Merry Christmas

It's raining in San Diego.

Apparently rainfall is correlated to the Sun's magnetic activity: Blame the Sun for a Cloudy Day?

An Australian researcher has linked the sun's magnetic activity to rainfall patterns in his country over the past century. The connection is solid enough that meteorologists might be able to use it to make better long-term weather predictions. But experts remain cautious about the wider implications of the findings.

Scientists have long known that the sun plays a key role in Earth's weather patterns. For example, the number of sunspots on its surface--dark zones of intense magnetic activity--peaks about every 11 years, followed by a period of dormancy. The cycle causes swings in sea-surface temperatures--more sunspots mean warmer oceans, and fewer mean chillier waters--but the effect is small. There's also a 22-year cycle, in which the sun changes the polarity of its magnetic field, but it's unclear how that phenomenon affects Earth.

Now geographer Robert Baker of the University of New England, Armidale, in Australia, has linked solar magnetic activity to Earth's climate--at least regionally. Using sunspot counts and Australian meteorological data, as well as NASA satellite data for more recent years, he tracked sunspots and rainfall in Australia from 1876 to 2006. In this month's issue of Geographical Research, Baker reports that the amount of rainfall in most regions of the country tracked the 22-year magnetic cycle almost exactly. "It was unbelievable," Baker says. At the height of magnetic activity, rainfall across most of the country was plentiful. At the other end of the cycle, many of those same regions experienced severe droughts. The findings are particularly compelling, Baker says, because even though the lengths of the magnetic cycles are not precise and can vary by several years, the rainfall patterns followed them.

So what's behind the connection? Baker thinks it has to do with the amount of ultraviolet (UV) radiation hitting Earth. When the reversing of polarity approaches, he explains, the sun's magnetic field weakens, allowing more UV energy to reach our planet. More UV radiation kills off some of the oceans' plankton, which produce dimethyl sulfide, one of the primary atmospheric chemicals involved in cloud formation, and fewer clouds mean less rainfall.

Based on the 130 years of data, Baker predicts that the current solar cycle, which reached a minimum in 2007, will continue a bit longer. In fact, he says, "there could be a 100-year minimum in solar activity," meaning much of Australia could experience a prolonged drought.

"This could be an important paper," says climatologist John Christy of the University of Alabama, Huntsville. He explains that current climate models don't give the solar effect much weight in general, because scientists think it is overwhelmed by the buildup of greenhouse gases in the atmosphere. But if there's a mechanism by which the sun's variations are tied directly to weather patterns, such as the effect of UV radiation on cloud formation, he says, the sun may have a greater impact than the models are showing. As a result, the models might not be creating an accurate picture for the future.