Warming in the Arctic is already affecting wildlife. It may be the reason some herds of Alaskan caribou, like these, have declined in numbers. This abundant and prolific species is expected to survive. But even they will be at risk (along with polar bears and many others) if greenhouse gas emissions grow uncontrolled to the end of this century and beyond.

Global warming is also measurably damaging alpine meadows (like this one in Colorado), coral reefs, and other important ecosystems. Many may be devastated within our lifetime — along with some equally vulnerable human communities — unless prompt action is taken.