Blue skies would fade to hazy white if geoengineers inject light-scattering aerosols into the upper atmosphere to offset global warming. Critics have already warned that this might happen, but now the effect has been quantified.

Releasing sulphate aerosols high in the atmosphere should in theory reduce global temperatures by reflecting a small percentage of the incoming sunlight away from the Earth. However, the extra particles would also scatter more of the remaining light into the atmosphere. This would reduce by 20 per cent the amount of sunlight that takes a direct route to the ground, and it would increase levels of softer, diffuse scattered light, says Ben Kravitz of the Carnegie Institution for Science in Stanford, California.

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