The amount of sunlight reflected back into the sky naturally averages 30 per cent - but in areas covered by snow or ice, it's a healthy 90 per cent. But the ice caps are melting fast.
To counter this, say scientists, we would need to lay vast sheets of reflective material across 1.2 million square miles of the Sahara Desert, at a cost of £20billion over ten years. (A similar idea is to float immense artificial islands of white plastic in the oceans.)
One flaw in the plan is that it would require polyester film to be manufactured in unprecedented quantities, which in itself would add to the planet's carbon emissions.
COST: Upwards of £50billion.
FEASIBILITY RATING: 1/10
Smoke and mirrors: Or a realistic way to save the planet?
A SUNSHIELD IN SPACE
We know that blocking sunlight can counter global warming, so scientists are considering the notion of shading the planet with mirrors.
This would mean sending trillions of ultra-thin lenses into space to deflect the sun's rays.
Arizona University astronomer Roger Angel suggests using electro-magnetically powered spacecraft (a concept NASA is already exploring) to blast the silicon lenses into orbit.
Once in place, they would create a 100,000 square-mile sunshade, reducing the sun's rays by about two per cent.
British-born Prof Angel has already produced the prototype of the lenses. Each is 3ft across, but wafer-thin and etched with a diffraction pattern that will cause the sun's rays to change direction.
In an experiment in a giant hangar, a laser representing the sun was aimed at a model of Earth, with a single lens to protect it. When the laser beam hit the lens, it split into a number of smaller beams that were successfully diffracted away from the model Earth.
COST: An annual bill of £70 billion over 50 years.
FEASIBILITY RATING: 5/10
ARTIFICIAL WEATHER
It has been a long-held dream - creating artificial weather. Now a Scottish engineer says it can be done, by spraying salt into the clouds.
Stephen Salter, of Edinburgh University, and John Latham, a physicist based at the National Center for Atmospheric Research in Colorado, suggest launching unmanned wind powered ships into the Atlantic and Pacific oceans, which would suck up saltwater and spray a fine mist into the air.
This would increase the density and whiteness of the cloud cover - making them reflect four per cent more solar energy back into space. Salter and Latham would like to see a flotilla of 1,500 ships roaming the world's oceans to 'salt' clouds and create the larger cloud droplets needed to reflect solar rays.
An additional immediate effect would be a local cooling of the sea surface, so the technique could be targeted around shrinking polar ice.
COST: Between £1million and £2million for each of the initial 1,500 ships. But this fleet would have to expand by an extra 50 ships a year to keep pace with the current rate of increase in carbon-dioxide levels.
FEASIBILITY RATING: 3/10
Out of this world: An artist's impression of a large asteroid approaching Earth
ARTIFICIAL VOLCANOES
It sounds scary, but a Nobel Prize winning scientist has suggested that we simulate volcanic eruptions to create a sun-blocking haze in the stratosphere.
Atmospheric chemist Paul Crutzen proposes using artillery shells or large balloons full of sulphur-dioxide to create a huge cloud in the atmosphere, ten miles above the Earth, thereby duplicating the effect of a major volcano erupting and blocking out the sun's rays.
He claims he has solid data to support his controversial theory that 'injecting one million tons of sulphur into the atmosphere a year would cool down the climate until the greenhouse effect is wiped out'.
Some environmentalists take the notion seriously. 'This idea should at least be examined,' said Steve Sawyer, a policy adviser for Greenpeace. 'As a last resort, it could buy us a few decades.'
COST: Crutzen says the bill for saving the planet might be £150 billion - though a 'relatively affordable' £20-£30 billion worth would cool the atmosphere for two years.
FEASIBILITY RATING: 2/10
FACTORY FILTERS
Coal-fired power stations pump tons of bad gas into the atmosphere - the 850 new ones planned by China and India by 2012 will produce five times as much pollution as the Kyoto Protocol aims to eliminate.
It has already been proved that super-filters fitted to factory smokestacks can turn pollution into harmless baking soda.
Plastic mesh filters being tested on the smokestacks of Texas power plants trap 90 per cent of the carbon dioxide emitted which, when mixed with sodium hydroxide, is turned into sodium bicarbonate, which can be used in everything from baking to industrial cleaning.
Mark Clayton, of the U.S. company Skyonic, which developed the process, says: 'With the value of the by-products produced (including chlorine and hydrogen), the process can almost pay for itself.'
COST: As little as zero.
FEASIBILITY RATING: 9/10
JELLYFISH FARMS
One way in which the planet might 'heal itself' is revealed in the TV natural history documentary Swarm.
The programme will demonstrate that animal swarms are not all bad news - as in the case of salps; transparent jellyfish-like creatures whose numbers are exploding due to the warming of the oceans.
A single swarm of these little fishy filters consumes 4,000 tons of carbon-absorbing phytoplankton plants every day, removing carbon from the oceans. Scientists propose creating vast oceanic processing plants to breed salps by the trillion, using them as biological filters to cleanse the atmosphere.
COST: £1million per farm, but maintenance costs would be minimal - because nature would do all the work for us.
FEASIBILITY RATING: 8/10
Jellyfish could help the planet 'heal itself' according to scientists
WRAPPING UP GREENLAND
We have been warned that continuing Arctic melting could cause catastrophic worldwide flooding. Could giant blankets be the first step in stemming that tide?
Nobel Prize-winning scientist Dr Jason Box believes we can prevent glaciers from melting by simply covering them with giant blankets that will reflect the power of the sun. The idea was inspired by similar shrouds already used in Alpine ski resorts.
Dr Box, of the Byrd Polar Research Center, Ohio State University, is convinced that his felt-like, man-made material is resilient enough for Arctic conditions - and is already testing the theory.
Ten thousand square metres of specially designed blankets were airlifted to Greenland to lay over more than two acres of the melt zone. The long-term results are awaited.
COST: Covering just the melt zone of Greenland alone would mean manufacturing 770 million blankets at a cost of £250billion.
FEASIBILITY RATING: 4/10
REPLANT FORESTS - FROM THE AIR
Forests are great carbon consumers and can absorb 15 per cent of global emissions - if we keep enough of them. But half of the world's forests have already been cut or burned down and it is estimated that only 20 per cent will remain by 2030.
But American scientists have devised an ambitious plan to speed up the replanting process. How successful this could be will be proved by an experiment taking place now.
Following the devastation of hurricanes Katrina and Rita, thousands of acres of Louisiana mangrove forests were left bare.
The huge swathe of land presented a perfect proving ground for the notion of using aircraft to re-establish the forest by dropping tens of thousands of canisters, each holding a tree seedling which will self-plant itself.
COST: £10million per forest area.
FEASIBILITY RATING: 7/10
MOVE HEAVEN AND EARTH!
We are now in the realm of science fiction, but why not cool down the Earth by moving the planet farther away from the sun? This idea, published in the professional journal Astrophysics And Space Science, involves shifting the Earth's orbit.
All that is needed to achieve this is a usefully sized asteroid, about 60 miles long, say the report's authors.
The asteroid, once located, would be used as a 'gravitational sling shot' by strapping a rocket to it and putting it in an orbit around the solar system. By having the asteroid pass close enough to Earth, it would slowly push the planet farther and farther away from the sun.
There are a couple of drawbacks, of course. Every time the asteroid passed by, it would cause super-storms and tsunamis. If it accidentally collided with Earth, it would eliminate all life on the planet. And if it didn't, it would take 6,000,000,000 years to finish the job.
COST: Out of this world.
FEASIBILITY RATING: 0/10.
