Dismissed a decade ago as far-fetched and dangerous, schemes to tame the effects of global warming by engineering the climate have migrated from the margins of policy debates towards centre stage.
"Plan A" refers to solving the problem at its source. But the UN's top climate science body has made it clear that slashing carbon pollution won't be enough to keep Earth from overheating. That has opened the door to a host of geoengineering schemes, from building underwater walls to shore up an Antarctic glacier the size of Britain to injecting a giant sunscreen into the stratosphere.