The modern concept of geoengineering (or climate engineering) describes deliberately manipulating the Earth's climate to counteract the effects of global warming from greenhouse gas emissions. Other uses of the word sometimes occur.
The National Academy of Sciences defined geoengineering as "options that would involve large-scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry." The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such as ocean fertilization to remove CO2 from the atmosphere, remained largely unproven. It was judged that reliable cost estimates for geoengineering had not yet been published.
Geoengineering accompanies mitigation and adaptation to form a three-stranded 'MAG' approach to tackling global warming, notably advocated by the Institution of Mechanical Engineers. Some geoengineering techniques are based on carbon dioxide removal. These techniques seek to reduce greenhouse gases in the atmosphere directly. These include direct methods (e.g. carbon dioxide air capture) and indirect methods (e.g. ocean iron fertilization). These techniques can be regarded as mitigation of global warming.
Alternatively, solar radiation management techniques do not reduce greenhouse gas concentrations, and can only address the warming effects of carbon dioxide and other gases; they cannot address problems such as ocean acidification, which are expected as a result of rising carbon dioxide levels. Examples of proposed solar radiation management techniques include the production of stratospheric sulfur aerosols, which was suggested by Paul Crutzen, space mirrors, and cloud reflectivity enhancement. Most techniques have at least some side effects.
To date, no large-scale geoengineering projects have been undertaken. Some limited tree planting and cool roof projects are already underway, and ocean iron fertilization is at an advanced stage of research, with small-scale research trials and global modelling having been completed. Field research into sulfur aerosols has also started. Some commentators have suggested that consideration of geoengineering presents a moral hazard because it threatens to reduce the political and popular pressure for emissions reduction. Scientists do not typically suggest geoengineering as an alternative to emissions control, but rather an accompanying strategy. Reviews of geoengineering techniques have emphasised that they are not substitutes for emission controls and have identified potentially stronger and weaker schemes.