<h2 class=" eplus-wrapper" id="crosshead">Sheer lunacy?</h2>

"Moonteam One to control," says the space captain, looking up from the lunar surface towards Earth. "BallisticsThe study of objects that are shot through the air, such as bullets. checks completed. We are ready to fire."

"Thank you, Moonteam One," says a voice from 239,000 miles away. "You are good to go." Taking a deep breath, the space captain 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 the latest idea from a branch of science known as geoengineering, which means "the deliberate large-scale intervention in the Earth's natural systems to counteract climate change".¹

The plan is to create a "solar shield" by mining millions of tons of dust from the Moon and firing them 750,000 miles into space. There they would deflect the Sun's rays, dimming them by 1.8% - equivalent to six days' sunlight a year. This would lower the Earth's temperature, reversing global warming.

"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.

Other geoengineering proposals have included spraying aerosolsLiquids kept in a pressurised container and then released as a spray. into the stratosphere and injecting sea salt into clouds so that they reflect more of the Sun's rays. Bromley argues that these could damage the environment, whereas his scheme would act as "a fine-tuned dimmer switch, leaving our planet untouched".

But there is considerable opposition to all types of solar geoengineering. "I would say the majority of scientists believe this is a crazy idea for a variety of reasons," 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 fuels now.' This debate threatens to derail current climate policies. It's a huge risk."

This is one of the objections outlined in an open letter signed by over 380 scientists. It calls for an international treaty banning solar geoengineering.

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.

Ballistics - The study of objects that are shot through the air, such as bullets.

Astrophysicists - The branch of astronomy looking at the physical laws of space.

Aerosols - Liquids kept in a pressurised container and then released as a spray.

Utrecht - A city in the Netherlands.