The first three proposed ideas featured in the film, look at reducing the power of the sun - thereby cooling the planet.

Professor Roger Angel from Arizona - the designer of the world's largest telescope - is proposing to put a giant glass sunshade in space.

Professor Angel's sunshade will deflect a small percentage of the sun's rays back into space.

Dutch Professor Paul Crutzen won the Nobel Prize for chemistry when he discovered the causes of the hole in the ozone layer. His plan is to fire hundreds of rockets loaded with tons of sulphur into the stratosphere creating a vast, but very thin sunscreen of sulphur around the earth.

British atmospheric physicist Professor John Latham and engineer Stephen Salter, have designed a fleet of remote-controlled yachts.

These will pump fine particles of sea water into the clouds, increasing the thickness of the clouds and reflecting the suns rays.

Carbon dioxide debate

The other two men in the programme want to tackle the problem of excess carbon dioxide - the cause of global warming.

Sydney engineer Professor Ian Jones proposes to feed plankton with gallons of fertiliser. This will make the plankton grow and absorb carbon dioxide from the air.

And New York-based Professor Klaus Lackner has designed a carbon dioxide capturing machine and his plan is to locate more of them across the globe. They would suck in carbon dioxide, turn it into a powder and he would bury it deep under the ocean in disused oil or gas fields.

Most of the scientists are reluctant advocates of these ideas, and all believe we should be cutting down on our use of fossil fuels to heat our homes and drive our cars.

But is time running out for planet earth?

Although these ideas might have unknown side effects, some scientists believe we may soon have no choice but to put these radical and controversial plans into action.

Five Ways To Save The World was broadcast on 19 February 2007 at 2100 GMT on BBC Two.

Producer/director: Jonathan Barker
Directors: Cecilia Hue & Anna Abbott
Executive producer: Karen O'Connor