Find out if we can cool the planet

OUR profligate greenhouse emissions are creating problems of planetary proportions for our descendants. Even in the best-case scenario, if we make drastic cuts in emissions soon, sea levels will rise by anything from 10 metres to 25 metres over the next few thousand years.

Faced by the loss of so much precious coastal land, it seems quite plausible that our descendants will resort to some kind of mega-project to cool the planet and stop the ice sheets melting. If so, why not do it sooner rather than later? It might save countless lives, not to mention the myriad species otherwise doomed to extinction.

There is no shortage of grand ideas for geoengineering. We could pump cooling sulphur into the atmosphere to disperse incoming sunlight, or generate reflective clouds by spraying seawater heavenwards from special ships. We might even launch an almighty flotilla of parasols into space to shade our planet from the sun.

The problem with all of these schemes is that we have little clue whether they would work. Some of the best evidence so far comes from the cataclysmic eruption of Mount Pinatubo in 1991, which obligingly conducted a large-scale experiment for us on the effect of injecting sulphur into the upper atmosphere. From a global cooling perspective, the results were encouraging: temperatures sank temporarily by up to 0.5 °C. It remains unclear, however, whether the effects of sulphur on global weather patterns can be predicted or controlled. The dangers include triggering severe regional droughts, and even destroying the ozone layer.

Faced with such dangers, it would be foolhardy to do anything yet. What we need is a concerted global research drive into the potential and pitfalls of geoengineering. It will take decades to establish which of the possibilities are feasible, effective and safe, what their costs would be, and for whom. Such a programme - encompassing modelling and small-scale experiments, as well as research into the international legal implications of such schemes - need not be expensive, says Steve Rayner of the University of Oxford. It would be small change compared with, say, what is needed to develop alternative energy technologies.

Despite that, resistance to geoengineering is considerable, and with good reason. In some quarters, geoengineering is already being promoted as an alternative to reducing greenhouse gas levels, rather than as a temporary measure for curbing warming while we get emissions under control. Cooling the planet without curbing carbon dioxide levels won't prevent ocean acidification, whose effects will include the loss of protective coral reefs as erosion outstrips reef-building.

What's more, by deploying geoengineering without also cutting emissions, we could land ourselves in a terrible trap. The higher levels of greenhouse gases rise, the more geoengineering would be required to counteract their warming effect and the longer it would have to go on for. We could suffer unexpected and disastrous side effects from geoengineering but be unable to stop for fear of worse consequences from rapid warming if we did.

That is just the kind of thing a coherent plan of research into geoengineering should investigate. Given the possibility that researchers have underestimated the scale and speed of climate change, and with emissions rising faster than ever, it would be foolish not to investigate what geoengineering might achieve. Is it our best bet for ensuring that Earth remains a benign home to future generations, or a dangerous delusion? We need to find out.