Since at least the early ’80s, the world has been getting greener. Satellite data show plants cover more and more land every year. That’s happening even as deforestation increases in the tropics. Scientists have attributed some of the greening to climate change and an effect called CO2 fertilization, where rising atmospheric carbon dioxide levels fuel photosynthesis, the process plants use to make food, and in turn help plants grow.

Now researchers have discovered that agriculture – expan