With worries about recession in many countries, does it make sense to try out some more radical ideas for fighting global warming, like placing mirrors in the sky to block the sun or fertilising the oceans to soak up greenhouse gases?

They sound like great proposals at first sight: simple,  probably cheaper and in some cases reversible. See a story about the technologies here. But there’s a lot of scepticism among scientists in the U.N. Climate Panel – there could be nasty side effects.

If you spew clouds of tiny particles, such as sulphur, into the upper atmosphere to block out some sunlight, for instance, they will eventually fall to earth and add to smog (think Beijing on bad days before the Olympics). Backers say that volcanoes do the same thing naturally — big eruptions can cool the planet. But who wants to breathe it in every day?

You can also dump iron filings into the seas to spur a bloom of algae which absorb carbon dioxide from the atmosphere and then sink to the bottom when they die and cut the amount of the greenhouse gas in the atmosphere. Sounds great, but it might also make the oceans more acidic — shellfish, crabs, lobsters, etc could then find it harder to build shells and so be more vulnerable to predators.

And if you put a giant barrier in space to reflect sunlight it might cause all sorts of havoc with the climate.

Time Magazine once called such “geo-engineering” options the “Hail Mary pass” - a desperate throw that just might win you the match. 

Is it worth the risk?