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 limiting nutrient such as iron in the hope of enhancing the oceanic carbon sink (which currently absorbs about 25% of man-made CO2 emissions). Small-scale ocean-fertilization experiments have produced artificial phytoplankton blooms through the addition of iron, but it is questionable whether this will translate into a long-term enhancement of the carbon sink. A major risk with this approach is that ocean currents make it impossible to contain the area over which ocean ecosystems are modified by the addition of nutrients.
A safer method of removing carbon dioxide is air capture, which involves chemical or physical extraction of CO2 from the air and burial of the carbon in geological stores. The storage part of this approach is similar to conventional carbon capture and storage, which aims to remove the CO2 from the exhaust gases of fossil-fuel power stations. Air capture can in principle be carried out at any location, although it is most useful close to the geological stores. Chemical methods of air capture typically involve the reaction of carbon dioxide with sodium hydroxide to produce sodium carbonate, whereas physical capture involves ion-exchange resins that are able to filter CO2 from the air, which can subsequently be washed from the filters with water. There are major advantages to air-capture techniques because they remove the primary cause of global warming and, unlike conventional mitigation, offer the possibility of reducing CO2 concentrations below current levels. However, these techniques are currently expensive and carry the associated difficulties of finding suitable stable geological stores for the carbon.
Blotting out the Sun or brightening the planet
An alternative to the removal of carbon dioxide is solar-radiation management, which involves reducing the amount of sunlight absorbed by the Earth as a whole. The global mean temperature of the planet is determined by the balance between the solar radiation absorbed and the infrared...