But the shift towards synthetic fuel has provoked criticism, because when such fuel is made from coal and then burned in an aircraft engine, more greenhouse gases are emitted overall than would be produced if the aircraft simply burned conventional fuel derived from oil.

ECONOMIST: Monitor

Some people think signals emitted by television and radio stations would be detectable from nearby stars, thus rendering the debate irrelevant.

ECONOMIST: An argument over whether to send messages to aliens

Last September a team led by researchers from the Scripps Institution of Oceanography, in La Jolla, California, looked at whether clouds could be whitened artificially (and thus caused to reflect more sunlight back into space) using particles emitted from a boat.

ECONOMIST: Geoengineering