We face three epoch-defining environmental problems: climate, extinction, and pestilence. Our climate is changing in ways that will have serious consequences for humans, and it may even profoundly affect the ability of the planet to support life. All around us, other species are disappearing at a rate between several hundred and several thousand times the normal background rate of extinction. The SARS-CoV-2 virus, which has wreaked social and economic havoc, is merely the latest model off a blossoming production line of newly emerging infectious diseases, many of which have the potential to be far worse. At the heart of these problems lies an ancient habit: eating animals. This habit is the most significant driver of species extinction and of newly emerging infectious diseases, and one of the most important drivers of climate change. This is a habit we can no longer afford to indulge. Breaking it will substantially reduce climate emissions. It will stem our insatiable hunger for land that is at the heart of both the problems of extinction and pestilence. Most importantly, breaking this habit will make available vast areas of land suitable for afforestation: the return of forests to where they once grew. Afforestation will significantly mitigate all three problems. But only if we stop eating animals will we have enough land for this strategy to work.