Computer simulations are showing how tiny variations in the Sun's brightness can have a big influence on weather above the Pacific Ocean.

The simulations match observations that show precipitation in the eastern Pacific varies with the Sun's brightness over an 11-year cycle. However, the model does not indicate a relationship between solar activity and the rise in global temperature over the past century.

"This is not a global warming thing," says Gerald Meehl, a modeller at the National Center for Atmospheric Research in Boulder, Colorado, and first author of the study. "But it does indicate that the Sun has a measurable impact on Earth's climate." The research is published this week in the journal Science1.

Although the Sun burns steadily, its shifting magnetic fields can lead to cooler, darker spots on the Sun's surface. The edges of these sunspots are much brighter than the rest of the Sun, and although this causes only a tiny increase in the Sun's total output of light over the 11-year cycle, researchers believe that it can influence Earth's climate. Many scientists think that a cold snap between 1645 and 1715, for example, may have been caused by an unusually spotless Sun. Researchers had also noticed that precipitation patterns in the Pacific Ocean seemed to vary with the 11-year sunspot cycle, with the average rainfall in the eastern Pacific seeming to drop during periods of high solar activity.

Solar puzzle

But how could such a tiny change in brightness influence weather over the world's largest ocean? Two theories have circulated in recent years. The first is that an increase in ultraviolet radiation associated with the brighter Sun changes the temperature of the upper echelons of the atmosphere. Those changes alter the winds over the tropics, and eventually lead to a drought in the east.

The second theory is that the increased brightness is heating the Pacific itself. The heating intensifies evaporation and rainfall in some regions, but creates cooling winds in the eastern part of the ocean, which prevents rain clouds from forming in those eastern areas.

Both theories seemed plausible, but when they were inserted separately into the models, neither produced an effect that was big enough to explain the observations, Meehl says. So he and his colleagues tried combining the two into a single model. "Sure enough, we got a much bigger response," he says.

The work is a good piece of modelling, but not all parts of the puzzle are in place, says Drew Shindell, a climate modeller at the NASA Goddard Institute for Space Studies in New York. Combining the two theories does seem to produce a model that replicates the magnitude of the sunspot cycle's impact. But the results of the simulation are far from being a perfect geographical match for real observations (see 'A model forecast'). "I think it's a nice step," Shindell says. "But there's clearly still a long way to go."

"I don't think we're claiming we've solved the problem," Meehl says. But he maintains that the model clearly replicates the general trends seen in the Pacific. He expects that as atmospheric scientists, oceanographers and others combine their different models in the coming years, their predictive power will only improve.