The Sun's Chilly Impact On Earth

During the Little Ice Age, access to Greenland was largely cut off by ice from 1410 to the 1720's. At the same time, canals in Holland routinely froze solid, glaciers advanced in the Alps, and sea-ice increased so much that no open water was present in any direction around Iceland in 1695.

Drew Shindell of NASA's Goddard Institute for Space Studies, New York, N.Y., and other researchers have used a computer model to reconstruct climate and atmospheric conditions from the present back to the Little Ice Age.

They determined that a dimmer sun reduced the model's westerly winds, cooling the continents during wintertime. Shindell's model shows large regional climate changes, unlike other climate models that show relatively small temperature changes on an overall global scale. Other models did not assess regional changes.

During the coldest part of the Little Ice Age, from 1645 to 1715, there is believed to have been a decrease in the total energy output from the sun, as indicated by little or no sunspot activity. Known as the Maunder Minimum, astronomers of the time observed only about 50 sunspots for a 30-year period as opposed to a more typical 40-50,000 spots. The sun normally shows signs of variability, such as its eleven-year sunspot cycle. Within that time, it goes from a minimum to a maximum period of activity represented by a peak in sunspots and flare activity.

Beginning in 1611, Galileo Galilei made drawings of lower sunspot activity before the Maunder Minimum. Records of sunspot activity during the Minimum from other astronomers confirm the lower number of sunspots over the70 year event.

During those periods of low solar activity, levels of the sun's ultraviolet radiation decrease, and can significantly impact ozone formation in the stratosphere. "The changes in ozone that we modeled were key in producing the enhanced response," Shindell said. "The changes in the upper atmosphere then feed down to the surface climate."

Between the mid-1600's and the early 1700's the Earth's surface temperatures in the Northern Hemisphere appear to have been at or near their lowest values of the last millennium. European winter temperatures over that time period were reduced by 1.8 to 2.7 degrees Fahrenheit (1-1.5 Celsius). This cool down is evident through derived temperature readings from tree rings and ice cores, and in historical temperature records, as gathered by the University of Massachusetts-Amherst and the University of Virginia.

Shindell noted that the effects of this period of a dimmer sun were concentrated more regionally than globally. "Global average temperature changes are small, approximately .5 to .7 degrees Fahrenheit (0.3-0.4C), but regional temperature changes are quite large." Shindell said that his climate model simulation shows the temperature changes occurring mostly because of a change in the Arctic Oscillation/North Atlantic Oscillation (AO/NAO).

This oscillation is basically a hemispheric-scale see-saw of atmospheric pressure and temperature between the mid latitudes and the Arctic which modulates the strength of the westerly jet stream winds. These winds are reduced as the AO/NAO shifts in response to a dimmer sun. Because the oceans are relatively warm during the winter due to their large heat capacity, the diminished flow creates cold land temperatures by reducing the transport of warm Pacific air to America, and warm Atlantic air to Europe. During this shift, winter temperatures cooling of as much as 2 to 4 degrees Fahrenheit (1-2C).

The paper, "Solar forcing of regional climate change during the Maunder Minimum," by authors Shindell, Gavin Schmidt and David Rind from NASA's Goddard Institute for Space Studies and co-authors Michael Mann and Anne Waple, from the Universities of Virginia and Massachusetts respectively, appears in the December 7 issue of the journal Science.

"The period of low solar activity in the middle ages led to atmospheric changes that seem to have brought on the Little Ice Age. However, we need to keep in mind that variations in solar output have had far less impact on the Earth's recent climate than human actions," Shindell said. "The biggest catalyst for climate change today are greenhouse gases," he added.