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March 27, 2009

Less Dust Responsible For N. Atlantic Warming, Hurricanes

A new study reported Thursday finds that lessening airborne dust has caused a rapid warming of the tropical North Atlantic in recent decades.

The sun-dimming dust, blown in from sandstorms in the Sahara or caused by volcanic eruptions, reflects a portion of the sun's rays back into space.  This decline in dust was responsible for 70 percent of the warming in the Atlantic since the early 1980s, according to the study.

"Since 1980 tropical North Atlantic Ocean temperatures have been rising at a rate of nearly 0.25 Celsius (0.45 F) per decade," wrote scientists from University of Wisconsin-Madison and the National Oceanic and Atmospheric Administration (NOAA) in the journal Science.

Warmer ocean temperatures may spur more hurricanes, which require sea surface temperatures of about 82.40F to develop. A sea temperature change of just one degree Fahrenheit separated 1994, a quiet year for hurricanes, from the record-setting storms of  2005, which included the devastating Hurricane Katrina.

In the past, the swift rise in North Atlantic water temperatures had been blamed on factors such as shifts in ocean currents or global warming. 

"We were surprised" by the significant role of dust on Atlantic water temperatures, said Ralf Bennartz, a University of Wisconsin-Madison and a co-author of the study.

In previous decades "there was much more dust blowing from (Africa) onto the Atlantic and cooling the sea and ... potentially suppressing hurricane intensity," he said during an interview with Reuters.

The Atlantic is the only ocean that receives so much dust.

A higher number of droughts in Africa during the 1980s, for example, caused more dust to be in the air, Bennartz said of the study of climate models and satellite data. 

Dust emissions from North Africa have been estimated at between 240 million and 1.6 billion tons per year.

Scientists are trying to determine, for instance, if wetter weather in North Africa could translate into less dust, which would in turn result in fewer hurricanes hitting the U.S. or Caribbean islands.

Bennartz said that the volcanic eruptions of El Chichon in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991 had both dimmed the sun.

The research suggests that just 30 percent of the warming of the Atlantic can be attributed to factors other than dust, such as global warming.

"This makes sense, because we don't really expect global warming to make the ocean (temperatures) increase that fast," said Amato Evan, the study's lead author and a researcher at the University of Wisconsin-Madison, in an Reuters interview.

Bennartz said it was not yet clear what impact climate change would have on overall dust amounts blown in from Africa this century.

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Image Caption: A dust storm off the coast of Morocco was imaged by NASA's MODIS Aqua meteorological satellite on March 12, 2009.  Photo: courtesy Amato Evan

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