June 5, 2006 - Cleaner air and more Atlantic hurricanes may come as a pair, according to a new study comparing rising global sea surface temperatures with sun-blocking pollution particles.

It turns out that the recent decline of small manmade pollution particles "called aerosols" in the North Atlantic might be allowing hurricane activity to catch up with the effects of global warming there, reported climate researchers Michael Mann and Kerry Emanuel in a new study in the journal Eos.

The newfound powerful role of aerosols throws out the need for a largely theoretical "Atlantic Multi-decadal Oscillation" (AMO) that's been called on by some researchers to explain North Atlantic temperature changes over the last century.

"It's kind of an Occam's razor argument," said Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology, referring to the maxim that the right explanation is usually the one that's least complicated, requiring the fewest assumptions. "It really fits beautifully."

Emanuel and Mann modeled the warming effects of accumulating greenhouse gases in the atmosphere along with changes in aerosols, which filter sunlight and cool the ocean surface. The researchers found the observed sea surface temperatures could be easily explained without any AMO.

Instead, according to this new view, the recent rise in hurricanes can be traced to the end of World War II, when the US, Europe and the Soviet Union kicked up industrial output.

"From the 1940s to the 1970s aerosols increased enormously in the Northern Hemisphere," confirmed Gavin Schmidt, a climate researcher at NASA's Goddard Institute for Space Studies. The aerosols essentially dimmed sunlight over the North Atlantic and masked the effect of global warming there over those decades.

Then in the 1970s and 1980s, the US and Europe instituted clean air regulations and the Soviet Union collapsed. These changes reduced or flattened aerosol emissions trends in the western Northern Hemisphere.

Since aerosols have a lifespan of only a few weeks in the atmosphere, the effect on the climate was almost immediate: Clearer air over the North Atlantic allowed more sunlight to reach and warm up surface waters.

Meanwhile, much longer-lived greenhouse gases have continued to mount -- also supplying more heat to the oceans.

"There is a very good correlation of North Atlantic hurricanes and the temperature of the North Atlantic in the early fall," Emanuel told Discovery News. The warmer the water at that time of the year, the rowdier the hurricane season appears to be. "It's an astoundingly good correlation."

In other words, ever since the air got cleaner over the North Atlantic, hurricanes have been playing catch up with global warming.

Elsewhere in the world, however, the aerosols are still holding sway, said Schmidt. While the West has been reducing aerosols, India and China have been increasing their emissions. In the Indian Ocean the cooling effects of aerosols has been recently tied to powerful Indian Monsoons.

Source: http://dsc.discovery.com/news/briefs/20060605/hurricane_pla.html