This year, Arctic sea ice shrank to its second-lowest extent in more than 40 years of satellite measurements. On 15 September, ice covered just 3.74 million square kilometres of Arctic waters at its annual summer minimum. In only one other year — 2012 — has the annual minimum Arctic sea-ice cover dropped below 4 million square kilometres (see ‘Thin ice’).
“In my lifetime, the sea ice at the end of summer has decreased by 50%,” said Cecilia Bitz, an atmospheric scientist at the University of Washington in Seattle, in a statement.
As global temperatures rise, the minimum Arctic sea-ice extent has shrunk by an average of 13.4% per decade since 1979. The ice that remains is often thinner and more fragile than before, making it more vulnerable to melting the following year.
This year’s near-record low — reported by the US National Snow and Ice Data Center (NSIDC) in Boulder, Colorado on 21 September — got its start last winter, when winds blowing offshore along Russia’s northern coast allowed only thin sea ice to form there; much of this ice melted away when spring came. And from May to August, Siberian heatwaves caused strong melting in the Kara and Laptev seas. The ambitious MOSAiC research expedition, which is examining the Arctic climate from the German icebreaker Polarstern, reached the North Pole on 19 August, in lighter-than-usual ice conditions.
The final blow came near the end of the melting season, in early September, when air temperatures over north-central Siberia soared as much as 6 ºC above normal. Some of this warm air leaked north, leading to further ice loss. For six days starting on 31 August, nearly 80,000 square kilometres of sea ice melted each day, a record loss rate for this period.
North of islands such as the Norwegian archipelago Svalbard, winds pushed the edge of the sea ice back as far as 85º north, the most extreme latitude seen in the era of satellite measurement.
Shrinking sea ice marks part of an overall transition to a new Arctic climate, researchers reported this month1. Warmer air temperatures are already causing a greater proportion of Arctic precipitation to fall as rain rather than snow — meaning a warmer and slushier environment than Arctic ecosystems are used to.
“In the Arctic, weather that used to be considered extreme is becoming the norm,” the NSIDC wrote in its report. “The summer of 2020 is clearly representative of this new Arctic.”