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Friday, March 20, 2009

Arctic sea ice shrinkage, Arctic amplification, Greenland ice sheet mass trend

from the blog, March 19, 2009

Arctic sea ice shrinkage

Very substantial decrease in Arctic Sea ice in 2007 from 2005 and also from 1979-2000 average

Greenland Ice Sheet Mass Trend

Arctic Temperature Trend 1987-2007

Arctic shrinkage is the shrinkage of the Arctic region (as marked by isotherms), due to changes in the regional climate. Effects of Arctic shrinkage include melting permafrost, leading to Arctic methane release, a marked decrease in Arctic sea ice and the observed increase in melt on the Greenland Ice Sheet in recent years. Whilst there is a periodic variation in sea-ice extent due to the Arctic oscillation, among other varying factors, there is a long-term negative trend in recent years, attributed to global warming. (It is notable that the Arctic oscillation itself is believed to be affected strongly by global warming.)

The Arctic also stores vast quantities of methane, a potent greenhouse gas in permafrost and clathrates. Arctic methane release is accelerating rapidly due to Arctic shrinkage. This has the potential to create a runaway climate change event, which may have features in common with the Great dying (a mass extinction), and with the Paleocene-Eocene thermal maximum (an abrupt climate change event), although both of these took place in a climate much warmer than today’s. It has also been suggested that there could be a shutdown of thermohaline circulation, similar to that which is believed to have driven the Younger Dryas, an abrupt climate change event. This is now thought unlikely in the near future. There is also potentially a possibility of a more general disruption of ocean circulation, which may lead to an ocean anoxic event, although these are believed to be much more common in the distant past. It is unclear whether the appropriate pre-conditions for such an event exist today.

Computer models predict that the sea ice area will continue to shrink in the future, though there is no consensus on when the Arctic Ocean might become ice-free in summer. Current climate models frequently underestimate the rate of shrinkage. In 2007 the IPCC reported that “the projected reduction is accelerated in the Arctic, where some models project summer sea ice cover to disappear entirely in the high-emission A2 scenario in the latter part of the 21st century.? There is currently no scientific evidence that a seasonally ice-free Arctic Ocean existed anytime in the last 700,000 years, although there were periods when the Arctic was warmer than it is today. Scientists are studying possible causal factors such as direct changes resulting from the greenhouse effect as well as indirect changes such as unusual wind patterns, rising Arctic temperatures, or shifting water circulation (such as increasing inflows of warm, fresh water to the Arctic Ocean from rivers.)

According to the Intergovernmental Panel on Climate Change, “warming in the Arctic, as indicated by daily maximum and minimum temperatures, has been as great as in any other part of the world.” Reduction of the area of Arctic sea ice means less solar energy is reflected back into space, thus accelerating the reduction.

The lowest extent of sea ice was in 2007; that of 2008 was slightly greater The area covered at a particular time is what determines the albedo of the Arctic region and thus has an important effect on future warming (or cooling) trends through ice-albedo feedback.

Sea ice may reach a tipping point and then be lost. The loss of the Arctic sea ice may represent a tipping point in global warming, when ‘runaway’ climate change starts. This is due to the release of methane from permafrost and clathrates in the region, and also because of ice-albedo feedback effects. Recent research, however, has challenged the notion of an imminent Arctic sea ice tipping point.

Several specific Arctic geoengineering schemes have been proposed to reduce Arctic Shrinkage. Further, scientists such as Paul Crutzen have argued for general geoengineering proposals such as using stratospheric sulfur aerosols to be used, which will affect the Arctic if deployed in or near this region.

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