It's the stuff of science fiction, but could mirrors in space or sea water sprayed in the air be shortcuts to halt global warming?
"It's Dr Strangelove. But it's the kind of Dr Strangelove you could see governments really using."
That's how one expert describes geo-engineering - the idea that we can use a kind of technical quick fix to cool the planet if global warming accelerates.
Plans for geo-engineering can sound bizarre.
Smoke and mirrors to cool the planet?
They range from placing millions of tiny mirrors in space to reflect back some of the sun's rays, to using rockets to launch tons of sulphur into the stratosphere to create a kind of planetary sun shade.
That plan was inspired by watching what happened after the eruption of the Mount Pinatubo volcano in the Philippines in 1991.
Sulphur ejected into the atmosphere spread around in subsequent months to create a layer believed to have had a temporary cooling effect as it blocked some of the sun's warmth.
Other suggestions include spraying sea water into the atmosphere to make it cloudier, or pumping carbon dioxide out of the atmosphere or out of the oceans.
Until recently, policymakers have dismissed this as science fiction, a complete distraction from the fight against global warming. Now, attitudes seem to be changing.
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"I think we're faced with such an enormous problem that we need to do all the research we can to see if there are any geo-engineering proposals which work through to the marketplace," says Professor Sir David King, until recently the government's chief scientific adviser.
There are still many scientific doubts about geo-engineering. What might the side effects be? Are such schemes irreversible?
But as there is now so much pessimism about whether governments will ever agree to reduce carbon emissions enough, more and more scientists say we need to know exactly what our other options are.
Wait till it's too late?
If we don't do any research, says Professor Brian Launder of Manchester University, who is editing a new study of geo-engineering for the Royal Society, "we won't have anything that we can bring into place in 2030 say, when suddenly the world is at a crisis point".
Some forms of geo-engineering are also surprisingly cheap. That leads to fears that governments facing particular climatic problems might go it alone.
China and India, which have growing scientific capabilities, could use geo-engineering as a way of challenging international climate policy if they saw it as too skewed towards the interests of Western countries.
Or, even more alarmingly, an individual might decide to play with the global climate.
Professor David Victor, of Stanford University, imagines a scenario in which someone is frustrated at the lack of international action. "[They] could buy the aircraft and buy the rockets and just start doing some geo-engineering off their own island."
James Bond films of the future, he adds, might not feature Goldfinger. It could be Greenfinger, hand hovering over the global thermostat.
Running on empty
So how can geo-engineering be policed? It's a major challenge.
While climate change treaties try to persuade everyone to do the same things and reduce emissions, agreements on geo-engineering would be about stopping something happening - something we don't yet understand.
Efforts to cut emissions falter
But Sir David King says we should at least begin to discuss it as part, and still a minor part, of the climate change policy debate.
"We need to make sure that there is control and validation over any of these procedures. But at the same time let's not take attention away from the major issue of removing our dependence on fossil fuels."
The dilemma is painful. Discuss a technical fix for future climate change, and people assume there's less need to cut carbon emissions now. Ignore it, and possibly face a kind of climate anarchy.
Others suggest geo-engineering should be embraced with enthusiasm, such as Julian Morris, of International Policy Network, a think tank sceptical about climate change and in favour of free market solutions.
"Investments in geo-engineering research are almost certainly the biggest bang for the buck that one could get in terms of addressing catastrophic climate change - a much, much bigger return than, for example, trying to control carbon emissions at the moment," he says.
"In fact diverting money into controlling carbon emissions and away from geo-engineering is probably morally irresponsible."
There is no one obvious solution
Most scientists and governments say geo-engineering remains hazardous and is only a partial fix. They hope it will never be needed. But if global warming becomes more and more threatening, some will see it as the lesser risk.
Professor Scott Barrett, of Johns Hopkins University in Baltimore and an expert on international environmental agreements, says he ignored geo-engineering for many years.
Now, faced with the failure to reduce emissions enough, he says there are no easy options.
"[Geo-engineering] is not something we're going to want to use as a first choice. I think the chances are likely that it will eventually be used, for better or worse, in circumstances in which the risks of not using it seem to be higher than the risks of using it."
Below is a selection of your comments.
It very much strikes me as an excuse for doing nothing. Hoping that some odd thought experiment will save the world in the manner of a Hollywood science fiction blockbuster. The answer itself is a much more simple than its made out to be. Healthier power supply options are readily available. We're just tangled in the grip of the fossil fuel industries. Carbon dioxide levels can be reduced by long-term forestation. As long as you don't cut down the trees they'll happily absorb and convert CO2 for centuries. Individuals are readily capable of change. In the face of rubbish mountains and the credit crunch, people are recycling and cutting energy use through simple, common-sense measures. We just have to wait for industry and governments to catch up.
Just move the planet a bit further out than its current orbit around the sun. When the Earth cools down a bit, it can be moved back in. All it needs is a powerful rocket to give a tow or a line attached to one of the large outer planets that can act as a winch anchor. So simple!
Mr Gaia, London
Bizarrely I was recently thinking of the ice aircraft carriers proposed during WWII and the potential for using pykrete to create artificial ice sheets to help reflect sunlight.
Andy, Brighton, UK
Is this news? Governments all around the world have already been permitting reflective chaff to be sprayed into the atmosphere for over ten years now. They were presumably informed that the spraying of heavy metals into the atmosphere, although creating a significant health risk, would great an "albedo" (whiteness) around the globe, reflecting a sufficient percentage of the sun's rays back into outer space to reduce "global warming". This operation (involving hundreds of small, silver, unmarked tanker-aircraft) can be witnessed by anyone in the UK (even those in far-out Wales and Scotland) should they care to look at their increasingly spray-obscured skies from time to time. In my experience it is clearly visible twice a week on average.
Timothy Johnson, Esher, UK
Though many in the green lobby are dead against even talking about geo-engineering, for fear of encouraging polluters, we need to have a look at the options. It only takes a problem with one main link in the planetary food chain for a real catastrophe, and it already looks like the eco-sphere may be more fragile than even the most pessimistic observers have been predicting. James Hansen, for one, thinks we've already gone beyond the point of no return. I'm concerned that the options we look at are holistic. If we shade the planet judiciously but leave acidic oceans, there are still likely to be major collapses. For this reason I favour encouraging algae to photosynthesise CO2 out of the atmosphere in areas of blue (biotically dead) marine space. It could even provide abundant food for fish that we have driven close to extinction. We need to start looking at the world as a deeply interconnected organism.
Daniel Johnston, Sheffield, England
The trouble with quick fixes, or even the suggestion of them, is that they do not address the core issues causing the problems and encourage a "business as usual" approach.
James Trevaskis, Taunton
We should be converting the sun's heat to power with massive plants in the world's deserts. As was recently proposed by a scientist in Time magazine. The very fact of that energy not heating the ground will reduce the temperature too.
Richard H, Essex
Many years ago I heard of a scheme to alter the climate of the Sahara by "planting" vast numbers of palm trees and similar made from recycled plastic waste. The idea being to create an instant forest under which real trees could be grown by altering the conditions under the canopy. Might be something to try that does not involve the sorting, cleaning and collection of PET plastic be semi-slave labour in India.
Adam Colledge, Czech Republic
One of the feedback loops making matters worse is the way a dark area absorbs heat whereas a white area reflects it. So the loss of snow cover in mountain ranges and ice cover at sea is helping the earth to warm faster. Could we put reflectors or white paint in place of snow and white surface covering in place of sea ice? Also, if we covered the Sahara with solar PV panels, apart from generating electricity, what effect would that have?
VivR, Bristol, UK
Do these people not watch Futurama? They invented this years ago.
No easy options? I beg to differ. I'm doing everything I can during my lifetime to minimise my environmental impact but most importantly, I'm not leaving any children to deal with the fallout of our selfish lifestyles. Can I recommend everyone else does the same? We're one species this planet really doesn't need.
None of that rubbish would work. We can't predict the current weather system beyond a couple of days, how the hell can we justify making major modifications when we have no way of knowing the result? Global warming doesn't bother me, pollution is my greatest concern.
Dan, Loughborough, UK