Climate

Ancient Icelandic Volcanoes May Have Hastened Ice Age Melting

Volcanic ash in the atmosphere cooled temperatures over Northern Europe, but once it fell to the surface, it sped up melting because dark ice absorbs incoming solar radiation.

A surge of volcanic eruptions from Iceland may have sped up the melting of the ice sheets that covered Scandinavia thousands of years ago as tons of soot fell from the sky.

That thaw would have taken place even as the cloud of ash released from the volcanoes was cooling northern Europe, paleoclimatologist Francesco Muschitiello told Seeker. And it’s a story that has some relevance to current climate concerns.

“You have his cooling effect of volcanic eruptions,” said Muschitiello of Columbia University’s Lamont-Doherty Earth Institute in New York and the Uni Research Climate center at Norway’s University of Bergen. “But at the same time, as you increase the deposition of dust and ash on the ice sheets, you increase the runoff. You increase the melting of the ice. You have these two counteracting effects.”

The story Muschitiello and his colleagues tell in the Oct. 24 issue of Nature Communications unfolded in the waning days of the last ice age, about 13,000 years ago. It’s a tale teased out of layers of sulfur compounds in the clay of what’s now southeastern Sweden.

At that time, Scandinavia was buried under an ice sheet that stretched from the British Isles to the Ural Mountains of present-day Russia. But as the glaciers began to recede, the volcanoes beneath them began to stir.

“The ice sheets are getting lighter and lighter, they relieve pressure on top of the Earth’s crust,” Muschitiello said. “And this lightening basically in turn triggers volcanic eruptions, especially in Iceland and North America, in Alaska and Kamchatka.”

Iceland is a volcanic hot spot even today. And at that time, its volcanoes belched sulfuric smoke, soot and ash across the Northern Hemisphere. The sulfur compounds Muschitiello recorded in core samples of Swedish clay match those found in ice cylinders extracted from Greenland’s ice sheets from the same era.

That cloud gradually would have driven temperatures down as they hung in the atmosphere, blocking more sunlight. But the portion that fell onto the ice sheets would have led to a faster melt, as the spots of dark carbon meant the ice absorbed solar radiation it otherwise would have reflected back into space.

Using computer models of the climate, Muschitiello’s team tried to reproduce the effects of those eruptions. They found an eruption in the summer would have cooled northern Europe by as much as 3.5 degrees Celsius (6.3 degrees Fahrenheit). Meanwhile, even small changes in the amount of sunlight could have sped up dramatically the amount of water they shed.

“Even though you have sort of a long-term cooling, you have an immediate response of the ice sheet, which increases melting,” he said.

That process may be relevant today, as researchers attempt to project the effects of climate change and its impact on future sea levels.

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The Arctic is already warming at twice the rate of the rest of the globe, with sea ice shrinking and Greenland’s ice sheet shedding water. Today’s thaw is also opening up the far North to more human activity, like shipping, which results in more soot falling on the remaining ice, fueling more melt. Those effects aren’t well represented in today’s climate projections, Muschitiello said.

“What I hope we’re showing in the study is it’s really important to figure out the physics, the links between atmospheric dynamics and ice-sheet melting, because there seems to be a very important relationship there,” he said.