Thinning ice leads to winter warming in the Arctic

Even when the Arctic goes dark and cold, thinning ice could keep the North Pole from cooling off.

The loss of insulating ice between the ocean and atmosphere increases the amount of heat-trapping water vapor and clouds in the Arctic air. That extra moisture keeps air temperatures relatively warm during fall and winter and melts even more ice, new climate simulations suggest. This self-reinforcing cycle could partially explain why Arctic warming has outpaced the global average over recent decades, researchers report online November 11 in the Journal of Climate.

The heat trapped by the extra moisture is about three times as much as is gained during summer when reflective sea ice gives way to dark, light-absorbing open ocean, the researchers estimate. Arctic temperatures, therefore, will continue rapidly rising “long after the summer ice is out of business,” says coauthor David Randall, an atmospheric scientist at Colorado State University in Fort Collins.

While the planet as a whole has warmed over the last few decades, the Arctic is heating up roughly twice as fast as lower latitudes (SN Online: 1/16/15). This warming imbalance weakens the winds that circle the top of the globe, affecting climates throughout the Northern Hemisphere. For instance, Arctic warming exacerbates summer heat waves in Europe (SN: 4/18/15, p. 13).

Climate scientists have blamed some of the Arctic’s rapid warming on increased moisture hovering above the Arctic Ocean. Where that water originated from is a point of contention, with some scientists arguing that moist air is blown in from the tropics.

Randall and colleagues analyzed a computer simulation of Earth’s climate for the next three centuries, keeping a close eye on changes in the Arctic air. They found that during autumn and winter, Arctic seawater is warmer than the overlying air. Under normal climate conditions, sea ice forms a boundary between the water and air. However, as the ice thins, the relatively warm seawater can heat up the overlying air, the team found.

Because warmer air can hold more moisture, the amount of water vapor above the Arctic Ocean nearly doubled during the simulation. This water vapor acts like a blanket, preventing some heat (in the form of infrared radiation) from escaping into space. The end result is that more heat stays close to the Arctic surface and melts more ice, forming a continuous cycle that keeps temperatures relatively warm.

Warming Arctic air does more than just hold more moisture, the researchers say. The simulation predicted that winter temperatures over the Arctic Ocean will eventually become warmer than those over the surrounding continents. This temperature difference would create an air pressure contrast between land and sea that generates low-altitude winds similar to those seen during winter monsoons in the tropics, though “kind of pathetic” in comparison, Randall says. Those winds whip seawater into the air that traps additional heat, the researchers propose.

The work “demonstrates that it’s really the Arctic that’s causing the Arctic to warm so fast,” says atmospheric scientist Jennifer Francis of Rutgers University in New Brunswick, N.J. “There’s a lot more going on up there than just this [summer sea ice melting] that we’ve been hearing about for so long.”