Geo-Engineering for Global Warming

This week’s Economist discusses “Plan B for global warming,” which is to implement measures directed at cooling the earth’s temperature.

It’s an idea which is pragmatic (it acknowledges that collectively we aren’t likely to take concerted enough action to stop, let alone reverse, the rising levels of greenhouse gases that are producing higher temperatures). But it’s also fraught with risk. One of the problems with the recent Intergovernmental Panel on Climate Change’s report is that weather is one of the most complicated modelling problems. And climate change is an even more complicated problem. We don’t have enough computing power, let alone enough understanding. So even thought these ideas may sound compelling, and worth implementing, the second order effects are unknown, and may be significant. It’s likely that their impact won’t be evenly distributed, that atmospheric or ocean heat transport patterns could be disrupted, which could in turn produce dramatic local effects.

Science fiction fans will note that this is a precursor to terraforming, the process by which alien planets are altered to make them more fit for human life. It’s a sad irony that the alpha version may be attempted on our own planet.

From the Economist:

If man is inadvertently capable of heating the entire planet, surely it is not beyond his wit to cool it down as well?….

Of all the schemes proposed, the most ambitious (and expensive) idea would be to place a giant sunshade in space at the inner Lagrange point, the position on the line between the Earth and the sun where the combination of centripetal and gravitational forces allows an object to maintain a constant position between the two. If the object is big enough, it could block out enough of the sun’s rays to cool the Earth. Roger Angel, an astronomer at the University of Arizona, has suggested assembling a cloud of millions of small, reflecting spacecraft less than a metre across at this point, where together they would block out 1.8% of the sun’s rays.

Dr Angel estimates that the total mass of the sunshade required would be around 20m tonnes. The shade would consist of individual craft around one metre across, put into position using a combination of magnetic launchers and ion propulsion. He believes the total cost of the project would be a few trillion dollars, or less than 0.5% of world GDP. Dr Angel admits that this is a somewhat far-fetched solution, and does not believe it would be attempted unless all other options had failed. But he has been given a small grant by NASA to explore the idea.

A less exotic approach, endorsed by Dr Crutzen, would be to spread tiny particles in the upper atmosphere to reflect the sun’s rays. This effect has already been shown to work in nature: fine sulphate particles, called aerosols, ejected by large volcanic eruptions like that of Mount Pinatubo in 1991, have produced periods of global cooling. And sulphate pollution from industry had similar consequences, helping to balance the warming effects of carbon dioxide until the 1990s, when pollution controls in many regions had the perverse effect of increasing warming.

Ken Caldeira, a scientist at the Carnegie Institution, suggests that this idea might be more suited to local rather than global application, at least at first. The Arctic, for example, is among the regions most affected by global warming, and keeping the polar sea-ice frozen would be a good thing: white ice reflects more heat back into space than dark ocean, and the scheme would also save a few polar bears from drowning.

The most down-to-earth idea is that proposed by John Latham, a scientist at the National Centre for Atmospheric Research in Colorado. He suggests that blasting tiny droplets of seawater into the air would stimulate the formation of highly reflective, low-lying marine cloud. Simulations suggest this would have a substantial cooling effect. The question is how to do it economically. Stephen Salter of the University of Edinburgh has designed an unmanned vessel which would produce these clouds using wind power. Just 50 vessels, he reckons, each costing a few million dollars and spraying around 10kg (22lb) of water per second, could cancel out a year’s worth of global carbon-dioxide emissions—though another 50 vessels would be needed every year until carbon-dioxide emissions were under control.

Dr Salter’s ships would be much more precise than other geo-engineering schemes—“like an artist’s paintbrush”, as he puts it. They could be deployed to the North Atlantic to cool the Greenland ice sheet during the northern summer and then migrate to Antarctica for the southern summer. Dr Caldeira even suggests that by cooling the sea, these ships could be used to combat hurricanes, since high sea-surface temperatures are linked to hurricane formation.

Other proposals include seeding the oceans to get them to absorb more carbon dioxide and building huge reflectors in desert regions to reflect sunlight back into space. This latter idea is impractical, says Dr Caldeira, who reckons that half the world’s deserts would have to be covered. Indeed, most geo-engineering schemes sound half-crazy and tend to have both technical and aesthetic complications. Deliberately polluting the stratosphere would make the sky less blue, although sunsets would probably be prettier. Blocking out the sun would help to cool the planet, but it would do little to address other nasty side-effects of high carbon-dioxide levels, such as the acidification of the oceans.

Many greens oppose the whole idea in principle. Ralph Cicerone, president of America’s National Academy of Sciences, has said that geo-engineering inspires opposition for “various and sincere reasons that are not wholly scientific”. But it does seem reasonable to worry that the illusory hope of a scientific fix might undermine the adoption of policy solutions, such as carbon caps and carbon quotas, designed to address the underlying cause of the problem. And then there is the danger of unintended consequences. Climate change is arguably an experiment which mankind has unwittingly found itself performing on the planet. To start a second experiment in the hopes of counteracting the first would be risky, to put it mildly.

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