While greenhouse gases include not only carbon dioxide (CO2) but also methane, nitrous oxide, ozone and CFCs, international political negotiations have focused on the need to reduce CO2 emissions. In three months' time, the 15th Conference of the Parties (CoP15), part of the United Nations Convention on Climate Change in Copenhagen, will aim to set binding targets for emission reductions (so-called conventional mitigation). But even if global CO2 emissions are cut by 50% by 2050, this now seems unlikely to be enough to keep global warming below 2 °C this century. Indeed, since the Kyoto protocol to limit greenhouse gases was established in 1997, global CO2 emissions have continued to climb despite growing concerns over climate change. Given that conventional mitigation now appears insufficient to avoid dangerous climate change, do we have a plan B? This is the motivation for geoengineering, a term that describes deliberate intervention in the climate system to counteract man-made global warming. This can be achieved in two ways, by direct removal of carbon dioxide and by solar-radiation management, which aims to cool the planet by reflecting more sunlight out into space.
Removing carbon dioxide
The most obvious approach to CO2 removal is to plant forests, but this is relatively inefficient and requires large areas of land. A more radical suggestion is to fertilize the ocean with a...