Researchers at this week’s conference discuss how Arctic sea ice is fundamentally changing air masses and weather systems in the Northern Hemisphere.
By Bob Berwyn
Sea ice around Greenland in September of 2015, near the peak of the melt season. (Photo: Bob Berwyn)
This winter’s persistent northern hemisphere weather pattern, characterized by a warm Arctic and cold continents, could be an indicator of how the rapid meltdown of Arctic sea ice will lead to more extreme winter conditions across North America and Eurasia.
The escalating warmth in the Arctic disrupts atmospheric circulation, with global ripple effects, “especially across the industrialized countries and population centers of the Northern Hemisphere,” according to some climate scientists meeting this week in Washington, D.C., at a conference about the connections between the Arctic and mid-latitude weather.
The new administration may reject overwhelming findings that heat-trapping greenhouse pollution is tilting Earth’s climate toward possibly catastrophic change, but that isn’t stopping the researchers at the conference from discussing the scientific reality that climate change’s most serious effects could happen much sooner than expected.
The dramatic decline in Arctic sea ice and snow is one of the most profound signs of global warming and has coincided with “a period of ostensibly more frequent events of extreme weather across the mid-latitudes, including extreme heat and rainfall events and recent severe winters,” according to the conference organizers, who are posting updates under the #arctic17 hashtag on Twitter.
The possible link between Arctic change and mid-latitude weather is the focus of the conference, and even if the researchers don’t have all the answers yet, there is an emerging consensus that melting Arctic sea ice is fundamentally changing the the way air masses and weather systems whirl around the Northern Hemisphere.
There is an emerging consensus that melting Arctic sea ice is fundamentally changing the the way air masses and weather systems whirl around the Northern Hemisphere.
Those changes are not easy to measure or reproduce with climate models, and some researchers think natural variability or changes in the tropics are more important drivers of weather extremes in the mid-latitudes.
But observations from recent years support the idea that the melting ice is a key factor in shaping the persistent pattern of warm temperatures over the Arctic that displaces bitter cold air toward North America and especially Eurasia, says conference co-chair Judah Cohen, a climate scientist at the Massachusetts Institute of Technology.
“I’m in the camp that the rapidly warming Arctic impacts mid-latitude weather. Even though there is some debate about this, no one would say that the melting ice has no impact,” he says. “But some say you can’t identify it above the noise of natural variability.”
It makes sense if you visualize the atmosphere above the Arctic as a giant lava lamp, says Karsten Haustein, a climate scientist at the University of Oxford: The vast new expanse of open water absorbs the sun’s heat all summer and into the fall. Just a few decades ago, most of that warmth would have bounced off the frozen ice cap and dissipated back into space; now, that heat creates a huge, rising mass of warm air over the Arctic in the fall and early winter, a bubble that pushes colder air farther south. Haustein says the pattern the past few months was a “textbook example” of the disruption, with record warmth centered over the Arctic and cold temperatures over the Eurasian landmass.
“This fall, the pattern was so persistent it was blowing everyone’s mind,” Haustein says. “Eventually, the models will be showing this.”
During the fall and early winter, that displaced cold air picks up huge amounts of moisture from the Arctic Ocean. As it’s pushed over the Eurasian land mass, it cools and falls back to the ground as snow. A thick early winter snow cover in the region refrigerates the air directly above, strengthening the temperature contrast with the Arctic Ocean and reinforcing the entire pattern, Haustein explains. “More snow means another feedback, which is more cold, and that is exactly what we’ve seen throughout October, November, and December.”
Haustein acknowledges that there’s still vigorous debate in the climate research community about the link. Some scientists say natural variability may still be masking the effects of melting sea ice. According to other researchers, shifts in tropical ocean regions have a stronger influence on mid-latitude weather than the dwindling Arctic sea ice.
One thing is for certain. Around 1990, the temperature trends for the Arctic and mid-latitudes started to diverge, with the Arctic warming at a much greater amplitude. Cohen says that has to affect mid-latitude weather.
“The difference is striking. … We’ve tried to make the connection through the polar vortex, which is a strong low-pressure system that sits right over the North Pole and spins really fast,” he says. “It locks the coldest air right over the Arctic. That was supposed to get stronger with greenhouse gases, but it didn’t. Increasing greenhouse gases should, by simple physics, strengthen the polar vortex, but what favors weakening is the loss of sea ice and more snow cover in Eurasia, Those work together to force this weakening of the polar vortex that we’re seeing.”
When it weakens, cold air spills down into lower latitudes, Cohen adds.
“Every year that this happens, it gets harder to say that it’s natural variability,” he says. “The Earth is getting warmer, the trend is clear, the system is just getting warmer and warmer. The climate change dice are loaded.”
