The plan sounds like science fiction — but could be fact within a decade; every day more than 800 giant aircraft would lift millions of tonnes of chalk dust to a height of 12 miles above the Earth’s surface and then sprinkle the lot high around the stratosphere.

In theory, the airborne dust would create a gigantic sunshade, reflecting some of the Sun’s rays and heat back into space, dimming those that get through and so protecting Earth from the worsening ravages of climate warming.

This is not the crackpot plan of a garden-shed inventor. The project is being funded by billionaire and Microsoft founder Bill Gates and pioneered by scientists at Harvard University.

Indeed, the plans are so well advanced that the initial ’sky-clouding‘ experiments were meant to have begun months ago.

This initial $3 million test, known as Stratospheric Controlled Perturbation Experiment (SCoPEx) would use a high-altitude scientific balloon to raise around 2kg of calcium carbonate dust — the size of a bag of flour — into the atmosphere 12 miles above the desert of New Mexico.

This would seed a tube-shaped area of sky half a mile long and 100 yards in diameter. For the ensuing 24 hours, the balloon would be steered by propellers back through this artificial cloud, its onboard sensors monitoring both the dust’s sun-reflecting abilities and its effects on the thin surrounding air.

SCoPEx is, however, on hold, amid fears that it could trigger a disastrous series of chain reactions, creating climate havoc in the form of serious droughts and hurricanes, and bring death to millions of people around the world.

One of the Harvard team’s directors, Lizzie Burns, admits: ‚Our idea is terrifying… But so is climate change.‘ An advisory panel of independent experts is to assess all the possible risks associated with it.

So where did the idea for such a mind-boggling scheme come from?

The inspiration was in part spawned by a natural disaster. When the volcano Mount Pinatubo in the Philippines exploded in 1991, it killed more than 700 people and left more than 200,000 homeless.

But it also gave scientists the chance to monitor the consequences of a vast chemical cloud in the stratosphere.

The volcano disgorged 20 million tonnes of sulphur dioxide high above the planet, where it formed droplets of sulphuric acid that floated around the globe for more than a year. These droplets acted like tiny mirrors to reflect sunlight.