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Scientists at the University of Colorado-Boulder have published a new paper with data suggesting that the so-called Little Ice Age (LIA), a global cooling period that started in the Middle Ages and lasted well into the 19th century, probably began abruptly, between the years 1275 and 1300.
While scientists have long believed the LIA began in the later Middle Ages, the exact time and causes have been harder to determine.
But now, according to the team led by Professor Gifford Miller, there's good reason to believe the major cause of the LIA was a half-century long period during which four massive volcanic eruptions filled the Earth's atmosphere with dust and triggered a sea ice-ocean feedback system that prevented the Earth's climate from quickly returning to warmer temperature.
“The dominant way scientists have defined the Little Ice Age is by the expansion of big valley glaciers in the Alps and in Norway,” said Miller, who is a fellow at Boulder's Institute of Arctic and Alpine Research. “But the time it took for European glaciers to advance far enough to demolish villages would have been long after the onset of the cold period.”
The series of volcanic eruptions is only part of the story, however. The climate models Miller and his team used in their study suggested the persistence of cold summers following the eruptions is best explained by a sea ice--ocean feedback system that began in the North Atlantic.
"If the climate system is hit again and again by cold conditions over a relatively short period -- in this case, from volcanic eruptions -- there appears to be a cumulative cooling effect,” he said.
In order to find evidence to support their dates, the team of scientists went to the retreating ice caps on Baffin Island, north of Hudson Bay in Canada, where they were able to collect about 150 samples of dead plant matter from newly exposed ground.
Miller radiocarbon-dated the samples and found a significant cluster of "kill dates", between 1275 and 1300. In his assessment, these "kill dates" represent the relatively short period during which the plants were frozen and engulfed by ice. This is too short a period, according to Miller, to be explained by the onset of glaciers.
In addition, the team examined sediment cores from a glacial lake in Iceland. Layers in these cores can be reliably dated, using tephra deposits from historic volcanic eruptions that have taken place on the island over the past thousand years and more.
They found that the layers which correspond to the late 13th century and also in the 15th century were thicker --due to increased erosion, and likely caused by the expansion of the ice cap as the overall climate cooled.
“That showed us the signal we got from Baffin Island was not just a local signal, it was a North Atlantic signal,” said 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 question arises, which volcanic eruptions triggered the LIA? Were there any contemporaneous records (in China, or India--for example) that revealed the occurrence of the eruptions from the same period.
"The volcanic record is derived by measuring the sulfur content of polar ice cores," Gifford told me in an email. "Only those eruptions with large volumes of sulfur have a lasting impact on the climate system. The 1452 major eruption is from Kuwae, a volcano in Vanuatu, the tropical Pacific."
"The big 13th-century eruptions are thought to be tropical also," he added, "because they appear at the same time in ice at both poles."
HT: Phil Plait.
