Are sunspots prime suspects in global warming?

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

Others claim to have linked shifts in levels of cosmic rays reaching deep into the atmosphere to changes in average cloud cover. Depending on how thick and how high they are, clouds either cool the planet by reflecting sunlight back into space or act as a blanket and trap heat. The valve controlling the flow of cosmic rays from deep space is the sun's magnetic field – which shifts with sunspot activity.

But this broad line of inquiry faces an enormous credibility problem, Dr. Shindell notes. From Herschel's day through the early 20th century, scientists have offered correlations that "fall apart the longer you look at them," he says.

Moreover, when scientists report a new correlation, some enthusiastic advocates go beyond what the data show and imbue it with too much significance. Such is the case with cosmic rays, many scientists say, whose poorly demonstrated ties to cloud formation have nevertheless been touted in the public arena – if not the scientific arena – as an explanation for most of the warming in the 20th century.

To say that current warming trends are "all cosmic rays and no carbon dioxide is totally ludicrous, in the same way that people say that it's all [human-induced] carbon dioxide and nothing natural. That is equally ludicrous," says Jasper Kirkby, a physicist who is actively exploring potential links between cosmic rays and clouds at CERN, Europe's center for high-energy physics research in Geneva.

"Climate is a cocktail," he explains. "The effect of cosmic rays on clouds – if there is a significant effect – will be part of the mix. The question is: Is it a significant part of the mix, or insignificant?"

Mainstream scientific skepticism about a strong direct link between changes in the sun's output and today's global warming stems from a tiny shift in sunlight.

Generally, peak periods of high sunspot activity deliver more sunlight to the top of the atmosphere than periods of minimum activity. Scientists measure this "total solar irradiance," which includes infrared and ultraviolet light as well as visible light.

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