Model shows polar ice caps can recover from warmer climate-induced melting

[OKIsItJustMe]

http://www.washington.edu/news/articles/model-shows-polar-ice-caps-can-recover-from-warmer-climate-induced-melting

Aug. 17, 2011

Model shows polar ice caps can recover from warmer climate-induced melting

By Vince Stricherz
News and Information

A growing body of recent research indicates that, in Earth’s warming climate, there is no “tipping point,” or threshold warm temperature, beyond which polar sea ice cannot recover if temperatures come back down. New University of Washington research indicates that even if Earth warmed enough to melt all polar sea ice, the ice could recover if the planet cooled again.

In recent years scientists have closely monitored the shrinking area of the Arctic covered by sea ice in warmer summer months, a development that has created new shipping lanes but also raised concerns about humans living in the region and the survival of species such as polar bears.

In the new research, scientists used one of two computer-generated global climate models that accurately reflect the rate of sea-ice loss under current climate conditions, a model so sensitive to warming that it projects the complete loss of September Arctic sea ice by the middle of this century.

However, the model takes several more centuries of warming to completely lose winter sea ice, and doing so required carbon dioxide levels to be gradually raised to a level nearly nine times greater than today. When the model’s carbon dioxide levels then were gradually reduced, temperatures slowly came down and the sea ice eventually returned.

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http://www.agu.org/journals/pip/gl/2011GL048739-pip.pdf

The reversibility of sea ice loss in a state-of-the-art climate model

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Rapid Arctic sea ice retreat has fueled speculation about the possibility of threshold (or ‘tipping point’) behavior and irreversible loss of the sea ice cover. We test sea ice reversibility within a state-of-the-art atmosphere–ocean global climate model by increasing atmospheric carbon dioxide until the Arctic Ocean becomes ice-free throughout the year and subsequently decreasing it until the initial ice cover returns. Evidence for irreversibility in the form of hysteresis outside the envelope of natural variability is explored for the loss of summer and winter ice in both hemispheres. We find no evidence of irreversibility or multiple ice-cover states over the full range of simulated sea ice conditions between the modern climate and that with an annually ice-free Arctic Ocean. Summer sea ice area recovers as hemispheric temperature cools along a trajectory that is indistinguishable from the trajectory of summer sea ice loss, while the recovery of winter ice area appears to be slowed due to the long response times of the ocean near the modern winter ice edge. The results are discussed in the context of previous studies that assess the plausibility of sea ice tipping points by other methods. The findings serve as evidence against the existence of threshold behavior in the summer or winter ice cover in either hemisphere.

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[JDPriestly]

1. if the planet cooled again

That is a big "if."

[kestrel91316]

2. Indeed. And with ever-increasing levels of CO2, one is hard-pressed to explain

exactly how such cooling could happen.

The greenhouse effect is not fantasy.

[OKIsItJustMe]

3. They’re not saying that it’s fantasy

http://www.washington.edu/news/articles/model-shows-polar-ice-caps-can-recover-from-warmer-climate-induced-melting

…

While the lack of a “tipping point” could be considered good news, she said, the increasing greenhouse gases leave plenty of room for concern.

“Climate change doesn’t have to exhibit exotic phenomena to be dangerous,” Bitz said, adding that while sea ice loss can have some positive effects, it is proving harmful to species such as polar bears that live on the ice and to some people who have been forced to relocate entire villages.

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[NickB79]

4. Their model required 9X today's atmospheric CO2 concentration

Funny, because while this sounds impossible, it's just about what we'll see if the permafrost and some of the methane hydrate fields thaw and collapse when you factor in the massive warming effects of methane compared to lowly CO2.

[OKIsItJustMe]

5. This is a climate model

Physical evidence suggests otherwise:
http://newsroom.ucla.edu/portal/ucla/last-time-carbon-dioxide-levels-111074.aspx

Last time carbon dioxide levels were this high: 15 million years ago, scientists report

By Stuart Wolpert October 08, 2009

You would have to go back at least 15 million years to find carbon dioxide levels on Earth as high as they are today, a UCLA scientist and colleagues report Oct. 8 in the online edition of the journal http://www.sciencemag.org/sciencexpress/">Science.

"The last time carbon dioxide levels were apparently as high as they are today — and were sustained at those levels — global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland," said the paper's lead author, Aradhna Tripati, a UCLA assistant professor in the department of Earth and space sciences and the department of atmospheric and oceanic sciences.

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"We are able, for the first time, to accurately reproduce the ice-core record for the last 800,000 years — the record of atmospheric C02 based on measurements of carbon dioxide in gas bubbles in ice," Tripati said. "This suggests that the technique we are using is valid.

"We then applied this technique to study the history of carbon dioxide from 800,000 years ago to 20 million years ago," she said. "We report evidence for a very close coupling between carbon dioxide levels and climate. When there is evidence for the growth of a large ice sheet on Antarctica or on Greenland or the growth of sea ice in the Arctic Ocean, we see evidence for a dramatic change in carbon dioxide levels over the last 20 million years.

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http://dx.doi.org/10.1126/science.1178296

Published Online October 8 2009
Science 4 December 2009:
Vol. 326 no. 5958 pp. 1394-1397
DOI: 10.1126/science.1178296

Report

Coupling of CO₂ and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years

Abstract

The carbon dioxide (CO₂) content of the atmosphere has varied cyclically between ~180 and ~280 parts per million by volume over the past 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, the partial pressure of CO₂ (pCO₂) is much less certain, and the relation between pCO₂ and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO₂ during major climate transitions of the past 20 million years. During the Middle Miocene, when temperatures were ~3° to 6°C warmer and sea level was 25 to 40 meters higher than at present, pCO₂ appears to have been similar to modern levels. Decreases in pCO₂ were apparently synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago) and Late Pliocene (~3.3 to 2.4 million years ago).

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These results provide some constraints on pCO₂ thresholds for the advance and retreat of continental ice sheets in the past, which is also relevant in the context of anthropogenic climate change because it is uncertain how continental ice sheets will respond over the coming centuries to increased levels of pCO₂. By comparing our reconstruction to the published data sets described above, we are able to estimate past thresholds for the buildup of ice in different regions. When pCO₂ levels were last similar to modern values (that is, greater than 350 to 400 ppmv), there was little glacial ice on land or sea ice in the Arctic, and a marine-based ice mass on Antarctica was not viable. A sea ice cap on the Arctic Ocean and a large permanent ice sheet were maintained on East Antarctica when pCO₂ values fell below this threshold. Lower levels were necessary for the growth of large ice masses on West Antarctica (~250 to 300 ppmv) and Greenland (~220 to 260 ppmv). These values are lower than those indicated by a recent modeling study, which suggested that the threshold on East Antarctica may have been three times greater than in the Northern Hemisphere.

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