Scientists have a revolutionary idea to save Earth from global warming. Geo-engineers from Harvard University believe that spraying "sun-dimming" chemicals high above the Earth could slow down global warming. They say this would be "remarkably inexpensive," costing about $2.25 billion a year over 15 years. The technique is called stratospheric aerosol injection (SAI). It is still yet to be proven. It would involve using huge hoses or high-powered cannons to spray chemicals into the stratosphere. This would be a reflective barrier against sunlight.

The scientists admitted that their idea is still in the planning stages. They said: "We make no judgment about the desirability of SAI. We simply show that a hypothetical deployment program, while both highly uncertain and ambitious, would indeed be technically possible." Not all scientists think the idea is a good one. Some think it is pie in the sky. They say 100 specially-designed planes would need to make 4,000 missions a year, crossing most of the globe's airspace. Over 100 governments would need to agree to this. There are concerns it could increase droughts and extreme weather.