Could Dust Weaken Hurricanes?

May 28, 2008 — -- The Atlantic hurricane season starts Saturday, and some scientists will try something new this year to unravel the extremely complex mosaic that creates storms of varying intensities. Is it possible, the scientists are asking, that dust storms in Africa might weaken those Atlantic storms before they reach the eastern seaboard?

For several years now, scientists have had evidence that dust from storms across the vast expanse of the Sahara Desert drifts out over the Atlantic where it reflects some solar radiation back into space, thus cooling the ocean waters that fuel hurricanes. Cooler waters should mean fewer, or less intense, storms, according to recent studies.

Amato Evan of the University of Wisconsin-Madison's Cooperative Institute for Meteorological Satellite Studies is hoping that this year could tell something about the effect of Africa's dust storms on Atlantic hurricanes. His computer model indicates a moderate level of dust storm activity for Africa this year, thus a moderate impact on Atlantic storms. It may take more extreme years to answer the question, but at least this is a start.

This year, he said, the amount of cooling will be only about average. But in February Evan came out with a startling claim. At least one third of the recent increase in Atlantic Ocean temperatures is due to a decrease in dust storms, he said. If he's right, dust could play a very important role in determining the strength of hurricanes.

Evan has compiled data from records covering more than a quarter of a century and found a distinct correlation between the amount of dust in the atmosphere and the severity of storms. Low dust years correlate with higher ocean temperatures leading to higher severity. The record-breaking year of 2005 had below-average dust over the Atlantic, very warm sea surface temperatures, and an unprecedented four hurricanes that reached category 5, the highest classification.

So was it the dust that made the difference? It should be pointed out that correlation does not prove causation, so the fact that there was less dust does not prove that dust was related to hurricanes.

It's safe to say that many scientists believe that even if dust contributed significantly, it probably wasn't a major player. One scientist noted that El Nino and La Nina events are likely much more dominant. Dust also tends to dry out the air, and dry air should diminish hurricanes, not intensify them. So there is likely to be much debate over this for several years.

The early predictions for this year's hurricane season are a little worrisome. Ocean temperatures are already one to two degrees higher than average off the coast of Africa this year, according to Phil Klotzbach of Colorado State University, so the storm season could be robust. In the section of the ocean where hurricanes most often spawn, temperatures were higher than they have been in all but three years since 1950.

So Klotzbach's team has predicted 15 named storms, including eight hurricanes. Half should be category 3 or higher, and there is a 70 percent chance that at least one will hit the U.S. coast.

But predicting the weather remains one of the great challenges of modern science. Weather forecasting is so complex, and subject to so many random variables, that it spawned a whole new field of science, called chaos. In other words, there's so many poorly understood things going on that we can't be sure what the heck is going to happen.

For example, dust may impact hurricanes, as Evan suggests, but dust has not been a major tool in storm predictions. There are just too many unanswered questions. And at a time when humans seem to be getting blamed for everything, it's really unclear just how much humans are contributing to the production of dust. In one recent study, scientists at the National Center for Atmospheric Research in Boulder concluded that the human contribution to dust from activities like cultivation and urbanization ranges from 60 to 24 percent of the dust present in the atmospheric. That's a huge gap, but at least we're not to blame for the rest of it.

It may seem logical that global warming would cause more land to dry out, and thus more dust could be blown across the Atlantic, diminishing hurricanes. But these researchers concluded just the opposite. Increased carbon dioxide, one of the principal drivers of global warming, should fuel more vegetation, thus retaining more dust in the soil. So if there's less dust with global warming, the Atlantic should grow even hotter, and the hurricane season more dreadful.

But even Evan isn't sure of that.

"This is just one study, so I wouldn't ring the alarm bells just yet," he said of the possible impact of global warming.

While there is some debate over the role of dust in big storms, there is much less debate about the fact that many conditions around the entire Earth are influenced by something as mundane as dust.

Scientists at NASA's Goddard Space Flight Center say about eight million tons of dust from the Sahara Desert reached as far west as Puerto Rico in one year alone. Or as one scientist put it, that's equivalent to eight million one-ton pickup trucks.

And all that dust does more than just modify the weather. Minerals in the dust, like iron, aluminum, calcium, magnesium and potassium, settles into the ocean to the benefit of some species and detriment of others. Tiny plants, like phytoplankton, should prosper, but that could lead to enormous growth in larger species that feed on the little guys, thus throwing off the balance of nature. And increased iron could cause red tides of toxic algae, killing birds and other wildlife.

Some scientists are very concerned about the possible presence of harmful bacteria in dust, and Florida, which gets about 4 percent of the dust that blows off the Sahara, isn't the only place where people need to worry. In recent years scientists have documented that dust from the great storms of China reach the west coast of North America, possibly carrying pathogens.

So it isn't just dust. It's a powerful, global force.

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.