December sea ice extent 2nd lowest
The Arctic sea ice extent for December averaged 4.54 million square miles, the second lowest in the satellite record, which began in 1979, the National Snow and Ice Data Center said Jan. 3.
The December Arctic sea ice extent was 420,900 square miles below the average for 1981 to 2010 and 108,100 square miles above the December 2016 record low extent, with both the Bering and Chukchi seas below average.
NSIDC said Arctic sea ice growth in December averaged 23,100 square miles per day, fairly close to the average rate of 24,800 square miles, with ice growth in the Chukchi close to average, although very late for the Chukchi compared to previous years.
Air temperature at the 925 hPa level, some 2,500 feet above sea level, was 2 to 6 degrees Celsius, 4 to 11 degrees Fahrenheit, above average in December, NSIDC said, with one prominent warm spot over central Alaska. The pattern was similar to November, “driven in part by the arrangement of high and low air pressure regions surrounding the Arctic,” the center said.
The linear rate of decline for sea ice in December was 18,300 square miles per year, 3.7 percent per decade, NSIDC said, with record low winter sea ice extent and higher than average temperatures in the 2016-17 winter.
Assessments of both sea ice thickness and sea ice age “indicate that Arctic sea ice remains very low in overall volume,” the center said, with ice extent especially low in the Chukchi and Bering seas at year end 2017. An unusually early seasonal ice retreat in the Chukchi Sea in the summer of 2017 “likely relates to a strong inflow of oceanic heat into the region via the Bering Strait,” NSIDC said.
Chukchi, Beaufort ice-over datesThe center said the National Oceanic and Atmospheric Administration’s National Weather Service assembled a time series of ice-over dates for the Chukchi and Beaufort seas, with ice-over defined as the first day that the ice concentration exceeds 95 percent in the region.
“The trends toward later freeze up in both areas is striking,” NSIDC said. “This has an impact on sea ice thickness as the growth season is shortened, which may lead to thinner ice by the end of winter.”
Another result of later freeze up is that there is less time for snow accumulation on the sea ice. “Since sea ice grows faster for a thinner snowpack, this may partially offset the impacts of late ice formation.”
NOAA has also used maps, ship reports and other records to create monthly estimates of Arctic sea ice extent from 1850 to 2013, NSIDC said. There is limited data for the earlier part of the record, but “the carefully constructed time series helps to put the more recent satellite record in a longer-term context,” and reveals an earlier period of unusually low summer sea ice extent, compared to the 1850-2013 average, from 1937 to 1943. That unusually low summer ice extent, however, “did not extend to the winter season, and was followed by a few years of significantly higher-than-average summer ice extents.”
“Early in the record (1850 to 1900), winter sea ice extent was not particularly elevated relative to the 1850 to 2013 average, but summer sea ice extent was quite a bit higher than the average.”
- KRISTEN NELSON