BY TRAPPING HEAT from the sun, greenhouse gases have kept Earth’s climate habitable for humans and millions of other species. But those gases are now out of balance and threaten to change drastically which living things can survive on this planet—and where.

Atmospheric levels of greenhouse gases such as carbon dioxide are now higher than at any time in the last 800,000 years, primarily because humans have released them into the air by burning fossil fuels. The gases absorb solar energy and keep heat close to Earth’s surface, rather than letting it escape into space. That trapping of heat is known as the greenhouse…