A portion of eastern Arctic Canada and neighboring Greenland has baked the past 30 years, heating up at a rate that's extraordinary, even for the rapidly warming Arctic.
Now a study led by University of Washington scientists identifies what they believe to be behind that unusual heat.
Up to half of the warming in that region over the last three decades can be attributed to natural forces unconnected to global warming -- namely, regularly spaced atmospheric waves that emanate from a region in the tropical Pacific Ocean. So said the study, published online Wednesday in the scientific journal Nature. The other half, the study says, can be attributed to manmade causes -- largely human-produced greenhouse gas emissions.
Putting the two together results in a double whammy heating up the eastern Canadian Arctic and Greenland, said one of the co-authors, with warming and melting building upon themselves to create yet more warming and melting.
"It's a little like a perfect storm," said John "Mike" Wallace, an atmospheric sciences professor at the University of Washington. "You're very likely to find that it's a super positioning of things that work together to create these extreme conditions."
Northeastern Canada and neighboring parts of Greenland have warmed about twice as fast as the Arctic as a whole. Over the last three decades, Greenland has warmed by 0.8 to 1 degree Celsius (1.4 to 1.8 degrees Fahrenheit) per decade, compared to the Arctic per-decade average of 0.4 to 0.5 degrees C (0.7 to 0.9 degrees F), said Qinghua Ding, a University of Washington atmospheric research scientist and the study's first author.
An earlier study led by University of Colorado scientists found that Canada's Baffin Island is warmer now than it has been in at least 44,000 years.
Such extraordinary warming led scientists to search for the forces behind it. The University of Washington-led study used sea-surface temperatures and other observations and combined them with modeling to parse out the global factors affecting that portion of the Arctic. The "tropical forcing" spot of origin is a particularly warm spot in the western Pacific that sends atmospheric waves, known as Rossby waves, that move in an arced pattern from the Arctic to the Atlantic before returning to the Pacific point of origin, according to the study.
In contrast to the rapidly heating eastern Canadian Arctic-Greenland area, the high Arctic north of Siberia and Alaska has seen a less-dramatic but steady warming trend that is highly correlated and "can be related directly" to significant reductions in sea ice, the study says.
Wallace said parts of Siberia that warmed dramatically in the 1980s -- a specific period of spiking temperatures -- may also have been similarly affected by natural variability that coupled with human-caused factors.
Natural forces can bring localized cooling as well, he said. "The natural variability goes both ways," he said, and regional cooling might "give you a false sense of security when it's happening."
A separate study, also published online by Nature on Wednesday, found that regional conditions -- not faraway atmospheric patterns -- will be responsible for dramatically increased Arctic precipitation expected in coming decades. The study, by scientists from the Royal Netherlands Meteorological Institute, used modeling to identify retreating winter sea ice as the cause of future increases in evaporation in the Arctic. That increased evaporation will amplify the entire Arctic hydrological cycle, putting into motion the forces that will result in a predicted 50 percent increase in Arctic precipitation, the study said.
"This demonstrates that increases in Arctic precipitation are firmly linked to Arctic warming and sea-ice decline," the study said. The model results are counter to previous theories that attributed increased Arctic rain and snow to atmospheric patterns moving north from temperate latitudes.
Increased Arctic precipitation will mean more river discharges and more snowfall over ice sheets.
Reach Yereth Rosen at firstname.lastname@example.org.
By YERETH ROSEN