Understanding the science of climate change provides only part of what is needed to decide what to do about the issue. Deciding how to proceed also requires information about the likely impacts of climate change on human society, the options available to respond to climate change, and the tradeoffs these options present in terms of effectiveness, benefits, costs, and risks. This chapter summarizes present knowledge and uncertainties on these matters.

The responses available to deal with climate change fall into two broad categories, called adaptation and mitigation, plus a third type of potential response, only recently receiving serious attention, called geoengineering. Adaptation measures target the impacts of climate change: they seek to adjust human society to the changing climate, to reduce the resultant harms. Examples include building sea walls or dikes to limit risks from higher sea levels or river flooding, or planting drought-resistant crops to deal with drier summers in agricultural regions. Mitigation measures target the causes of climate change: they seek to slow or stop climate change by reducing the emissions of greenhouse gases that are responsible.

In early climate debates, many advocates treated mitigation versus adaptation as an either-or choice, perhaps because they imagined attention and support for the response they favored would be weakened by acknowledging the need for the other. Fortunately, debate has moved past this false dichotomy, and it is now widely understood that both adaptation and mitigation are needed.