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More research is needed on solar geoengineering techniques that could be used to avoid dangerous climate warming, write more than 60 researchers in published 27 February.

��“While reducing emissions is crucial, no level of reduction undertaken now can reverse the warming effect of past and present greenhouse gas emissions,” says the letter, which was organised by at the University of Washington in Seattle.

There are many possible methods of removing atmospheric carbon, which the UN Intergovernmental Panel on Climate Change has found is needed to limit warming this century to 2°C.

The letter says interventions through what it terms “solar radiation modification” are the fastest way to counter climate warming in the short term, and more research should be carried out to understand their effectiveness and potential consequences for different regions and for the planet as a whole. It says any decision to implement any techniques would require “cooperative international decision-making” based on science.

“We think it’s important that the public understand what is motivating scientists to do research on this topic,” said Doherty in a press statement.

The letter describes three different approaches to solar radiation modification, each of which “slightly change the energy entering and leaving the planet” to have a cooling effect.

Stratospheric aerosol injection would involve spraying aerosols that scatter light, such as sulphur dioxide or calcium carbonate, into the stratosphere. Another technique, called marine cloud brightening, would add sea salt to low clouds over the ocean to make them more reflective. Cirrus cloud thinning would add aerosols to high-altitude cirrus clouds to reduce the infrared radiation they reflect back to the surface.

The letter says none of these approaches have been found to be safe and effective enough to use, and each would come with risks and benefits. It calls for computer model simulations, observations, analytical studies and small-scale field experiments to better understand them.

Sixty-seven researchers had signed the letter as of 27 February, including the physicist James Hansen, well-known for his 1980s testimony to Congress on climate change. Also among the signatories were researchers from a group at Harvard University that had a planned solar geoengineering-related experiment cancelled when it met opposition in Sweden.

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