The Little Ice Age - the centuries-long period of cooling that began in the Middle Ages - appears to have been triggered by four massive volcanic eruptions in the tropics.
Scientists at the University of Colorado Boulder and the National Center for Atmospheric Research (NCAR) say the eruptions took place between 1275 and 1300 AD, with the subsequent cooling explained by an expansion of sea ice and a related weakening of Atlantic currents.
The study was based on analyses of patterns of dead vegetation, ice and sediment core data, and powerful computer climate models.
"Our simulations showed that the volcanic eruptions may have had a profound cooling effect," says NCAR scientist Bette Otto-Bliesner.
"The eruptions could have triggered a chain reaction, affecting sea ice and ocean currents in a way that lowered temperatures for centuries."
The effects of the Little Ice Age became most apparent in Europe from the 15th century onwards. Advancing glaciers in mountain valleys destroyed towns, and 'frost fairs' were held on the ice of the River Thames in London. But it started before that, says the team.
"The dominant way scientists have defined the Little Ice Age is by the expansion of big valley glaciers in the Alps and in Norway," says lead author Gifford Miller of the University of Colorado Boulder.
"But the time in which European glaciers advanced far enough to demolish villages would have been long after the onset of the cold period."
The team radiocarbon-dated around 150 samples of dead plant material with roots intact, collected from beneath receding margins of ice caps on Baffin Island in the Canadian Arctic. They found a large cluster of 'kill dates' between 1275 and 1300 AD, indicating the plants had been frozen and engulfed by ice during a relatively sudden event.
A second spike in plant kill dates followed around 1450 AD.
To broaden the study, the researchers analyzed sediment cores from a glacial lake linked to the 367-square-mile Langjökull icecap in the central highlands of Iceland.
The annual layers in the cores — which can be reliably dated by using tephra deposits from known historic volcanic eruptions on Iceland going back more than 1,000 years — suddenly became thicker in the late 13th and 15th centuries, due to increased erosion caused by the expansion of the ice cap as the climate cooled.
"That showed us the signal we got from Baffin Island was not just a local signal, it was a North Atlantic signal," says Miller.
"This gave us a great deal more confidence that there was a major perturbation to the Northern Hemisphere climate near the end of the 13th century."
The team then used the Community Climate System Model to test the effects of volcanic cooling on Arctic sea ice extent and mass. The model, which simulated various sea ice conditions from about 1150 to 1700 AD, showed several large, closely spaced eruptions could have cooled the Northern Hemisphere enough to trigger the expansion of Arctic sea ice.
Sustained cooling from volcanoes would have sent some of the expanding Arctic sea ice down along the eastern coast of Greenland until it eventually melted in the North Atlantic. Since sea ice contains almost no salt, when it melted the surface water became less dense, preventing it from mixing with deeper North Atlantic water.
This would have weakened heat transport back to the Arctic and created a self-sustaining feedback, long after the effects of the volcanic aerosols subsided.