Scientists think they’ve figured out why Greenland’s climate is out of sync with the rest of the Northern Hemisphere — but they had to go way back in time to find the proposed culprit.
From the 1970s through the early 1990s, Greenland kept its cool even as the Northern Hemisphere warmed. Reconstructing and examining 2,100 years of Greenland temperatures using ice-trapped air bubbles, researchers propose that periodic fluctuations in the sun’s activity can desynchronize Greenland’s climate and cause changes decades later. The study, to be published in Geophysical Research Letters, suggests that, based on current solar activity, Greenland could warm significantly faster over the coming decades than previously thought.
Whenever the seesawing solar activity peaks, as it did in the 1950s, the North Atlantic sea surface warms. The researchers postulate that this extra heat slows the ocean current that ferries warm water north from the tropics. The cooling effect of this slowdown wouldn’t be felt until 30 to 40 years later, the researchers note. If the sun’s activity diminishes as expected over the coming decades, this mechanism could reverse and accelerate warming in Greenland in the 2020s and 2030s, says Takuro Kobashi, a climate scientist at the University of Bern in Switzerland.
“Greenland could experience not only global warming, but also declining solar activity could introduce additional warming,” he says. “This could cause Greenland’s ice sheets to melt faster than expected.” Since Greenland resumed warming in the mid-1990s, meltwater from the icy island has contributed about a third of global sea level rise.
Kobashi and colleagues collected a 500-meter-long cross section of ice from a mountain glacier and sliced off sections that formed as long ago as 2,100 years ago. Glacial ice grows under a layer of snow, often trapping pockets of air. Heavier argon and nitrogen atoms in the air congregate toward the bottom of the snow when air temperatures are warmer than the ice. As temperatures cool, the atoms rise to the top of the snow layer. Comparing the relative number of heavier atoms in different layers provides a record of the climate when the ice formed.
The temperature record revealed that Greenland warms long after the sun’s activity reaches a minimum and cools long after a solar maximum. In each case, the temperature change lags around 10 to 40 years after the change in the sun’s activity.
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Kobashi proposes that rises in solar heating warm the North Atlantic and dump extra rainwater into the ocean. These changes could cause the sea surface to become less dense and slow the ocean circulation that carries warm water up from lower latitudes, keeping Greenland cool.
“Rises and falls in solar variability can have the opposite effect on Greenland’s temperature decades later,” Kobashi says. “This is something we can predict and potentially use to improve predictions of future climate change in Greenland.”
While the new Greenland temperature record is useful, climate scientist Didier Swingedouw of the University of Bordeaux in France isn’t convinced that the sun caused Greenland’s recent oddball climate. In the July 2 Nature, Swingedouw and colleagues report that solar variations don’t cause changes in the circulation of North Atlantic currents. “The idea seems reasonable,” he says, “but the link is just not really there.”