Climate change is happening. Its impacts and costs will be large, serious, and unevenly spread. The impacts may be reduced by adaptation, and moderated by mitigation, especially by reducing emissions of greenhouse gases. However, global efforts to reduce emissions have not yet been suffi ciently successful to provide confi dence that the reductions needed to avoid dangerous climate change will be achieved. It is hoped that post-2012 emission reduction targets will stimulate greater action through more effective mechanisms, but there is a serious risk that suffi cient mitigation actions will not be introduced in time, despite the fact that the technologies required are both available and affordable. It is likely that global warming will exceed 2°C this century unless global greenhouse gas emissions are cut by at least 50% of 1990 levels by 2050, and by more thereafter. There is no credible emissions scenario under which global mean temperature would peak and then start to decline by 2100. Unless future efforts to reduce greenhouse gas emissions are much more successful then they have been so far, additional action may be required should it become necessary to cool the Earth this century. Such action might involve geoengineering, defi ned as the deliberate large-scale intervention in the Earth’s climate system, in order to moderate global warming.