A new study suggests that more sunspots mean less intense hurricanes on Earth. But many hurricane experts are cool on the idea.

James Elsner, a climatologist at Florida State University in Tallahassee, has analyzed hurricane data going back more than a century. He says he has identified a 10- to 12-year cycle in hurricane records that corresponds to the solar cycle, in which the Sun's magnetic activity rises and falls.

Solar activity varies on a roughly 11-year cycle, in which its magnetic activity waxes and wanes. Credit: NASA/TRACE

The idea is that increased solar activity - associated with sunspots - means more ultraviolet radiation reaching the Earth's upper atmosphere. That warms the airs aloft and decreases the temperature differential between high and low elevations that otherwise would fuel hurricanes.

"Our results indicate that there is an effect in the intensity of storms due to the higher temperatures aloft," says Elsner, who published the results on 19 September in Geophysical Research Letters1.

He says the statistical analysis suggests a 10% decrease in hurricane intensity for every 100 sunspots. At the peak of its cycle, the Sun might exhibit around 250 sunspots.

Hot debate

Establishing such a relationship would be enormously valuable, providing researchers, meteorologists and insurance companies with another tool for predicting storms and assessing financial liabilities. Several hurricane experts called the study provocative but raised questions about statistical analysis, as well as the physical processes at play.

Ka-Kit Tung, a climate researcher at the University of Washington in Seattle who has studied the solar cycle, says ultraviolet rays do most of their heating when they are absorbed by ozone higher in the stratosphere. But hurricanes don't make it past the tropopause, the boundary layer between the troposphere and the stratosphere that is about 16 kilometres up at storm latitudes. "We have not established that there is heating at the tropopause due to the solar cycle," Tung says.

For the new study, Elsner and a post-doctoral student, Thomas Jagger, used more than a century of records on hurricanes that reached land in the United States as a proxy for records on hurricane intensity. The assumption is that the effect on overall hurricane intensity would make itself evident as more or fewer hurricane-strength storms made land. But not everybody buys the idea that frequency can be reliably translated into intensity.

"This is something worth investigating, but they made too many assumptions for me to just accept their conclusion at this point," says Judy Curry, a hurricane researcher at the Georgia Institute of Technology in Atlanta. She and others questioned whether the influence of solar cycles would be strong enough to appear in such data sets.

In search of a mechanism

Earlier this month Elsner published a paper in Nature2 providing more evidence that rising ocean temperatures will increase the intensity of major storms around the globe. He says the solar-cycle pattern emerged after his team had already taken into account major factors such as sea-surface temperatures.

The team also found a correlation when comparing sunspot records with daily intensity values for storms dating back to 1944, using "best-track" records from the National Hurricane Center. "It gives us some faith that we are probably on to something here," Elsner says.

The effect would be felt in areas where sea-surface temperatures are high enough to produce big storms, he says. In marginal areas, where waters are cooler, solar activity could promote storms by providing the initial heat to seed a hurricane.

"It's an interesting statistical finding, but I'm unclear on the mechanisms here," says Thomas Knutson, a hurricane researcher with the US National Oceanic and Atmospheric Administration in Princeton, New Jersey. "There's more work needed to see if there really is a physical effect and to flesh out the mechanism."