The planet's temperature has been rising over the last century in large part, evidently, because of anthropogenic emissions of greenhouse gas (GHG). The prospects for future emissions depend on a host of variables, including population growth, economic development, technological advances, and the efforts and policies that will be undertaken to mitigate climate change. The consequences for the atmospheric concentrations of GHG and the global average temperatures depend on the responses of various natural mechanisms, including both amplifying and restraining feedbacks. Potential tipping points in the climate system are a major concern. It is difficult to assess the likely outlook for emissions and temperature change. Weighing the uncertainties surrounding possible outcomes is an even more daunting task, but an essential one.