Arctic warming bolsters summer heat waves

Sweltering summertime heat waves are on the rise across the Northern Hemisphere because of atmospheric changes brought on by Arctic warming, new research shows.

After examining 35 years of weather data, researchers spotted a decline in the strength of summer storms that carry cool, moist air across the northern continents. The sagging of these storms is the result of wind pattern changes induced by the rapidly warming Arctic, the researchers report online March 13 in Science.

Without the relief offered by these storms, the Northern Hemisphere will face longer bouts of intense summer heat, notes lead author Dim Coumou, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany.

“It’s the duration of heat waves that makes them devastating,” he says. “If you have several weeks of extremely high temperatures, then you tend to see massive damage to crops and heat-related deaths.”

The difference in temperature between the Arctic and the rest of the Northern Hemisphere powers high-altitude winds that blow west to east around the North Pole. Riding along this polar jet stream like eddies in a flowing river are the smaller, temporary wind patterns that make up storms.

While the Northern Hemisphere as a whole has heated up over the last few decades, the Arctic is warming twice as fast at lower latitudes due to the disappearance of reflective sea ice and snow cover (SN Online: 1/16/15). The rapid Arctic warming reduces the temperature disparity that drives the polar winds, weakening the jet stream.

Most previous studies investigating the impacts of the dwindling jet stream on lower-latitude weather have focused on the autumn and winter, when the Arctic Ocean warms up the overlying air. Coumou and colleagues instead focused on weather changes during the overlooked summer months. Pulling together meteorological data from 1979 through 2013, the team found changes in the atmosphere’s summer behavior.

The summertime jet stream slowed by 5 percent between 1979 and 2013, the researchers discovered. This drop caused a 10 percent decline in the energy available to power summertime storms. Climate simulations predict similar decreases in jet stream speed by the end of the century, the researchers point out.

Fewer storms dragging cool, wet air over the continents bolsters the likelihood of dangerously warm summers such as the 2003 heat wave (SN: 7/3/04, p. 10) that killed an estimated 70,000 people in Europe, Coumou says.

“We can’t say for sure that Arctic warming caused a particular heat wave,” he says, “but we have seen extreme heat waves happening more and more often lately.”

The impact of Arctic warming on summertime storms is surprisingly dramatic and unlikely to change course in the near future, says atmospheric scientist Jennifer Francis of Rutgers University in New Brunswick, N.J.

“We don’t expect the Earth to start cooling anytime soon, and certainly not the Arctic,” Francis says. “So this increasing frequency and intensity of heat waves that we’ve been seeing over both North America and Eurasia is probably only going to intensify.”