Thursday, 2 February 2012

Volcanoes Indicted for Europe's Long, Big Chill

For years scientists have debated what could have plunged Europe into the half-millennium-long cold spell that ended only a century ago. Was it the temporarily spotless and therefore faint sun, or did a burst of volcanic eruptions loft debris that shaded out a normal sun? Or were the sun and volcanoes in cahoots? Researchers analyzing plants killed in the Little Ice Age's opening years are now pinning the blame on volcanoes alone.

Solving this climatic whodunit has been hampered by the uncertain timing of the Little Ice Age's onset. So geologist Gifford Miller of the University of Colorado, Boulder, and his colleagues went to the best climate record they could find: intact plants emerging from beneath the retreating ice cap on Canada's Baffin Island. Carbon dating showed that most of the plants died between 1275 and 1300 as Arctic ice suddenly expanded across once-green terrain. The same signal of sudden cooling turned up in sediment from the late 1200s deposited in a glacier-fed lake in Iceland.

Both high-latitude cooling signals coincide with an exceptional burst of activity from four tropical volcanoes. Each of them tossed more than a million tons of sulfurous debris into the stratosphere, according to ice core records, where it could block sunlight and cool the surface. Gifford and his colleagues take the coincidence of the Little Ice Age's onset and massive eruptions as evidence that the one caused the other.

The other problem with linking either the sun or volcanoes to the cooling, however, has been the relative impotence of either chilling mechanism. Estimates of the dimming of the sun when sunspots become rare—as during the centuries around 1700—have been falling of late. Now there is no good evidence that the sun would have faded enough to cause the Little Ice Age. And the cooling following even a huge eruption lasts only until the debris falls out of the stratosphere after a few years.
But Miller and his colleagues did some climate modeling to see what closely spaced eruptions might do to climate. In a climate model that included Arctic sea ice, repeated volcanic cooling sent sea ice southward along the east coast of Greenland. Its melting made surface waters less salty, reducing ocean mixing and thus chilling the waters that return to the Arctic. There the colder water completed a feedback loop by encouraging the formation of more sea ice. In at least some model runs, that feedback loop maintained an icy chill directly upwind of Europe for centuries.

"I think it is likely that the abrupt start to the Little Ice Age in the late 13th century is volcanic in origin," says climate scientist Gavin Schmidt of NASA's Goddard Institute for Space Studies in New York City, who was not involved in the new work published today in Geophysical Research Letters. The agreement of the Baffin Island and Iceland climate records is convincing, he says, but he finds the modeling less so. "I'm far less convinced that this is the cause of the subsequent centuries of climate change," he says. Too few runs of a single model have been made, he explains, to do more than hint at how a volcanic cooling could become locked in for centuries.

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