The latest report from the UN Intergovernmental Panel on Climate Change includes geoengineering – a sure sign that the idea has become respectable. But research and field trials are needed before we know whether influencing the climate like this is a viable option for cooling Earth.

This is easier said than done. In 2010, the UN Convention on Biological Diversity (CBD) forbade any form of geoengineering that could affect biodiversity – which is effectively all of them. And any geoengineering that involves adding chemicals to the ocean, to increase the carbon sequestered, for example, is prohibited by the London Convention and Protocol (LCP). However, some small-scale trials have gone ahead.