The urgency of acting to limit climate change has nothing to do with politics or economics. Instead, it arises directly from the physics and chemistry of the climate system. Carbon dioxide, once it is added to the atmosphere, will remain there a long time. Some of it will remain in the atmosphere for centuries until natural processes remove it. Thus, it will be there essentially forever, if we think in terms of the implications on human time scales. The only known way to prevent atmospheric carbon dioxide amounts from increasing further is simply to cease emitting carbon dioxide into the atmosphere. That is why acting swiftly to make large reductions in global emissions, in order to limit climate change, is urgent. Yet very little significant progress has occurred toward actually making the large cuts in global emissions of heat-trapping gases that would be needed to stabilize climate. Without drastic and rapid cuts in emissions, our children and their descendants, and ultimately all living things, will be faced with the consequences of more severe climate disruption.