Originally created 12/16/01

How a dimmer sun cools Earth

Starting in 1611, Galileo Galilei and other astronomers noticed there were fewer sunspots on our sun, and the coldest part of what's known as the Little Ice Age began soon after.

Now, NASA scientists have modeled how the reduced solar activity chilled much of Earth by changing the atmospheric circulation of the Northern Hemisphere from the 1400s into the early 18th century.

During the Little Ice Age, access to Greenland was largely cut off by ice, ending Viking colonization there, canals in Holland routinely froze solid, Londoners ice-skated on the Thames and glaciers advanced down the Alps.

A team led by Drew Shindell of NASA's Goddard Institute for Space Studies and the Center for Climate Systems Research at Columbia University in New York used a computer model to determine that a dimmer sun reduced westerly winds, cooling the continents during winters. They reported their findings Friday in the journal Science.

Based on temperature readings gleaned from tree rings and a limited number of recorded temperature readings, researchers believe that between the mid-1600s and the early 1700s, winter surface temperatures in the Northern Hemisphere appear to have been 2 to 3 degrees Fahrenheit lower than average, the coldest times of the millennium, scientists believe.

"This period of low solar activity in the Middle Ages led to atmospheric changes that seem to have brought on the Little Ice Age," Shindell said. "However, we need to keep in mind that variations in solar output have had far less impact on Earth's recent climate than human actions."

Other climate scientists, including one of Shindell's own colleagues at Columbia, Richard Willson, have reported controversial evidence that increased brightening of the sun in recent decades has played a significant role in global warming.

Until the Space Age, the only yardstick scientists had for solar activity was to count the number and size of sunspots, which coincide with solar flares and a more energetic sun.

Galileo made drawings of lower sunspot activity beginning in 1611, and records from other astronomers a bit later describe an episode called the Maunder Minimum, when there were only about 50 sunspots over 30 years, when typically there would be 40,000 to 50,000 spots over three decades.

Ordinarily, solar flares rise and fall in numbers over an 11-year cycle, but the sun also has longer patterns of brightening and dimming that can run for hundreds or even thousands of years, other researchers have found.

With ground-based instruments unable to really tell how much energy the sun was putting out, only satellite-mounted instruments can determine just how fluky the sun is. "Recent evidence from satellite data has shown that the term 'solar constant' is a misnomer," said British solar researcher Joanna Haigh in a commentary on the new study.

During periods of lower solar activity, levels of the sun's ultraviolet radiation decrease, and cause ozone to build up in the upper atmosphere and block more of the energy received from the sun, further cooling things down, the researchers said. "The changes in the upper atmosphere then feed down to the surface climate," explained Shindell.

But Shindell's team found that the effects of the dimmer sun were concentrated more regionally than globally, and particularly seemed to affect the seasonal jet-stream patterns that keep the coldest Arctic air out of North America and Europe and moderates winter by pumping warmer ocean air over the continents.

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