Climate change is considered one of the most important environmental issues of our time. This concern reflects the reality that so much of human activity is sensitive to climate change, and that adapting to current and projected rates of climate change could be very challenging. It also reflects the understanding that human perturbation of the climate system is essentially irreversible, for many centuries at least. How much climate change future generations are exposed to will be determined by the actions that we take over the coming decades to reduce human impacts on the climate system.