© Gifford Miller, University of ColoradoUniversity of Colorado, Boulder Professor Gifford Miller collects dead plant samples from beneath a Baffin Island ice cap. Miller led a new study, to be published in in Geophysical Research Letters, which indicates the Little Ice Age began roughly A.D. 1275 and was triggered by repeated, explosive volcanism that cooled the atmosphere.
Washington, DC - New evidence from northern ice sheets suggests that volcanic eruptions triggered the multiple-century cool spell known as the Little Ice Age, and pinpoints the start of the climate shift to the final decades of the 13th century. Researchers have long known that the Little Ice Age began sometime after the Middle Ages and lasted into the late 19th century. But, estimates of its onset have ranged from the 13th to the 16th century.
According to the new study, the Little Ice Age began abruptly between 1275 and 1300 A.D., triggered by repeated, explosive volcanism and sustained by a self- perpetuating sea ice-ocean feedback in the North Atlantic Ocean, according to Gifford Miller, a geological sciences professor at the University of Colorado, Boulder (CU-Boulder), who led the study. The primary evidence comes from radiocarbon dates from dead vegetation emerging from rapidly melting icecaps on Baffin Island, combined with ice and sediment core data from the poles and Iceland, and from sea-ice climate model simulations, said Miller.
He and his colleagues will publish their findings on 31 January in Geophysical Research Letters,
a publication of the American Geophysical Union (AGU).
During the cool spell, advancing glaciers in mountain valleys in northern Europe destroyed towns. Famous paintings from the period depict people ice-skating on the Thames River in London and canals in the Netherlands, places that were ice-free before and after the Little Ice Age. There is evidence also that the Little Ice Age affected places far from Europe, including South America and China.
While scientific estimates regarding the onset of the Little Ice Age extend from the 13th century to the 16th century, there has been little consensus, said Miller. "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. "But the time in which European glaciers advanced far enough to demolish villages would have been long after the onset of the cold period," said Miller, a Fellow at his university's Institute of Arctic and Alpine Research.
Most scientists think the Little Ice Age was caused either by decreased summer solar radiation, erupting volcanoes that cooled the planet by ejecting shiny aerosol particles that reflected sunlight back into space, or a combination of both, said Miller.
The new study suggests that the onset of the little Ice Age was caused by an unusual, 50-year-long episode of four massive tropical volcanic eruptions. Climate models used in the new study showed that the persistence of cold summers following the eruptions is best explained by sea-ice ocean feedbacks originating in the North Atlantic Ocean.
"This is the first time anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age," said Miller. "We also have provided an understandable climate feedback system that explains how this cold period could be sustained for a long period of time. 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 who contributed to the study include scientists and students from CU-Boulder, the National Center for Atmospheric Research in Boulder, the University of Iceland in Reykjavik, the University of California Irvine and the University of Edinburgh in Scotland. The study was funded in part by the National Science Foundation and the Icelandic Science Foundation.
As part of the study, Miller and his colleagues radiocarbon-dated roughly 150 samples of dead plant material with roots intact collected from beneath receding ice margins of ice caps on Baffin Island in the Canadian Arctic. There was a large cluster of "kill dates" between 1275 and 1300 A.D., indicating the plants had been frozen and engulfed by ice during a relatively sudden event.
Both low-lying and higher altitude plants all died at roughly the same time, indicating the onset of the Little Ice Age on Baffin Island - the fifth largest island in the world - was abrupt. A more gradual start of the Ice Age would have affected the upper-altitude vegetation first, since it would have been colder to start with at those elevations. The team saw a second spike in plant kill dates at about 1450 A.D., indicating the quick onset of a second major cooling event.
To broaden the study, the team analyzed sediment cores from a glacial lake linked to the 367-square-mile Langjökull ice cap in the central highlands of Iceland that reaches nearly a mile high. The annual layers in the cores - which can be reliably dated by using volcanic deposits, called tephra, from known historic eruptions on Iceland going back more than 1,000 years - 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, he said.
"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." Average summer temperatures in the Northern Hemisphere did not return to those of the warmer medieval times until the 20th century, he said.
The team used NCAR's 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 1150-1700 A.D., showed several large, closely spaced eruptions could have cooled the Northern Hemisphere enough to trigger Arctic sea-ice growth.
The simulations showed 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. Without mixing taking place, the water that flowed back to the Arctic was colder, helping sustain large areas of sea ice and creating a self-sustaining feedback loop long after the effects of the volcanic aerosols subsided, he said.
The researchers set the solar radiation at a constant level in the simulations, and Miller said the Little Ice Age likely would have occurred without decreased summer solar radiation at the time. "Estimates of the sun's variability over time are getting smaller; it's now thought by some scientists to have varied little more in the last millennium than during a standard 11-year solar cycle," he said.