A hot time in the Arctic
In late February, a large portion of the Arctic Ocean near the North Pole experienced an alarming string of extremely warm winter days, with the surface temperature exceeding 25 degrees above normal.
These conditions capped nearly three months of unusually warm weather in a region that has seen temperatures rising over the past century as greenhouse gas concentrations (mostly carbon dioxide and methane) have increased in the atmosphere. At the same time, the extent of frozen seawater floating in the Arctic Ocean reached new lows in January and February in 40 years of satellite monitoring.
In recent years, the air at the Arctic Ocean surface during winter has warmed by over 5 degrees above normal. So was this recent spate of warm weather linked to longer-term climate change, or was it, well, just the weather?
What we can say is this: Weather patterns that generate extreme warm Arctic days are now occurring in combination with a warming climate, which makes extremes more likely and more severe. What’s more, these extreme temperatures have had a profound influence on sea ice, which has become thinner and smaller in extent, enabling ships to venture more often and deeper into the Arctic.
This sea ice loss, in turn, has created a feedback loop. Thinner, receding ice allows the ocean’s heat to escape more readily into the atmosphere, which the Arctic’s highly stable lower atmosphere traps near the surface during the polar night. As a result, winter surface air temperatures have warmed more in the Arctic compared with the global average.
This warming is posing major threats to coastal communities and wildlife in the Arctic. Thinner sea ice is more mobile and deforms more easily from winds and currents. In February, southerly surface winds blew sea ice away from the northern shore of Greenland, causing the largest expanse of open ocean in winter ever recorded by satellites. Similarly, the Bering Sea experienced extreme sea ice retreat this winter. With more open ocean, major wind storms generate large waves that cause damaging coastal erosion, endangering oceanside towns.
A warmer Arctic also means that Greenland’s land mass sheds its vast blanket of ice faster, currently at about 70 trillion gallons of water per year, which contributes to rising sea levels.
The extremely warm Arctic days occurred at about the same time as an atmospheric pattern known as sudden stratospheric warming dominated the Northern Hemisphere. During a sudden stratospheric warming, the air temperature rapidly rises by at least 45 degrees at altitudes above where transcontinental aircraft fly, at roughly 30,000 to 150,000 feet.
The sudden stratospheric warming in February caused a major disruption to the atmospheric jet stream of the Northern Hemisphere — the strong westerly winds that encircle the Arctic — by driving the jet stream to teeter and temporarily slow. Without westerly winds to transport weather from west to east, cold air streamed in over Europe and parts of North America from the northeast. As a result, Europe was freezing while the Arctic was extremely warm.
We don’t know yet whether the sudden stratospheric warming pattern drove warming in the Arctic this year and whether climate change influenced this weather pattern. But we do know from paleoclimate reconstructions that Arctic warming rates in the industrial era are happening faster than at any time in the past 12,000 years and that the Arctic’s sea ice decline is now greater than at any time in at least the past 1,450 years. Recent warming and ice loss are inevitably linked to climate change caused by human activity. But more research is needed to understand events this winter because the conditions were so unusual.
Both the sudden stratospheric warming and the Arctic warming were forecast with outstanding accuracy in timing and magnitude about two weeks in advance, which allowed parts of Europe hit by the unusually frigid weather plenty of time to prepare. American scientists are now developing new computer models to improve forecasts so that they go beyond two weeks by taking into account ocean temperature, circulation and sea ice cover.
Preparing for future shifts in weather extremes also requires a better understanding of how the climate is changing. This will require long-term government investment in surface-based and satellite observations, and in the continued development of new computer models for improved predictions. Even as funding for climate change research has become highly politicized and is under threat, record numbers of talented students are applying at our universities to do graduate work in climate science. Given enough support, they could improve our future by helping us prepare for it.
The extreme Arctic warming this winter is a foreshadowing of things to come. On our current greenhouse emissions trajectories, the Arctic Ocean is expected to be icefree in late summer by about midcentury or possibly as early as 2030, depending on natural variability. The impact will extend beyond the Arctic, adding to warming and sea level rise throughout the Northern Hemisphere.