Volcanoes might have triggered the Little Ice Age

A new international study might answer longstanding questions about the Little Ice Age. Scientists have announced that four massive tropical volcanoes – which appear to have erupted between 1275 and 1300 AD – might have set into motion a planet-wide cooling that ultimately caused the Little Ice Age. These scientists have evidence for the volcanic eruptions, which they say triggered a chain reaction, affecting sea ice and ocean currents in a way that lowered temperatures for centuries. Their results are in contrast to the work of other scientists who contend that decreased radiation from the sun is what caused the Little Ice Age.

The Frozen Thames, a painting by Abraham Hondius from 1677 

The Little Ice Age was not a true ice age, but it was a period of widespread cooling on Earth. Northern Europe felt the cooling temperatures and advancing glaciers in mountain valleys destroyed towns. Paintings from the 1600s 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. Places as far away as South America and China might also have cooled.

Although the Little Ice Age is generally agreed to have lasted into the 19th century, its beginning is less certain. Some sources suggest it began as early as the 13th century; others say it was more like the 16th century.

Scientists at University of Colorado Boulder with co-authors at the National Center for Atmospheric Research (NCAR) and other organizations say their evidence favors an early start for the Little Ice Age. They suggest that an unusual, 50-year-long episode of four massive tropical volcanic eruptions triggered the Little Ice Age between 1275 and 1300 AD. They used a computer model to show that subsequent expansion of sea ice and a related weakening of Atlantic currents caused the persistence of cold summers following the eruptions.

 Gifford Miller collected vegetation samples on Baffin Island. He and other scientists analyzed patterns of dead vegetation, and ice and sediment core data, at high northern latitudes to retrieve evidence for four massive volcanoes that might have triggered the Little Ice Age. 

The scientists analyzed patterns of dead vegetation, and ice and sediment core data, at high northern latitudes to retrieve evidence for the volcanoes. Miller and his colleagues radiocarbon-dated roughly 150 samples of dead plant material with roots intact, collected from beneath receding margins of ice caps on Baffin Island in the Canadian Arctic. They found a large cluster of “kill dates” between 1275 and 1300 AD, indicating the plants had been frozen and engulfed by ice during a relatively sudden event.

The team saw a second spike in plant kill dates at about 1450 AD, indicating the quick onset of a second major cooling event. 

The researchers also 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 suddenly became thicker in the late 13th century, they said, and again in the 15th century as the climate cooled.

The team used the Community Climate System Model, which was developed by scientists at NCAR and the Department of Energy with colleagues at other organizations, 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 to 1700 A.D., showed several large, closely spaced eruptions could have cooled the Northern Hemisphere enough to trigger the expansion of Arctic sea ice.

The model showed that 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. This weakened heat transport back to the Arctic and created a self-sustaining feedback on the sea ice long after the effects of the volcanic aerosols subsided, according to the simulations.

This figure summarizes sunspot number observations. During the long Maunder Minimum, almost no sunspots were observed. Some contend that the absence of sunspots correlated to a decrease in solar radiation, which caused the Little Ice Age. 

Meanwhile, there is still the idea of decreased radiation from the sun, as evidenced by, for example, a decline in visible spots on the sun during the period of the Little Ice Age. The researchers addressed that question by setting solar radiation at a constant level in their climate models. They said the simulations indicated that the Little Ice Age likely would have occurred without decreased summer solar radiation at the time.