These days, it seems a safe bet to blame everything from melting glaciers to mixed-up bird migrations on human-induced global warming. But a new study serves as a reminder that not every weather development is our fault. An international team has linked rainfall intensity in East Africa to the 11-year sunspot cycle. If confirmed, the findings would represent an example of a long-standing climate pattern that remains unaffected by greenhouse gas buildup.
The sun may seem to shine steadily, but its activity actually ebbs and surges (ScienceNOW, 25 April). Solar radiation intensity follows an 11-year cycle that has persisted ever since Galileo began tracking it some 400 years ago; other evidence from the geologic record suggests that the cycle goes back thousands of years. Its peak, called the solar maximum, can produce up to 250 solar eruptions--also known as sunspots--per year, whereas at solar minimum the annual sunspot number can drop almost to zero. However, no one knows exactly how variation in solar radiation affects weather patterns on Earth, particularly rainfall.
Now, U.S. and British researchers have discovered that the sunspot cycle correlates with the intensity of rainfall in East Africa and with the rise and fall of water levels in the region's largest body of water, Lake Victoria. As they report in this week's Journal of Geophysical Research—Atmospheres, over the entire 20th century the three measurements match statistically: About every 11 years, rainfall amounts spike in East Africa. The curious thing, however, is that the peaks of East African rainfall consistently precede the solar maximum by 1 year.
"The timing of the rainfall peaks does seem rather odd," admits geophysicist and lead author J. Curt Stager of Paul Smith's College in Paul Smiths, New York. Ecologist and co-author Piet Verburg of the University of Georgia, Augusta, says the spike in solar radiation that precedes a solar maximum might somehow cause a complex mix of atmospheric changes and warming in the Indian Ocean that intensifies East African rainfall, an effect that diminishes before the actual solar maximum arrives. "At this point we can only guess," he says.
If the evidence holds up, sunspots could prove a valuable tool for predicting local weather patterns. For example, with the next solar maximum due sometime in 2011 or 2012, East Africans should expect more rain in 2010 or 2011, along with an accompanying surge in insect-borne diseases triggered by wetter weather.
The paper "makes a convincing argument" for some kind of interaction between the solar cycle and variation in Victoria's water levels, says limnologist Robert Hecky of the University of Waterloo in Canada. But NASA climatologist Gavin Schmidt doesn't buy the connection. "None of the correlations are significant over the whole period [studied]," he says, because the peaks of rainfall and sunspots don't coincide exactly. "Without a predisposition to want to find a solar signal, this would convince no one."