A testable model addressing the issue of East Antartica ice sheet stability?

[zankaon]

A considerable portion of E. Antarctica ice (average of 1.6 km height) rests on bedrock of 2000+ ft above sea level. https://www.physicsforums.com/showthread.php?t=344123" Would such a core suffice to cover enough glacial and inter-glacial periods; giving an adequate sample? Would any evidence of oceanic sediment, current flows, foraminifora etc. be consistent with breaching of such isthmus; hence consistent with significant sea level elevation, consistent with E. Antarctica dissolution? Would there be any other possible causes of significant sea level elevation? Might we thus have a historical record of whether or not E. Antarctica is stable; using the Isthmus of Panama as our proxy?

 

[Xnn]

There was a recent news release about the discovery of another link between declining CO2 levels and formation of the Antarctic Ice Cap 34 million years ago. They found the tipping point to be around 760 ppm. Of course, when the ice cap formed it was at lower elevation, so it will probably require higher levels of CO2 for the ice cap to become unstable.

My understanding regarding the Isthmus of Panama is that it gradually formed over millions of years and was essentially intact by about 3 million years ago. It's formation changed the flow of ocean currents, forcing warm water towards the polar regions, in particular Europe. This in turn has allowed the periodic formation and melting of large ice caps in step with orbital and greenhouse gas forcings. That is the Earth has a bi-stable climate.

Don't believe there has been a significant breaching of the Isthmus during the last 3 million years. However, the Antarctic ice cap formed before the formation of the Isthmus. So, it's future is dictated by long term greenhouse gas levels and not by the Isthmus. For example, the antarctic continent was located near the south pole during the creataceous period (65 to 145 million year ago) and yet there was no isthmus or ice cap since CO2 levels around 1500 ppm.


http://www.physorg.com/news172072921.html

A team of scientists from Cardiff, Bristol and Texas A&M universities braved the lions and hyenas of a small East African village to extract microfossils in samples of rocks which show the level of CO2 in the Earth's atmosphere at the time of the formation of the ice-cap.
Geologists have long speculated that the formation of the Antarctic ice-cap was caused by a gradually diminishing natural greenhouse effect.
The study's findings, published in Nature online, confirm that atmospheric CO2 declined during the Eocene - Oligocene climate transition and that the Antarctic ice sheet began to form when CO2 in the atmosphere reached a tipping point of around 760 parts per million (by volume).
Professor Paul Pearson from Cardiff University's School of Earth and Ocean Sciences, who led the mission to the remote East Africa village of Stakishari said: "About 34 million years ago the Earth experienced a mysterious cooling trend. Glaciers and small ice sheets developed in Antarctica, sea levels fell and temperate forests began to displace tropical-type vegetation in many areas.
"The period, known to geologists as the Eocene - Oligocene transition, culminated in the rapid development of a continental-scale ice sheet on Antarctica, which has been there ever since.
"We therefore set out to establish whether there was a substantial decline in atmospheric carbon dioxide levels as the Antarctic ice sheet began to grow."
The team mapped large expanses of bush and wilderness and pieced together the underlying local rock formations using occasional outcrops of rocks and stream beds.
Eventually they discovered sediments of the right age near a traditional African village called Stakishari. By assembling a drilling rig and extracting hundreds of meters of samples from under the ground they were able to obtain exactly the piece of Earth's history they had been searching for.
Co-author Dr Gavin Foster from the University of Bristol Earth Sciences Department said: "By using the rather unique set of samples from Tanzania and a new analytical technique that I developed, we have, for the first time, been able to reconstruct the concentration of CO2 across the Eocene-Oligocene boundary - the time period about 34 million years ago when ice sheets first started to grow on Eastern Antarctica. "
The new findings offer important lessons for the future and will add to the debate around rising CO2 levels in the Earth's atmosphere as the world's attention turns to on UN Climate Conference, which opens in Copenhagen later this year.
Co-author Dr Bridget Wade from Texas A&M University Department of Geology and Geophysics added: "This was the biggest climate switch since the extinction of the dinosaurs 65 million years ago.
"Our study is the first to provide a direct link between the establishment of an ice sheet on Antarctica and atmospheric carbon dioxide levels and therefore confirms the relationship between carbon dioxide levels in the atmosphere and global climate."

Here's a link to a paper in Nature, which also illustrates that the antarctic climate cooling was forced by falling CO2 levels as opposed to tectonic shifts.

http://www.nature.com/nature/journal/v421/n6920/abs/nature01290.html

The sudden, widespread glaciation of Antarctica and the associated shift towards colder temperatures at the Eocene/Oligocene boundary (34 million years ago) (refs 1–4) is one of the most fundamental reorganizations of global climate known in the geologic record. The glaciation of Antarctica has hitherto been thought to result from the tectonic opening of Southern Ocean gateways, which enabled the formation of the Antarctic Circumpolar Current and the subsequent thermal isolation of the Antarctic continent5. Here we simulate the glacial inception and early growth of the East Antarctic Ice Sheet using a general circulation model with coupled components for atmosphere, ocean, ice sheet and sediment, and which incorporates palaeogeography, greenhouse gas, changing orbital parameters, and varying ocean heat transport. In our model, declining Cenozoic CO2 first leads to the formation of small, highly dynamic ice caps on high Antarctic plateaux. At a later time, a CO2 threshold is crossed, initiating ice-sheet height/mass-balance feedbacks that cause the ice caps to expand rapidly with large orbital variations, eventually coalescing into a continental-scale East Antarctic Ice Sheet. According to our simulation the opening of Southern Ocean gateways plays a secondary role in this transition, relative to CO2 concentration.

 

[zankaon]

If the Isthmus of Panama were opened by significant rising sea level, then Gulf of Mexico waters might egress to Pacific. This would then seem to disrupt the Gulf Stream. Hence one could also obtain sea bed cores from south more proximal aspect of Stream to see if there has been disruption of flow. This could be compared to Panama Isthmus findings, and also could serve as a proxy for significant sea level increase from East Antarctica dissolution.

 

[mheslep]

If the Isthmus of Panama were opened by significant rising sea level, then Gulf of Mexico waters might egress to Pacific. This would then seem to disrupt the Gulf Stream. Hence one could also obtain sea bed cores from south more proximal aspect of Stream to see if there has been disruption of flow. This could be compared to Panama Isthmus findings, and also could serve as a proxy for significant sea level increase from East Antarctica dissolution.

The Isthmus is not old enough in place (~10 million years) to have seen the Eocene/Oligocene boundary and the onset of the Antarctic ice sheet. See e.g. http://www.springerlink.com/content/wr28854hr7n1535t/

 

[zankaon]

Additional background articles:

Links between climate and sea levels for the past three million years

Kurt Lambeck, Tezer M. Esat, Emma-Kate Potter

Nature 419, 199-206 12 September 2002. Insight Review http://www.nature.com/nature/journal/v419/n6903/abs/nature01089.html"

Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation

Gerald H. Haug, Ralf Tiedemann

Nature 393, 673-676 18 June 1998. http://www.nature.com/nature/journal/v393/n6686/abs/393673a0.html

 

[zankaon]

Re: A testable model addressing the issue of East Antarctica ice sheet stability?

As above: If the Isthmus of Panama were opened by significant rising sea level, then Gulf of Mexico waters might egress to Pacific. This would then seem to disrupt the Gulf Stream. Hence one could also obtain sea bed cores from south more proximal aspect of Stream to see if there has been disruption of flow. This could be compared to Panama Isthmus findings, and also could serve as a proxy for significant sea level increase from East Antarctica dissolution.

For record of foraminifera for < 3 million years, one has oscillations of glaciation cycles. This approximately corresponds to geological closing of Isthmus of Panama at ~3 M yrs. http://www.nature.com/nature/journal...ture01089.html" Thus a history of cycles of N. ice sheets would seem consistent with persistence of Gulf Stream; and hence consistent with persistence of closure of Isthmus of Panama. Such persistence of Isthmus of Panama (84 ft elevation) would seem consistent with no sea surface elevation of ~180 ft. Hence cycles of glaciation, persistence of Gulf Stream, would seem consistent with no complete melting or dissolution of East Antarctica. Thus via indirect evidence, East Antarctica would seem to be historically stable.

 

[Xnn]

The Antarctic ice sheet began to form 34 million years ago when CO2 in the atmosphere reached a tipping point of around 760 parts per million.

So, as long as CO2 levels remain below that level, the Ice Sheet should be stable.

 

[Andre]

There were also assumed to be no ice sheets in the Paleocene when fossil leaf stomata indicated similar CO2 levels with nowadays http://www.sciencemag.org/cgi/content/abstract/292/5525/2310?ck=nck.

 

[Skyhunter]

There were also assumed to be no ice sheets in the Paleocene when fossil leaf stomata indicated similar CO2 levels with nowadays http://www.sciencemag.org/cgi/content/abstract/292/5525/2310?ck=nck.

The Paleocene covers 350 million years. [edit] My mistake I am looking at the Paleozoic. [/edit]

During the Carboniferous, so named for the enormous amount of carbon sequestered, CO₂ levels did drop to those of the present day, and global temperatures were ~12C on average.

http://www.geocraft.com/WVFossils/Carboniferous_climate.html

 

[Skyhunter]

Royer 2001 conclusion.

If our low SI-based temperature predictions are correct, additional factors such as paleogeography, enhanced meridional heat transport, and high latitude vegetation feedbacks are required to explain this warm period,

Here is a paper that outlines one possible factor.

http://www.nature.com/nature/journal/v439/n7072/full/nature04386.html

An exceptional analogue for the study of the causes and consequences of global warming occurs at the Palaeocene/Eocene Thermal Maximum, 55 million years ago. A rapid rise of global temperatures during this event accompanied turnovers in both marine and terrestrial biota as well as significant changes in ocean chemistry and circulation. Here we present evidence for an abrupt shift in deep-ocean circulation using carbon isotope records from fourteen sites. These records indicate that deep-ocean circulation patterns changed from Southern Hemisphere overturning to Northern Hemisphere overturning at the start of the Palaeocene/Eocene Thermal Maximum. This shift in the location of deep-water formation persisted for at least 40,000 years, but eventually recovered to original circulation patterns. These results corroborate climate model inferences that a shift in deep-ocean circulation would deliver relatively warmer waters to the deep sea, thus producing further warming9. Greenhouse conditions can thus initiate abrupt deep-ocean circulation changes in less than a few thousand years, but may have lasting effects; in this case taking 100,000 years to revert to background conditions.

 

[Xnn]

That's interesting.

Greenhouse conditions can thus initiate abrupt deep-ocean circulation changes in less than a few thousand years, but may have lasting effects; in this case taking 100,000 years to revert to background conditions.

So, if we start to witness changes in deep-ocean circulation, then we may be in for a whole lot more changes than anticipitated.