They are the ultimate technological fixes: schemes that will span our planet and involve scientists in reshaping our world to save it from global warming. Yet only a few years ago, such projects - giant space mirrors, flotillas of artificial cloud makers and ocean fertilisation programmes - were dismissed as the stuff of science fiction.
Today many engineers and researchers - fearful of the rate at which our planet is warming - say geo-engineering projects are now mankind's only hope of saving itself from the impact of climate change. A major report and a new exhibition at the Science Museum starting next week will resurrect the debate.
Despite 10 years of international negotiations aimed at reducing carbon dioxide levels by between 60-80 per cent, global emissions are still rising. The only hope, say geo-engineers, is to change the planet, alter its oceans and reshape its cloud cover.
It is a point highlighted by Brian Launder, professor of mechanical engineering at Manchester University, who was once 'neutral' about these great geo-engineering projects but who has come to believethat current attempts to reduce CO2 emissions are doomed to failure.
'As time has gone on I have become increasingly concerned about the lack of progress on climate change and [although] they once seemed a last resort, I have to say we're going to need to do this.'
Launder is now editing a forthcoming issue of the Philosophical Transactions of the Royal Society which will be devoted to the subject of geo-engineering schemes. 'We're moving, but I think we need to go a lot further.'
An exhibition - Can Algae Save The World? - opening at the Science Museum will also focus on hi-tech projects aimed at saving the planet.
The latest assessment report by the Intergovernmental Panel on Climate Change, published earlier this year, considered three major techniques to reduce sunlight reaching the Earth: orbiting mirrors, sulphur particle schemes and projects for enhancing cloud cover.
The ideas 'could have beneficial consequences' by increasing agricultural productivity and forestry, the panel concluded. Carbon dioxide would be left in the atmosphere, stimulating plant growth, while reductions in sunlight would stop temperatures from rising even as CO2 levels continued to increase.
'Geo-engineering is one of the types of thing that are worth investigating,' says Ken Caldeira, of the Carnegie Institution of Washington. 'If we can generate 100 ideas, and 97 are bad and we land up with three good ones, then the whole thing will have been worthwhile.'
Opponents to such schemes point out that it is technology that got mankind in its current fix. An even bigger dose of technology is therefore the last thing the planet needs. Schemes for fertilising the oceans with iron compounds pose immense risks to marine life, for example. Geo-engineers defend their schemes by pointing out that emissions of greenhouse gases are already bringing huge changes to natural ecosystems.
It is a point stressed by the distinguished ecologist James Lovelock: there are dangers in intervening but the risks posed by doing nothing are worse. 'There may be all sorts of ecological consequences,' he said. 'But then the stakes are terribly high.'
Ocean pumps
Two of Britain's leading environmental thinkers, Chris Rapley, head of the Science Museum, and James Lovelock, creator of the Gaia concept, suggest vertical pipes could pump deep cold water to the sea surface. Cold ocean water is considered to be more 'productive' than warmer water because it contains more lifeforms. And these lifeforms are vital for absorbing CO2.
Using special valves, cold water would be made to flow up floating pipes and out on to the ocean surface, bringing increased numbers of lifeforms into contact with the atmosphere and its carbon dioxide. These lifeforms would absorb CO2, die and then sink to the ocean floor, storing the carbon away for millennia.
Marine biologists point out that the scheme could pose major problems for sea life, in particular for creatures such as whales and porpoises.
Chance of success: 3/5 Impact on marine life could count against the scheme.
Sulphur blanket
During major volcanic eruptions, the Earth often undergoes significant cooling. For example, when Mount Pinatubo in the Philippines erupted in 1991 the average temperature across the Earth decreased by 0.6C. Scientists pointed the finger of blame at the 10 million tonnes of sulphur that the volcano ejected into the stratosphere. So why not copy Pinatubo? That is the suggestion of Professor Paul Crutzen who won a Nobel prize in 1995 for his work on the ozone layer.
He has proposed creating a 'blanket' of sulphur that would block the Sun's rays from reaching Earth; to do this, he envisages hundreds of rockets filled with sulphur being blasted into the stratosphere. About one million tonnes of sulphur would be enough to create his cooling blanket, he says.
The idea alarms other scientists, who fear such a massive input of sulphur into the upper atmosphere could increase acid rain or damage the ozone layer. Crutzen believes his idea may still be necessary if Earth continues to warm up at its current rate. 'I am prepared to lose some bit of ozone if we can prevent major increases of temperature, say beyond two degrees or three degrees,' he says.
Chance of success: 1/5 Risks of acid rain and ozone depletion will provoke opposition.
Mirrors
Radiation from the Sun heats our planet and sustains life here. But as Earth warms up, scientists want to cut that radiation and one of the most ambitious ideas involves firing giant mirrors into its orbit.
Physicist Lowell Wood, at the Lawrence Livermore National Laboratory in California, has put forward the idea of using a mesh of aluminium threads, a millionth of an inch in diameter. 'It would be like a window screen made of exceedingly fine metal wire,' he explains. The screen wouldn't completely block sunlight but would filter infra-red radiation.
However, such mirrors would be expensive to make and put into orbit. To produce a 1 per cent cut in solar radiation would require mirrors with surface areas of 600,000 square miles. But once in space such mirrors would be extremely cheap to operate.
'It's very hi-tech,' said John Shepherd, professor of marine science at the National Oceanographic Centre at Southampton University. 'Who knows whether they can really do it? And it's going to cost a lot of money to find out.'
Chance of success: 1/5 Incredibly expensive.
Cloud shield
John Latham, at the National Centre for Atmospheric Research in Colorado, and Stephen Salter, of Edinburgh University estimate that increasing cloud cover using a seawater spray 'seeding' process could increase cloud cover by 4 per cent - enough to counter a doubling of carbon dioxide in the atmosphere by shielding Earth from solar radiation.
Their plan is one of the cheaper ideas for countering rising carbon dioxide levels and is relatively low-tech, leading to hopes that, if computer simulations give good results, a field trial could start in five years.
Latham acknowledges there are dangers in changing weather patterns. 'We certainly shouldn't implement [it] in any global sense until we've done our best to examine what implications it might have,' he says.
'But if one felt that there are unlikely to be any implications that are more severe than the damage global warming is causing, then I think we'd begin.'
Chance of success: 2/5 Will need major global commitment to succeed.
Synthetic trees
Planting trees that absorb carbon dioxide has become a major eco-industry. But now scientists are proposing a surprise technological variant: synthetic trees. These trees would not grow or flower or leaf - but they would absorb carbon dioxide.
This startling concept is the brainchild of Klaus Lackner of Columbia University who first outlined his proposal at an annual meeting of the American Association for the Advancement of Science. He describes his synthetic trees as looking like 'goal posts with Venetian blinds'.
Lackner has calculated that one of his synthetic trees could remove about 90,000 tonnes of carbon dioxide in a year - the output of more than 15,000 cars and a thousandfold improvement on the natural behaviour of a real, living tree.
Lackner's concept is a variant of carbon sequestration technology which involves the seizing of carbon and storing it underground. Already schemes exist for removing carbon dioxide produced by burning coal, gas or oil at power plants before it reaches the atmosphere. Other projects are investigating ways to liquefy this carbon dioxide and store it in old mines or oilfields.
However, the process does not work for all polluters, in particular cars and lorries - hence Lackner's synthetic trees which would act like filters, removing carbon dioxide from that atmosphere. An absorbent coating, such as limewater, on slats would capture carbon dioxide so that it could be removed and then buried. However, critics say the scheme suffers from the fact that engineers could end up expending more energy in capturing carbon dioxide than they would save.
Chance of success: 4/5 Carbon sequestration is likely to play a major role in the world's battle against climate change, though perhaps not in the form of synthetic trees.
Forests of the seas
Blooms of plankton and algae are the grasslands and prairies of the oceans. They absorb carbon dioxide, die and then sink to the seabed carrying the carbon dioxide they absorbed during their lifetimes. Increase such blooms and you could take out more and more carbon dioxide from the atmosphere, scientists argue - an idea that formed the core of a recent meeting of experts at the Woods Hole Oceanographic Institution in the US.
The favoured method for stimulating plankton growth is to use iron fertilisers. It is known that tiny amounts of iron are critical in stimulating phytoplankton growth in seas. However, in many parts of the world iron in seawater is virtually non-existent and plankton levels correspondingly low.
Several groups of US entrepreneurs have begun experiments aimed at correcting this problem by pumping tonnes of soluble iron compounds into sea areas. Several trial schemes are now under way. But some critics warn that very little carbon dioxide would be removed from the atmosphere this way, while there is a danger such schemes could cause dangerous pollution.
Chance of success: 2/5 Method already in trials, but faces considerable opposition over potential damage to marine life.
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