Scientists are to copy the effects of a massive volcanic eruption to block the Sun’s rays and combat global warming.

As part of a £2.3million ($3million) experiment partly-funded by Microsoft’s Bill Gates, a team from Harvard University will spray tiny chalk particles into the atmosphere 12 miles above the Earth to reflect some of the Sun’s rays back into space.

They hope this will have a similar effect to an erupting volcano releasing sulphur dioxide. In 1991, Mount Pinatubo erupted in the Philippines, releasing 20million tons of sulphur dioxide. That cooled the planet by 0.5C for 18 months.

As part of a £2.3million ($3million) experiment partly-funded by Microsoft’s Bill Gates (pictured), a team from Harvard University will spray tiny chalk particles into the atmosphere

The goal is to reduce the worst effects of climate change in the hope that this could save coral reefs and polar ice sheets.

The team hopes to launch a steerable balloon over the southwest United States before next July. It will release jets of calcium carbonate: chalk dust. Scientists will measure how this affects the Sun’s light.

Opponents say spraying particles in such a way might damage the ozone layer and disrupt rainfall patterns, which could cause drought in some areas, according to The Times.

And they say such geo-engineering diverts attention from cutting greenhouse gas emissions.