As human activities raise levels of the greenhouse gas carbon dioxide in the atmosphere, researchers are thinking up ways of counteracting its warming effects through methods known collectively as geoengineering. Although many theoretical studies and small-scale experiments have been carried out, scientists and the wider public are wary of going ahead with geoengineering — defined as deliberate large-scale intervention in the Earth’s natural systems to counteract climate change. As investigations by authorities such as the US National Academy of Sciences and Britain’s Royal Society have found, the risk of action in the short term is still too great.
But they advocate wide-ranging research to lay the foundations for geoengineering in case it is needed to fight a global emergency in the more distant future. This could be required if international efforts to hold down carbon dioxide emissions fail or the climate turns out to be more sensitive than expected to rising CO2 levels. Though many different geoengineering techniques have been proposed, they fall broadly into two main categories. One is based on the removal of excess CO2 from the atmosphere. The other depends on reflecting more of the sun’s energy back into space. The second category, solar radiation management, is riskier but could be implemented more quickly in an emergency. Several futuristic schemes have been proposed that include the building of giant sunshades in Earth orbit. But the two most realistic ideas are to inject aerosols of tiny reflective particles into the upper atmosphere — mimicking the cooling effect of a large volcanic eruption — and to increase reflective cloud cover over the oceans.