The Little Ice Age, a centuries-long spell of cold summers in Europe and elsewhere, began suddenly late in the 13th century, a new study finds. A string of volcanic explosions may have set off this change in climate by belching particles that reflected sunlight and allowed Arctic sea ice to reach epic proportions, researchers report online January 31 in Geophysical Research Letters.
“We’ve been able to identify the beginning of the Little Ice Age, something that’s been very difficult to do in the past,” says Gifford Miller, a paleoclimatologist and geologist at the University of Colorado Boulder. “This cooling wasn’t gradual; it was an abrupt shift.”
It’s long been known that much of the globe became chillier during the Renaissance. By the 17th century, temperatures in the Northern Hemisphere had fallen by half a degree Celsius compared with medieval times. Ice skating on London’s frozen River Thames became popular.
To pin down when this climate change began, Miller’s team traveled to Baffin Island on the northern fringes of Canada. Small glaciers in this region tend to respond quickly to temperature changes. Carbon dating of moss entombed in Baffin’s ice revealed two sudden advances of the snow line that killed off the vegetation: a sudden cold spell between 1275 to 1300, followed by intensifying cold between 1430 and 1455.
Testing whether this chill extended beyond Canada took the researchers to the Langjökull glacier, the second largest ice cap in Iceland. Layered sediments from a nearby lake appeared progressively thicker in the 14th century — exactly what would be expected if the glacier expanded and ground away the landscape.
These chillier conditions began during an especially active time for volcanoes. “The second half of the 13th century had the most volcanism of any period of the past 1,500 years,” says Alan Robock, an atmospheric scientist at Rutgers University in New Brunswick, N.J.
Volcanoes have been blamed for the Little Ice Age before. But the cooling produced by an eruption tends to be short-lived. The atmosphere recovers from the junk spewed into the sky within a few years. The challenge is to explain how the Little Ice Age persisted for many centuries.
“It’s been hard to understand how volcanism could lead to such long-lasting cooling,” says Stephen Vavrus, a climate scientist at the University of Wisconsin–Madison.
Sea ice may have been the secret to keeping Earth frosty. In simulations of global climate run by Miller’s team, volcanic eruptions stimulated the growth of Arctic ice. Normally, this ice would melt back during summer months. But a series of four explosions, each within a decade of the last, could have expanded the ice enough to make it stable, says Miller.
Polar ice samples have revealed just such a series of eruptions, says Robock: an especially big explosion somewhere in the world in 1258, and three smaller ones in 1268, 1275 and 1284.
Whether these events could have kicked off the Little Ice Age still isn’t certain. Two occurred in the Southern Hemisphere, which may have muted their impact. And some of the simulations run by Miller’s team didn’t lead to supersized sea ice cover, even with four eruptions.
Still, the new study offers a plausible chain of events, says Robock, ready to be put to the test by other climate simulations.