Friday, November 20, 2009

Less dust from Africa warms the tropical North Atlantic Ocean, creating conditions for more hurricanes

Less dusty air warms Atlantic, may spur hurricanes

by Alister Doyle, Environment Correspondent, Reuters, March 26, 2009

OSLO (Reuters) -- 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% 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 °C (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 °C (82.4 °F) to form.

A sea temperature difference of just one degree 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.

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% 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)

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