Geoengineering would turn blue skies whiter

June 1, 2012

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”.

(Journal reference: Geophysical Review Letters, DOI: 10.1029/2012gl051652)

Source = New Scientist

 Source = BBC News

Geoengineering for Global Warming: Increasing Aerosols in Atmosphere Would Make Sky Whiter

May 31, 2012

One idea for fighting global warming is to increase the amount of aerosols in the atmosphere, scattering incoming solar energy away from Earth’s surface. But scientists theorize that this solar geoengineering could have a side effect of whitening the sky during the day. New research from Carnegie’s Ben Kravitz and Ken Caldeira indicates that blocking 2% of the sun’s light would make the sky three-to-five times brighter, as well as whiter. Their work is published June 1st in Geophysical Research Letters, a journal of the American Geophysical Union. Carbon dioxide emissions from the burning of coal, oil, and gas have been increasing over the past decades, causing Earth to get hotter and hotter.

Large volcanic eruptions cool the planet by creating lots of small particles in the stratosphere, but the particles fall out within a couple of years, and the planet heats back up. The idea behind solar geoengineering is to constantly replenish a layer of small particles in the stratosphere, mimicking this volcanic aftermath and scattering sunlight back to space. Using advanced models, Kravitz and Caldeira — along with Douglas MacMartin from the California Institute of Technology — examined changes to sky color and brightness from using sulfate-based aerosols in this way. They found that, depending on the size of the particles, the sky would whiten during the day and sunsets would have afterglows. Their models predict that the sky would still be blue, but it would be a lighter shade than what most people are used to looking at now.

The research team’s work shows that skies everywhere could look like those over urban areas in a world with this type of geoengineering taking place. In urban areas, the sky often looks hazy and white. “These results give people one more thing to consider before deciding whether we really want to go down this road,” Kravitz said. “Although our study did not address the potential psychological impact of these changes to the sky, they are important to consider as well.” There are several larger environmental implications to the group’s findings, too.

Because plants grow more efficiently under diffuse light conditions such as this, global photosynthetic activity could increase, pulling more of the greenhouse gas carbon dioxide out of the atmosphere. On the other hand, the effectiveness of solar power could be diminished, as less sunlight would reach solar-power generators. “I hope that we never get to the point where people feel the need to spray aerosols in the sky to offset rampant global warming,” Caldeira said. “This is one study where I am not eager to have our predictions proven right by a global stratospheric aerosol layer in the real world.”

Source = Science Daily