David Keith - Environmental Scientist
Few branches of science divide opinion as sharply as that championed by Professor David Keith.
He's one of the pre-eminent geo-engineers striving to find new ways to combat global warming, by altering the way the sea, the clouds and the atmosphere work.
This isn't a replacement, he says, for the political drive to reduce emissions.
But is geo-engineering a cost-effective, emergency stop-gap, before the effects of climate change become irreversible, or a dangerous leap into the unknown?