Plans to whiten the clouds over the worlds oceans so that they reflect some of the suns powerful rays may fail to delay global warming, according to British and Finnish researchers.
Scientists from the Universities 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.
Study author Professor Ken Carslaw from the University of Leeds, said: "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."
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," said Prof 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," said study author Dr Hannele Korhonen, from the Finnish Meteorological Institute.
Prof Carslaw added: "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."
For more information
A copy of the paper entitled: 'Enhancement of marine cloud albedo via controlled sea spray injections: a global model study of the influence of emission rates, microphysics and transport', is available online doi:10.5194/acp-10-4133-2010.
Contact Hannah Isom in the University of Leeds press office on 0113 343 4031 or email h.isom@leeds.ac.uk.
Notes to editors
This work was funded by the Academy of Finland's Centre of Excellence programme, the Natural Environment Research Council (UK-SOLAS programme), and Maj and Tor Nessling Foundation.