Geoengineering would turn blue skies whiter

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.

That would have knock-on effects for life – and human technology. The reduction in direct sunlight would impact the solar industry, which relies on direct sunlight to generate much of its power. But the increased indirect sunlight would boost photosynthesis beneath tree canopies. The most visible effect, though, would be above us.

The blue colour of the clear sky comes from light being scattering by molecules in the air. The scattering is much stronger for short blue wavelengths than for longer red wavelengths. Aerosol particles are much larger than molecules in the air, however, and they scatter red light more strongly, which washes out the blue light scattered by smaller molecules and makes the sky brighter and whiter.

Kravitz calculated how scattering from particles ranging from 0.1 to 0.9 micrometers in diameter would affect the spectrum of the scattered light, and how that would affect the colour of the sky. He found the sky would appear paler for all potential diameters. Particles with diameters in the middle of the range would make for much whiter skies.

The effect would be most visible in the countryside, where air pollution is generally lower, says Kravitz. "All you'd have to do to see it is to step outside."

Important uncertainties remain, including what size aerosols would be used for geoengineering and how their sizes might change over time as particles stick together. But Craig Bohren, a meteorologist and expert in atmospheric scattering at Pennsylvania State University in University Park, who was not involved in the research, says "it's difficult to argue against the claim that increasing the concentration of particles in the atmosphere will change the colour and brightness of the sky".