A solar shield that reflects some of the Sun's radiation back into space would cool the climate within a decade and could be a quick-fix solution to climate change, researchers say.

Because of their rapid effect, however, they should be deployed only as a last resort when "dangerous" climate change is imminent, they warn.

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.

The 1991 eruption of Mount Pinatubo in the Philippines cooled Earth by a few tenths of a degree for several decades.
Quick-acting

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.

"So if you have the shield up there and it fails - or, for example, the Republicans put up a shield and then the Democrats come in to power and turn it down - then you effectively compress into a decade or two the warming that would have happened while the shield was up," Caldeira explains.
Poorly understood

A solar shield would not necessarily stunt plant growth. In fact, there is some evidence that plants grew more vigorously after Mt Pinatubo erupted because the sulphate particles increased the amount of diffuse light and boosted growth in shaded areas. But if a shield was suddenly removed, a portion of the CO2 stored in plants would be suddenly released as the plants respire faster in warmer temperatures.

"Personally, as a citizen not a scientist, I don't like geo-engineering because of the high environmental risk," Caldeira told New Scientist. "It's toying with poorly understood complex systems."

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."

But he also believes it is time to consider solar shields seriously. On 1 June, James Hansen, head of NASA's Institute for Space Studies in the US, published a paper stating that Earths' climate system has reached a tipping point (Atmospheric Chemistry and Physics, vol 7, p 2287).
Lesser of two evils?

Hansen's study suggests that only moderate additional warming is likely to trigger the disintegration of the west Antarctic and Arctic ice sheets - events which would be near-impossible to reverse.

"If this is the case, then I am not clear on what the 'greenest' path is," says Caldeira. "Is it better to let the Greenland ice sheet collapse and let the polar bears drown their way to extinction, or to spray some sulphur particles in the stratosphere?"

He says that if forced to consider deploying a solar shield, "we would need to be confident that we would not be creating bigger problems than we are solving. Therefore, it is important both to understand the mess we are in today - how close are we to making irreversible changes, how fast can we alter our energy system - and to understand what might happen should we try to avoid some of the worst outcomes by engineering our climate".

Caldeira and Matthews also found that a solar shield would not correct abnormalities in rainfall. Most notably, the tropics would receive less rain than in the absence of the greenhouse effect, as predicted by climate change models.

It is especially remarkable that the practical conception of freedom is based upon the transcendental idea, and that the question of the possibility of the former is difficult only as it involves the consideration of the truth of the latter. Freedom, in the practical sense, is the independence of the will of coercion by sensuous impulses. A will is sensuous, in so far as it is pathologically affected (by sensuous impulses); it is termed animal (arbitrium brutum), when it is pathologically necessitated. The human will is certainly an arbitrium sensitivum, not brutum, but liberum; because sensuousness does not necessitate its action, a faculty existing in man of self-determination, independently of all sensuous coercion. It is plain that, if all causality in the world of sense were natural—and natural only—every event would be determined by another according to necessary laws, and that, consequently, phenomena, in so far as they determine the will, must necessitate every action as a natural effect from themselves; and thus all practical freedom would fall to the ground with the transcendental idea. For the latter presupposes that although a certain thing has not happened, it ought to have happened, and that, consequently, its phenomenal cause was not so powerful and determinative as to exclude the causality of our will—a causality capable of producing effects independently of and even in opposition to the power of natural causes, and capable, consequently, of spontaneously originating a series of events. Here, too, we find it to be the case, as we generally found in the self-contradictions and perplexities of a reason which strives to pass the bounds of possible experience, that the problem is properly not physiological, but transcendental. The question of the possibility of freedom does indeed concern psychology; but, as it rests upon dialectical arguments of pure reason, its solution must engage the attention of transcendental philosophy. Before attempting this solution, a task which transcendental philosophy cannot decline, it will be advisable to make a remark with regard to its procedure in the settlement of the question. If phenomena were things in themselves, and time and space forms of the existence of things, condition and conditioned would always be members of the same series; and thus would arise in the present case the antinomy common to all transcendental ideas—that their series is either too great or too small for the understanding.