Effects of Arctic shrinkage include a marked decrease in Arctic sea ice; melting permafrost, leading to the release of methane, a potent greenhouse gas; the release of methane from clathrates, leading to longer time-scale methane release; the observed increase in melt on the Greenland Ice Sheet in recent years; and potential changes in patterns of ocean circulation. Scientists worry that some of these effects may cause positive feedbacks which could accelerate the rate of global warming. Current research shows arctic sea ice cover is shrinking somewhat more rapidly than predicted by the IPCC, while recent atmospheric methanes levels show little increase, and current atmospheric methane concentrations are far below IPCC projections.

The sea ice in the Arctic region is in itself important in maintaining global climate due to its albedo (reflectivity). Melting of this sea ice will therefore exacerbate global warming due to positive feedback effects, where warming creates more warming by increased solar absorption. An important feedback in the Arctic currently is ice-albedo feedback.

April 3, 2007, the National Wildlife Federation urged the U.S. Congress to place polar bears under the Endangered Species Act. Four months later, the United States Geological Survey completed a year-long study which concluded in part that the floating Arctic sea ice will continue its rapid shrinkage over the next 50 years, consequently wiping out much of the polar bear habitat. The bears would disappear from Alaska, but would continue to exist in the Canadian Arctic Archipelago and areas off the northern Greenland coast. Secondary ecological effects are also resultant from the shrinkage of sea ice; for example, Polar Bears are denied their historic length of seal hunting season due to late formation and early thaw of pack ice.

Sea ice loss has melting effects on permafrost, both in the sea, and on land and consequential effects on methane release, and wildlife. Some studies imply a direct link, as they predict cold air passing over ice is replaced by warm air passing over the sea. This warm air carries heat to the permafrost around the Arctic, and melts it. This thawing of the permafrost might accelerate methane release from areas like Siberia.

Sea ice serves to stabilise methane deposits on and near the shoreline, preventing the clathrate breaking down and outgassing methane into the atmosphere. Any methane released to the atmosphere will then causing further warming. This positive feedback, known as the clathrate gun effect, has the potential to lead to runaway climate change

Greenland's ice sheet contains enough fresh water as ice to raise sea level worldwide by 7 metres (23 ft). Models predict a sea-level contribution of about 5 centimetres (2 in) from melting in Greenland during the 21st century. It is also predicted that Greenland will become warm enough by 2100 to begin an almost complete melt during the next 1,000 years or more. It is viewed by some scientists that wholly inadequate attention is being given to this issue.

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.

The phenomenon of Arctic shrinkage is leading some scientists to fear that a runaway climate change event may be imminent, and may even have started. Estimates of the size of the total carbon reservoir in Arctic permafrost and clathrates vary widely. It is suggested that is at least 900 gigatonnes of carbon in permafrost worldwide. Further, there are believed to be around and another 400 gigatonnes of carbon in methane clathrates in permafrost regions alone. However, Buffett and Archer predict a much higher release of between 2,000 and 4,000 gigatonnes as a result of expected human-induced warming, as they include some deep-ocean clathrate stores in the expected release albeit over a millennial timescale. Should the more conservative estimate be correct or at least too low, then 1300 gigatonnes of carbon may potentially be released as methane from permafrost and clathrates as a result of human activity. As methane is a potent greenhouse gas, this Arctic methane release will have a very severe impact on global warming

Geological records indicate that runaway climate change is not unprecedented. It has been speculated that the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum were caused by such a runaway effect.

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For more information (background, pictures, experiments and references): Global Warming, Arctic Shrinkage