartim27,
Perhaps if you read the following Reporting Climate Science summary this matter will become more clear (as well as looking at the attached image & associated caption).  During cold glacial periods, the Southern Ocean sucks more CO₂ out of the atmosphere than during interglacial periods (such as now).  This is not good news for the long-term wellbeing of future societies.  However, an abrupt calving of the WAIS this century would temporarily change these long-term findings:

http://www.reportingclimatescience.com/news-stories/article/ocean-synched-antarctic-temperatures-with-co2.html

Extract: "Scientists have struggled for the past few decades to understand why air temperatures around Antarctica over the past one million years were almost perfectly in synch with atmospheric CO2 concentrations. Both dipped down during glacial ice ages and back up again during warm interglacials."

Caption: "How ocean currents changed during a) modern oceans and b) oceans during glacial periods."

Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #173 on: October 05, 2015, 09:37:48 PM »
How high was sea level during the middle Pliocene?
Interesting discussion in Geology on contrasting findings:
http://geology.gsapubs.org/content/43/10/943.full

Austermann, Pollard, Mitrovica, Raymo et al 2015 think it could have been relatively high:
http://geology.gsapubs.org/content/43/10/927.abstract?ijkey=c11579fec9b3174252ab2ae3df7b16b32399a1e1&keytype2=tf_ipsecsha

Winnick & Caves 2015 think it could have been relatively low:
http://news.stanford.edu/news/2015/september/sea-level-rise-090315.html

So what to think of Winnick & Caves 2015, in particular?


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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #175 on: October 05, 2015, 11:04:20 PM »
Lennart,

Thanks for all of the links.  My brief comments on the Winnick & Caves (2015) findings are:
 
(1)  They have no bearing on the stability of the WAIS this century, nor on Greenland's marine terminating glaciers this century, nor on key East Antarctic marine glaciers (like Totten) this century;
(2) These findings seem to have most bearing on the stability of the land-based portions of the Antarctic Ice Sheet from a paleo point of view, that cannot serve as a direct comparison with future SLR forecasts for the next few centuries, as the forcing scenarios, and the Earth System state conditions, are both different.
(3) Paleo-data has a lot of inherent uncertainties and their interpretation can change with time.

Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #176 on: October 06, 2015, 09:17:32 AM »
ASLR, thanks for the comment. I agree, and I'm trying to find out what some of the experts think, like Eelco Rohling. Will let you know if they respond.

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #177 on: October 07, 2015, 10:24:54 PM »
DeConto gave a lecture tonight at Rutgers Climate Institute:
http://climatechange.rutgers.edu/events/all-climate-events/icalrepeat.detail/2015/10/07/1690/84%7C96%7C115/using-the-past-to-predict-the-future-of-the-antarctic-ice-sheet

Wish I could have been there, and can't wait for his new paper with Pollard to come out.

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #178 on: February 16, 2016, 11:26:52 PM »
The linked (open access) article indicates, based on physical evidence, that for the past 1.4 million years the WAIS contribution to global sea level during interglacial periods has been limited to 3.3m.  The first image shows the location of the location of the study area, and the second image shows the minimum WAIS ice sheet extend over that duration:

Andrew S. Hein, John Woodward, Shasta M. Marrero, Stuart A. Dunning, Eric J. Steig, Stewart P. H. T. Freeman, Finlay M. Stuart, Kate Winter, Matthew J. Westoby & David E. Sugden (2016), "Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years", Nature Communications, Volume: 7, Article number: 10325, doi:10.1038/ncomms10325


http://www.nature.com/ncomms/2016/160203/ncomms10325/full/ncomms10325.html

Abstract: "Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial–interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #179 on: March 11, 2016, 06:16:32 PM »
The linked reference indicates that the Antarctic Ice Sheet is more sensitivity to atmospheric GHG concentrations than indicated by current ice sheet models; thus indicating that the AR5 sea level rise projections err significantly on the side of least drama:

Simone Galeotti, Robert DeConto, Timothy Naish, Paolo Stocchi, Fabio Florindo, Mark Pagani, Peter Barrett, Steven M. Bohaty, Luca Lanci, David Pollard Sonia Sandroni, Franco M. Talarico & James C. Zachos (10 Mar 2016), "Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition", Science, DOI: 10.1126/science.aab0669


http://science.sciencemag.org/content/early/2016/03/09/science.aab0669


Abstract: "About 34 million years ago (Ma) Earth’s climate cooled and an ice sheet formed on Antarctica as atmospheric CO2 fell below ~750 ppm. Sedimentary cycles from a drill core in western Ross Sea provide the first direct evidence of orbitally-controlled glacial cycles between 34–31 Ma. Initially, under atmospheric CO2 levels ≥ 600 ppm, a smaller Antarctic Ice Sheet, (AIS) restricted to the terrestrial continent, was highly responsive to local insolation forcing. A more stable, continental-scale ice sheet calving at the coastline, did not form until ~32.8 Ma coincident with the first time atmospheric CO2 levels fell below ~600 ppm. Our results provide new insights into the potential of the AIS for threshold behavior, and its sensitivity to atmospheric CO2 concentrations above present day levels."

See also:
https://www.washingtonpost.com/news/energy-environment/wp/2016/03/11/the-more-we-learn-about-antarcticas-past-the-scarier-the-present-looks/

Extract: "In this context, the new research further tunes our understanding of how much carbon dioxide allows the ice sheet to grow or causes it to melt. “The ice sheet was particularly vulnerable between 33.6 and 32.8 [million years ago], with the CO2 level between 750 and 600,” Galeotti said.
That might still sound pretty far off from where we are now, but here’s the catch. When the Antarctic ice sheet initially formed, scientists don’t believe that it had one of its most distinctive current features — large portions, particularly in West Antarctica but also in key regions of East Antarctica, where ice is grounded deep below sea level.



Still, the research suggests that the Antarctic ice sheet can do things that our computer simulations, alone, may not capture, said Thomas Wagner, program scientist for the cryosphere at NASA, who is familiar with the new study."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #180 on: March 13, 2016, 05:59:08 PM »
The linked article discusses efforts to obtain Antarctic ice cores that extend back beyond a million years ago (current ice cores only go back about 0.8 million years ago).  As climate cycles appear to have experienced a fundamental change around 1 million years ago, obtaining such new Antarctic ice cores would significantly assist in the effort to calibrate climate models (particularly w.r.t. the role of CO₂):

http://www.climatecentral.org/news/climate-scientists-search-for-million-year-old-ice-20130

Extract: "We know what temperatures were like on Earth going back millions of years. That’s because the air temperatures leave a unique signature in the sediment at the bottom of the ocean, as well as in other “proxy” systems like ice and tree rings.
Examining that sediment, scientists have found that a strange thing happened about one million years ago. The cycle of ice ages sped up. Rather than happening every 100,000 years, they suddenly started happening every 40,000 years.
“The fact that we can’t fully explain why that change occurred tells us that we still don’t know all that we’d like to know about the climate system,” says Tas van Ommen from the Antarctic Climate and Ecosystems Cooperative Research Centre at the University of Tasmania.
“And we think carbon dioxide could well be the key to what drove the change.”"
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #181 on: March 18, 2016, 02:09:37 PM »
The linked paleo reference indicates that while the WAIS could collapse sooner, the entire AIS would likely become unstable at atmospheric C02 levels greater than 600ppm (although it would take the AIS thousands of years to disappear at that forcing level).  As our CO₂-e level is currently above 490ppm we should not feel complacent that this is a problem for our great grandchildren to deal with.


Simone Galeotti, Robert DeConto, Timothy Naish, Paolo Stocchi, Fabio Florindo, Mark Pagani, Peter Barret, Steven M. Bohaty, Luca Lanci, David Pollard, Sonia Sandroni, Franco M. Talarico &James C. Zachos (10 Mar 2016), "Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition", Science, DOI: 10.1126/science.aab0669

http://science.sciencemag.org/content/early/2016/03/09/science.aab0669

Abstract: "About 34 million years ago (Ma) Earth’s climate cooled and an ice sheet formed on Antarctica as atmospheric CO2 fell below ~750 ppm. Sedimentary cycles from a drill core in western Ross Sea provide the first direct evidence of orbitally-controlled glacial cycles between 34–31 Ma. Initially, under atmospheric CO2 levels ≥ 600 ppm, a smaller Antarctic Ice Sheet, (AIS) restricted to the terrestrial continent, was highly responsive to local insolation forcing. A more stable, continental-scale ice sheet calving at the coastline, did not form until ~32.8 Ma coincident with the first time atmospheric CO2 levels fell below ~600 ppm. Our results provide new insights into the potential of the AIS for threshold behavior, and its sensitivity to atmospheric CO2 concentrations above present day levels."


See also:
Wendel, J. (2016), Scientists find the point of no return for Antarctic ice cap, Eos, 97, doi:10.1029/2016EO047929. Published on 10 March 2016.

https://eos.org/articles/scientists-find-the-point-of-no-return-for-antarctic-ice-cap

Extract: "“When CO2 dropped below 600 ppm is when we see first evidence of the ice sheet expanding and growing into the ocean and onto the continental shelf,” said coauthor Timothy Naish, a glaciologist and director of the Antarctic Research Centre at the Victoria University of Wellington in New Zealand.
“This paper strengthens our understanding that CO2 affects climate including ice and sea level, that smaller ice masses respond more rapidly, and that large ice-sheet changes affect other aspects of the climate and in turn influence themselves,” said Richard Alley, a glaciologist at the Pennsylvania State University in University Park, who wasn’t involved in the new research.
The current concentration of atmospheric CO2 sits well below this threshold; it recently surpassed 400 ppm. Although recent research has found signs that the West Antarctic ice sheet is already in irreversible decline, today’s research reveals that the entire ice sheet could revert back to its unstable roots if the 600-ppm threshold is crossed once again—like replaying the ice sheet’s evolution backward through time.
Models run by the Intergovernmental Panel on Climate Change show that this threshold could be reached by the end of the century. However, Naish emphasized that even if we were to reach this crucial threshold, it would be thousands of years before the ice sheet melted completely. Although many other greenhouse gases contribute to warming, he added, they would contribute little to this potential deterioration of the Antarctic ice cap because they remain so briefly in the atmosphere compared with CO2, which can last for centuries.
At the 600-ppm threshold, Naish continued, we start to commit to continent-wide ice loss that “we can’t stop.”"
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #182 on: April 14, 2016, 07:47:26 PM »
I have provided selected abstracts from the linked EGU April 2016 Conference Session on Antarctica Paleoclimate & SLR & Ice Dynamics in Past Warm Episodes: Marrying Models & Data.  Calibration of state-of-the-art models allows us to apply this models to future evaluations with greater confidence, and in general terms the cited abstracts indicate that paleo-evidence indicates that the Antarctic ice sheet was more sensitivity to ice mass loss then previously realized:

http://meetingorganizer.copernicus.org/EGU2016/posters/20048

CL1.12/CR1.17/OS1.11 Media
Antarctic palaeoclimates, sea level change and ice dynamics in past warm episodes: marrying models and data (co-organized)
Convener: Peter Bijl 
Co-Conveners: Carlota Escutia , Aisling Dolan 
Evidence from field observations of sedimentological records alongside geochemical, microfossil and seismic data analysis suggests that the entire Cenozoic Antarctic ice sheet witnessed several episodes of dramatic waxing and waning in concert with evidence for climates moderately warmer than today. In contrast, numerical modelling studies have not always been able to predict such dynamic behaviour given reasonable climate forcings. In general, the causes and consequences of major ice sheet volume and sea level changes in the past are often poorly understood.

This session aims to bring together research fields of numerical ice sheet, climate and oceanographic modelling and field/proxy data, as a way to foster model-data comparison. We invite submissions that aim to present new insights from improved numerical modelling experiments of ice sheet, oceanographic and sea ice dynamics as well as those presenting new field data from sedimentary records around the Antarctic Margin (e.g., those from Integrated Ocean drilling program Leg 318 to the Wilkes Land Margin, ANDRILL and their predecessors) or proxy data pertaining to conditions in the Southern Ocean. We welcome research from all areas related to ice sheet dynamics, e.g. bedrock responses to ice sheet changes, the gravitational isostatic responses to glaciation, potential thresholds in climate (induced by orbit or carbon dioxide changes). Submissions considering both proxy-evidence and modelling studies are encouraged.


Geophysical Research Abstracts
Vol. 18, EGU2016-3577, 2016
EGU General Assembly 2016
Large-Ensemble modeling of past and future variations of the Antarctic
Ice Sheet with a coupled ice-Earth-sea level model
David Pollard (1), Robert DeConto (2), and Natalya Gomez (3)

To date, most modeling of the Antarctic Ice Sheet’s response to future warming has been calibrated using recent and modern observations. As an alternate approach, we apply a hybrid 3-D ice sheet-shelf model to the last deglacial retreat of Antarctica, making use of geologic data of the last ~20,000 years to test the model against the large-scale variations during this period.  The ice model is coupled to a global Earth-sea level model to improve modeling of the bedrock response and to capture ocean-ice gravitational interactions.  Following several recent ice-sheet studies, we use Large Ensemble (LE) statistical methods, performing sets of 625 runs from 30,000 years to present with systematically varying model parameters. Objective scores for each run are calculated using modern data and past reconstructed grounding lines, relative sea level records, cosmogenic elevation-age data and uplift rates. The LE results are analyzed to calibrate 4 particularly uncertain model parameters that concern marginal ice processes and interaction with the ocean. LE’s are extended into the future with climates following RCP scenarios. An additional scoring criterion tests the model’s ability to reproduce estimated sea-level high stands in the warm mid-Pliocene, for which drastic retreat mechanisms of hydrofracturing and ice-cliff failure are needed in the model. The LE analysis provides future sea-level-rise envelopes with well-defined parametric uncertainty bounds. Sensitivities of future LE results to Pliocene sea-level estimates, coupling to the Earth-sea level model, and vertical profiles of Earth properties, will be presented.

Geophysical Research Abstracts
Vol. 18, EGU2016-16572, 2016
EGU General Assembly 2016
Marine ice sheet collapse and terrestrial climate stability in Pliocene East
Antarctica
Daniel Hill and Yvonne Smith
New marine evidence is emerging of ice sheet collapses in vulnerable marine basins of the East Antarctica during warm periods of the Pliocene. This contrasts the long-standing terrestrial evidence from the Dry Valleys and wider Transantarctic Mountains, showing landscape and climatic stability since the middle Miocene. This terrestrial evidence has been used to infer that the East Antarctic Ice Sheet has been large and in a similar state to today for at least 10 million years.  Here we present a series of sensitivity experiments using the HadCM3 General Circulation Model, simulating the impact of ice sheet retreats on Pliocene climate. Major collapses in the marine basins cause changes in the atmospheric circulation around East Antarctica and propagate warmer and wetter air masses into the interior of the ice sheet. However, remaining areas of upland ice sheet act to protect areas of the interior from increases in temperature and precipitation. Only when ice retreats from the upland areas between the subglacial basins and the Transantarctic Mountains of Northern Victoria Land are the Dry Valleys exposed to mean summer temperatures significantly above freezing and the full increases in modelled precipitation. This suggests that collapses of the marine portions of the Wilkes Subglacial Basin and Aurora Subglacial Basin would not have significantly altered the palaeoenvironmental record of the Dry Valleys. These results provide a reconciliation of the records of East Antarctic ice sheet retreat and climate stability and further corroborate the findings from marine cores.  We also present the results of an iceberg modelling study that shows that observed losses of Wilkes Land IRD in the Prydz Bay region cannot be explained by climate induced changes in melting or iceberg trajectory, but probably requires the loss of the marine margin of the Wilkes Land sector of the East Antarctic Ice Sheet.


Geophysical Research Abstracts
Vol. 18, EGU2016-9773, 2016
EGU General Assembly 2016
Modeling the oxygen isotopic composition of the Antarctic ice sheet and
significance to Pliocene sea level
Edward Gasson (1), Robert DeConto (1), and David Pollard (2)
In addition to measuring the elevations of paleoshorelines, attempts to constrain past sea level and changing ice volume have made use of oxygen isotope records from benthic foraminifera. These reconstructions either rely on partitioning the temperature and ice volume components of these records or make use of independent temperature records. A number of recent studies have also suggested that the interpretation of these records may be complicated by changes in the isotopic composition of the ice sheets due to changing climate and the isotopic composition of precipitation. This may have led to an overestimate of sea level change when using constant oxygen isotope to sea level calibrations. Here we simulate the isotopic composition of the Antarctic ice sheet for a range of proposed mid-Pliocene configurations. Our simulations account for ice flow and the incorporation of the surface isotopic signal within the ice sheet. We also discuss the magnitude of the mid-Pliocene isotope excursion and significance to Pliocene sea level estimates.
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #183 on: April 19, 2016, 09:44:29 PM »
The linked reference discusses GHG measurements from Antarctic ice cores indicating that during the last deglaciation, abrupt & natural emissions of both CO₂ and CH4 occurred:

Thomas K. Bauska, Daniel Baggenstos, Edward J. Brook, Alan C. Mix, Shaun A. Marcott, Vasilii V. Petrenko, Hinrich Schaefer, Jeffrey P. Severinghaus, and James E. Lee (2016), "Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation", PNAS, vol. 113 no. 13, pp 3465–3470, doi: 10.1073/pnas.1513868113


http://www.pnas.org/content/113/13/3465.abstract

Significance
Antarctic ice cores provide a precise, well-dated history of increasing atmospheric CO2 during the last glacial to interglacial transition. However, the mechanisms that drive the increase remain unclear. Here we reconstruct a key indicator of the sources of atmospheric CO2 by measuring the stable isotopic composition of CO2 in samples spanning the period from 22,000 to 11,000 years ago from Taylor Glacier, Antarctica. Improvements in precision and resolution allow us to fingerprint CO2 sources on the centennial scale. The data reveal two intervals of rapid CO2 rise that are plausibly driven by sources from land carbon (at 16.3 and 12.9 ka) and two others that appear fundamentally different and likely reflect a combination of sources (at 14.6 and 11.5 ka).

Abstract
An understanding of the mechanisms that control CO2 change during glacial–interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO2 (δ13C-CO2) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO2 from 17.6 to 15.5 ka, these data demarcate a decrease in δ13C-CO2, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO2 rise of 40 ppm is associated with small changes in δ13C-CO2, consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in δ13C-CO2 that suggest rapid oxidation of organic land carbon or enhanced air–sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO2 coincident with increases in atmospheric CH4 and Northern Hemisphere temperature at the onset of the Bølling (14.6–14.3 ka) and Holocene (11.6–11.4 ka) intervals are associated with small changes in δ13C-CO2, suggesting a combination of sources that included rising surface ocean temperature.
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AbruptSLR

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The linked (open access) reference provides new paleo-information on the response of the Ross Sea portion of the WAIS to past climate change.  This information can be used to help calibrate from ice-sheet models:

Halberstadt, A. R. W., Simkins, L. M., Greenwood, S. L., and Anderson, J. B.: Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica, The Cryosphere, 10, 1003-1020, doi:10.5194/tc-10-1003-2016, 2016.

http://www.the-cryosphere.net/10/1003/2016/

Abstract. Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage.

During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role.

Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.
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AbruptSLR

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The linked reference provides paleo-evidence that climate models which projected increased snowfall in West Antarctica with increasing global warming may be wrong and that future models should make a greater effort to identify weather driven atmospheric variations (such as atmospheric river events) that can alter projections:

T. J. Fudge et al, Variable relationship between accumulation and temperature in West Antarctica for the past 31,000 years, Geophysical Research Letters (2016). DOI: 10.1002/2016GL068356


http://onlinelibrary.wiley.com/doi/10.1002/2016GL068356/abstract

Abstract: "The Antarctic contribution to sea level is a balance between ice loss along the margin and accumulation in the interior. Accumulation records for the past few decades are noisy and show inconsistent relationships with temperature. We investigate the relationship between accumulation and temperature for the past 31 ka using high-resolution records from the West Antarctic Ice Sheet (WAIS) Divide ice core in West Antarctica. Although the glacial-interglacial increases result in high correlation and moderate sensitivity for the full record, the relationship shows considerable variability through time with high correlation and high sensitivity for the 0–8 ka period but no correlation for the 8–15 ka period. This contrasts with a general circulation model simulation which shows homogeneous sensitivities between temperature and accumulation across the entire time period. These results suggest that variations in atmospheric circulation are an important driver of Antarctic accumulation but they are not adequately captured in model simulations. Model-based projections of future Antarctic accumulation, and its impact on sea level, should be treated with caution."

See also:
http://phys.org/news/2016-05-antarctica-offset-seas-dont.html

Extract: "Many factors related to warming will conspire to raise the planet's oceans over coming decades—thermal expansion of the world's oceans, melting of snow and ice worldwide, and the collapse of massive ice sheets.
But there are a few potential brakes. One was supposed to be heavier snowfall over the vast continent of Antarctica. Warmer air will hold more moisture and thus generate more snow to fall inland and slightly rebuild the glacier, according to climate model projections.
Not so fast, says a University of Washington study published in Geophysical Research Letters, a journal of the American Geophysical Union. The authors looked at evidence from the West Antarctic Ice Sheet Divide ice core to get a first clear look at how the continent's snowfall has varied over 31,000 years."

Caption for image: "In the more recent part of the record, at the top, the Antarctic air temperature (orange) and annual snow accumulation (purple) follow similar paths. But in the earlier part of the record, at the bottom, shifts in temperature and snowfall are often unrelated. Credit: T.J. Fudge/University of Washington"
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Adam Ash

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Tricky that the temperature scales on the two charts are different, but I get the point.

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I have provided selected abstracts from the linked EGU April 2016 Conference Session on Antarctica Paleoclimate & SLR & Ice Dynamics in Past Warm Episodes: Marrying Models & Data.  Calibration of state-of-the-art models allows us to apply this models to future evaluations with greater confidence, and in general terms the cited abstracts indicate that paleo-evidence indicates that the Antarctic ice sheet was more sensitivity to ice mass loss then previously realized:


Geophysical Research Abstracts
Vol. 18, EGU2016-3577, 2016
EGU General Assembly 2016
Large-Ensemble modeling of past and future variations of the Antarctic
Ice Sheet with a coupled ice-Earth-sea level model
David Pollard (1), Robert DeConto (2), and Natalya Gomez (3)



At an EGU press conference DeConto said this work implied tipping points for major sea level rise occur between 2 and 2.7C above pre-industrial.

http://client.cntv.at/egu2016/press-conference-8 (DeConto starts about 22:10)

AbruptSLR

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The following series of linked references & an associated video address both field and modeling efforts to study the mid-Miocene dynamics of the Antarctic ice sheet (focused on the Ross Ice Shelf).  Considering that if we use a GWP100 for methane of 35 that we are already at a CO₂-e of 517ppm, these findings should be cause for alarm:
The first linked reference is:
Edward Gasson, Robert M. DeConto, David Pollard and Richard H. Levy (2016), "Dynamic Antarctic ice sheet during the early to mid-Miocene", PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1516130113

http://www.pnas.org/content/113/13/3459

Significance: "Atmospheric concentrations of carbon dioxide are projected to exceed 500 ppm in the coming decades. It is likely that the last time such levels of atmospheric CO2 were reached was during the Miocene, for which there is geologic data for large-scale advance and retreat of the Antarctic ice sheet. Simulating Antarctic ice sheet retreat is something that ice sheet models have struggled to achieve because of a strong hysteresis effect. Here, a number of developments in our modeling approach mean that we are able to simulate large-scale variability of the Antarctic ice sheet for the first time. Our results are also consistent with a recently recovered sedimentological record from the Ross Sea presented in a companion article."

Abstract: "Geological data indicate that there were major variations in Antarctic ice sheet volume and extent during the early to mid-Miocene. Simulating such large-scale changes is problematic because of a strong hysteresis effect, which results in stability once the ice sheets have reached continental size. A relatively narrow range of atmospheric CO2 concentrations indicated by proxy records exacerbates this problem. Here, we are able to simulate large-scale variability of the early to mid-Miocene Antarctic ice sheet because of three developments in our modeling approach. (i) We use a climate–ice sheet coupling method utilizing a high-resolution atmospheric component to account for ice sheet–climate feedbacks. (ii) The ice sheet model includes recently proposed mechanisms for retreat into deep subglacial basins caused by ice-cliff failure and ice-shelf hydrofracture. (iii) We account for changes in the oxygen isotopic composition of the ice sheet by using isotope-enabled climate and ice sheet models. We compare our modeling results with ice-proximal records emerging from a sedimentological drill core from the Ross Sea (Andrill-2A) that is presented in a companion article. The variability in Antarctic ice volume that we simulate is equivalent to a seawater oxygen isotope signal of 0.52–0.66‰, or a sea level equivalent change of 30–36 m, for a range of atmospheric CO2 between 280 and 500 ppm and a changing astronomical configuration. This result represents a substantial advance in resolving the long-standing model data conflict of Miocene Antarctic ice sheet and sea level variability."

The second linked reference is:
Richard Levy, David Harwood, Fabio Florindo, Francesca Sangiorgi, Robert Tripati, Hilmar von Eynatten, Edward Gasson, Gerhard Kuhn, Aradhna Tripati, Robert DeConto, Christopher Fielding, Brad Field, Nicholas Golledge, Robert McKay, Timothy Naish, Matthew Olney, David Pollard, Stefan Schouten, Franco Talarico, Sophie Warny, Veronica Willmott, Gary Acton, Kurt Panter, Timothy Paulsen, Marco Taviani & SMS Science Team (2016), "Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene", PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1516030113

http://www.pnas.org/content/113/13/3453

Significance: "New information from the ANDRILL-2A drill core and a complementary ice sheet modeling study show that polar climate and Antarctic ice sheet (AIS) margins were highly dynamic during the early to mid-Miocene. Changes in extent of the AIS inferred by these studies suggest that high southern latitudes were sensitive to relatively small changes in atmospheric CO2 (between 280 and 500 ppm). Importantly, reconstructions through intervals of peak warmth indicate that the AIS retreated beyond its terrestrial margin under atmospheric CO2 conditions that were similar to those projected for the coming centuries."

Abstract: "Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23–14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene."

http://phys.org/news/2016-02-antarctic-ice-sheet-vulnerable-co2.html


See also, the third link which leads to a 2015 video focused on the findings of the Andrill project to drill through the Ross Ice Shelf into the seafloor.  I suspect that these findings likely gave DeConto & Pollard incentive to produce their ground-breaking 2016 paper modeling the impact of cliff failures and hydrofracturing on marine glaciers (primarily in Antarctica):

NOVA's: Antarctica Meltdown - Secrets Beneath the Ice Antarctic Drilling Project
https://www.youtube.com/watch?v=LPEGzPv7quQ

Also see:
http://phys.org/news/2016-02-antarctic-ice-sheet-vulnerable-co2.html

&

http://sites01.lsu.edu/faculty/swarny/wp-content/uploads/sites/30/2016/03/Levy-et-al.-2016-Antarctic-ice-sheet-sensitivity-to-atmospheric-CO2-variations-in-the-early-to-mid-Miocene.pdf
« Last Edit: June 05, 2016, 10:10:21 AM by AbruptSLR »
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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The linked reference discusses both model and paleo findings to evaluate "Southern Ocean deep convection as a driver of Antarctic warming events".  As most of the pre-conditions for the first stage (see attached image and the second linked article) of such an Antarctic warming event are currently happening today, it is possible that we may see more rapid warming of Antarctica with continued global warming than was previously (AR5) expected:

J. B. Pedro, T. Martin, E. J. Steig, M. Jochum, W. Park & S. O. Rasmussen (12 March 2016), "Southern Ocean deep convection as a driver of Antarctic warming events", Geophysical Research Letters, DOI: 10.1002/2016GL067861


http://onlinelibrary.wiley.com/doi/10.1002/2016GL067861/abstract;jsessionid=FA93F2EAE8AAFF6CE6C82E0AADC2FF5C.f02t03

Abstract: "Simulations with a free-running coupled climate model show that heat release associated with Southern Ocean deep convection variability can drive centennial-scale Antarctic temperature variations of up to 2.0°C. The mechanism involves three steps: Preconditioning: heat accumulates at depth in the Southern Ocean; Convection onset: wind and/or sea ice changes tip the buoyantly unstable system into the convective state; and Antarctic warming: fast sea ice-albedo feedbacks (on annual-decadal time scales) and slow Southern Ocean frontal and sea surface temperature adjustments to convective heat release (on multidecadal-century time scales) drive an increase in atmospheric heat and moisture transport toward Antarctica. We discuss the potential of this mechanism to help drive and amplify climate variability as observed in Antarctic ice core records."

See also:
https://ice2ice.b.uib.no/2016/03/29/pin-the-tail-on-the-donkey-southern-ocean-deep-convection-and-the-global-dansgaard-oeschger-variation/

Extract: "In our paper we show that heat loss from the convective zone in the Weddell Sea ultimately causes warming of up to 2°C on the Antarctic continent. Four factors are important (in almost equal measure) in driving the Antarctic warming: ocean to atmosphere heat flux from the convective zone; sea-ice loss and albedo feedback; southward migration of the ACC; and increased heat and moisture transport to Antarctica. The southward migration of the ACC, which takes around 50 years, was a surprise. We put this migration down to heat (i.e. buoyancy) loss from the convective zone dragging the outcropping isopycnals southward. A southward-shifted ACC can also be viewed as a southward shift in the sub-tropical front, which in turn means warmer sea surface temperatures in the mid-latitudes. In response to the mid-latitude warming the atmosphere fluxes more heat toward Antarctica."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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In the linked (open access) reference the authors use advanced statistical techniques to try to improve large ensemble modeling of the last deglacial retreat of the WAIS.  Such calibration efforts will soon help to quantify future sea-level rise projections (see the second reference in preparation):

David Pollard, Won Chang, Murali Haran, Patrick Applegate, and Robert DeConto (2016), "Large ensemble modeling of the last deglacial retreat of the West Antarctic Ice Sheet: comparison of simple and advanced statistical techniques", Geosci. Model Dev., 9, 1697–1723, doi:10.5194/gmd-9-1697-2016

http://www.geosci-model-dev.net/9/1697/2016/gmd-9-1697-2016.pdf
www.geosci-model-dev.net/9/1697/2016/


Abstract. A 3-D hybrid ice-sheet model is applied to the last deglacial retreat of the West Antarctic Ice Sheet over the last ~20 000 yr. A large ensemble of 625 model runs is used to calibrate the model to modern and geologic data, including reconstructed grounding lines, relative sea-level records, elevation–age data and uplift rates, with an aggregate score computed for each run that measures overall model–data misfit. Two types of statistical methods are used to analyze the large-ensemble results: simple averaging weighted by the aggregate score, and more advanced Bayesian techniques involving Gaussian process-based emulation and calibration, and Markov chain Monte Carlo. The analyses provide sea-level-rise envelopes with well-defined parametric uncertainty bounds, but the simple averaging method only provides robust results with full-factorial parameter sampling in the large ensemble. Results for best-fit parameter ranges and envelopes of equivalent sea-level rise with the simple averaging method agree well with the more advanced techniques.  Best-fit parameter ranges confirm earlier values expected from prior model tuning, including large basal sliding coefficients on modern ocean beds.


Also keep an eye out for the release of:

Pollard, D., DeConto, R., Chang, W., Haran, M., and Applegate, P.: Large ensemble modeling of the Antarctic Ice Sheet: Contributions to past and future sea-level rise, The Cryosphere Discuss., in preparation, 2016.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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While the linked reference is a bit old, the paleo data is still valuable for quantifying the sensitivity of ice mass loss from Antarctica with continued warming:

TIM R. NAISH AND GARY S. WILSON, (2009), "Constraints on the amplitude of Mid-Pliocene (3.6–2.4 Ma) eustatic sea-level fluctuations from the New Zealand shallow-marine sediment record", Phil. Trans. R. Soc. A,  367, 169–187, doi:10.1098/rsta.2008.0223

http://rsta.royalsocietypublishing.org/content/roypta/367/1886/169.full.pdf

Abstract: "Ice-volume calibrations of the deep-ocean foraminiferal d18O record imply orbitally influenced sea-level fluctuations of up to 30 m amplitude during the Mid-Pliocene, and up to 30 per cent loss of the present-day mass of the East Antarctic Ice Sheet (EAIS) assuming complete deglaciation of the West Antarctic Ice Sheet (WAIS) and Greenland.  These sea-level oscillations have driven recurrent transgressions and regressions across the world’s continental shelves. Wanganui Basin, New Zealand, contains the most complete shallow-marine Late Neogene stratigraphic record in the form of a continuous cyclostratigraphy representing every 41 and 100 ka sea-level cycle since ca 3.6 Ma. This paper presents a synthesis of faunally derived palaeobathymetric data for shallow-marine sedimentary cycles corresponding to marine isotope stages M2–100 (ca 3.4–2.4 Ma). Our approach estimates the eustatic sea-level contribution to the palaeobathymetry curve by placing constraints on total subsidence and decompacted sediment accumulation. The sea-level estimates are consistent with those from d18O curves and numerical ice sheet models, and imply a significant sensitivity of the WAIS and the coastal margins of the EAIS to orbital oscillations in insolation during the Mid-Pliocene period of relative global warmth. Sea-level oscillations of 10–30 m were paced by obliquity."

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #192 on: August 18, 2016, 05:34:11 PM »
I have provided copious extracts (& the four attached images) from the linked reference that studies four deep East Antarctic ice cores, with time resolutions as fine as 200-years, specifically to study the sensitivity of the WAIS to collapse during the Eemian peak (circa 128 ka ago).  While the study provides valuable (& useful) scientific information, the authors seem to lean over backwards to demonstrate that under peak Eemian conditions (with 300ppm CO₂) that the entire WAIS did not entirely (but very likely partially) collapsed within the 200-year period resolution offered by their ice cores.  They offer an alternate hypothesis (to rapid WAIS collapse), which matches their observations, and that likely requires invocations of the bipolar seesaw mechanism (driven by ice mass loss from the GIS) to warm the Southern Ocean, resulting in relatively rapid sea ice coverage loss around Antarctica.  However, I emphasize that none of their findings run counter to the findings of either Hansen et. al. (2016) or DeConto & Pollard (2016); nor do they preclude DeConto's EGU finding that the WAIS could largely collapse abruptly if/when the modern GMST anom reaches/exceeds 2.7C:

Max D. Holloway, Louise C. Sime, Joy S. Singarayer, Julia C. Tindall, Pete Bunch & Paul J. Valdes (August 16, 2016), "Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse", Nature Communications, Volume: 7, Article number: 12293, doi:10.1038/ncomms12293

http://www.nature.com/ncomms/2016/160816/ncomms12293/full/ncomms12293.html?WT.ec_id=NCOMMS-20160817&spMailingID=52079988&spUserID=ODkwMTM2NjQyNgS2&spJobID=983091385&spReportId=OTgzMDkxMzg1S0

Abstract: "Several studies have suggested that sea-level rise during the last interglacial implies retreat of the West Antarctic Ice Sheet (WAIS). The prevalent hypothesis is that the retreat coincided with the peak Antarctic temperature and stable water isotope values from 128,000 years ago (128 ka); very early in the last interglacial. Here, by analysing climate model simulations of last interglacial WAIS loss featuring water isotopes, we show instead that the isotopic response to WAIS loss is in opposition to the isotopic evidence at 128 ka. Instead, a reduction in winter sea ice area of 65±7% fully explains the 128 ka ice core evidence. Our finding of a marked retreat of the sea ice at 128 ka demonstrates the sensitivity of Antarctic sea ice extent to climate warming."

Extract: "During the last interglacial (LIG; 130,000–115,000 years ago) global climate was warmer than today and global mean sea level was 6-9 m higher. This LIG sea-level high stand was mainly driven by ice-sheet loss. Recent ice core results indicate that the Greenland ice sheet likely provided a modest 2 m contribution towards the global sea-level rise5, with estimates ranging from +1.4 m to +4.3 m. This implies that ice loss from the West Antarctic Ice Sheet (WAIS) must have contributed to the LIG sea-level maxima: loss of the entire WAIS would contribute 3–4 m of global sea-level rise. Coral records from Western Australia indicate that the sea level rose late in the interglacial, ~118,000 years ago (118 kyr ago). However, Seychelles coral has been interpreted as indication of a +5 m global sea level at 128 ka. These differing interpretations prevent constraint on the timing of WAIS loss, thus reducing the potential to use the LIG to inform the debate on the likelihood of future WAIS loss. We therefore turn to the ice core records to push forward the WAIS loss debate.

The recent ice core drilled at WAIS Divide does not extend back through the LIG; ice that may have been present during the LIG has since been lost through basal melt. However, ice cores extending back throughout the LIG, at a resolution of <200 years per metre of ice, are available from four locations on the East Antarctic Ice Sheet …

We have explored only complete WAIS loss, rather than WAIS reduction, scenarios here. Our results thus do not preclude some loss of the WAIS by 128 ka, or that the WAIS may have been lost later in the LIG, possibly preconditioned by the early retreat of Southern Hemisphere sea ice. Indeed, loss of the WAIS between 128 and 125 kyr ago and a meltwater driven build-up of Southern Hemisphere sea ice may provide an explanation for the late LIG δ18O drop observed in ice core records; the δ18O trend throughout the early LIG, with a significant peak and subsequent drop, is distinct from the isotope record of the present interglacial (Fig. 1). Our results indicate that the LIG isotope trend may be consistent with a WAIS collapse and sea ice build-up in the following few thousand years of the isotope maximum.

The difference between an isotope record from Mt. Moulton and East Antarctic ice core records may also be consistent with a slow loss of the WAIS, which could have been mostly melted after another 2,000 years, by ~126 kyr ago. Lower isotope anomalies in the Mt. Moulton record relative to isotope records from East Antarctica suggest a local cooling anomaly, which is consistent with climate model simulations of WAIS collapse driven by pre-industrial boundary conditions27. The low isotope values in the Mt. Moulton record, relative to the other ice core sites, persists throughout the LIG, but the difference is greatest after ~126 kyr ago, perhaps coinciding with maximum retreat of the WAIS. Considering the reasonable agreement between the observed peak-to-trough δ18O anomalies and those calculated between our sea ice retreat and the WAIS loss experiments (Supplementary Fig. 5), we suggest that a large sea ice retreat best explains the early isotope maximum and a subsequent retreat of the WAIS, and sea ice build-up could provide an explanation for the observed pattern of isotope anomalies following the LIG maximum.

The bipolar seesaw mechanism proposes that a slowdown in northwards ocean heat transport, particularly in the Atlantic, tends to warm the Southern Ocean. This mechanism is consistent with a recent bipolar re-interpretation of the early LIG, alongside a recent synthesis of sea surface temperature reconstructions between 40 and 60° S (ref. 3). These all support Southern Ocean warming at 128 ka, providing a partial explanation for why Southern Hemisphere sea ice retreated at 128 ka. In future work, we will investigate whether the bipolar seesaw can provide the mechanism to cause a major Southern Hemisphere sea ice retreat and thus reconcile the 128 ka δ18O maximum. Further simulations, including WAIS loss and North Atlantic meltwater input, could provide insight into the non-linear interactions between the bipolar seesaw, the WAIS and Southern Hemisphere sea ice.

Finally, we note the similarity between the wintertime sea ice reduction of up to 58% forecast for the end of the 21st century and our 58–72% decrease suggested for 128 ka. This implies that the 128 ka sea ice retreat may prove a crucial model–data target for the sea ice modelling community. Currently, the most recent Coupled Model Intercomparison Project Phase 5 multi-model simulations do not simulate a reduction in September sea ice area >13% between the LIG and the present interglacial (Supplementary Discussion; Supplementary Table 2). Considering the disagreement between modelled and observed Antarctic sea ice during the satellite era, a number of studies have called for improvements in the modelling of climate and climate change in the Antarctic region. Whether this recent discrepancy is a function of natural variability or represents a failing of current climate models is still a matter of debate. If the currently observed increase in Antarctic sea ice is robust, a major reduction at 128 ka could indicate a tipping point in the sea ice system. There is clearly a need for more (and more robust) data for Antarctica and the surrounding sea ice edge during the LIG. If it is possible to correctly simulate the 128 ka sea ice reduction, it would improve the low confidence associated with future predictions of Southern Hemisphere sea ice change and, subsequently, improve projections of Antarctic temperature, precipitation and mass balance."

See also:
http://www.independent.co.uk/news/science/west-antarctic-ice-sheet-sea-level-rise-global-warming-climate-change-british-antarctic-survey-a7195481.html

Extract: "She said their results suggested the ice sheet had not melted within about 50 to 200 years – a ‘sudden’ change in these terms. “It probably took longer than that,” she said.

The broader lesson from 128,000 years ago is somewhat concerning. The temperature was higher than today even though the level of carbon dioxide in the atmosphere reached just 300 parts per million (ppm), compared to 400ppm today.

“The climate [today] hasn’t had time to respond fully to the CO2 we’ve put into the atmosphere,” Dr Sime said.

“We are now heading for a time that will be almost certainly warmer than the last interglacial. It’s just we haven’t got there yet.
“It’s really interesting to look at the past because the climate had long enough to adjust to higher levels of CO2.”

Dr Sime said she and others in the US were now planning to do more research to try to work out how long it took the Western Antarctic ice sheet to melt.
She stressed that the past was only an indication of what might happen in the future, not what would certainly happen."

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #193 on: October 08, 2016, 05:50:02 PM »
The linked reference provides paleo evidence that subglacial gas hydrates within zones of the basal sediment served to reduce ice flow velocities in the marine glaciers of the Barents-Sea-Fennoscandian ice sheet approximately 20,000 years ago.  As there are projected to be large amounts of gas hydrates in the marine sediments beneath the WAIS, these findings may help to partially explain why the ice flow velocities of key WAIS marine glaciers like the PIG and Thwaites have plateaued recently (rather than continuing to accelerate rapidly).  However, if this is the case and if cliff failures and hydrofracturing occurs in the WAIS before GMST anoms reach 2.7C (as forecast by DeConto 2016), then such postulated basal hydrates could release significant volumes of methane if/when the WAIS collapses:

Monica Winsborrow, Karin Andreassen, Alun Hubbard, Andreia Plaza-Faverola, Eythor Gudlaugsson, Henry Patton. Regulation of ice stream flow through subglacial formation of gas hydrates. Nature Geoscience, 2016; DOI: 10.1038/ngeo2696

http://www.nature.com/ngeo/journal/v9/n5/full/ngeo2696.html


Abstract: "Variations in the flow of ice streams and outlet glaciers are a primary control on ice sheet stability, yet comprehensive understanding of the key processes operating at the ice-bed interface remains elusive. Basal resistance is critical, especially sticky spots-localized zones of high basal traction-for maintaining force balance in an otherwise well-lubricated/high-slip subglacial environment. Here we consider the influence of subglacial gas-hydrate formation on ice stream dynamics, and its potential to initiate and maintain sticky spots. Geophysical data document the geologic footprint of a major palaeo-ice-stream that drained the Barents Sea-Fennoscandian ice sheet approximately 20,000 years ago. Our results reveal a ∼250 km2 sticky spot that coincided with subsurface shallow gas accumulations, seafloor fluid expulsion and a fault complex associated with deep hydrocarbon reservoirs. We propose that gas migrating from these reservoirs formed hydrates under high-pressure, low-temperature subglacial conditions. The gas hydrate desiccated, stiffened and thereby strengthened the subglacial sediments, promoting high traction-a sticky spot-that regulated ice stream flow. Deep hydrocarbon reservoirs are common beneath past and contemporary glaciated areas, implying that gas-hydrate regulation of subglacial dynamics could be a widespread phenomenon."

https://www.sciencedaily.com/releases/2016/04/160413084735.htm

Extract: " One of the major questions today is: What are the ice sheets going to do in an ever-warming climate? Ice sheets of Greenland and Antarctica are major contributors to the sea level rise, which can make life difficult for many coastal nations in the near future.

To understand the ice sheets we need to understand their drainage system -- a key component of this is ice streams, fast-flowing rivers of ice, that deliver ice from the centre of the ice sheet to the oceans. Many of these ice streams are speeding up, which may be seen as the logical consequence of the warming climate. But some are slowing down, even stopping, examples of this may be found in the Ross ice streams of West Antarctica.

A new study in Nature Geoscience suggests that a 250km2 sticky spot made up of sediments with gas hydrates in them, slowed down an ice stream in the Barents Sea. This happened sometime during the last ice age, 20,000 years ago, when the Barents Sea was covered with an ice sheet.

Gas hydrate sticky spots under ice streams are a potentially widespread feature also today.
"If there are gas hydrates under today's ice sheets, they can slow the ice streams. There are studies indicating that there may be vast reservoirs of hydrates under the West Antarctic Ice sheet. Anywhere you have a hydrocarbon reservoir, water, high pressure and low temperature, you will get gas hydrate." says Winsborrow.

Ice streams of today are extensively monitored with GPS tracking systems, but it is very difficult to gaze beneath three kilometres of ice to see what is going on at the bottom. But scars left by the Barents Sea Ice sheet are visible on the ocean floor today. That makes this ancient ice sheet an important analogue, especially for the modern West Antarctica Ice Sheet, as both are based in marine environments.

"We need these analogies from the past. Understanding what is happening at the base of ice streams is important for modelling and predicting the future of the ice sheets.""
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #194 on: October 12, 2016, 08:23:52 AM »
Evidence of CO2-rise connected to Antarctic ice loss at start of the Miocene 23 million years ago:
http://www.ldeo.columbia.edu/news-events/historic-shrinking-antarctic-ice-sheet-linked-co2-spike?platform=hootsuite

Reichgelt et al 2016, Abrupt plant physiological changes in southern New Zealand at the termination of the Mi-1 event reflect shifts in hydroclimate and pCO2:
http://www.sciencedirect.com/science/article/pii/S0012821X16305064

Abstract
A rise in atmospheric CO2 is believed to be necessary for the termination of large-scale glaciations. Although the Antarctic Ice Sheet is estimated to have melted from ∼125% to ∼50% its modern size, there is thus far no evidence for an increase in atmospheric CO2 associated with the Mi-1 glacial termination in the earliest Miocene. Here, we present evidence from a high-resolution terrestrial record of leaf physiological change in southern New Zealand for an abrupt increase in atmospheric CO2 coincident with the termination of the Mi-1 glaciation and lasting approximately 20 kyr. Quantitative pCO2 estimates, made using a leaf gas exchange model, suggest that atmospheric CO2 levels may have doubled during this period, from View the MathML source to View the MathML source, and subsequently returned back to View the MathML source. The 20-kyr interval with high pCO2 estimates is also associated with a period of increased moisture supply to southern New Zealand, inferred from carbon and hydrogen isotopes of terrestrial leaf waxes. The results provide the first high-resolution record of terrestrial environmental change at the Oligocene/Miocene boundary, document a ∼20 kyr interval of elevated pCO2 and increased local moisture availability, and provide insight into ecosystem response to a major orbitally driven climatic transition.
« Last Edit: October 12, 2016, 08:30:51 AM by Lennart van der Linde »

AbruptSLR

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The linked reference examines the abrupt collapse of the Last Weichselian Icelandic ice sheet to better understand the risk for future collapses of existing marine glaciers / ice sheets.  They find that the geothermal conditions beneath such ice sheets can control the rate of ice mass loss particularly during phases of rapid retreat (& I note that the lithosphere beneath the Thwaites Glacier is exceptionally thin):

Henry Patton, Alun Hubbard, Tom Bradwell, Anders Schomacker. The configuration, sensitivity and rapid retreat of the Late Weichselian Icelandic ice sheet. Earth-Science Reviews, 2017; 166: 223 DOI: 10.1016/j.earscirev.2017.02.001

http://www.sciencedirect.com/science/article/pii/S001282521630246X

Abstract: “The fragmentary glacial-geological record across the Icelandic continental shelf has hampered reconstruction of the volume, extent and chronology of the Late Weichselian ice sheet particularly in key offshore zones. Marine geophysical data collected over the last two decades reveal that the ice sheet likely attained a continental shelf-break position in all sectors during the Last Glacial Maximum, though its precise timing and configuration remains largely unknown. Within this context, we review the available empirical evidence and use a well-constrained three-dimensional thermomechanical model to investigate the drivers of an extensive Late Weichselian Icelandic ice-sheet, its sensitivity to environmental forcing, and phases of deglaciation. Our reconstruction attains the continental shelf break across all sectors with a total ice volume of 5.96 × 105 km3 with high precipitation rates being critical to forcing extensive ice sheet flow offshore. Due to its location astride an active mantle plume, a relatively fast and dynamic ice sheet with a low aspect ratio is maintained. Our results reveal that once initial ice-sheet retreat was triggered through climate warming at 21.8 ka BP, marine deglaciation was rapid and accomplished in all sectors within c. 5 ka at a mean rate of 71 Gt of mass loss per year. This rate of ice wastage is comparable to contemporary rates observed for the West Antarctic ice sheet. The ice sheet subsequently stabilised on shallow pinning points across the near shelf for two millennia, but abrupt atmospheric warming during the Bølling Interstadial forced a second, dramatic collapse of the ice sheet onshore with a net wastage of 221 Gt a−1 over 750 years, analogous to contemporary Greenland rates of mass loss. Geothermal conditions impart a significant control on the ice sheet's transient response, particularly during phases of rapid retreat. Insights from this study suggests that large sectors of contemporary ice sheets overlying geothermally active regions, such as Siple Coast, Antarctica, and NE Greenland, have the potential to experience rapid phases of mass loss and deglaciation once initial retreat is initiated.”

See also:
 https://www.sciencedaily.com/releases/2017/04/170424093950.htm


Extract: “"We found that, at certain times, the Icelandic ice sheet retreated at an exceptionally fast rate -- more than double the present-day rate of ice loss from the much larger West Antarctic ice sheet -- causing global sea level to rise significantly."
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AbruptSLR

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History has a habit of repeating itself:

Derouin, S. (2017), Déjà vu? Ocean warmth melted ancient West Antarctic Ice Shelf, Eos, 98, https://doi.org/10.1029/2017EO066787

https://eos.org/articles/deja-vu-ocean-warmth-melted-ancient-west-antarctic-ice-shelf

Extract: "The new results indicate that the similar and seemingly unstoppable melting of huge swaths of the West Antarctic Ice Sheet (WAIS) today by relatively warm ocean waters has precedent in an earlier era, members of the research team and other scientists said. Prior to this new drilling, scientists lacked evidence of such melting from undersea warmth in WAIS’s past."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #197 on: August 01, 2017, 11:47:58 PM »
The linked reference discusses state of the art surface temperature at the West Antarctic Divide for the past ~ 40,000 years (see image bottom panel).  Findings indicate that current climate models are challenged to hind cast the observed findings and that models with low climate sensitivities can be eliminated from consideration.  Furthermore, they find that an Antarctic Amplification of 2 to 3 time GMSTA.  These findings do not bode well for the stability of the WAIS with continued global warming:

Kurt M. Cuffey, Gary D. Clow, Eric J. Steig, Christo Buizert, T. J. Fudge, Michelle Koutnik, Edwin D. Waddington, Richard B. Alley, and Jeffrey P. Severinghaus (2016), "Deglacial temperature history of West Antarctica", PNAS, vol. 113 no. 50, 14249–14254, doi: 10.1073/pnas.1609132113

http://www.pnas.org/content/113/50/14249

Abstract: "The most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how Earth’s climate responds to various forcings, including a rise in atmospheric CO2. This transition has left a direct thermal remnant in the polar ice sheets, where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings. For Antarctica, the deglacial warming has previously been constrained only by the water isotopic composition in ice cores, without an absolute thermometric assessment of the isotopes’ sensitivity to temperature. To overcome this limitation, we measured temperatures in a deep borehole and analyzed them together with ice-core data to reconstruct the surface temperature history of West Antarctica. The deglacial warming was 11.3±1.8 ∘  11.3±1.8∘ C, approximately two to three times the global average, in agreement with theoretical expectations for Antarctic amplification of planetary temperature changes. Consistent with evidence from glacier retreat in Southern Hemisphere mountain ranges, the Antarctic warming was mostly completed by 15 kyBP, several millennia earlier than in the Northern Hemisphere. These results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on Antarctic temperature. Although climate models perform well on average in this context, some recent syntheses of deglacial climate history have underestimated Antarctic warming and the models with lowest sensitivity can be discounted."

Extract: "Of greatest immediate interest, however, is our demonstration that the global deglacial temperature change was amplified by a factor of 2–3 in the Antarctic, that Antarctic warming was largely achieved by 15 ka in coherence with records from Southern Hemisphere mountain ranges, and that climate models of the deglaciation perform well on average, but that the ones with lowest sensitivity can be discounted. The early warming of the Southern Hemisphere, which our study helps to quantify, arose from combined effects of reduced northward oceanic heat transport, increased insolation, and increasing atmospheric CO2. Quantitative simulation of this phenomenon could provide an illuminating challenge for model studies."
« Last Edit: August 02, 2017, 12:05:49 AM by AbruptSLR »
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #198 on: August 10, 2017, 08:45:28 PM »
The linked 2015 reference provides details about how abruptly the bipolar seesaw can change climate:

Joel B. Pedro, Helen C. Bostock, Cecilia M. Bitz, Feng He, Marcus J. Vandergoes, Eric J. Steig, Brian M. Chase, Claire E. Krause, Sune O. Rasmussen, Bradley R. Markle, Giuseppe Cortese. The spatial extent and dynamics of the Antarctic Cold Reversal. Nature Geoscience, 2015; DOI: 10.1038/ngeo2580

http://www.nature.com/ngeo/journal/v9/n1/full/ngeo2580.html?foxtrotcallback=true
&
https://www.researchgate.net/publication/284104371_The_spatial_extent_and_dynamics_of_the_Antarctic_Cold_Reversal

Abstract: "Antarctic ice cores show that a millennial-scale cooling event, the Antarctic Cold Reversal (14,700 to 13,000 years ago), interrupted the last deglaciation. The Antarctic Cold Reversal coincides with the Bølling–Allerød warm stage in the North Atlantic, providing an example of the inter-hemispheric coupling of abrupt climate change generally referred to as the bipolar seesaw. However, the ocean–atmosphere dynamics governing this coupling are debated. Here we examine the extent and expression of the Antarctic Cold Reversal in the Southern Hemisphere using a synthesis of 84 palaeoclimate records. We find that the cooling is strongest in the South Atlantic and all regions south of 40° S. At the same time, the terrestrial tropics and subtropics show abrupt hydrologic variations that are significantly correlated with North Atlantic climate changes. Our transient global climate model simulations indicate that the observed extent of Antarctic Cold Reversal cooling can be explained by enhanced northward ocean heat transport from the South to North Atlantic, amplified by the expansion and thickening of sea ice in the Southern Ocean. The hydrologic variations at lower latitudes result from an opposing enhancement of southward heat transport in the atmosphere mediated by the Hadley circulation. Our findings reconcile previous arguments about the relative dominance of ocean and atmospheric heat transports in inter-hemispheric coupling, demonstrating that the spatial pattern of past millennial-scale climate change reflects the superposition of both."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

VeliAlbertKallio

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #199 on: August 13, 2017, 04:24:01 AM »
In Western Europe and Greenland, the Younger Dryas is an extremely well-defined synchronous cool period as illustrated in the first diagram (Image 1, enclosed) that shows 30 Celcius degree mid-winter cooling across parts of North Atlantic, Greenland, Iceland and Scandinavia due to aggressive sea ice and snow formation. The severe cooling brought about high temperature gradients causing strong precipitation in Greenland. Alley, Richard B.; et al. (1993). "Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event". Nature. 362 (6420): 527–529.

The severe cold impact is widely seen across the entire Northern Hemisphere - even China where well-expressed Younger Dryas events in the Chinese δ18 O records of Termination III are seen in stalagmites from high-altitude caves in Shennongjia area, Hubei Province, China. Chen, S., Y. Wang, X. Kong, D. Liu, H. Cheng, and R.L. Edwards. (2006) A possible Younger Dryas-type event during Asian monsoonal Termination 3. Science China Earth Sciences. 49(9):982–990.

While Younger Dryas is clear in NH GCM (Image 1), same cannot be said about Southern Hemisphere and tropics (Image 2).

The Younger Dryas cooling, in the Northern Hemisphere, began while the Antarctic Cold Reversal (ACR) was still ongoing, and the ACR ended in the midst of the Younger Dryas. Blunier, Thomas; et al., "Phase Lag of Antarctic and Greenland Temperature in the last Glacial...," in Abrantes, Fatima; Mix, Alan C., eds. (1999). Reconstructing Ocean History: A Window into the Future. New York: Kluwer Academic. ISBN 0-306-46293-1., pp. 121–138.

The GCM (Image 2) interestingly shows freezing conditions in estuaries of the Equatorial rivers (the Amazon and the Congo) while GCM delivers overall warming for SH and tropics. It is assumed from YD images 1 and 2 that the ACR had ended and thus SH SST had largely increased, but isolated pockets of cold existed because there were grounded ice bergs that were still melting - or SH GCM model is overstating the SH warmth and more of it was cooler.

Notable exceptions to overall SH warmth (Image 2) are the large cold regions (i.e. the Chilean Coast of South America and Western and Southern Shores of Australia):

It is here proposed that GCM probably overstates the SH warming, or there were significant persistent pile ups of ice bergs along the coast lines of South America (i.e. the Amazon cold anomaly), or the Western Equatorial Africa (i.e. the Congo anomaly). The failure of Ross Ice Shelf would be behind the Chilean cold anomaly, the Brazilian coastal anomaly (i.e. the Amazon cold spot), the Equatorial African anomaly (i.e. the Congo cold spot) and the failure of the Ronne Ice Shelf on the Weddell Sea and other Antarctic shelves were behind the ice pile up on the southern and western shores of Australia.

The rapid cooling within one summer season is indicative of ice sheet / melt water lake collapse to cool the Northern Atlantic Ocean and its associated climate just within few weeks (Image 3). Teleconnections of such rapid cooling of ocean require large ice volume dumping which should have rapidly increased the sea level projecting a tongue of water beneath continental ice shelves world wide and making them to calve rapidly on both hemispheres due to a bending effect. German sea level fears may be well placed and correct in seeing coastal nuclear reactors as risk.

« Last Edit: August 13, 2017, 04:36:25 AM by VeliAlbertKallio »