“This albedo effect is really translated into melting of sea ice in the summer and early fall,” says Chylek. “Because of this evaporation of water, and because a larger area of water is open at that time, we get this water vapor into the atmosphere and formation of low-level clouds.” And clouds, as it turns out, also play a role in boosting temperatures. They bounce some of the sun’s radiation back into space, sure, but they also absorb some of it, like insulation. The low-level clouds stick around through the winter, trapping heat against the landscape. So even though in some parts of the Arctic the sun doesn’t come out at all during the depths of winter, warmer summers and autumns prime the coldest months to get hotter.
All the extra warmth of summer is also getting trapped in the Arctic Ocean, then released throughout the winter. “The greatest warming in the Arctic is happening in winter, which maybe surprises people, because you have the greatest sea ice melt in summertime,” says Hahn, of the University of Washington. “That’s when you have incoming sunlight. But the idea is that there’s seasonal ocean heat storage.” It’s like a giant radiator that warms up a room even after it’s been turned off.
Simultaneously, storms have been transporting moisture from lower latitudes into the Arctic, further encouraging the development of clouds. And injections of warmer water from the south, brought north by ocean currents, further melt sea ice. “As it melts, water evaporates and increases atmospheric humidity, which causes an increase in cloudiness in winter, and we have infrared radiation coming from these clouds to the surface,” says Chylek. “This is one feedback loop that can cause increased Arctic temperature, and we believe that it is one of the reasons why we see this increase in temperature around 2000.”
University of Washington climate scientist Cecilia Bitz, who studies Arctic amplification but wasn’t involved in the new research, points out that there has been a lag in how areas at high latitudes have responded to greenhouse gases, compared to the rest of the planet. It’s taken time for sea ice to melt, but now that it’s doing so, the heat feedback loop in the Arctic has worsened, and the rate of change has become much more noticeable. “The tropics warmed faster first, and now the poles are catching up, and that’s why you see a trend,” she says.
The consequences are already massive and far-reaching. First and foremost, more melting—particularly in Greenland, which is losing a quarter trillion tons of ice each year—means higher sea levels. Plus, warmer waters get physically bigger, a phenomenon known as thermal expansion, further raising sea levels.
The landscape is also suffering literal and metaphorical upheaval. Warming temperatures are thawing frozen soil known as permafrost. When that permafrost loses water, it collapses, dragging down any infrastructure in or above it, like pipelines, roads, and buildings. “There are people in the Arctic,” says Bitz. “They did very little to deserve to live in this environment that is dangerous.”