The Dutch meteorologist showed what he calls the positive cooling effect of adding a layer of sulphates to the atmosphere at a global warming conference at the Porter School for Environmental Studies in Tel Aviv.

He said new, detailed calculations carried out since August showed the project would indeed lower global temperatures.

"Our calculations using the best models available have shown that injecting 1 million tonnes of sulphur a year would cool down the climate so the greenhouse effect is wiped out," Crutzen told Reuters.

An added layer of sulphates in the stratosphere, some 10 miles (16 km) above the earth, would reflect sunlight into space and reduce solar radiation reaching the earth's surface, Crutzen said.

He said he envisioned giant cannons or balloons dispersing the sulphur to offset the build-up of greenhouse gases such as carbon dioxide, largely released by burning fossil fuels in power plants, factories and vehicles.

The world has struggled for decades to reduce sulphur pollution, a component of acid rain that kills forests and fish, mainly through tighter controls on burning coal.

"We are now entering a very intensive period of model calculations and following that we will conduct small experiments to test the sulphur oxidation mechanisms that we calculated," Crutzen said.

NO LONGER TABOO

Crutzen said he planned to publish the new findings in a few months' time in one of the major scientific journals.

The idea of using sulphur to combat global warming -- which most scientists say will bring more floods, desertification, heatwaves and rising sea levels -- is not new.

Scientists noticed that large volcanic eruptions had similar effects and the 1991 eruption on Mount Pinatubo in the Philippines lowered temperatures around the world for two years.

For decades the theory was dismissed as dangerous until Crutzen, who won the 1995 Nobel Prize in Chemistry for research on ozone, published his paper.

"Until August this was a taboo issue. But the paper I published really set off some movement in this area. It never hit the level of seriousness which it has taken in the past months. It may have had to do with the Nobel Prize, but I hope that's not all," Crutzen said.

Some critics say the project is too risky and will have negative effects on the earth's water supply and increase acid rain.

Crutzen said it was necessary to study the negative consequences, but he did not expect a rise in acid rain because the amount of sulphur injected would be a small percentage of the sulphates polluting the lower atmosphere today.

Some environmental groups, wary of geo-engineering projects, say the idea should at least be looked at.

"The fact that the top experts in the field are saying it's necessary shows it's a sad state of affairs," said Steve Sawyer, a policy adviser for Greenpeace International.

"This idea should be examined and as a last resort it can buy us a few decades," Sawyer said.