The modern concept of geoengineering is usually taken to mean proposals to deliberately manipulate the Earth's climate to counteract the effects of global warming from greenhouse gas emissions. 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." Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (199… (More on Geoengineering)