Are sunspots prime suspects in global warming?

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

from the September 27, 2007 edition

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This mechanism may account for plunging winter temperatures in the Little Ice Age (1450 to 1850) – at least over land in the Northern Hemisphere, he says.

Another possibility: cosmic rays

But if changes in ultraviolet light tied to sunspot cycles merely stir the climate pot, might something else affect long-term global average temperatures?

Enter galactic cosmic rays. In 1997, Danish researcher Henrik Svensmark and a colleague at the Danish Meteorological Institute injected new life into this debate with the first in a set of papers that suggested a strong correlation between an increase in galactic cosmic rays reaching Earth's surface during low points in the sunspot cycle and increased cloud cover.

The idea of a big effect on climate from cosmic rays is controversial. For instance, the team that studied sunspots and cloud cover over North America found that average cloudiness rose and fell with the sunspot cycle, but didn't track with cosmic ray trends.

Still, a study published last year in Britain showed a small but statistically significant ef­­fect from cosmic rays, notes Rasmus Ben­­estad, who specialized in solar-climate interactions at the Norwegian Meteorological Institute in Oslo. He is highly skeptical that cosmic rays play a big role in climate, he says. But, he adds, the phenomenon is worth exploring.

Dr. Kirkby and colleagues at several institutions aim to do just that. They've designed an aerosol chamber to test how cosmic rays might affect cloud formation and how significant the effect might be. "You really can't settle the issue by more heated debate," he says. "You need experimental data."

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