Scientists have said that global warming in the Arctic is occurring more quickly 1.2 miles (2 kilometers) above the surface than at the ground level.
According to a report in the National Geographic News, a team led by Rune Graversen of Stockholm University came to this conclusion.
His team drew on decades of weather data, mostly from satellites and trans-Arctic airplane flights, to examine temperature changes at various elevations across the Arctic.
Scientists had earlier predicted that the rapid warming of the air higher up was an effect of solar heating, in which melting snow reveals darker underlying land and water that better absorb heat—an effect that escalates as snow and ice continue to shrink.
But, what Graversen's team found was that during the summer, the rate at which the upper atmosphere was warming up was two times faster each decade.
The finding meant that solar heating can't be the only cause, because that should produce the greatest warming close to the surface.
"Retreating snow and ice cannot explain the vertical structure of the warming that we show," Graversen said. "So snow and ice retreat is not as important as we previously thought," he added.
According to the research team, the upper-elevation warming is linked to changes in atmospheric circulation, such as heat flow northward each month.
Such patterns can be calculated using weather balloon data for winds and air temperature data from above meteorological stations in Europe and Canada.
"This assumption is that if this heat flow has increased—which we see in the data that it has—then it has contributed to the warming in the Arctic, not only at the surface, but higher in the atmosphere," said Graversen.
"Increased moisture in northward-moving air also plays a role, because when the water vapor condenses into clouds and snow, it releases energy, warming the air," he added. Nobody knows how much of this change is the result of human emissions of planet-warming gases such as carbon dioxide, but it's likely that they play a role.
"Many models suggest an increase in energy transport when more greenhouse gases are introduced into them," the journal quoted Graversen as saying. "Changes in the circulation in the atmosphere might have had a much larger effect than previously thought, but these changes may also have been induced by greenhouse gases," he added.
"Were it not for this increase in poleward heat transport, the Arctic might have warmed less and the middle latitudes might have warmed more than actually happened over the past several decades," said Graversen.