NCAR study finds permafrost threatened by rapid retreat of Arctic sea ice

Climate scientists are warning that if we do not reduce atmospheric CO2 levels to around 350ppm (down from the current 387ppm), we might witness dangerous ecological 'tipping points': events at which a slow gradual change becomes irreversible and then proceeds with gathering pace.

An example of such a tipping point would be the thawing of permafrost, which could trigger the release of large amounts of additional greenhouse gases, speeding up global warming even further. According to a new study led by the National Center for Atmospheric Research (NCAR), key conditions that could lead to the emergence of such an irreversible event are under way. It found that the rate of climate warming over northern Alaska, Canada, and Russia could more than triple during periods of rapid sea ice loss.

Lead author David Lawrence said the study suggests that, if sea-ice continues to contract rapidly over the next several years, Arctic land warming and permafrost thaw are likely to accelerate. The study, by scientists from NCAR and the National Snow and Ice Data Center, will be published Friday in Geophysical Research Letters.

Accelerated Arctic warming (click to enlarge). Simulations by global climate models show that when sea ice is in rapid decline, the rate of predicted Arctic warming over land can more than triple. The image at left shows simulated autumn temperature trends during periods of rapid sea-ice loss, which can last for 5 to 10 years. The accelerated warming signal (ranging from red to dark red) reaches nearly 1,000 miles inland. In contrast, the image at right shows the comparatively milder but still substantial warming rates associated with rising amounts of greenhouse gas in the atmosphere and moderate sea-ice retreat that is expected during the 21st century. Most other parts of the globe (in white) still experience warming, but at a lower rate of less than 1 degree Fahrenheit (0.5 Celsius) per decade. Courtesy: Steve Deyo, ©UCAR.

The research was spurred in part by events last summer, when the extent of Arctic sea ice shrank to more than 30 percent below average, setting a modern-day record. From August to October last year, air temperatures over land in the western Arctic were also unusually warm, reaching more than 4 degrees Fahrenheit (2 degrees Celsius) above the 1978-2006 average and raising the question of whether or not the unusually low sea-ice extent and warm land temperatures were related.

To investigate this question, Lawrence and his colleagues analyzed climate change simulations generated by the NCAR-based Community Climate System Model. Previous analysis of these simulations suggested that a sustained period of rapid ice loss lasting roughly 5 to 10 years can occur when the ice thins enough. During such an event, the model revealed, the minimum sea-ice extent can drop by an area greater than the size of Alaska and Colorado combined.

The team found that during episodes of rapid sea-ice loss, the rate of Arctic land warming is 3.5 times greater than the average 21st century warming rates predicted in global climate models. While this warming is largest over the ocean, the simulations suggest that it can penetrate as far as 900 miles inland. The simulations also indicate that the warming acceleration during such events is especially pronounced in autumn. The decade during which a rapid sea-ice loss event occurs could see autumn temperatures warm by as much as 9 degrees F (5 degrees C) along the Arctic coasts of Russia, Alaska, and Canada.

Lawrence and his colleagues then used the model to study the influence of accelerated warming on permafrost and found that in areas where permafrost is already at risk, such as central Alaska, a period of abrupt sea-ice loss could lead to rapid soil thaw:
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This situation, when summer thaw extends more deeply than the next winter’s freeze, can lead to a talik, which is a layer of permanently unfrozen soil sandwiched between the seasonally frozen layer above and the perennially frozen layer below. A talik allows heat to build more quickly in the soil, hastening the long-term thaw of permafrost.

Potential impacts on greenhouse gases
Arctic soils are believed to hold 30 percent or more of all the carbon stored in soils worldwide. Although researchers are uncertain what will happen to this carbon as soils warm and permafrost thaws, one possibility is that the thaw will initiate significant additional emissions of carbon dioxide or the more potent greenhouse gas, methane.

About a quarter of the Northern Hemisphere's land contains permafrost, defined as soil that remains below 32 degrees F (0 degrees C) for at least two years. Recent warming has degraded large sections of permafrost, with pockets of soil collapsing as the ice within it melts. The results include buckled highways, destabilized houses, and "drunken forests" of trees that lean at wild angles.

An important unresolved question is how the delicate balance of life in the Arctic will respond to such a rapid warming. Will we see, for example, accelerated coastal erosion, or increased methane emissions, or faster shrub encroachment into tundra regions if sea ice continues to retreat rapidly? - David Lawrence, lead author

The study sheds light on how interconnected the Arctic system is, says co-author Andrew Slater, a scientist at the National Snow and Ice Data Center (NSIDC). "The loss of sea ice can trigger widespread changes that would be felt across the region."

References:
National Center for Atmospheric Research: Permafrost Threatened by Rapid Retreat of Arctic Sea Ice, NCAR Study Finds - June 10, 2008.

David Lawrence, Andrew Slater, Robert Tomas, Marika Holland, and Clara Deser, "Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss", Geophysical Research Letters, June 13, 2008.

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