With a solar shield, temperatures would be roughly the same as in 1900 (c), but precipitation would drop (d). Without the shield, temperatures would rise dramatically (a), and precipitation would increase in some regions and drop in others (b) (Image: PNAS/Caldeira/Matthews)
Solar shields are not a new idea - such "geoengineering" schemes to artificially cool the Earth's climate are receiving growing interest, and include proposals to inject reflective aerosols into the stratosphere, deploying space-based solar reflectors and large-scale cloud seeding.
The shields are inspired by the cooling effects of large volcanic eruptions that blast sulphate particles into the stratosphere. There, the particles reflect part of the Sun's radiation back into space, reducing the amount of heat that reaches the atmosphere, and so dampening the greenhouse effect.
Ken Caldeira at the Carnegie Institution of Washington, in California, US, and Damon Matthews at Concordia University, Canada, used computer models to simulate the effects that a solar shield would have on the Earth's climate if greenhouse gas emissions continued to rise along a "business as usual" scenario.
"We have been trying to pinpoint the one really bad thing that argues against geoengineering the climate," says Caldeira. "But it is really hard to find."
His computer models simulated a gradually deployed shield that would compensate for the greenhouse effect of rising carbon dioxide concentrations. By the time CO2 levels are double those of pre-industrial times - predicted to be at the end of the 21st century - the shield would need to block 8% of the Sun's radiation.
The researchers found that a sulphur shield could act very quickly, lowering temperatures to around early 20th-century levels within a decade of being deployed.
"The trouble is, the decadal timescale works both ways," says Caldeira. A sulphate shield would need to be continuously replenished, and the models show that failing to do so would mean the Earth's climate would suddenly be hit with the full warming effect of the CO2 that has accumulated in the meantime
And the ease with which they could work is also risky, he says: "These schemes are almost too cheap and easy. Just one fire hose spraying sulphur dioxide into the atmosphere would do the job for a century. That would cost about $100 million - nothing in comparison to the hundreds of billions it would take to transform our energy supply."
A previous article on other climate modification proposals
Caldeira has also looked at Gregory Benfords proposal
Benford has a proposal that possesses the advantages of being both one of the simplest planet-cooling technologies so far suggested and being initially testable in a local context. He suggests suspension of tiny, harmless particles (sized at one-third of a micron) at about 80,000 feet up in the stratosphere. These particles could be composed of diatomaceous earth. "That's silicon dioxide, which is chemically inert, cheap as earth, and readily crushable to the size we want," Benford says. This could initially be tested, he says, over the Arctic, where warming is already considerable and where few human beings live. Arctic atmospheric circulation patterns would mostly confine the deployed particles around the North Pole. An initial experiment could occur north of 70 degrees latitude, over the Arctic Sea and outside national boundaries. "The fact that such an experiment is reversible is just as important as the fact that it's regional," says Benford.
Is Benford's proposal realistic? According to Ken Caldeira, a leading climate scientist at Stanford University and the Carnegie Institution's Department of Global Ecology, "It appears as if any small particle would do the trick in the necessary quantities. I've done a number of computer simulations of what the climate response would be of reflecting sunlight, and all of them indicate that it would work quite well." He adds, "I wouldn't look to these geoengineering schemes as part of normal policy response, but if bad things start to happen quickly, then people will demand something be done quickly."