January 30, 2012
New Study Sheds Light On Little Ice Age
University of Colorado researchers report that they have answered some questions surrounding Earth's Little Ice Age, which started between A.D. 1275 and 1300, and lasted into the late 19th century.
According to the new study, the Little Ice Age was triggered by repeated, explosive volcanism and sustained by a self-perpetuating sea ice-ocean feedback system in the North Atlantic Ocean.Professor Gifford Miller, who led the study, said the team's evidence from radiocarbon dates from dead vegetation that emerged from melting icecaps on Baffin Island in the Canadian Arctic.
Scientists widely believe the Little Ice Age was caused by decreased summer solar radiation, erupting volcanoes that cooled Earth by ejecting aerosol particles, or a combination or both, according to Miller.
The new study suggests the Little Ice Age was caused by a 50-year-long episode of four massive tropical volcanic eruptions.
The climate models that the researchers used in the study show that the persistence of cold summers following the eruptions is best explained by a sea ice-ocean feedback system originating in the North Atlantic Ocean.
"We also have provided an understandable climate feedback system that explains how this cold period could be sustained for a long period of time," Miller said in a statement.
"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."
The researchers radiocarbon-dated about 150 samples of dead plant material with roots intact, collected from beneath receding ice margins of ice caps on Baffin Island.
A large number of "kill dates" indicated the plants had been frozen and engulfed by ice during a relatively sudden event between A.D. 1275 and 1300.
They found that both low-lying and higher altitude plants all died out at about the same time, indicating the onset of the Little Ice Age on Baffin Island was abrupt.
The team saw a second spike in plant kill dates at about A.D. 1450, which indicated another major cooling event.
The scientists then analyzed sediment cores from glacial lake linked to the 367-square-mile LangjÃ¶kull ice cap in the central highlands of Iceland.
According to Miller, the annual layers in the cores suddenly became thicker in the late 13th century and again in the 15th century 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," 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."
He said the average summer temperatures in the Northern Hemisphere did not return to the same as the middle ages until the 20th century.
The team used NCAR-based Community Climate System Model to test the effects of volcanic cooling on Arctic sea ice extent and mass.
The models used showed several large, closely spaced eruptions that could have cooled the Northern Hemisphere enough to trigger Arctic sea ice growth. The models showed sustained cooling from volcanoes would have sent some of the expanding Arctic sea ice down along the eastern coast of Greenland until it melted in the North Atlantic.
Miller said 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.
He said this weakened heat transport back to the Arctic, creating a self-sustaining feedback system on the sea ice after the effects of the volcanic aerosols subsided.
"Our simulations showed that the volcanic eruptions may have had a profound cooling effect," NCAR scientist Bette Otto-Bliesner, a co-author of the study, said in a press release. "The eruptions could have triggered a chain reaction, affecting sea ice and ocean currents in a way that lowered temperatures for centuries."
Miller said one of the primary questions pertaining to the Little Ice Age is how unusual the warming of Earth is today. A previous study led by Miller found that temperatures today are the warmest when compared to the last 2,000 years.
The new study is to be published in Geophysical Research Letters on January 31.
Image Caption: Gifford Miller collects vegetation samples on Baffin Island. (Photo courtesy University of Colorado Boulder.)
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