The Amazon rainforest is in trouble as well, largely because farmers and loggers are cutting it down so rapidly.

A 2008 study projected that humans would clear away 31% of the Amazon by 2030. Another 24% would be damaged by drought or logging, the study found.

People have already cut down 20% of the Amazon. If another 20% disappears, that could trigger a feedback loop known as a "dieback," in which the forest could dry out and become a savannah.