Sheer lunacy?

"Moonteam One to control," says the space captain, looking up from the lunar surface. "BallisticsThe study of objects that are shot through the air, such as bullets. checks completed. We are firing now." Taking a deep breath, he presses the button. The ground shakes as thousands of tons of moondust hurtle towards the Sun.

This is the scene imagined by three astrophysicistsThe branch of astronomy looking at the physical laws of space. at Harvard University. It is a form of geoengineering, which means "the deliberate large-scale intervention in the Earth's natural systems to counteract climate change."¹

The plan is to mine millions of tons of dust from the Moon and fire them 750,000 miles into space. There they would deflect the Sun's rays, dimming them by 1.8% and lowering the Earth's temperature.

"A really exciting part of our study was the realisation that the natural lunar dust grains are just the right size and composition for efficiently scattering sunlight away from Earth," explains the leader of the team, Ben Bromley.

And, he notes, "It takes much less energy to launch these grains from the Moon's surface as compared with an Earth launch." It would nevertheless be a major project, possibly involving a new space station which would help spread the particles in the right direction.

The firing would also need to happen on a regular basis to maintain the dust cloud. If the cloud suddenly dispersed, the Earth could suffer "termination shock" and heat up again rapidly.

But there is considerable opposition to all types of solar geoengineering. "I would say the majority of scientists believe this is a crazy idea," says Frank Biermann, a professor at UtrechtA city in the Netherlands. University.

"Soon, everyone who is dependent on coal, oil and gas will jump on the solar engineering bandwagon and say: 'We can continue for 40 years with fossil fuelsFuels made from decomposing plants and animals, including coal, natural gas and oil. These fuels release carbon dioxide, causing global warming. now.' This debate threatens to derail current climate policies."

An open letter signed by over 380 scientists calls for solar geoengineering to be banned.

The scientists argue that no one can be sure what the effect of plans like Bromley's might be: "There are uncertainties about the effects on weather patterns, agriculture and the provision of basic needs of food and water."

They also worry about governments going ahead without other countries' agreement. If this resulted in some parts of the world benefiting while others suffered, it would be a recipe for conflict.

Yes: There are too many variables. The dust cloud could end up in the wrong place and be completely ineffective. Or it could be much more effective than anticipated, with catastrophic consequences.

No: With governments dragging their feet over reducing emissions, a radical alternative is needed. Even if solar geoengineering did not solve the problem completely, it would buy us vital extra time.

Or... A better solution would be to inject sulphur into the Earth's atmosphere to reflect the Sun's rays. Professor David Keith of Harvard University estimates that this could cool the planet by 1C.