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Are sunspots prime suspects in global warming?

Climate-change 'optimists' say complex natural cycles may be at the heart of global warming.

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It's a modern-day climate scuffle William Herschel would recognize. He should. He helped trigger it.

In 1801, the eminent British astron­­omer reported that when sunspots dotted the sun's surface, grain prices fell. When sunspots waned, prices rose.

He suggested that shifts in grain prices were a stand-in for shifts in climate. Large numbers of sunspots led to a warmer sun, he reasoned. With more warmth reaching Earth, crop yields would increase, depressing grain prices.

With that, a 200-year hunt began for links between shifts in the sun's output and changes in climate.

No one doubts that the sun drives Earth's climate. Nor do researchers doubt that over long time spans, changes in the level of sunlight reaching Earth's surface leave their imprints on climate.

The vast bulk of research to date, however, points to greenhouse gases – mainly carbon dioxide from burning coal, oil, and natural gas – as the main force behind the current warming trend, most climate scientists say.

Still, over the past decade some researchers say they've found puzzling correlations between changes in the sun's output and weather and climate patterns on Earth. These links appear to rise above the level of misinterpreted data or faulty equipment.

"There are some empirical bits of evidence that show interesting relationships we don't fully understand," says Drew Shindell, a researcher at NASA's Goddard Institute for Space Studies in New York.

For example, he cites a 2001 study in which scientists looked at cloud cover over the United States from 1900 to 1987 and found that average cloud cover increased and decreased in step with the sun's 11-year sunspot cycle. The most plausible cause, they said: changes in the ultraviolet (UV) light the sun delivers to the stratosphere.

Clouds can cool, or clouds can heat

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