| April 6, 2010 |  |
Arctic Sea Ice is Shrinking, Despite Contrarian Claims
In what is becoming an annual occurrence, climate science contrarians recently pointed to routine seasonal freezing as "evidence" that Arctic sea ice is growing. Long-term trends, however, show that Arctic sea ice, which plays a key role in moderating the planet's temperature, is shrinking.
In particular, contrarians are citing the "extent" of Arctic sea ice, a measure of the area encompassed by the furthest edges of ice coverage. Indeed, sea ice extent increases every winter as temperatures drop and water turns into ice. Extent usually peaks in late February or March. The ice melts away during the Arctic summer and typically reaches its lowest extent in September.
However, over at least the last six years, the maximum extent of sea ice has remained well below the 1979 to 2000 average extent, both in the winter and the summer. This past winter is no exception. Sea ice extent is still below average, despite a cold spring.
It's also worth noting that the medium-term Arctic Oscillation (similar to the more familiar Pacific El Nino-La Nina cycle) is contributing to colder weather this year. The Arctic Oscillation operates on a time scale of between 10 and 40 years. According to the National Snow and Ice Data Center, the Arctic Oscillation contributed to this spring having the latest date for the maximum Arctic sea ice extent since satellite records started in 1979.
A better measure of the stability and health of Arctic sea ice than area is the sea ice thickness and its persistence over time. The new 2010 melt season is about to begin with more thin ice than usual. Thin ice likely will melt quickly in the summer.
According to the 2009 Copenhagen Diagnosis report (pdf) from the United Nations Environment Programme, "The thickness of Arctic sea ice has also been on a steady decline over the last several decades. For example, Lindsay et al. (2009) estimated that the September sea ice thickness has been decreasing at a rate of 57 centimeters per decade since 1987. Similar decreases in sea-ice thickness have been detected in winter. For example, within the area covered by submarine sonar measurements, Kwok and Rothrock (2009) show that the overall mean winter thickness of 3.64 meters in 1980 decreased to only 1.89 meters by 2008—a net decrease of 1.75 meters, or 48 percent. By the end of February 2009, less than 10 percent of Arctic sea ice was more than two years old, down from the historic values of 30 percent."
The National Snow and Ice Data Center also has tracked the decline in multiyear Arctic sea ice.
It would be great news if the Arctic were not so vulnerable to climate change. Because Arctic sea ice reflects sunlight back into space, it acts as a giant air conditioner for the planet. Losing Arctic sea ice is one of several changes brought about by global warming that can further exacerbate warming and other shifts in the climate system. Climate projections that assume a high level of warming are in part based on the assumption that such "enhancing feedbacks" will boost temperatures above what would be expected from burning fossil fuels and destroying forests.
This season's annual winter increase in sea ice extent is not evidence of an Arctic recovery. The National Snow and Ice Data Center concludes that "a true recovery would continue over a longer time period than two years…but also [a] return to [sea ice extent] within the range of natural variation. In a recovery, scientists would also expect to see a return to an Arctic sea ice cover dominated by thicker, multiyear ice."
Unfortunately, we're not seeing that.
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