A new study led by INSTAAR Fellow Gifford Miller appears to answer contentious questions about the onset and cause of Earth’s Little Ice Age, a period of cooling temperatures that began after the Middle Ages and lasted into the late 19th century.
According to the study, the Little Ice Age began abruptly between A.D. 1275 and 1300, triggered by repeated, explosive volcanism and sustained by a self-perpetuating sea ice-ocean feedback system in the North Atlantic Ocean. 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.
While scientific estimates regarding the onset of the Little Ice Age range from the 13th century to the 16th century, there is little consensus. 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. There is evidence the Little Ice Age affected places as far away as South America and China, although it was particularly evident in northern Europe. Advancing glaciers in mountain valleys destroyed towns, and paintings from the period depict people ice skating on the Thames River in London and canals in the Netherlands, which were ice-free in winter before and after the Little Ice Age.
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 a sea ice-ocean feedback system originating in the North Atlantic Ocean.
Radiocarbon dates obtained from samples of dead plant material collected from beneath receding ice margins of ice caps on Baffin Island show a large cluster of “kill dates” between A.D. 1275 and 1300, 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. The team saw a second spike in plant kill dates at about A.D. 1450, indicating the quick onset of a second major cooling event.
“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.”
A paper on the study is being published Jan. 31 in Geophysical Research Letters. The paper was authored by scientists and students from CU-Boulder, the National Center for Atmospheric Research 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.
The paper in Geophysical Research Letters is being actively discussed in Andy Revkin’s Dot Earth blog on the New York Times web site.
One of the primary questions pertaining to the Little Ice Age is how unusual the warming of Earth is today, he said. A previous study led by Miller in 2008 on Baffin Island indicated temperatures today are the warmest in at least 2,000 years.