Fewer cold snaps in the forecast

Bitter bouts of unusually cold days will become less frequent in much of the Northern Hemisphere as the planet warms, two new studies suggest.

Some scientists had suggested that winter cold snaps would become more common in the future as Arctic warming caused frigid polar winds to waver and dip southward. But that wind waviness is minor compared with other expected climate changes, researchers report in the March issue of the Journal of Climate. Using climate simulations, they found that the shrinking temperature difference between the Arctic and lower latitudes will stabilize temperatures in the hemisphere.

In a separate study published online March 30 in Environmental Research Letters, researchers estimate that the frequency of cold snaps will actually decrease by about 20 percent by the 2030s.

“Cold spells are not caused by global warming,” says climate scientist Tapio Schneider of ETH Zurich, lead author of the Journal of Climate paper. “What these people hypothesized is just not backed up in the data.”

Cold snaps can be just as dangerous as their heat wave counterparts. An early 2012 cold snap killed more than 800 people in Central and Eastern Europe. Such cold spells can happen when the winds that flow around the North Pole wobble southward and blow frosty Arctic air into the lower latitudes. These winds are powered by the difference in temperature between the Arctic and the rest of the hemisphere.

Over the last few decades, the Arctic has warmed nearly twice as fast as lower latitudes due to the disappearance of sunlight-reflecting ice and snow (SN Online: 1/16/15). With the temperature difference shrinking, the polar winds have slowed and become more prone to southward dips. This wind change led some scientists to predict that cold snaps would become more common as the Arctic continued warming.

But the shrinking temperature difference also reduces the variation in day-to-day temperatures, Schneider says. Using climate simulations, Schneider and colleagues investigated how the rapidly warming Arctic affected the number of extremely cold days throughout the Northern Hemisphere. As the Arctic became less cold relative to the rest of the hemisphere, temperatures throughout the hemisphere deviated less from the average temperature. This temperature stabilization reduced the frequency of cold snaps in the simulations more than the changing polar winds increased it.

In the other study, climate scientist Ruby Leung of the Pacific Northwest National Laboratory in Richland, Wash., and colleagues examined simulations of future global climate. They found that while cold snaps became less frequent, the extremely cold days didn’t disappear altogether. The top five most extreme cold outbreaks under current climate conditions would still occur in a warmed-up world, Leung says.

“The frequency may be reduced, yes, but if people aren’t accustomed to cold outbreaks, then they might not be as prepared when one does happen and the impacts could be more severe,” she says.

While a decrease in cold snap frequency makes sense, an outstanding question is whether the duration of cold snaps will change as well, says atmospheric scientist Jennifer Francis of Rutgers University in New Brunswick, N.J. “This is a difficult metric because the definition of ‘cold’ will change as we get used to a warmer climate,” she says. “Colder than a new normal will still be disruptive, especially if it tends to stick around longer.”