Evidence of Arctic Warming


Comiso’s new study presents some striking trends. When compared to longer- term, ground-based surface temperature data, the rate of warming in the Arctic from 1981 to 2001 is eight times larger than the rate of Arctic warming over the last 100 years. There have also been some remarkable seasonal changes. Arctic spring, summer, and autumn have each warmed, lengthening the seasons when sea ice melts by 10 to 17 days per decade. Temperatures increased on average by almost one and a quarter (1.22) degrees Celsius (C) per decade over sea ice in the Arctic summer. Conversely, Arctic winters cooled from the 1980s to the 1990s. The study finds that winters were almost 1 (0.89) degree C cooler per decade.

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Graph of Arctic Temperature Trends

The summer warming and longer sea ice melt season appear to be affecting the volume and extent of perennial sea ice, the study suggests. Also, a longer melt season could in turn cause thinner sea ice at the end of winter, making it even more susceptible to early thawing. Winter cooling, on the other hand, caused sea ice to actually advance in the Bering Sea, Greenland Sea, and Baffin Bay.

  The rapid warming trend in the Arctic over the last 25 years has dramatically reduced the region’s sea ice extent. Comparing this more recent trend with long-term data, scientists are trying to determine to whether this 25-year warming trend will continue, or is part of a longer-term cycle of ups and downs. (Graph by Larry Stock and Josefino Comiso, NASA GSFC)
Graph of Melt Period in Arctic Regions

Researchers observed varied temperatures across different regions within and near the Arctic Circle. Average temperature trends increased by one-third of 1 degree C per decade over sea ice, and they also rose half a degree C per decade over the lands of Eurasia. Temperatures over North America experienced the highest regional warming, increasing by 1.06 degrees C per decade. Greenland cooled by less than one-tenth of a degree C per decade. The cooling found over Greenland was mainly at high elevations, while warming trends were observed around its periphery. These results are consistent with a National Snow and Ice Data Center study that found record loss of sea ice around Greenland’s periphery in 2002.

The surface temperature records from 1981 to 2001 were based on thermal infrared data from the Advanced Very High Resolution Radiometer (AVHRR) onboard NOAA satellites. The AVHRR sensor detects radiation in the visible and thermal infrared range of the electromagnetic spectrum. These satellite temperature data were compared with accurate ground-based data taken during the one-year-long Arctic ice station project called the Surface Heat Budget in the Arctic (SHEBA) from 1997 to 1998. Comiso found that the two datasets are extremely consistent with each other.

“This study is unique in that previously, similar studies made use of data from very few points scattered in various parts of the Arctic region,” says Comiso. “These results show the large regional and seasonal differences in the trends that only satellite data can provide.”

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This graph charts the number of days per year in which air temperature was warm enough to melt ice on the surface. (Multiply the number by 36.5 to determine the melt period for a given year.) Since 1980, the melt period has been growing longer in the Northern Hemisphere. Scientists suspect global warming is the reason. (Graph adapted by Robert Simmon from Larry Stock and Josefino Comiso, NASA GSFC)