An understanding of the science of climate change provides only part of what is needed to decide what to do about the issue. We also need information about the likely impacts of climate change on human society, the options for responding to climate change, and the relevant tradeoffs among policy choices with their associated effectiveness, benefits, risks, costs. This chapter summarizes present knowledge and uncertainties on these matters.

The responses available to deal with the threat of climate change can be grouped into three broad categories. Adaptation measures target the impacts of climate change, seeking to adjust human society to the changing climate and so reduce the resultant harms. Building seawalls would be one way to adapt to sea level rise; planting drought-resistant crops would be one way to adapt to drier agricultural regions. Mitigation measures – an odd use of the term, but one too well established in the policy debate to resist – target the causes of climate change, seeking to reduce the emissions of greenhouse gases that are causing the climate to change.

Most proposals to address climate change revolve around mitigation and adaptation. A third class of potential responses involves actively manipulating the climate system to offset the climatic effects of greenhouse-gas emissions, making it possible to break the linkage between emissions and climate change. This approach, sometimes called geoengineering, has received less attention than mitigation and adaptation, and present understanding of its potential benefits and associated costs and risks is in its infancy.