Tackling climate change by planting trees has an intuitive appeal. …The suggestion that you can plant trees to offset your carbon emissions is widespread. Many businesses, from those selling shoes to booze, now offer to plant a tree with each purchase, and more than 60 countries have signed up to the Bonn Challenge, which aims to restore degraded and deforested landscapes. However, expanding tree cover could affect the climate in complex ways. Using models of the Earth’s atmosphere, land and oceans, we have simulated widescale future forestation. Our new study shows that this increases atmospheric carbon dioxide removal, beneficial for tackling climate change. But side-effects, including changes to other greenhouse gases and the reflectivity of the land surface, may partially oppose this. Our findings suggest that while forestation – the restoration and expansion of forests – can play a role in tackling climate change, its potential may be smaller than previously thought.