A break in the clouds

Published online on May 9, 2012, in Nature.

Clouds and aerosol particles have bedevilled climate modellers for decades. Now researchers are starting to gain the upper hand.

Seen from space, Earth can look dressed up or downright dowdy, depending on the location. In some spots, swathes of cloud cloak the dark ocean, offering a stunning contrast of hues. In others, power plants spew out plumes of grey haze and desert storms cover vast regions in palls of dust. Together, those clouds and the fine particles, which are known as aerosols, do more than just obscure the planet’s surface. By reflecting, absorbing and emitting radiation, they have a major role in setting Earth’s temperature and have proved maddeningly difficult to simulate in atmospheric models. For decades, they have been the biggest sources of uncertainty in forecasts of future climate.

But researchers say they are beginning to turn a corner in simulating clouds and aerosols. In recent months, climate scientists have started rolling out initial results from the newest generation of models, which represent atmospheric chemistry and microphysics in much more sophisticated ways than previous incarnations. These models allow clouds and aerosols to evolve as they interact with each other and respond to factors such as temperature, relative humidity and air currents. And early results suggest that such processes have a much greater impact on regional climate than scientists had realized. Recent studies have shed light on the roles that clouds and aerosols might have in triggering major African droughts, altering Arctic climate and weakening the monsoon in southern Asia.

“This is fundamentally new science,” says Ben Booth, a climate modeller at the UK Met Office Hadley Centre in Exeter, who is investigating how aerosols influence surface temperatures in the North Atlantic Ocean and affect the weather on the surrounding continents. “The new generation of models is changing the kinds of questions we face as scientists.”

And more science is coming soon. Leading climate-modelling groups around the world are racing to work up their latest results for the Intergovernmental Panel on Climate Change (IPCC), which is due to release its fifth report section by section in 2013 and 2014. It is already clear that the issue of aerosols and clouds will provide some of the biggest surprises. “This is the real wild card,” says Ron Stouffer, a climate researcher at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey.

The drought-makers

Each day, the winds that sweep east across North America stir up a witch’s brew of atmospheric refuse. Power plants belch out sulphur dioxide gas, which evolves into sulphate particles that reflect sunlight and serve as seeds for clouds. Microscopic specks of carbon rise from vehicles, steel smelters, agricultural fires and other sources. The brighter carbon particles scatter the Sun’s rays and dark ones absorb them, processes known as the direct aerosol effect. As the particles ride the air currents eastward, they collide with each other and mix with natural dust and ocean spray to form the load of atmospheric aerosols. Over time, they can build up chemical coatings or merge to form new particles with different properties.

The prevailing winds carry this aerosol stew on a long horseshoe-shaped route around the Atlantic basin (see graphic). The particles are first transported eastward across the ocean, then take a right turn down the coast of France, gathering up more pollution from Europe. The aerosol-laden air curves towards the west coast of North Africa before veering westward and riding tropical air currents back towards America.

Scientists have proposed that this arc of aerosols could block enough sunlight to cool sea surface temperatures in the Atlantic Ocean and alter the regional climate. So Booth and fellow researchers at the Hadley Centre tested the idea with their newest model, which simulates not only the direct aerosol effect but also many of the indirect effects that aerosols have on cloud properties.

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