29 January 2010

Ozone hole closure not so cool

by Kate Melville

Using a new global aerosol model and two decades of meteorological data, University of Leeds climatologists have discovered a feedback effect related to the ozone hole's closure that could actually increase warming in the southern hemisphere.

The hole in the ozone layer above the Antarctic was once regarded as one of the biggest environmental threats, but the new discovery shows that it has instead helped to shield this region from carbon-induced warming over the past two decades.

The new Leeds University study, published in Geophysical Research Letters, shows that high-speed winds in the area beneath the hole led to the formation of brighter summertime clouds, which reflect more of the sun's powerful rays.

"These clouds have acted like a mirror to the sun's rays, reflecting the sun's heat away from the surface to the extent that warming from rising carbon emissions has effectively been cancelled out in this region during the summertime," said study co-author Ken Carslaw. "If, as seems likely, these winds die down, rising CO2 emissions could then cause the warming of the southern hemisphere to accelerate."

Key to this newly-discovered feedback is aerosol - tiny reflective particles suspended within the air that are known by experts to have a huge impact on climate. Greenhouses gases absorb infrared radiation from the Earth and release it back into the atmosphere as heat, causing the planet to warm up over time. Aerosol works against this by reflecting heat from the sun back into space, cooling the planet as it does so.

Beneath the Antarctic ozone hole, high-speed winds whip up large amounts of sea spray, which contains millions of tiny salt particles. This spray then forms droplets and eventually clouds, and the increased spray over the last two decades has made these clouds brighter and more reflective. As the ozone layer recovers it is believed that this feedback mechanism could decline in effectiveness, or even be reversed.

"Our research highlights the value of today's state-of- the-art models and long-term datasets that enable such unexpected and complex climate feedbacks to be detected and accounted for in our future predictions," concluded Carslaw.

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Source: University of Leeds