A new University of Colorado (CU) Boulderled study appears to answer contentious questions about the onset and cause of Earth's Little Ice Age, a period of cooling that began after the Middle Ages and lasted into the late 19th century.
According to the new study, the Little Ice Age began abruptly between AD 1275 and 1300, triggered by repeated, explosive volcanism and sustained by a self-perpetuating sea ice-ocean feedback system in the North Atlantic Ocean, according to Gifford Miller, the CU Boulder professor who led the study. The primary evidence comes from radiocarbon dates from dead vegetation emerging from rapidly melting icecaps on Baffin Island in the Canadian Arctic, combined with ice and sediment core data from the poles and Iceland and from sea ice climate model simulations, says Miller.
Though scientific estimates regarding the onset of the Little Ice Age range from the 13th century to the 16th century, there is little consensus, says Miller. Evidence suggests the Little Ice Age affected places as distant as South America, China, and, in particular, northern Europe. Advancing glaciers in mountain valleys destroyed towns, and famous paintings from the period depict people ice skating on the Thames River in London and canals in the Netherlands, waterways that were ice-free in winter before and after the Little Ice Age.
"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 Miller. "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," says Miller, a fellow at CU Boulder'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, says Miller.
The new study suggests that the onset of the Little Ice Age was caused by an unusual, 50-year 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 a sea ice-ocean feed-back system 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," says 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."
A paper on the subject is published in Geophysical Research Letters, a publication of the American Geophysical Union. The paper was authored by scientists and students from CU Boulder; the National Center for Atmospheric Research (NCAR) in Boulder; the University of Iceland; 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. …