The race to find a solution to a rapidly warming world is one of the most pressing challenges facing our planet.

One proposal to try to halt this warming is literally out-of-this-world: a giant, space-based sunshade. We’re already modifying our climate by accident, so why not do it by deliberate geoengineering?

It’s a radical idea, and it just might just work. Reducing the amount of light reaching our planet could cool the Earth quickly, even with rising carbon dioxide levels. While the asteroid which helped wipe out the dinosaurs blocked out 90% of the Sun’s rays, we would need to divert just 2-4%, it’s believed, to take the Earth back to its pre-industrial climate.

Space sunshades have support in high places, from the Royal Society to Nasa, to the European Union. It’s even roused the interest of the most respected authority on global warming, the Intergovernmental Panel on Climate Change (IPCC).

By far the greatest challenge is getting the sunshade into outer space

The concept may be increasingly mainstream, but how we’d do it sounds more far-fetched. To uniformly cool the planet in a system that’s always on the move, the shade would be installed in an area of outer space that’s balanced between the gravity of the Earth and the Sun – the L1 point – about a million miles away.

First conceived by engineer James Early in 1989, the original design was a vast, 2,000 km-wide glass shield – a structure so heavy, it would need to be constructed on the Moon. More recent suggestions include clouds of Moon dust, 55,000 wire-mesh mirrors or a planet-girdling ring of tiny umbrellas. And just when you thought they couldn’t get more ambitious: how about moving the Earth further away from the sun, with anexplosion equivalent to five thousand million million hydrogen bombs.

By far the greatest challenge is getting the sunshade into outer space. It currently costs at least $10,000 (£6,930) to launch a pound of payload into orbit, and we haven’t put a man on the Moon since 1972. To be a viable option, the technology would need to be wafer-light and it would have to be assembled here on Earth.

Astronomer Roger Angel believes he has the answer: 16 trillion flying space robots. Each would weigh about a gram – the same as a large butterfly – and deflect sunlight with a transparent film pierced with tiny holes. To keep the burden low, the lenses would be less than a hundredth of the thickness of a human hair.  “You can’t stop sunlight with anything thinner than that,” he says.

The robots would steer themselves into orbit by solar-powered ion propulsion, a technology already used by the European Space Agency’s Smart-1 Moon orbiter, to form a cylindrical cloud 60,000-miles wide. After that they’d need regular nudges from ‘shepherd dog satellites’ to stop them crashing into each other or being blown off course by the sunlight they’re deflecting. “If you leave them alone they’ll drift off and eventually fall back to Earth,” he says.

Electromagnetic cannon

In all we’d need to fire 20-million-tonnes-worth into outer space – still too heavy to be feasible by chemical rocket. Angel’s solution is so outrageous, for years it was thought to be impossible because it defied the laws of physics: a giant electromagnetic gun embedded in a mountain.

By far the easiest – and cheapest – option would be to emulate a natural disaster

The system would accelerate cargo to launch at the mountain’s summit using a form of electromagnetic energy to convert electricity into thrust. Known as the Lorentz force, it already powers magnetic levitation, Maglev, trains, and the US Navy’s latest weapon. By side-stepping the need for fuel, the cost of launch may be as low as $20 (£13.90) per pound, enough to catapult the shade into orbit for just a few trillion dollars.

There’s just one problem: the technology doesn’t yet exist.

By far the easiest – and cheapest – option would be to emulate a natural disaster. The chilling effect of asteroid impacts and volcanic eruptions – like those which helped wipe out the dinosaurs – is mostly down to the emission of sulphur, generated by vaporising sulphur-rich rock.

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