As the season for Arctic sea ice melt comes to a close, no new record has been set. Even so, the minimum extent, which it has either reached or will reach in a matter of days, is still the second lowest on the books since satellites began tracking the ice.
“It’s roughly tied for second lowest with 2007 – actually very slightly lower than 2007, but likely within the margin of error,” said Walt Meier, a research scientist and sea ice specialist at NASA’s Goddard Space Flight Center in Greenbelt, MD. “It could still drop further, though at this point that seems unlikely.”
Sea ice extent – the area of the Arctic Ocean with at least 15 percent sea ice – has been tracked by satellites since 1979. The record low during this time was in September 2012.
On September 10, 4.137 million square km (1.59 million square miles) of sea ice remained in the Arctic Ocean, according to the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado. The 2007 minimum reached 4.154 million square km (1.6 million square miles) on September 18 that year.
This year’s minimum is roughly the same level seen during the past decade, but lower than what was seen prior to 2000, said Rasmus Tonboe, a sea ice remote sensing expert at the Danish Meteorological Institute. “It has found a new lower level.”
A warm winter helped thin the ice and contributed to its early breakup in the Beaufort and Kara seas. “The first-year ice was very thin at the beginning of the melt season, making it vulnerable to rapid loss,” said Christian Haas, an Arctic sea ice geophysicist at York University in Toronto.
Cool, cloudy conditions prevailed during the summer, slowing down the ice melt and reducing the likelihood of a record. But the melt sped up again in late August and early September, allowing it to “catch up to 2007,” said Meier. Two major storms also rolled through the region in August, breaking apart the ice and encouraging its melt.
“Interestingly, we got all the way to second-lowest this year even though the summer weather pattern was cool, cloudy and stormy, which should have limited the summer ice loss,” said Mark Serreze, director of the NSIDC.
The early spring melt also meant that Arctic waters took in a lot of heat this year. “The water had a long time to warm up,” said Tonboe. The sea surface temperature in many parts of the ice-free Arctic Ocean and its adjacent seas was more than 4C (7.6F) warmer than normal in early September, Tonboe wrote on the DMI website (in Danish). The late summer warm water will slow the formation of the winter ice, and leave it thinner and less resilient to future melting.
“It may be that the ice is so thin now that it does not matter so much anymore what weather patterns we get. It may also be that there was a lot of heat in the upper ocean that hastened the melt,” said Serreze. “We’ll know more in the coming months after we analyze all the data.”
Scientists would like to have better data on Arctic sea ice thickness. Since 1979, satellites have provided information about sea ice extent, but it is far more difficult to get sea ice thickness measurements. Most have come from submarines or on-ice missions that have drilled holes into the ice. More recently, thickness measurements have been done from airplanes and the European Space Agency’s CryoSat-2.
“These flights are very expensive and we don’t have mechanisms to fund them extensively,” said Haas, who carried out an electromagnetic sounding measurement of the ice thickness over the Beaufort Sea and Northwest Passage in April, where he found first-year ice was less than a meter thick – half what it was in 2015.
And scientists remain unsatisfied with the thickness and volume data satellites are providing. “Satellite missions such as CryoSat are still struggling with large uncertainties and biases, and the parameters, which are needed for reducing the biases, are not available,” said Tonboe.
But that may change in 2018, when NASA is scheduled to launch ICESat-2. The satellite will use lasers to measure the height of the surface relative to the water and scientists will calculate its thickness (roughly 90 percent of the sea ice lies below the surface).
“Our ability to assess thickness is, and will be, improving,” said Serreze.