Volcanic eruptions emerge as lead cause for Little Ice Age

The team gathered additional evidence from sediment cores taken from a glacial lake in central Iceland. The team found two bands of sediment that quickly thickened: one dated to the 1300s, and the other to the 1400s. The sediment entered the lake when an ice cap that feeds the lake expanded, increasing the amount of erosion created.

The sediment confirmed the onset of cooling for the periods in question. And it established the cooling as hemisphere-wide.

The team then turned to climate models to figure out how eruptions could effect such a change. With successive eruptions over the course of a decade or so, enough SO2 would get kicked into the stratosphere to cool the temperatures over a longer period than a one-off blast. Modeling indicated that this would chill the Arctic Ocean, allowing sea-ice to thicken in winter. Thicker ice is more likely to survive the summer melt season, reflecting sunlight back into space and preventing the ocean from warming as much as it does when sea-ice cover shrinks. Because the ocean heats and cools very slowly, once a cooling trend sets in, it's hard to reverse.

Even so, more ice also would be available to move into the North Atlantic during the spring and summer. The addition of cold fresh water would have chilled and slowed ocean currents that otherwise would warm eastern North America and Europe in winter.

The new research is consistent with other recent studies of the Little Ice Age, notes Michael Mann, a climate scientist at Pennsylvania State University, in State College. The global cooling was attributable to volcanic activity. But the regional distribution and intensity of cold temperatures owed much to the sun's weakened output, he adds in an e-mail.

The sun produces most of its radiation as ultraviolet light, which sees the largest swings in intensity with the rise and fall of the sunspot cycle. Most of the ultraviolet light is intercepted by ozone in the stratosphere; the UV light builds ozone molecules there. Ozone is a greenhouse gas. When solar radiation declines, so does stratospheric ozone, cooling the stratosphere. This changes wind patterns there. These changes eventually work their way into wind patterns in the lower atmosphere, as well.

In a paper in the journal Science in 2009, Dr. Mann and colleagues showed how these shifting patterns during the Little Ice Age – when the sun entered a pronged period of weakened radiation – would have thrown natural climate swings into one phase more often than the other.

For instance, regional temperature reconstructions from the period indicated that La Niña was appearing more frequently and persisting longer than might otherwise be the case – one sign a weaker sun was shuffling warm and cold regions, rather than contributing significantly to a worldwide cooling.

Indeed, as if to underscore a regional, rather than global, effect from reduced solar radiation, Miller's team didn't vary the sun's output in the modeling phase of its study; they held it constant – implying that the Little Ice Age would have occurred in some form even with the sun's weaker output.

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