LiveScience Yahoo News 26 Mar 09;
The warming of Atlantic Ocean waters in recent decades is largely due to declines in airborne dust from African deserts and lower volcanic emissions, a new study suggests.
Since 1980, the tropical North Atlantic has been warming by an average of a half-degree Fahrenheit (a quarter-degree Celsius) per decade.
While that number may sound small, it can translate to big impacts on hurricanes, which are fueled by warm surface waters, said study team member Amato Evan of the University of Wisconsin-Madison. For example, the ocean temperature difference between 1994, a quiet hurricane year, and 2005's record-breaking year of storms (including Hurricane Katrina), was just 1 degree Fahrenheit.
Evan and his colleagues had previously shown that African dust and other airborne particles can suppress hurricane activity by reducing how much sunlight reaches the ocean and keeping the sea surface cool. Dusty years predict mild hurricane seasons, while years with low dust activity - including 2004 and 2005 - have been linked to stronger and more frequent storms.
In the new study, the researchers investigated the exact effect of dust and volcanic emissions on ocean temperatures. They combined satellite data of dust and other particles with existing climate models and calculated how much of the Atlantic warming observed during the last 26 years could be accounted for by simultaneous changes in African dust storms and tropical volcanic activity, primarily the eruptions of El Chichón in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991.
The results: More than two-thirds of this upward trend in recent decades can be attributed to changes in African dust storm and tropical volcano activity during that time.
This was a surprisingly large amount, Evan said.
The results, detailed in the March 27 issue of the journal Science, suggest that only about 30 percent of the observed Atlantic temperature increases are due to other factors, such as a warming climate.
"This makes sense, because we don't really expect global warming to make the ocean [temperature] increase that fast," Evan said.
This adjustment brings the estimate of global warming's impact on the Atlantic more in line with the smaller degree of ocean warming seen elsewhere, such as the Pacific.
Of course, this doesn't discount the importance of global warming, Evan said, but indicates that newer climate models will need to include dust storms as a factor to accurately predict how ocean temperatures will change.
Satellite research of dust-storm activity is relatively young, and no one yet understands what drives dust variability from year to year. And volcanic eruptions are still relatively unpredictable.
"We don't really understand how dust is going to change in these climate projections, and changes in dust could have a really good effect or a really bad effect," Evan said.
More research and observations of the impact of dust will help answer that question.
Less Dusty Air Warms Atlantic, May Spur Hurricanes
Alister Doyle, PlanetArk 26 Mar 09;
OSLO - A decline in sun-dimming airborne dust has caused a fast warming of the tropical North Atlantic in recent decades, according to a study that might help predict hurricanes on the other side of the ocean.
About 70 percent of the warming of the Atlantic since the early 1980s was caused by less dust, blown from Saharan sandstorms or caused by volcanic eruptions, U.S.-based scientists wrote in the journal Science.
Clouds of dust can be blown thousands of kilometers (miles) and reflect some of the sun's rays back into space.
"Since 1980 tropical North Atlantic Ocean temperatures have been rising at a rate of nearly 0.25 Celsius (0.45 F) per decade," they wrote on Thursday.
In the past, the rapid temperature rise had been blamed on factors such as global warming or shifts in ocean currents. Warmer temperatures may spur more hurricanes, which need sea surface temperatures of about 28 Celsius (82.40F) to form.
A sea temperature difference of just one Fahrenheit separated 1994, a quiet hurricane year, from a record 2005 when storms included Hurricane Katrina, which devastated New Orleans, according to a University of Wisconsin-Madison statement.
"We were surprised" by the big role of dust on Atlantic temperatures, said Ralf Bennartz, a professor at the university and a co-author of the study written with experts at the National Oceanic and Atmospheric Administration.
COOLER
In past decades "there was much more dust blowing from (Africa) onto the Atlantic and cooling the sea and ... potentially suppressing hurricane intensity," he told Reuters. No other ocean receives so much dust.
More droughts in Africa in the 1980s, for instance, meant more dust in the air, he said of the study of satellite data and climate models. Annual emissions of dust from North Africa have been estimated at between 240 million and 1.6 billion tonnes.
Bennartz said the scientists were trying to work out, for instance, if wet weather in North Africa could mean less dust and in turn point to fewer hurricanes battering the United States or Caribbean islands.
Big volcanic eruptions were El Chichon in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991. Both dimmed the sun.
The study suggests that only 30 percent of the warming of the Atlantic can be explained by factors other than dust, for instance global warming blamed by the U.N. Climate Panel on emissions of greenhouse gases from burning fossil fuels.
"This makes sense, because we don't really expect global warming to make the ocean (temperatures) increase that fast," said Amato Evan, a researcher at the University of Wisconsin-Madison who was lead author.
Bennartz said it was unclear how climate change might affect overall dust amounts blown from Africa this century.
(Editing by Janet Lawrence)
