Little Ice Age: What caused it?

The Little Ice Age is the name for a period of widespread cooling on Earth. Scientists don’t agree on when it started and ended, but it’s generally agreed to have lasted into the 19th century. Its beginning point is less certain. Still, it’s known that northern Europe felt cooling temperatures. Advancing glaciers in mountain valleys destroyed European towns. Paintings from the 1600s depict people ice-skating on the Thames River in London and on 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. Scientists don’t know exactly what caused the Little Ice Age – but there are theories.

A recent theory came in early 2012. It’s the idea that an unusual, 50-year-long episode of four massive tropical volcanic eruptions triggered the Little Ice Age between 1275 and 1300 A.D.

Those dates might correspond with the beginning of the Little Ice Age, which might have begun as early as the 13th century. Meanwhile, others say the beginning was more like the 16th century.

The scientists with the volcano theory – at University of Colorado Boulder with co-authors at the National Center for Atmospheric Research (NCAR) and other organizations – have evidence for volcanic eruptions between 1275 and 1300 AD. They say these volcanoes 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. Their study was published in January 2012 in Geophysical Research Letters.

These UC Boulder and NCAR scientists 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.

The scientists also analyzed patterns of dead vegetation, and ice and sediment core data, at high northern latitudes to retrieve evidence for the volcanoes. Lead author Gifford Miller of the University of Colorado Boulder said:

This is the first time anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age. 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.

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 A.D., 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 A.D., 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.

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.

Bottom line: What caused the Little Ice Age, a period of cooling that’s generally agreed to have ended in the 19th century. One idea is that decreased radiation from the sun caused this period of widespread cooling on Earth. In early 2012, scientists at University of Colorado Boulder with co-authors at the National Center for Atmospheric Research (NCAR) and other organizations announced evidence suggesting that volcanoes caused the Little Ice Age. They used radiocarbon-dating of samples of dead plant material, collected from high northern latitudes, in combination with a computer model, to show that four massive volcanoes could have triggered the widespread cooling. Their study was being published in Geophysical Research Letters in January 2012.

August 9, 2012

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