What would happen if every single patch of farmland in the tropics, from Brazil through Congo, India and Indonesia, was abandoned overnight and left to turn back into forests? That's the question we investigated in our new research. Trees and forests have become increasingly important in plans to tackle the climate emergency, yet our work shows that once you factor in how the soil, oceans and other parts of the Earth system would respond, tree planting is not as potent a solution as it may first seem.