Sunspots May Drive Heavy Rains in Africa

Richard A. Lovett
for National Geographic News

August 10, 2007

Heavy rain, flooding, and even outbreaks of bug-borne disease in East Africa may be linked to sunspots, scientists say.

The unusual finding is the result of a new study that found a correlation between peak sunspot activity and severe, mosquito-breeding rains in East Africa.

What drives the dynamic is not clear, said the study's lead author, J. Curt Stager of Paul Smith's College in New York.

The key factor is likely not the sunspots themselves but the slight brightening of the sun that goes along with them, he said.

This brightening isn't severe—only a fraction of a percent—but in the intense sunlight of the tropics, it may be enough to warm the oceans slightly, putting more moisture in the air, Stager said.

At the same time, the land may also warm up, causing the moist air to rise and producing more rain clouds.

"So you get a double-whammy amplifying effect from what would otherwise be a weak solar signal," Stager said.

The solar cycle, which runs in 11-year phases, is also known to affect winds in the upper atmosphere, he noted.

"That can affect the flow of air that makes it rain," Stager said. "So it could be little things that hit certain parts of the world just the right way.

"The basic story is that the rhythm has been steady for more than a hundred years," Stager said of the pattern his team observed.

"It hasn't skipped a beat. If it was just coincidence, it should have drifted off, but it's been right on."

Old Theory Revived

A proposed link between sunspots and African rains is not a new theory.

Early in the 20th century, scientists observed that water levels in Africa's Lake Victoria appeared to fluctuate with the 11-year sunspot cycle (see a map of Lake Victoria region).

But by the middle of the century the relationship weakened, and scientists dismissed the pattern as coincidence.

Theories about what fueled Africa's fluctuating rain cycle turned to El Niño, a periodic Pacific Ocean warming that affects weather worldwide.

(Read related story: "Hurricane Secrets May Be Revealed by African Thunderstorms" [August 3, 2006].)

Stager arrived at the sunspot correlation based on studies of African lakebed sediments. He first noticed a pattern while researching historical records to explain the climate shifts he saw in the sediment layers, he said.

"I got the lake-level data from the last century and saw these pulses that matched the sunspot cycle," he said.

In the new study, Stager and an international team of researchers found the same lake-level fluctuations in two other African lakes, as well as corresponding swings in rainfall.

His team reports its findings in the current issue of the Journal of Geophysical Research-Atmospheres.

J. Marshall Shepherd, a meteorologist at the University of Georgia who was not involved in the study, said Stager's study is credible but added that he's "on the fence" regarding the sunspot connection.

"There are so many other confounding factors to consider," Shepherd said in an email. "More research is required, but they are on the right track."

Stager acknowledges that the mechanism behind the correlation remains unclear.

But whatever the cause, knowing the pattern exists could allow the affected regions to prepare years in advance, he said.

If the pattern holds, he said, heavy rains will likely peak in East Africa about a year ahead of the next spike in sunspot activity in 2011 and 2112.

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