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. . . . 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. . . . 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.
But, why did the cold triggered by the volcanos last? Enlarged ice caps might explain this:
“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.
From Science News relying upon G. Miller et al., "Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks." Geophysical Research Letters. Vol. 39, January 31, 2012, p. L02708. doi:10.1029/2011GL050168.
Comments to the article at the Science News website suggest solar variability could also have been involved, for example, stating:
Solar variability during this period began after the Medieval Maximum with the Wolf Minimum (which started around 1280) followed by the deeper 90 year long Spörer Minimum (which started around 1460) and the 70 year long Maunder Minimum in 1645 and the milder Dalton Minimum which ended about 1820.
Another comment suggests comet and asteroidal debris as a possible driver for the climate changes that took place.
Of course, simply knowing how quickly colder temperatures took hold is itself valuable because it puts boundaries on what would and would not have been possible for flora and fauna to do in order to adapt to the climate shifts.
The Little Ice Age, in turn, has been fingered as a major driving force behind some of the grand political trends of the Middle Ages.
Links between volcanic activity and major climate changes are also not new. One such spat of volcanic activity took place around the time that almost all of the Neanderthal population went extinct. An earlier spat of volcanic activity, associated with the Toba erruption, has been proposed as a driver of the expansion of modern humans into Eurasia.