John Holdren

Global warming is changing the Earth's climate so fast that influential scientists fear it is already outrunning our ability to prevent the planet from overheating.

John Holdren, US President Barack Obama's chief scientific adviser, is only the latest to suggest radical geo-engineering measures may have to be considered to keep the Earth habitable.

"It's got to be looked at. We don't have the luxury of taking any approach off the table," he said. He recently compared the crisis with being "in a car with bad brakes driving towards a cliff in the fog".