Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas. The warming of the climate system is unequivocal, as is now evident from observations of increasing global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level. To stabilize the concentration of CO2 in the atmosphere, emissions need to peak and then decline thereafter. In the long term, energy conservation, efficiency improvements in energy conversion, lower carbon fuels such as natural gas and renewable energy sources are the most promising alternatives. For lower stabilization targets, scenarios put more emphasis on the use of low-carbon energy sources, such as renewable energy and nuclear power, and the use of CO2 capture and storage; however, the transition from the current dependence on fossil fuels would take many decades.