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Author Topic: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios  (Read 80682 times)

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #100 on: January 15, 2014, 10:59:54 PM »
I like the attached image from Michael J. O’Leary*, Paul J. Hearty,William G. Thompson, Maureen E. Raymo, Jerry X. Mitrovica and Jody M.Webster (2013) Ice sheet collapse following a prolonged period of stable sea level during the last interglacial, so I thought that I would post it here, as it provides a lower bound on how quickly sea level could rise in the future:
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Laurent

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #101 on: January 16, 2014, 09:32:17 AM »
Hello abruptslr,

Can you explain what you think ?
When did that collapse begin ? between 116Ky and 117ky ?
Why does the level drop very fast ?
Do you think there was no more Arctic during that periode ?

Laurent

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #102 on: January 16, 2014, 11:07:37 AM »
Laurent,
First, here are: (a) links to the paper; (b) the doi number for the article and (c) the abstract:

http://www.nature.com/ngeo/journal/v6/n9/full/ngeo1890.html
http://www.nature.com/ngeo/journal/v6/n9/fig_tab/ngeo1890_F1.html

doi:10.1038/ngeo1890

Abstract
"During the last interglacial period, 127–116 kyr ago, global mean sea level reached a peak of 5–9  m above present-day sea level. However, the exact timing and magnitude of ice sheet collapse that contributed to the sea-level highstand is unclear. Here we explore this timing using stratigraphic and geomorphic mapping and uranium-series geochronology of fossil coral reefs and geophysical modelling of sea-level records from Western Australia. We show that between 127 and 119 kyr ago, eustatic sea level remained relatively stable at about 3–4 m above present sea level. However, stratigraphically younger fossil corals with U-series ages of 118.1±1.4 kyr are observed at elevations of up to 9.5 m above present mean sea level. Accounting for glacial isostatic adjustment and localized tectonics, we conclude that eustatic sea level rose to about 9 m above present at the end of the last interglacial. We suggest that in the last few thousand years of the interglacial, a critical ice sheet stability threshold was crossed, resulting in the catastrophic collapse of polar ice sheets and substantial sea-level rise."

Thus, a marine ice sheet collapse sometime between 119 kyr and 118.1+/- 1.4 kyr could explain the indicated abrupt sea level rise.  One possible explanation is that the paleo-WAIS went past a physical tipping point (see my discussions in the PIG/Thwaites thread) in that timeframe and became unstable and collapsed abruptly, even if the mean global temperature was decreasing at the time. The indicated rapid drop in sea level following 117 kyr could be due to significant accumulations of snowfall in both the NH and the SH in that timeframe due to cooling global temperatures (due to the Milankovitch cycle).  Yes, the Arctic was ice/snow free at that time, and due to polar amplification (or in this case de-amplification), sea levels can drop rapidly when water/snow accumulates on land, as we saw during the 2011 La Nina event.
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #103 on: January 17, 2014, 12:29:03 AM »
The following reference and link (with a free access pdf) provides insight into the evolution of the Antarctic Ice Sheet since the last interglacial:

Maris, M. N. A., Ligtenberg, S. R. M., Crucifix, M., de Boer, B., and Oerlemans, J., (2014), "Modelling the evolution of the Antarctic Ice Sheet since the last interglacial", The Cryosphere Discuss., 8, 85-120, 2014, www.the-cryosphere-discuss.net/8/85/2014/; doi:10.5194/tcd-8-85-2014

"Abstract. We present the effects of changing two sliding parameters, a deformational velocity parameter and two bedrock deflection parameters on the evolution of the Antarctic Ice Sheet over the period from the last interglacial until the present. These sensitivity experiments have been conducted by running the ice-dynamical model ANICE forward in time. The climatological forcing over time is established by interpolating between two climate states from a regional climate model over time. The interpolation is done in such a way that both temperature and surface mass balance follow the Epica Dome C ice-core proxy record for temperature. We have determined an optimal set of parameter values, for which a realistic grounding line retreat history and present-day ice sheet can be simulated, the simulation with this set of parameter values is defined as the reference simulation. An increase of sliding with respect to this reference simulation leads to a decrease of the Antarctic ice volume due to enhanced ice velocities on mainly the West Antarctic Ice Sheet. The effect of changing the deformational velocity parameter mainly yields a change in East-Antarctic ice volume. Furthermore, we have found a minimum in the Antarctic ice volume during the mid-Holocene. This is a robust feature in our model results, where the strength and the timing of this minimum are both dependent on the investigated parameters. More sliding and a slower responding bedrock lead to a stronger minimum which emerges at an earlier time. From the model results we conclude that the Antarctic Ice Sheet has contributed 10.7 ± 1.3 m of eustatic sea level to the global ocean from the Last Glacial Maximum (about 16 kyr ago for the Antarctic Ice Sheet) until the present."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #104 on: January 17, 2014, 12:51:20 AM »
The following reference (& link to a free access pdf) indicates that polar amplification (e.g. amplification by surface feedbacks) is relatively strong and has played a dominant role in the paleo advance and retreat of ice sheets (including the AIS):

C. R. Tabor, C. J. Poulsen, and D. Pollard, (2014) "Mending Milankovitch’s theory: obliquity amplification by surface feedbacks", Clim. Past, 10, 41–50, doi:10.5194/cp-10-41-2014

http://www.clim-past.net/10/41/2014/cp-10-41-2014.pdf

"Abstract. Milankovitch’s theory states that orbitally induced changes in high-latitude summer insolation dictate the waxing and waning of ice sheets. Accordingly, precession should dominate the ice-volume response because it most strongly modulates summer insolation. However, early Pleistocene (2.588–0.781Ma) ice-volume proxy records vary almost exclusively at the frequency of the obliquity cycle. To explore this paradox, we use an Earth system model coupled with a dynamic ice sheet to separate the climate responses to idealized transient orbits of obliquity and precession that maximize insolation changes. Our results show that positive surface albedo feedbacks between high-latitude annual-mean insolation, ocean heat flux and sea-ice coverage, and boreal forest/tundra exchange enhance the ice-volume response to obliquity forcing relative to precession forcing. These surface feedbacks, in combination with modulation of the precession cycle power by eccentricity, help explain the dominantly 41 kyr cycles in global ice volume of the early Pleistocene."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #105 on: January 19, 2014, 07:41:18 PM »
I would just like to note that in the "Selected Forcing Factors" thread I cite a number of forcing factors that have only recently been identified/verified; all of which significantly increase the probability that mean global temperatures will increase significantly by 2100, possibly with temperatures somewhere between 6 degrees C and 8 degrees C.  Here I would like to note that large temperature increases would significant increase the probability of the transition to an equable climate (see replies #56 to #78 in this thread) by the end of this century.  If this were to occur society would be simultaneously impacted by abrupt climate change and abrupt sea level raise, a combination that would cause synergistic consequences that society is not prepared to address.
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #106 on: January 20, 2014, 05:28:28 PM »
In addition to the discussion on chaos theory and climate change that I posted at the beginning of this thread, the following link provides a free pdf related to the significance of quasiresonant lorenzian attractor states and climate sensitivity:

http://karman3.elte.hu/doc/Chaos222.pdf

Also note the following discussion, by Ghoti of Lod, of this matter

"This pattern seems reminiscent of the 4 lobe Lorenzian attractor that Tim Palmer discussed this year at the AGU fall meetings. I can't seem to find the reference but maybe someone else recalls it.
Posted by: Ghoti Of Lod | January 20, 2014 at 15:35

I finally found the video on demand of Palmer's 15 minute AGU talk. Unfortunately you need to register to get access (I had registered for free at the time of the meetings). The talk was about how stochastic parameterization drastically improves the models so they actually reflect the regimes seen in nature.
More relevant to the high pressure that's been blocking the western US for over a year is the Petouhkov paper on quasiresonant planetary waves.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3619331/

Posted by: Ghoti Of Lod | January 20, 2014 at 17:04 "
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #107 on: January 25, 2014, 11:03:08 AM »
The linked reference provides "…seismostratigraphic analysis of the Amundsen Sea Embayment shelf and slope of West Antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition, erosion and transport reflecting the history from pre-glacial times to early glaciation and to the late Pleistocene glacial-interglacial cycles.":

Gohl, Karsten; Uenzelmann-Neben, Gabriele; Larter, Robert D.; Hillenbrand, Claus-Dieter; Hochmuth, Katharina; Kalberg, Thomas; Weigelt, Estella; Davy, Bryan; Kuhn, Gerhard; Nitsche, Frank O.. 2013 Seismic stratigraphic record of the Amundsen Sea Embayment shelf from pre-glacial to recent times: Evidence for a dynamic West Antarctic Ice Sheet. Marine Geology, 344. 115-131. 10.1016/j.margeo.2013.06.011

http://www.sciencedirect.com/science/article/pii/S0025322713001321
http://nora.nerc.ac.uk/502396/

"Abstract/Summary
Studies of the sedimentary architecture and characteristics of the Antarctic continental margin provide clues about past ice sheet advance-retreat cycles and help improve constraints for paleo-ice dynamic models since early glacial periods. A first seismostratigraphic analysis of the Amundsen Sea Embayment shelf and slope of West Antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition, erosion and transport reflecting the history from pre-glacial times to early glaciation and to the late Pleistocene glacial-interglacial cycles. The shelf geometry consists of a large pre- and syn-rift basin in the middle shelf region between basement cropping out on the inner shelf and buried basement ridge and highs on the outer shelf. A subordinate basin within the large basin on the mid-shelf may be associated with motion along an early West Antarctic Rift System branch. At least 4 km of pre-glacial strata have been eroded from the present inner shelf and coastal hinterland by glacial processes. Six major sedimentary units (ASS-1 to ASS-6) separated by five major erosional unconformities (ASS-u1 to ASS-u5) are distinguished from bottom to top. Unconformity ASS-u4 results from a major truncational event by glacial advance to the middle and outer shelf, which was followed by several episodes of glacial advance and retreat as observed from smaller-scale truncational unconformities within the units above ASS-u4. Some of the eroded sediments were deposited as a progradional wedge that extends the outer shelf by 25 to 65 km oceanward of the pre-glacial shelf-break. We compare the observed seismic characteristics with those of other Antarctic shelf sequences and assign an Early Cretaceous age to bottom sedimentary unit ASS-1, a Late Cretaceous to Oligocene age to unit ASS-2, an Early to Mid-Miocene age to unit ASS-3, a Mid-Miocene age to unit ASS-4, a Late Miocene to Early Pliocene age to unit ASS-5, and a Pliocene to Pleistocene age to the top unit ASS-6. Buried grounding zone wedges in the upper part of unit ASS-5 on the outer shelf suggest pronounced warming phases and ice sheet retreats during the early Pliocene as observed for the Ross Sea shelf and predicted by paleo-ice sheet models. Our data also reveal that on the middle and outer shelf the flow-path of the Pine Island-Thwaites paleo-ice stream system has remained stationary in the central Pine Island Trough since the earliest glacial advances, which is different from the Ross Sea shelf where glacial troughs shifted more dynamically. This study and its stratigraphic constraints will serve as a basis for future drilling operations required for an improved understanding of processes and mechanisms leading to change in the West Antarctic Ice Sheet, such as the contemporary thinning and grounding line retreat in the Amundsen Sea drainage sector."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #108 on: January 31, 2014, 10:49:11 PM »
The linked (free access pdf) article discusses the observed Antarctic temperatures since the late 1950s.  This indicates that the West Antarctic are both currently losing ice mass; and this article implies that as the ozone hole heals itself, it is probable that ice mass loss from the ASE glaciers will accelerate:

Nicolas, J. P., and D. H. Bromwich, 2014: Antarctic temperatures since the late 1950s: SAM cooling, background warming, and West Antarctica heating up. J. Climate, submitted.

Abstract: "A reconstruction of Antarctic near-surface air temperatures spanning 1958-2012 is presented.  It primarily aims to take advantage of a recently revised key temperature record from West Antarctica (Byrd Station) to shed further light on long-term temperature changes in this region. The spatial interpolation method builds upon a kriging technique previously employed and uses temperature data from the ERA-Interim Reanalysis as the kriging field. Comparison against independent observations and ERA-Interim demonstrates the high skill of the reconstruction, but also highlights important issues in this reanalysis. For 1958-2012, the reconstruction shows statistically significant warming in the Antarctic Peninsula  (0.42+/-0.21 degree C per decade), West Antarctica (0.21+/-0.10 degree C per decade), and Antarctica as a whole (0.12+/-0.08 degree C per decade). Apart from confirming earlier evidence of West Antarctic warming, the results highlight its occurrence in all seasons except austral fall in the climatically sensitive Amundsen-Bellingshausen Sea sector. Furthermore, the reconstruction allows a re-examination of the influence of the Southern Annular Mode (SAM) on Antarctic temperatures. The strengthening of the SAM in austral summer and fall in recent decades has mitigated an otherwise stronger background warming of Antarctica. This phenomenon amounts to a 0.80+/-1.11 degrees C increase in the annual mean temperature per 50 years, which is close to the warming of the entire Southern Hemisphere. The study concludes by showing that a future weakening of the SAM or future changes in the SAM-temperature relationship in austral summer would likely expose the Amundsen Sea coast of West Antarctica to enhanced surface melting, which could lead to greater mass loss from West Antarctica.

http://polarmet.osu.edu/PMG_publications/nicolas_bromwich_jc_2014.pdf
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #109 on: February 05, 2014, 12:55:03 AM »
The linked document discusses the Past Antarctic Ice Sheet Dynamics (PAIS) Implementation Plan; which aims to improve understanding of the sensitivity of East, West, and Antarctic Peninsula Ice Sheets to a broad range of climatic and oceanic conditions:

http://www.scar.org:8000/researchgroups/progplanning/PAIS_Implementation_Plan.pdf

Much of the cited research is on-going.
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #110 on: February 20, 2014, 11:38:26 PM »
The following quote comes from the linked reference about new evidence from the BAS that the PIG thinned rapidly about 8,000 years ago due to natural causes:

"Lead author Joanne Johnson from the British Antarctic Survey (BAS) said: "Our geological data show us the history of Pine Island Glacier in greater detail than ever before. The fact that it thinned so rapidly in the past demonstrates how sensitive it is to environmental change; small changes can produce dramatic and long-lasting results. Based on what we know, we can expect the rapid ice loss to continue for a long time yet, especially if ocean-driven melting of the ice shelf in front of Pine Island Glacier continues at current rates,""


J. S. Johnson, M. J. Bentley, J. A. Smith, R. C. Finkel, D. H. Rood, K. Gohl, G. Balco, R. D. Larter, J. M. Schaefer. Rapid thinning of Pine Island Glacier in the early Holocene. Science, 20 February 2014 DOI: 10.1126/science.1247385


http://www.sciencedaily.com/releases/2014/02/140220141721.htm

http://www.antarctica.ac.uk/press/press_releases/press_release.php?id=2514

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sidd

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #111 on: February 21, 2014, 12:52:31 AM »
8K yr is too late for MWP1A, but wasn't there a cold snap then ? Mebbe related to draining of Agassiz ?

sidd

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #112 on: February 21, 2014, 01:55:25 AM »
sidd,

The attached image of temperature variations in the Holocene indicates that 8K yrs ago was a high point for Holocene temperatures, and was about equal to the global temperatures in 2004.

Best,
ASLR
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Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #113 on: April 17, 2014, 04:33:32 PM »
A new paper by Rohling et al in Nature on Sea-level and deep-sea-temperature variability over the past 5.3 million years:
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13230.html

Abstract:
Ice volume (and hence sea level) and deep-sea temperature are key measures of global climate change. Sea level has been documented using several independent methods over the past 0.5 million years (Myr). Older periods, however, lack such independent validation; all existing records are related to deep-sea oxygen isotope (δ18O) data that are influenced by processes unrelated to sea level. For deep-sea temperature, only one continuous high-resolution (Mg/Ca-based) record exists, with related sea-level estimates, spanning the past 1.5 Myr. Here we present a novel sea-level reconstruction, with associated estimates of deep-sea temperature, which independently validates the previous 0–1.5 Myr reconstruction and extends it back to 5.3 Myr ago. We find that deep-sea temperature and sea level generally decreased through time, but distinctly out of synchrony, which is remarkable given the importance of ice-albedo feedbacks on the radiative forcing of climate. In particular, we observe a large temporal offset during the onset of Plio-Pleistocene ice ages, between a marked cooling step at 2.73 Myr ago and the first major glaciation at 2.15 Myr ago. Last, we tentatively infer that ice sheets may have grown largest during glacials with more modest reductions in deep-sea temperature.

Some more info here:
https://news.anu.edu.au/2014/04/17/ancient-sea-levels-give-new-clues-on-ice-ages/

Co-author Gavin Foster from UoS said the research for the first time found long-term trends in cooling and continental ice-volume build-up cycles over the past 5.3 Million years were not the same.

“In fact, for temperature the major step toward the ice ages of the past two million years was a cooling event at 2.7 million years ago,” he said.

“But for ice-volume, the crucial step was the development of the first intense ice age at around 2.15 million years ago. Before our results, these were thought to have occurred together at about 2.5 million years ago.”

Professor Rohling said the findings will help scientists better understand the nature of ice ages and development of coastal sediment.

“The observed decoupling of temperature and ice-volume changes provides crucial new information for our understanding of how the ice ages came about,” he said.

It seems they found some sort of hysteresis of over half a million years between decreasing temperatures and glaciation at the end of the Pliocene, but maybe I don't understand correctly.

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #114 on: April 20, 2014, 04:08:57 PM »
As the linked reference indicates that the GIS is more stable than previously expected, this implies that the WAIS is less stable than previously expected, in order to match the paleo-SLR record:

Paul R. Bierman, Lee B. Corbett, Joseph A. Graly, Thomas A. Neumann, Andrea Lini, Benjamin T. Crosby, and Dylan H. Rood, (2014), "Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet", Science, DOI: 10.1126/science.1249047

http://www.sciencemag.org/content/early/2014/04/16/science.1249047.abstract

Abstract: "Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been non-erosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets."
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sidd

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #115 on: April 20, 2014, 09:24:37 PM »
DIsagree that "linked reference indicates that the GIS is more stable than previously expected"

i suppose it's in the definition of "previously"

That the center of greenland was probably ice covered for a few million years was not in question, i recall a paper by Otto-Bliesner (?) from a few years ago indicating that ice existed on the north dome but perhaps not under the south dome in the Eemian, which means it was probably there as long ago as the Holsteinian if not before. this, i think has been known for several years.

That said, it is now becoming fairly clear that all of Greenland never usually (say in the Pleistocene) melts away and that both Greenland and Antarctica must melt to some extent to reproduce sea level high stands.

sidd

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #116 on: April 21, 2014, 03:38:49 AM »
sidd,

I will admit that my prior post is controversial at best.  That said the following linked discussion of the article indicates that the discovered/preserved tundra layer was found under 80% of the GIS:

http://io9.com/researchers-have-found-a-3-million-year-old-landscape-f-1564648460
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sidd

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #117 on: April 21, 2014, 05:40:57 AM »
That is a good paper, but i suppose further discussion should b on the Greenland thread. I did however wan to thank you for the reference

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #118 on: April 23, 2014, 03:49:12 PM »
The linked reference indicates that during the Eocene polar amplification was so strong that the climate in portions of Antarctica were like Florida:

Pronounced zonal heterogeneity in Eocene southern high-latitude sea surface temperatures by Peter M. J. Douglas, Hagit P. Affek, Linda C. Ivany, Alexander J. P. Houben, Willem P. Sijp, Appy Sluijs, Stefan Schouten, and Mark Pagani published in PNAS doi: 10.1073/pnas.1321441111

http://www.pnas.org/content/early/2014/04/16/1321441111.abstract?sid=416fb7a8-dbf3-43d0-9dba-8b4bbcf0c1a0
Abstract: " Paleoclimate studies suggest that increased global warmth during the Eocene epoch was greatly amplified at high latitudes, a state that climate models cannot fully reproduce. However, proxy estimates of Eocene near-Antarctic sea surface temperatures (SSTs) have produced widely divergent results at similar latitudes, with SSTs above 20 °C in the southwest Pacific contrasting with SSTs between 5 and 15 °C in the South Atlantic. Validation of this zonal temperature difference has been impeded by uncertainties inherent to the individual paleotemperature proxies applied at these sites. Here, we present multiproxy data from Seymour Island, near the Antarctic Peninsula, that provides well-constrained evidence for annual SSTs of 10–17 °C (1σ SD) during the middle and late Eocene. Comparison of the same paleotemperature proxy at Seymour Island and at the East Tasman Plateau indicate the presence of a large and consistent middle-to-late Eocene SST gradient of ∼7 °C between these two sites located at similar paleolatitudes. Intermediate-complexity climate model simulations suggest that enhanced oceanic heat transport in the South Pacific, driven by deep-water formation in the Ross Sea, was largely responsible for the observed SST gradient. These results indicate that very warm SSTs, in excess of 18 °C, did not extend uniformly across the Eocene southern high latitudes, and suggest that thermohaline circulation may partially control the distribution of high-latitude ocean temperatures in greenhouse climates. The pronounced zonal SST heterogeneity evident in the Eocene cautions against inferring past meridional temperature gradients using spatially limited data within given latitudinal bands."
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AbruptSLR

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

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

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


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


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


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


Abstract: "Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum (26,000–19,000 years ago) is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise."

See also (with extract):

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

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

"The Antarctic Ice Sheet had been considered to be fairly stable and kind of boring in how it retreated," said study co-author Peter Clark, a climate scientist at Oregon State University. "This shows the ice sheet is much more dynamic and episodic, and contributes to rapid sea-level rise.""
“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 following extract from the linked article about the Weber et al (2014) paper cited in my last Reply #119, not only reinforces the importance of AIS SLR contribution to Meltwater Pulse 1A, but more importantly that the fresh melt water causes a stratification of ocean water with a cool surface and warmer deep waters that creates a positive feedback mechanism that accelerates the rate of grounding-line retreat of Antarctic marine glaciers, particularly like those in the ASE.  Thus we can add this positive feedback mechanism to the list of factors that will likely cause accelerating ice mass loss from the WAIS this century (and well beyond):

http://www.abc.net.au/science/articles/2014/05/29/4014978.htm


Extract: "Major meltdown

The authors found the largest of these pulses occurred between 14,800 and 14,400 years ago, which overlaps with a period of global sea level increases known as melt water pulse 1A, when levels rose by an average of four metres per century.

This information provides the long-sought confirmation of Antarctic contributions to this major jump in sea level rise.

The core data also shows that most of the Antarctic deglaciation occurred earlier than the 11,000 to 9000 years ago previously estimated.

"Early models suggested this melting was caused by thermodynamic effects such as changes in precipitation and air temperature," says Menviel.

"But those are fairly small and would mean deglaciation of the Antarctic would have occurred later than the data shows."

Feedback system

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

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

"So what starts as a small melting can be amplified leading to more rapid melting than just through changes in atmospheric temperature," says Menviel."
“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 following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm

I have investigated the collapse of the Paleo-Pine Island glacier, and while not confirmed it is highly likely that this collapse event occurred during Meltwater Pulse 1A, which would indicate that the Paleo-Pine Island glacier may have contributed up to 2m of the 4m total SLR that occurred over a 100-year period during MWP 1A, and the following extract from Schroeder et al 2014 indicates that the Thwaites Glacier is primed to degrade in a similar character and pacing:

70A0914
Radar-sounding observations of basal water, sediments and geothermal heat flux and their implications for the past and future sea-level contribution of the Amundsen Sea sector of West Antarctica
 
Dustin SCHROEDER, Donald BLANKENSHIP, Duncan YOUNG, Enrica QUARTINI, John ANDERSON, Alexandra WITUS
Corresponding author: Dustin Schroeder
Corresponding author e-mail: dschroed@gmail.com

Abstract: "The basal morphology, lithology and hydrology of ice sheets and glaciers can exert strong even dominating control on their evolution, stability and sea-level contribution. However, the scales at which the physical processes and observable signatures of this control occur are typically smaller than the spatial resolutions achievable using ice-penetrating radar. Further, the strength of radar bed echo returns is a combination of both the material and geometric properties of the ice–bed interface as well as englacial attenuation. This ambiguity makes definitive assessment of basal conditions from echo strengths difficult. To address these challenges in interpreting geometric and material bed properties at glaciologically relevant scales, we apply a new algorithmic approach to measuring the radar scattering function of the ice–bed interface in terms of the relative contribution of angularly narrow specular energy and isotropically scattered diffuse energy. We compliment this specularity analysis with a coupled radar echo strength/subglacial water routing model to constrain the distribution of basal melt. We present the application of these techniques to a radar-sounding survey of Thwaites Glacier. We show that it can be used to assess the extent and geometry of distributed water across the catchment and detect the transition of the water system from distributed canals to concentrated channels. We also constrain the morphology of basal bedforms to infer the distribution of deformable sediments and crystalline bedrock. Finally, we compare our model of radar return strength and subglacial water routing with models of basal melting from ice flow to infer the distribution of geothermal heat flux and interpret its observed heterogeneity in the context of regional volcanism. These observed basal conditions provide new context for the past and potential evolution, stability and sea-level contribution of the rapidly changing Amundsen Sea sector of West Antarctica. We compare the contemporary configuration of Thwaites Glacier with that of the deglaciated Paleo Pine Island ice stream and the sedimentary record of its meltwater intensive retreat. We conclude that a transition in the basal hydrology of Paleo Pine Island was characteristic of its relatively rapid retreat across exposed bedrock on the inner continental shelf and that Thwaites Glacier may be currently configured for a retreat that is similar in character and pacing."
“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 following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm

The Whipple et al 2014 reference provides additional support for the case that the WAIS at least partially collapsed (beyond today's WAIS configuration) during the LIG (Eemian), which changed precipitation patterns about Antarctica:

70A0963
Assessing the extent of the Last Interglacial Antarctic ice sheet
Matthew WHIPPLE, Mark SIDDALL, Joy SINGARAYER, Sarah BRADLEY, Eric WOLFF, Glenn MILNE, Dan LUNT
Corresponding author: Matthew Whipple
Corresponding author e-mail: matt.whipple@bristol.ac.uk

Abstract: "Understanding the response of the Antarctic ice sheet (AIS) to warmer climate, such as during the Last Interglacial (LIG), is of vital importance to understand future changes in sea levels. Kopp and others (2009) gives a 50% chance of LIG sea levels exceeding 8.5 m above current levels, with other studies resulting in a modelled 4.2–5.8 m from Antarctic sources, assuming hemispheric synchronicity. Previous modelling work has not modelled warming to the degree suggested by the ice-core proxy data. Here we attempt to account for part of the Antarctic component of the additional sea level during the LIG through climate sensitivity testing by HadCM3 climate modelling for different ice-sheet scenarios, comparing with precipitation-weighted temperature to stable water isotope data in six East Antarctic ice-core records. We look at the effect of changing ice-core site elevation during the LIG, through Glacial Isostatic Adjustment (GIA) modelling, by adapting the ice-sheet models of Bradley and others (2012, 2013) for use in HadCM3 climate simulations. A collapse of the marine-based West Antarctic ice sheet (WAIS) provides changes in the precipitation regime over the East Antarctic ice sheet (EAIS), meaning that some ice-core temperature reconstructions using precipitation-weighted temperature show warming on the order seen in ice-core isotope data. Other EAIS ice-core isotope data show absolute temperature offsets, which we are unable to resolve within reasonable glaciological limits. However, the suggestion of reduced thickening over the LIG EAIS would come closer to resolving the offset temperature offset, and also to resolving the sea-level budget."
“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 following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm

It will be interesting when this fully historically calibrated model is run forward in time, and a probability distribution for the future evolution of the AIS is generated.

70A0976
Development towards a full Bayesian calibration of a 3-D glacial systems model of the Antarctic ice sheet over the last glacial cycle
Benoit LECAVALIER, Rob BRIGGS, Lev TARASOV
Corresponding author: Benoit Lecavalier
Corresponding author e-mail: b.lecavalier@mun.ca

Abstract: "How did the Antarctic ice sheet (AIS) evolve over the last glacial cycle? How much do current uncertainties in the thermomechanical present-day state of the ice sheet affect projections of its future evolution? Without the associated quantification of uncertainties, answers to these questions have little value. A developing technique for explicit uncertainty quantification of glacial systems is large-ensemble Bayesian calibration of models against large observational datasets. The foundation for a Bayesian calibration of a 3-D glacial systems model (GSM) for Antarctica has recently been completed. Bayesian calibration thoroughly samples model uncertainties against fits to observational data through Markov Chain Monte Carlo methods using Bayesian artificial neural network emulators of the full GSM. For the first time, this methodology will generate a probability distribution for the AIS deglaciation with explicit and well-defined confidence intervals. Past work has shown the GSM to have likely inadequate range of grounding line migration in certain sectors as well as persistent ice thickness biases in topographically complex regions. To advance towards full calibration, these deficiencies will be addressed through the following. First, basal drag representation will be improved. This will include improved subgrid treatment of the thermomechanical impacts of high basal roughness, examination of the impact of inclusion of fully coupled basal hydrology, and re-evaluation of uncertainties in basal drag parametric representation for regions that are presently marine. Second, an expanded climate forcing using statistical correction of simplified climate models will be added to better capture the uncertainty in past climate. Thirdly, the impact of past changes in ocean temperature on sub-ice-shelf melt will be explicitly incorporated. Finally, the calibration will also incorporate uncertainties in Earth rheology in the context of isostatic adjustment. We will outline the improvements currently being implemented, solicit further data constraints, and outline the remaining steps towards full calibration of the model. This research will improve our understanding of the present-day thermomechanical state of the AIS and its contemporary mass balance through the re-interpretation of geodetic data. By also running the calibrated GSM forward in time, a probability distribution for the future evolution of the AIS will be generated."
“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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The following link leads to the University of Alaska Fairbank's website focused on Lake Elgygytgn research, and the extract following the link is from an article Posted on February 4th, 2014 by Laura Nielsen on "Inter-hemispheric climate coupling". The extract emphasizes that in the paleo-past the Antarctic generally responded more quickly to orbital induced solar insolation variations, and that repeatedly paleo-collapses of the WAIS resulted in subsequent Arctic amplification, due both to changes in ocean currents, and to increases in sea level pushing more warm Pacific water through the Bering St. into the Arctic Ocean.  If the WAIS collapses this century, we may soon see a marked increase in Arctic amplification:


http://frontierscientists.com/tag/lake-elgygytgyn/

Extract: "Antarctica and the Arctic
Climate at the North and South pole are connected. Sediment records from Antarctica show that the West Antarctic ice sheet melted at various times in history. Following many of those events, the Arctic warmed. These recurring intervals of paired warming show that climate in the two hemispheres is linked – it’s called inter-hemispheric climate coupling.
“When the West Antarctic ice sheet pulls back we see a corresponding warmth in the high lattitudes again, probably affecting the size of the Greenland ice sheet with major implications for changes in sea level,” says Julie Brigham-Grette. “Our results mesh with what glaciologists are seeing today. Seven of the 12 major ice shelves around the Antarctic are melting or are gone. We suspect the tipping point for the gradual de-glaciation of Greenland and the Arctic may be lower than glaciologists once thought.”
Complex systems
Earth is a complicated place. We can’t explain past warming using only orbital dynamics or levels of Carbon Dioxide. Scientists affiliated with the project outlined some past events that might explain the rapid warming the sediment records show occurred in both Antarctica and the Arctic around similar times.
When you imagine Antarctica, the picture includes large ice shelves that hang off the rocky edge of the ice-covered continent. Normally that ice keeps nearby ocean water very cold. The cold water travels along currents toward the north Pacific where it wells up to the surface. Ocean circulation can be affected, though. If Antarctic ice sheets disintegrate or melt away, they no longer enforce cold water currents that journey to the Arctic. Instead, surface ocean waters in the Arctic become warmer.
When Antarctica’s ice sheets disintegrate the ocean gains more water and sea levels rise globally. The Bering Strait usually restricts how much warm surface water approaches the Arctic from the south, but higher sea levels would mean warm surface water didn’t have to squeeze through such a narrow space, letting more warm water past the Bering Strait into the Arctic Ocean.
Either way, a warmer ocean means higher temperatures and more rainfall for the Arctic, which impacts paleoclimatology and sea ice history. Grasping the climate connections between the hemispheres gives us insight into our near future."
“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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During the Pliocene the tropics may have been warmer than previously thought:
http://phys.org/news/2014-06-hot-tropical-oceans-pliocene-greenhouse.html

What, if anything, could this mean for climate sensitivity and/or polar amplification?

AbruptSLR

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Lennart,

Thanks for the reference (for which I provide details below).  To me this research indicates that those who were optimistically hoping that ECS would be towards the low end of the AR5 range, were wrong.

Best,
ASLR

Charlotte L. O’Brien, Gavin L. Foster, Miguel A. Martínez-Botí, Richard Abell, James W. B. Rae & Richard D. Pancost, (2014), "High sea surface temperatures in tropical warm pools during the Pliocene", Nature Geoscience, doi:10.1038/ngeo2194


http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2194.html

Abstract: "The western warm pools of the Atlantic and Pacific oceans are a critical source of heat and moisture for the tropical climate system. Over the past five million years, global mean temperatures have cooled by 3–4 °C. Yet, present reconstructions of sea surface temperatures indicate that temperature in the warm pools has remained stable during this time. This stability has been used to suggest that tropical sea surface temperatures are controlled by a thermostat-like mechanism that maintained consistent temperatures. Here we reconstruct sea surface temperatures in the South China Sea, Caribbean Sea and western equatorial Pacific Ocean for the past five million years, using a combination of the Mg/Ca-, TEX86H- and  -surface-temperature proxies. Our data indicate that during the period of Pliocene warmth from about 5 to 2.6 million years ago, the western Pacific and western Atlantic warm pools were about 2 °C warmer than today. We suggest that the apparent lack of warmth seen in the previous reconstructions was an artefact of low seawater Mg/Ca ratios in the Pliocene oceans. Taking this bias into account, our data indicate that tropical sea surface temperatures did change in conjunction with global mean temperatures. We therefore conclude that the temperature of the warm pools of the equatorial oceans during the Pliocene was not limited by a thermostat-like mechanism."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

Wipneus

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During the Pliocene the tropics may have been warmer than previously thought:
http://phys.org/news/2014-06-hot-tropical-oceans-pliocene-greenhouse.html

What, if anything, could this mean for climate sensitivity and/or polar amplification?


It is already indicated in the article you linked, there was an apparent conflict between models and data. This research indicates the conflict is caused by the data in error, I don't think it is the first.

This will at least increase the confidence in model results like climate sensitivity and polar amplification. It will also make future improvements in those models possible, when it is known that an unknown forcing does not have to be assumed.


Lennart van der Linde

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ASLR, Wipneus,

Thanks for the link and comments. The abstract of the paper by O'Brien et al. says:
"Over the past five million years, global mean temperatures have cooled by 3–4 °C. Yet, present reconstructions of sea surface temperatures indicate that temperature in the warm pools has remained stable during this time... Our data indicate that during the period of Pliocene warmth from about 5 to 2.6 million years ago, the western Pacific and western Atlantic warm pools were about 2 °C warmer than today."

Maybe this only means resolving a conflict between models and data. But to me the question that has not been answered yet is: if this paper is correct, does it imply global temperature in the Pliocene was higher than previously thought (based on models or proxiex), or does it mean polar amplification was less than previously thought (based on temperature proxies), or a little bit of both? Or: are global and regional Pliocene temperatures estimated independently?

And how certain are we of those 3-4 degrees higher temperature? IPCC 2013 WG1 Ch.5 says about the Mid-Pliocene Warm Period:
"General circulation model (GCM) results... produce a range of global mean SAT of +1.9°C and +3.6°C relative relative to the 1901–1920 mean (Haywood et al., 2013). Weakened meridional
temperature gradients are shown by all GCM simulations, and have significant implications for the stability of polar ice sheets and sea level (see Box 5.1 and Section 5.6)."

That 1.9-3.6 seems quite a large range, larger than and different from 3-4 degrees C, which could mean maybe a doubling or halving of sea level sensitivity to global temperature, depending on assumptions. Or: is it more likely that SLR for circa 1-4 degrees warmer than today is closer to say 5 meter/degree C or closer to say 10 meter/degree C, assuming sea level was up to about 20m higher during the Mid-Pliocene, as IPCC estimates (some think it could have been more)?

AbruptSLR

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Lennart,

First, you may want to review my Reply #188 in the "Forcing" thread (see link below):

http://forum.arctic-sea-ice.net/index.php/topic,41.150.html

In my opinion this new research implies that the mean global temperature 2.6 to 5 million years ago was not less than about 2.3 degree C and got as hot at 4 degrees C warmer than today.  In other words, this research effectively implies that the lower possible paleo-climate sensitivity values are not possible.  Clear polar amplification was very effective during the Pliocene.

Best,
ASLR
“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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I do not mean to over-generalize, but the Pliocene is considered by many researchers to represent a paleo-condition that the Earth is rapidly headed towards, and the linked reference indicates that there was considerable surface ice sheet melting in this period (2.6-5.3 Ma) in the WAIS, which was sufficient to feed major subglacial hydrological meltwater systems that resulted in considerable amount of subglacial erosion.  As in the "WAIS Collapse" thread I have provided evidence indicating that the WAIS could again experience widespread surface ice melting circa 2070, this research supports the concern the extensive subglacial meltwater networks may well contribute to the substantial collapse of the WAIS before the end of this century:


Kathryn Rose, Neil Ross, Robert Bingham, Hugh Corr, Fausto Ferraccioli, Tom Jordan, Anne LeBrocq, David Rippin, and Martin Siegert, (2014), "Major Subglacial Meltwater Channels Reveal Former Dynamic Ice Sheet in West Antarctica", Geophysical Research Abstracts, Vol. 16, EGU2014-7128, 2014


http://meetingorganizer.copernicus.org/EGU2014/EGU2014-7128.pdf

Abstract: "The Eocene-Oligocene boundary (ca. 34 Ma) marks the onset of widespread, continental-scale glaciation in Antarctica, due to declining atmospheric carbon dioxide levels and the opening of the Drake Passage. The marine-based West Antarctic Ice Sheet (WAIS) is considered highly susceptible to change, experiencing numerous oscillations since its formation. In order to assess how past changes to the WAIS are relevant for understanding its future behaviour, it is important to comprehend the glaciological processes involved in those changes. Central to this is an appreciation of climate and ice flow regimes, in particular the extent to which former ice sheets have experienced surface melting (as in Greenland today). Geomorphic analysis of subglacial topography has played a key role in reconstructing the nature of former ice masses in Antarctica, as landscape form can be linked to glacial process. While radio-echo sounding (RES) is the primary tool used to map boundary conditions beneath ice sheets, recent developments have demonstrated that satellite imagery of the ice surface can provide insights into subglacial topography, where RES is unavailable.
 
Using this combination of datasets, we have identified a series of major, elongate subglacial features, which we interpret as preserved subglacial channels, developed through the action of water. They are incised into a subglacial plateau in the region between the Möller and Foundation ice streams (MIS and FIS, respectively), in West Antarctica. The channels are observed across an area of ~17,700 km2 and extend 200 km inland from the grounding line. They are located below sea level and track over present-day reverse slopes, indicating a subglacial (rather than pre-glacial) fluvial origin. In order to form, these channels require significant, probably periodic (seasonal), meltwater inputs to the base of the ice sheet. We suggest the channels are the result of meltwater inputs to the subglacial environment from the ice surface, in a setting analogous to present-day Greenland. This allows us to bracket the most recent date at which this may have occurred.

The Pliocene (2.6-5.3 Ma) represents the most recent period in the geologic past when atmospheric temperatures for West Antarctica were high enough to generate surface melt comparable to that observed on the Greenland Ice Sheet today. These features provide evidence for temperate basal thermal conditions and thus, a former ice flow regime that differs markedly from the present-day polar ice sheet conditions of West Antarctica.  We envisage bed channel formation occurred under temperate ice sheet conditions, when the subglacial plateau was overridden by a temperate ice mass. If this interpretation is correct, it means that ice was still present (at least periodically) in this location, during the warm conditions of the Pliocene. The discovery of these channels also highlights what little was known about this large region of West Antarctica, prior to the Institute-Möller geophysical survey."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #131 on: August 20, 2014, 05:44:30 PM »
The linked reference provides a possible explanation of the abrupt Dansgaard-Oeschger (DO) events related to small changes in the height of Northern Hemisphere ice sheets, that then influence the Southern Hemisphere via the bipolar seesaw.

Xu Zhang, Gerrit Lohmann, Gregor Knorr & Conor Purcell, (2014), "Abrupt glacial climate shifts controlled by ice sheet changes", Nature, doi:10.1038/nature13592


http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13592.html


Abstract: "During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard–Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere–ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses—including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw—are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere–ocean–sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere–ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #132 on: August 28, 2014, 10:12:37 PM »
Per the linked reference, before the ice covered Antarctica, the Earth had a tropical climate. At that time in the Southern Ocean, plankton diversity was high until glaciation 33.6 million years ago reduced the populations leaving only those capable of surviving in the new climate.  It will be interesting to see what happens in the Southern Ocean as global warming continues.

Alexander J. P. Houben, Peter K. Bijl, Jörg Pross, Steven M. Bohaty, Sandra Passchier, Catherine E. Stickley, Ursula Röhl, Saiko Sugisaki, Lisa Tauxe, Tina van de Flierdt, Matthew Olney, Francesca Sangiorgi, Appy Sluijs, Carlota Escutia Henk Brinkhuis and the Expedition 318 Scientists. Reorganization of Southern Ocean Plankton Ecosystem at the Onset of Antarctic Glaciation. Science. DOI: 10.1126/science.1223646

http://www.sciencemag.org/content/340/6130/341

Abstract: "The circum-Antarctic Southern Ocean is an important region for global marine food webs and carbon cycling because of sea-ice formation and its unique plankton ecosystem. However, the mechanisms underlying the installation of this distinct ecosystem and the geological timing of its development remain unknown. Here, we show, on the basis of fossil marine dinoflagellate cyst records, that a major restructuring of the Southern Ocean plankton ecosystem occurred abruptly and concomitant with the first major Antarctic glaciation in the earliest Oligocene (~33.6 million years ago). This turnover marks a regime shift in zooplankton-phytoplankton interactions and community structure, which indicates the appearance of eutrophic and seasonally productive environments on the Antarctic margin. We conclude that earliest Oligocene cooling, ice-sheet expansion, and subsequent sea-ice formation were important drivers of biotic evolution in the Southern Ocean."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #133 on: September 12, 2014, 09:44:05 PM »
The following linked reference provides useful paleo data (see attached plot [where GL = grounding line]) about the ASE glaciers:

James A. Smith , Claus-Dieter Hillenbrand, Gerhard Kuhn, Johann Phillip Klages, Alastair G.C. Graham, Robert D. Larter, Werner Ehrmann, Steven G. Moreton, Steffen Wiers, & Thomas Frederichs, (2014), "New constraints on the timing of West Antarctic Ice Sheet retreat in the eastern Amundsen Sea since the Last Glacial Maximum", Global and Planetary Change, DOI: 10.1016/j.gloplacha.2014.07.015


http://www.sciencedirect.com/science/article/pii/S0921818114001489


Abstract: "Glaciers flowing into the Amundsen Sea Embayment (ASE) account for > 35% of the total discharge of the West Antarctic Ice Sheet (WAIS) and have thinned and retreated dramatically over the past two decades. Here we present detailed marine geological data and an extensive new radiocarbon dataset from the eastern ASE in order to constrain the retreat of the WAIS since the Last Glacial Maximum (LGM) and assess the significance of these recent changes. Our dating approach, relying mainly on the acid insoluble organic (AIO) fraction, utilises multi-proxy analyses of the sediments to characterise their lithofacies and determine the horizon in each core that would yield the most reliable age for deglaciation. In total, we dated 69 samples and show that deglaciation of the outer shelf was underway before 20,600 calibrated years before present (cal. yr BP), reaching the mid-shelf by 13,575 cal. yr BP and the inner shelf to within c.150 km of the present grounding line by 10,615 cal. yr BP. The timing of retreat is broadly consistent with previously published radiocarbon dates on biogenic carbonate from the eastern ASE as well as AIO 14C ages from the western ASE and provides new constraints for ice sheet models. The overall retreat trajectory – slow on the outer shelf, more rapid from the middle to inner shelf – clearly highlights the importance of reverse bedslopes in controlling phases of accelerated groundling line retreat. Despite revealing these broad scale trends, the current dataset does not capture detailed changes in ice flow, such as stillstands during grounding line retreat (i.e., deposition of grounding zone wedges) and possible readvances as depicted in the geomorphological record."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #134 on: September 13, 2014, 06:00:54 PM »
The linked reference provides paleo information about the accumulation of snow in the Pine Island Glacier basin:

Karlsson, Nanna B.; Bingham, Robert G.; Rippin, David M.; Hindmarsh, Richard C.A.; Corr, Hugh F.J.; Vaughan, David G., (2014), "Constraining past accumulation in the central Pine Island Glacier basin, West Antarctica, using radio-echo sounding", Journal of Glaciology, Volume 60, Number 221, June 2014, pp. 553-562(10)

http://www.ingentaconnect.com/content/igsoc/jog/2014/00000060/00000221/art00013

Abstract: "The potential for future dynamical instability of Pine Island Glacier, West Antarctica, has been addressed in a number of studies, but information on its past remains limited. In this study we use airborne radio-echo sounding (RES) data acquired over Pine Island Glacier to investigate past variations in accumulation pattern. In the dataset a distinctive pattern of layers was identified in the central part of the glacier basin. We use these layers as chronological identifiers in order to construct elevation maps of the internal stratigraphy. The observed internal layer stratigraphy is then compared to calculated stratigraphy from a three-dimensional ice-flow model that has been forced with different accumulation scenarios. The model results indicate that the accumulation pattern is likely to have changed at least twice since the deposition of the deepest identified layer. Additional RES data linked to the Byrd ice core provide an approximate timescale. This timescale suggests that the layers were deposited at the beginning of or during the Holocene period. Thus the widespread changes occurring in the coastal extent of the West Antarctic ice sheet at the end of the last glacial period could have been accompanied by changes in accumulation pattern."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #135 on: September 26, 2014, 05:04:47 AM »
The linked reference provides paleo-evidence that abrupt sea level rise events under conditions similar to our current conditions are not uncommon in a new paleo-record from the Red Sea.  This supports the idea that after the 150-years of anthropogenic global warming that the Earth has already experienced, the WAIS is likely primed to collapse:

K. M. Grant, E. J. Rohling, C. Bronk Ramsey, H. Cheng, R. L. Edwards,   F. Florindo, D. Heslop, F. Marra, A. P. Roberts, M. E. Tamisiea & F. Williams, (2014), "Sea-level variability over five glacial cycles", Nature Communications 5, Article number: 5076 doi:10.1038/ncomms6076

http://www.nature.com/ncomms/2014/140925/ncomms6076/abs/ncomms6076.html

Abstract: "Research on global ice-volume changes during Pleistocene glacial cycles is hindered by a lack of detailed sea-level records for time intervals older than the last interglacial. Here we present the first robustly dated, continuous and highly resolved records of Red Sea sea level and rates of sea-level change over the last 500,000 years, based on tight synchronization to an Asian monsoon record. We observe maximum ‘natural’ (pre-anthropogenic forcing) sea-level rise rates below 2 m per century following periods with up to twice present-day ice volumes, and substantially higher rise rates for greater ice volumes. We also find that maximum sea-level rise rates were attained within 2 kyr of the onset of deglaciations, for 85% of such events. Finally, multivariate regressions of orbital parameters, sea-level and monsoon records suggest that major meltwater pulses account for millennial-scale variability and insolation-lagged responses in Asian monsoon records.
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sidd

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #136 on: September 27, 2014, 12:07:58 AM »
Re:Grant(2014)

"This supports the idea that after the 150-years of anthropogenic global warming that the Earth has already experienced, the WAIS is likely primed to collapse:"

Disagree. This paper points out that large sea level rise rates only occur when the ice  volume is about twice present day (approx 65 m SLR below present). Also  points out the lag after onset is on the order of kiloyears.

That said, i do not disagree that WAIS may be "primed for collapse", just disagree that this paper supports the hypothesis.

sidd

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #137 on: September 27, 2014, 12:44:57 AM »
sidd,

Sometimes I get a little bit lazy, and I appreciate your efforts to encourage me to be more precise.  I was lazy to use the term "abrupt sea level rise" without defining this term in this context.  In this regard:

(a) The first extract from the Science Daily link below emphasizes that the researcher's data averages SLR over 500-year periods, so it cannot exclude brief pulses of faster SLR.

(b) The second extract is a partial quote from Dr Katharine Grant, indicating that for paleo-conditions with modern amounts of ice on Earth, the data shows 500-year averaged periods of up to 1 to 1.5 meters of SLR per century (one would expect periods of both higher, and lower, rates of SLR during such 500-year periods).

(c) The third extract provides a quote from Professor Rohling indicating that after 150-years of anthropogenic warming that ice mass loss from both the Antarctic and Greenland ice sheet is significant, and that once initiated (as we know that the PIG, and Thwaites Glacier, are) this ice mass loss will continue for centuries.

http://www.sciencedaily.com/releases/2014/09/140925082223.htm

Extract1: "The researchers emphasise that their values for sea-level change are 500-year averages, so brief pulses of faster change cannot be excluded."

Extract2: "Those with close to the modern amount of ice on Earth, show rates of up to 1 to 1.5 metres per century."

Extract3: "Professor Rohling speculates that there may be an important lesson for our future: "Man-made warming spans 150 years already and studies have documented clear increases in mass-loss from the Antarctic and Greenland ice sheets. Once under way, this response may be irreversible for many centuries to come.""

While my post above sounds a little less dramatic than my original post on Grant et al (2014); it sounds more dramatic to me than your post.  Thus I believe that Grant et al (2014) does provide evidence that our risk of ASLR is higher than many realized before they published their paper (but I agree that they do not provide definitive proof that the WAIS will partially collapse this century).

Best,
ASLR
« Last Edit: September 27, 2014, 01:00:51 AM by AbruptSLR »
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sidd

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #138 on: September 27, 2014, 04:11:53 AM »
Don't get me wrong, i really like the paper.

a) actually the data are at about 250 yr intervals, and the look at various averages, but since differentiating the data amplifies noise, they went with a 500 yr smoothing

b) agreed

c) is supported by the paper

There are other nice bits such as the connection to the Asian monsoon.

However, as has been pointed out in another thread, forcings today are much, much faster and stronger than those in the paper.

sidd 

Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #139 on: September 27, 2014, 08:11:16 AM »
ASLR, Sidd,

Thanks for bringing this paper up here as well: certainly the most appropriate place. I put it in the Consequences/Sea-level thread for its potential implications for future SLR.

As also added there, Rohling gave this presentation this week at a big conference in Rotterdam, Delta's in Time of Climate Change, based in part on the new paper:
http://edepot.wur.nl/314954

On the lag-time he says that in 50% of cases it's less than 500 yrs, if I understand correctly, based on Rohling et al 2013. At least the 2 ky lag is a maximum, not an average, mean or minimum response time for the ice sheets. And the current rate of SLR seems to indicate they are responding now after 150-200 years of poking them with our CO2-stick.

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #140 on: September 27, 2014, 08:19:54 AM »
Also, in this article for a wider audience Rohling says maximum rate of retreat is reached within 400 yrs in 68% of cases:
http://theconversation.com/why-ice-sheets-will-keep-melting-for-centuries-to-come-32171

So within the coming two centuries we could very well reach a very high rate of SLR, is my conclusion.

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #141 on: September 27, 2014, 10:12:40 AM »
Do not forget that we were protected by the thermal inertia of the oceans (because of the downward trend at the beginning the industrial revolution) and that the quantity of green house gazes released has nothing to do with what was released in the last million years. So these numbers give us an idea of what is waiting for future generations but the rate of melting will certainly be higher.

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #142 on: September 27, 2014, 05:15:58 PM »
Do not forget that we were protected by the thermal inertia of the oceans (because of the downward trend at the beginning the industrial revolution) and that the quantity of green house gazes released has nothing to do with what was released in the last million years. So these numbers give us an idea of what is waiting for future generations but the rate of melting will certainly be higher.

Laurent,

While I agree that the thermal inertia of the oceans delays most of the anthropogenic radiative forcing impacts on ice mass loss from the AIS, I think that it is a major mistake to overly discount the risks of abrupt sea level rise from the AIS, this century (which I believe that treating the paleo-record, and our current GCM projections, as any thing but lower bound estimates, tend to do).  A very partial list of things that are occurring now (besides much higher rates of GHG emissions) that did not occur in the past include:
- The ozone hole over Antarctica has increased wind velocities that have increased upwelling of warm CDW, that means that the thermal inertia of the ocean is actually a large detriment as it represents a massive bank of heat that will contribute to basal ice melting and grounding line retreat.
- GCM projections show that over the next 30-years, the expected synchronization of the North Pacific, and North Atlantic, Oceans will keep the Tropical Pacific in an El Nino-like condition that will telecommunicate extensive amounts of energy directly to the WAIS (possibly beginning as soon as this austral Spring).
- The extremely high rates of ice shelf basal ice melting (due to ozone hole driving upwelling) is keeping sea levels unusually high in the Southern Ocean; which serves to reduce the stability of the marine glaciers in Antarctica.
- The extremely rapid increase of wild fires in the North Hemisphere is introducing unusually high amounts of black carbon on the surface of the GIS that will resulting in historically high rates of sea level rise contributions from the GIS, which will also rise sea level in the Southern Ocean, that will also reduce the stability of marine glaciers in Antarctica.
- Recent increases in Antarctic sea ice area (as well as the freshening of the Southern Ocean surface waters) is contributing to a slowing of the rate of Antarctic Bottom Water formation; which historically used to carry heat away from the CDW into the deep ocean, but now that incremental heat remains in the CDW which is advected to the grounding lines of marine glaciers.

I could go on, but as current GCM/RCM/LCM projections cannot correctly account for the influence of even then new factors, I guess that we will all need to wait to see what actually happens in the coming years/decades (we are all leaving the coming centuries to future generations), but I still believe that 3m of global mean sea level rise by 2100 is the most likely (mean) probability.

Best,
ASLR
« Last Edit: September 27, 2014, 06:10:44 PM by AbruptSLR »
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #143 on: September 27, 2014, 06:51:32 PM »
Per the links that Lennart provided: (a) the following quotes two from Rohling from the following "The Conversation" website; and (b) the attached image from Rohling's PowerPoint presentation:

http://theconversation.com/why-ice-sheets-will-keep-melting-for-centuries-to-come-32171

Quote from Rohling: "We seem with modern sea level rise to be at the upper 1 sigma bound of "natural style" sea level rises (see Rohling et al., Scientific Reports 2013 - open access). So rise seems to be fast, but not (yet) statistically distinct from past rise rates. It may be that dynamic processes of ice mass loss are rate limited. Alternatively, modern rates may accelerate, but we don't know why and how yet. I would not want to second guess that."
Quote from Rohling: "We worked out in Foster and Rohling (PNAS 2013) that the "natural equlibrium" sea level at about 400 ppmv CO2 concentrations was 9-31 m above present at 68%probability, with the modal value at 24 m.
So that would be quite a large disequilibrium, with at least 9 m of sea level rise to come. But that would not be such a worry if it were to take 10,000 years, so the rate of rise is crucial. That is what we have quantified in the present paper in Nature Comms, and what we had estimated (now supported by the Nature Comms data) and used to look at the comparison between current changes and "natural style" changes (Rohling et al. Scientific Reports 2013). That suggested that we're on a fast pathway within the geologically precedented distribution of rates, along the upper 1 sigma boundary). So nothing 'exceptional' yet in terms of processes at work. We're still within what nature has done before and so clearly has the mechanisms for, and it has previously gone up to +8 to 9 m like that, so I see no reason to assume that modern change will not continue to behave in a similar way. This pathway extrapolates to about +1 m in 2100, and then +2.5 m in 2200."
In truth, I am not opposed to scientific reticence to second guess how much modern rates of SLR contributions from Antarctica may (or may not) accelerate this century (thus I appreciate allow Rohling's comments and scientific contributions).  But I do object to readers (the public or decision makers) of scientific reports who take the lower bound cases (note the IPCC AR5 does not present any upper bound projections for SLR by 2100) presented by the authors as if they are hard forecasts of future events.  I believe that it is the reader's responsibility to bear in mind all of the caveats explicit, or implicit, about the limits of that given research findings.  As one minor example, in both quotes above, when Rohling talks about "… modern sea level rise to be at the upper 1 sigma bound of "natural style" sea level rises …", it is the reader's responsibility to consider such factor as that beyond a 2 degree mean global temperature rise (which we are on track to exceed by about 2035, or sooner) most positive feedback factors becoming increasing non-linear; which due to polar amplification will like result in a collapse of Antarctic Sea Ice extent by (or before 2060-2070), which will likely promote wind-driven currents to advect more ocean heat toward Antarctic marine glaciers.
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #144 on: November 24, 2014, 10:44:04 AM »
The linked article discusses Antarctic contribution to abrupt sea-level rise during Meltwater Pulse 1A, due to reduced Southern Ocean overturning:

N. R. Golledge, L. Menviel, L. Carter, C. J. Fogwill, M. H. England, G. Cortese & R. H. Levy, (2014), "Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning", Nature Communications 5, Article number: 5107 doi:10.1038/ncomms6107

http://www.nature.com/ncomms/2014/140929/ncomms6107/full/ncomms6107.html

Abstract: "During the last glacial termination, the upwelling strength of the southern polar limb of the Atlantic Meridional Overturning Circulation varied, changing the ventilation and stratification of the high-latitude Southern Ocean. During the same period, at least two phases of abrupt global sea-level rise—meltwater pulses—took place. Although the timing and magnitude of these events have become better constrained, a causal link between ocean stratification, the meltwater pulses and accelerated ice loss from Antarctica has not been proven. Here we simulate Antarctic ice sheet evolution over the last 25 kyr using a data-constrained ice-sheet model forced by changes in Southern Ocean temperature from an Earth system model. Results reveal several episodes of accelerated ice-sheet recession, the largest being coincident with meltwater pulse 1A. This resulted from reduced Southern Ocean overturning following Heinrich Event 1, when warmer subsurface water thermally eroded grounded marine-based ice and instigated a positive feedback that further accelerated ice-sheet retreat."
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Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #145 on: December 05, 2014, 09:02:24 AM »
New paper by Stap et al 2014:
http://www.clim-past.net/10/2135/2014/cp-10-2135-2014.pdf

Interaction of ice sheets and climate during the past 800000 years
L. B. Stap, R. S. W. van de Wal, B. de Boer, R. Bintanja, and L. J. Lourens

Abstract
During the Cenozoic, land ice and climate interacted on many different timescales. On long timescales, the effect of land ice on global climate and sea level is mainly set by large ice sheets in North America, Eurasia, Greenland and Antarctica. The climatic forcing of these ice sheets is largely determined by the meridional temperature profile resulting from radiation and greenhouse gas (GHG) forcing. As a response, the ice sheets cause an increase in albedo and surface elevation, which operates as a feedback in the climate system. To quantify the importance of these climate–land ice processes, a zonally averaged energy balance climate model is coupled to five one-dimensional ice sheet models, representing the major ice sheets. In this study, we focus on the transient simulation of the past 800000 years, where a high-confidence CO2 record from ice core samples is used as input in combination with Milankovitch radiation changes. We obtain simulations of atmospheric temperature, ice volume and sea level that are in good agreement with recent proxy-data reconstructions. We examine long-term climate–ice-sheet interactions by a comparison of simulations with uncoupled and coupled ice sheets. We show that these interactions amplify global temperature anomalies by up to a factor of 2.6, and that they increase polar amplification by 94%. We demonstrate that, on these long timescales, the ice-albedo feedback has a larger and more global influence on the meridional atmospheric temperature profile than the surface-height-temperature feedback. Furthermore, we assess the influence of CO2 and insolation by performing runs with one or both of these variables held constant. We find that atmospheric temperature is controlled by a complex interaction of CO2 and insolation, and both variables serve as thresholds for northern hemispheric glaciation.

AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #146 on: December 10, 2014, 12:29:49 AM »
The linked reference about Heinrich events demonstrates a dynamic linkage between the Arctic and Antarctic climates.  Thus as the Arctic warms in an accelerated manner, we can expect this to drive surface temperature increases in the Antarctic:

Xiao Yang, J. A. Rial and Elizabeth P. Reischmann, (2014), "On the bipolar origin of Heinrich events", Geophysical Research Letters, DOI: 10.1002/2014GL062078

http://onlinelibrary.wiley.com/doi/10.1002/2014GL062078/abstract

Abstract: "Evidence obtained from ice cores from Greenland and Antarctica indicates the presence of interactions between the polar climates, but, until recently [Wunsch, 2010], it has not been clear what the interactions are. Here, we show that analysis under the previously established hypothesis of polar synchronization potentially connects the presence and possible energy source of the Heinrich (H) events and ice-rafted debris (IRD) events. These events appear to be related to the dynamic linkage between the polar climates, as they are not revealed in analysis of the records from a single pole. The H events and IRDs discovered in the North Atlantic along with coeval Southern Ocean events [Sachs and Anderson, 2005; Whittaker et al., 2011] appear to drive or be driven by bipolar climate oscillations."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #147 on: December 14, 2014, 08:05:48 PM »
Based on paleo research the linked reference projects that nature positive feedback mechanisms will increase nitrous oxide production as global warming continues; which, will not only directly increase the Earth System Sensitivity, but indirectly will also increase the Global Warming Potential, GWP, of both methane and nitrous oxide:

Adrian Schilt, Edward J. Brook, Thomas K. Bauska, Daniel Baggenstos, Hubertus Fischer, Fortunat Joos, Vasilii V. Petrenko, Hinrich Schaefer, Jochen Schmitt, Jeffrey P. Severinghaus, Renato Spahni & Thomas F. Stocker, (11 December 2014), "Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation", Nature 516, 234–237,  doi:10.1038/nature13971

http://www.nature.com/nature/journal/v516/n7530/full/nature13971.html

Abstract: "Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources1. The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial–interglacial and millennial timescales. It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations. Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming."
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AbruptSLR

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #148 on: December 16, 2014, 01:58:12 AM »
The linked article reported on the findings from cores that indicated two carbon pulses during the PETM, with the first one smaller than the second; which raises the possibility the with strong forcing, positive feedback mechanisms (particularly from methane hydrates) may be stronger than previously thought:

http://www.nbcnews.com/science/environment/earths-future-ancient-warming-gives-ominous-peek-climate-change-n268721

Quote: "Intriguingly, Bowen and his colleagues determined that there were actually two releases of carbon into the atmosphere, one before the PETM and one shortly after it started.
And that may be a sign of scary things to come.
"One possible explanation is that the first, the smaller one, caused some climate change that triggered a second one," Bowen said. "So it's possible that the current pulse we are adding to the atmosphere may trigger unanticipated feedbacks that might lead to warming that could last hundreds of thousands of years."
That first release of carbon could have been the result of volcanism, Bowen says. And that might have caused the oceans to warm, which could have led to the melting of methane that lies in frozen deposits on the sea floor. And that could have accounted for the second pulse.
"We don't need a ton of warming for that to happen," Bowen said. "That's a little scary.""

See:
http://www.nature.com/articles/ngeo2316.epdf?referrer_access_token=8qL2xHzOIEYqtOXeHGWNHNRgN0jAjWel9jnR3ZoTv0MFms2cyCBGVzLm4qXkc0yPPRqtmlhoybdEeLtzJY_dafXV2xa9tGePtpL1D8YTJOU%3D
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Lennart van der Linde

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Re: Supporting Paleo-Evidence/Calibration for WAIS Collapse Hazard Scenarios
« Reply #149 on: December 20, 2014, 12:58:07 PM »
Nice AGU-presentation by Richard Alley on the risk of WAIS-collapse as potential/probable Dragon King event, based on growing understanding of past Heinrich events:
https://virtualoptions.agu.org/media/PP11E-02.+To+Hose+or+Not+to+HoseA+Mechanisms+of+Abrupt+Climate+Variability+I%2C+Presented+By+Richard+Alley/0_ykuvxkh3

He ends with: "I'm not positive". Meaning he's not sure, or not optimistic, or both?