The two main forces that conspire to destroy Earth's massive polar ice sheets are heat, which melts their surfaces via sunlight and warm air, and gravity, which drives glaciers to slide to the sea. But a new analysis of the Greenland Ice Sheet points to an underappreciated culprit that could accelerate its demise: wind.
The study shows how local weather extremes might have driven the 2015 melting of the ice sheet, which holds enough water to cause global sea levels to rise 7 meters. And some researchers say it documents weather patterns that support, but don't prove, a controversial hypothesis that links melting Arctic sea ice with shifting weather patterns across the Northern Hemisphere.
“This study highlights how the many elements of the system are working together,” says Marco Tedesco, a glaciologist at Columbia University and the first author of the study, published today in Nature Communications. “It's crucial we understand how the different parts are driving Greenland's melting.”
Tedesco started the study after he noticed weird melting patterns on Greenland last summer. Overall, the amount of melting wasn't a blockbuster—2012 was a more impressive year for ice loss. Still, on more than half the days in June, July, and August, the spatial extent of surface melt exceeded the 1981–2010 average.
He and his colleagues were most puzzled, however, by the pattern of the melting. Northwest Greenland experienced record melt, setting summer records for surface temperatures, production of meltwater, and total runoff. In contrast, southern Greenland was unusually cold, with below average melting. Tedesco himself had trudged through snow in June in the town of Ilulissat, in the southern half of the island. “People were complaining about all that snow so late in the season,” he says.
Weather data also revealed some oddities. That year, high-altitude winds in northern Greenland tended to flow east to west—a reversal of the usual west to east pattern. Those winds also blustered at record speeds.
Another anomaly concerned the jet stream—the high-altitude, west-to-east river of air that drives weather patterns across the globe. In July the river was extremely curvy, meandering farther north than it has since 1948, beyond 76° north.
When the team consulted air pressure charts—maps of the moving atmosphere, essentially—the picture came together. From June through July 2015, that wavy jet stream had spun off an eddy, known as a “cut-off high.” The eddy lumbered past the northern side of Greenland, its winds swirling clockwise.
Weather maps show those winds pulled warm air up from the south toward north Greenland. The eddy also provided clear, sunny days in northwest Greenland for several weeks, Tedesco says. Meanwhile, on the cut-off high's eastern side, winds heading south drove cold air from the Arctic Ocean toward Greenland's southern tip, bringing the lower than usual temperatures there.
That pattern may be here to stay because of climate change, says co-author Edward Hanna, an earth scientist at the University of Sheffield in the United Kingdom. Cut-off highs, also called blocks because they disrupt the movement of west-to-east air, are becoming more prevalent in the Arctic, he says. Using an index that measures blocking, he calculated in a recent paper that seven of the top 11 blocking events over Greenland since 1851 have occurred since 2007. Hanna says that trend is “clearly related” to rising temperatures in the region, which is warming twice as fast as the rest of the planet—a trend known as Arctic amplification.
Some researchers see the new study as offering support to a controversial hypothesis regarding amplification developed by meteorologist Jennifer Francis of Rutgers University, New Brunswick, in New Jersey. Francis argues that the melting of sea ice is causing the Arctic to reflect less sunlight, warming it up. That, in turn, is causing the jet stream to meander more—and making it more likely to spin off blocking events like cut-off highs. Although Francis’s amplification hypothesis has many critics, she says the implication of the new paper “is that continued human-induced loss of Arctic sea ice is likely to contribute to additional melt of the Greenland Ice Sheet.”
Tedesco doesn't go that far in the new paper. And atmospheric scientist James Screen of the University of Exeter in the United Kingdom warns that the “link between blocking and Arctic amplification is still very much open to debate.”
It could take years to collect the data required to decide who is right—and how complex changes in the Arctic will play out.