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Cool the climate with cloud whitening?

U. LEEDS (UK)—Plans to “whiten” the clouds over the world’s oceans so that they reflect some of the sun’s powerful rays may fail to delay global warming, according to British and Finnish researchers.

Scientists from the University of Leeds and the Finnish Meteorological Institute found that injecting sea spray into the atmosphere to generate brighter clouds may actually hinder natural cloud formation in some areas.

Human manipulation of the climate—known as geoengineering—has been suggested by scientists as a way of slowing down global warming while efforts to reduce greenhouse gas emissions continue.

Cloud whitening (or cloud seeding) is one proposed form of geoengineering that would use off-shore vessels to pump sea spray into the atmosphere. The salt particles in the spray would in theory help to form brighter clouds that reflect more sunlight back into space, thus having a cooling effect on climate.

Previous studies using climate models have suggested that whitening the clouds could cool the planet enough to allow for a doubling of current CO2 levels. However, this latest study, published in Atmospheric Chemistry and Physics, shows that the sea spray emitted could interfere with existing cloud-forming particles known as aerosol.

The researchers used a global computer model to look at how the artificial emissions of sea spray would impact on the concentration of cloud drops, which is what controls changes in cloud brightness.

“Our research suggests that attempts to generate brighter clouds via sea spray geoengineering would at best have only a tiny effect and could actually cause some clouds to become less bright,” says study author Ken Carslaw, a professor at the University of Leeds.

Natural cloud drops form on tiny particles, or aerosols, suspended in the atmosphere. The higher the concentration of cloud drops the more they reflect the sun’s rays back to space.

Over oceans much of this aerosol is in the form of tiny sea spray particles blown off the surface. The geoengineering proposal is to enhance this natural aerosol source to make clouds brighter.

“The formation of clouds from artificial sea spray is particularly sensitive to background levels of aerosol,” says Carslaw. “This means that injecting spray around coastal areas where there is a lot of air pollution from land may not produce enough extra cloud drops to stave off global warming. In fact, in some locations the artificial spray particles may hinder natural drop formation and could have an opposite effect on climate to that intended.”

“Another problem with the proposed technology is that the efficiency of artificial sea spray generation decreases as the wind speed drops, so you can’t emit much spray where you really need it,” says study author Hannele Korhonen from the Finnish Meteorological Institute.

Carslaw adds: “While the concept of geoengineering may be appealing, in practice it appears that generating a uniform covering of reflective clouds over large regions of the world’s oceans would be extremely challenging.”

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