FEW scientists like to say so, but cutting greenhouse-gas emissions is not the only way to solve the problem of global warming. If man-made technologies are capable of heating the planet, they are probably capable of cooling it down again. Welcome to "geo-engineering", which holds that, rather than trying to change mankind's industrial habits, it is more efficient to counter the effects, using planetary-scale engineering.

This general approach has been kicking around for decades. A paper on climate change prepared for President Lyndon Johnson in 1965 made no mention of cutting greenhouse-gas emissions. It nonchalantly proposed dealing with the results by dumping vast quantities of reflective particles into the oceans, to increase the amount of sunlight reflected into space.

That school of thinking has since fallen out of fashion. As scientists have accumulated evidence for global warming and its possible consequences, so the scientific and political consensus has favoured attempts at reducing carbon emissions through taxes and regulations and subsidies, many of them directed at factories and motor-cars.

More needs to be done. Greenhouse-gas levels have gone on rising. The rapid industrialisation of China and India means they are going to rise even more.

This gloomy outlook has encouraged new interest in a technological fix. A scientific journal, Climatic Change, published a series of papers on the subject in August, including one by Paul Crutzen, a Nobel-prize-winning atmospheric chemist. Other journals followed up. In November the Carnegie Institution and NASA held a conference.

Many big ideas for global cooling have been suggested over the years. They include seeding the skies with compounds to encourage the formation of low-lying, cooling clouds; building a giant sun-shade in space; and dumping iron in the oceans to encourage the growth of algae that would take in carbon when alive and trap it in on the sea floor when dead.

Ken Caldeira, a scientist at the Carnegie Institution, says the most promising idea may be to spray tiny sulphate particles into the upper atmosphere, where they will reflect incoming sunlight. Nature has already done the proof-of-concept work: volcanic eruptions spew such particles into the air, and the cooling effect is well documented.

Schemes of this kind may sound half-crazy; and, admittedly, they do tend to have some technical and aesthetic complications. Deliberately polluting the stratosphere would make the sky less blue, although sunsets would probably be prettier. Blocking out the sun might keep the planet cool, but it would do little to address other effects of high carbon-dioxide levels, such as the acidification of the oceans.

Deliberately polluting the stratosphere would make the sky less blue, but sunsets would probably be prettier

A more fundamental objection is that the models used in geo-engineering are similar to those used in forecasting climate change. Which is to say, they rely similarly on assumptions and extrapolations.

Still, the basic science seems sound. "I started doing this work in an attempt to show that geo-engineering was a bad idea," says Mr Caldeira. "I still think it's a bad idea, but every simulation we do seems to shows it could be made to work."

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”. Others might say the same about its support. One early enthusiast was Edward Teller, an émigré Hungarian physicist known in America as the “father of the hydrogen bomb”, and often cited as an inspiration for Dr Strangelove.

Scientists tend now to see geo-engineering research as a form of insurance policy against the effects of continued global warning, not as an excuse for downplaying the problem, nor for tolerating more carbon emissions in the meantime.

You might expect green groups to applaud this belt-and-braces approach. More often, they resist it in principle, and have little time for the research involved. At worst they seem to see it as a scheme by devious scientists to thwart Nature's just revenge.

Still, there is a reasonable fear here that an illusory hope of a scientific fix might undermine the sort of dogged and grubby policy solutions, such as carbon caps and carbon quotas, needed for taking the fight against climate change to its source.

The precautionary principle, which calls for extra prudence in areas of scientific uncertainty, also applies. You can look at climate change as an experiment which mankind has―to its horror―found itself performing on the planet. To start a second experiment, in the hopes of counteracting the first, would be, to put it mildly, rather risky.