A blue Arctic Ocean and a climate tipping point
In a paper soon to be published in the Geophysical Review Letters (GRL), I reported (together with some colleagues at NASA) that the Arctic perennial ice cover this year underwent a phenomenal decline. It was about 38 percent less extensive in 2007 than the long-term average and 27 percent less than the previous record low observed in 2005. The perennial ice is sea ice that survives the summer melt and consists mainly of multiyear ice floes that are the mainstay of the Arctic sea ice cover. The decline in perennial ice from 2006 to 2007 is equal if not greater than that from 1979 (when the satellite time series started) to 2006. Such a big change in a single year is thus intriguing if not puzzling. During the satellite era, the perennial ice cover went through periods of large variability and there were times when unusually low perennial ice cover in a year would be followed by unusually extensive perennial ice cover the following year (e.g., 1995 and 1996). Interannual variations in atmospheric effects are known and have been attributed to the Arctic Oscillation (AO), which was highly correlated with the ice cover until the late 1990s. The AO has since became neutral while the perennial ice stayed at low levels with a record low in 2002, another record low in 2005 and a relatively low value in 2006 preceding this year’s big anomaly.
Our analysis of ancillary surface data indicates that much of the Arctic region went through anomalously warm temperatures in winter and spring of 2007, especially in February and April. Relatively warm temperatures in winter and spring keep the ice from gaining the normal thickness that would enable ice to survive the summer melt. Moreover, there were strong southerly winds that may have carried warm air from low latitudes and caused enhanced melting and advection of ice to the north. Also, strong cyclones in the region in August may have caused ice breakup and further ice decline. In a 2002 article, published also in GRL, I predicted that we would lose the perennial ice cover within this century, using statistics based on satellite data that was then showing a nine percent per decade decline. With the perennial ice cover so low in 2007 and the rate of decline currently at 11.4 percent per decade, similar analysis would yield an earlier disappearance. The accelerated decline is likely caused by a process called the “ice-albedo feedback,” explained as follows. Because sea ice has high reflectivity (albedo) compared to that of liquid water, much of the solar heat energy gets reflected back to outer space in sea ice covered areas while such energy gets absorbed in ice free areas of the ocean. Thus, as the ice retreats, the net amount of heat absorbed by the
A question of interest is, have we reached the “tipping point” or the “point of no-return” in which the
A blue
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Joey Comiso is a senior scientist at NASA Goddard Space Flight Center. He received his BS and PhD in physics at UP and UCLA, respectively, and was the chief scientist in several NASA aircraft missions in the
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