Can planting trees fix climate change, or is protecting forests the bigger prize?

Planting trees helps, but it is not a quick fix, and scientists disagree about how much it can do. Forests are real carbon sinks: they store large amounts of carbon in biomass and sequester approximately 25 percent of human carbon emissions annually, which is why afforestation, planting trees where there was no recent tree cover, counts as climate change mitigation. So more trees, planted well, can lock up carbon. The disagreement is about how much, how fast, and whether planting beats protecting forests that already stand.

The optimistic case comes from a 2019 study in Science led by Jean-François Bastin and Thomas Crowther: Earth has room for an additional 0.9 billion hectares of tree canopy, which would sequester 205 billion tons of carbon, about 20 years of current global emissions. Headlines hailed tree planting as the best climate solution. Critics led by Joseph Veldman replied that the study overestimated the potential about five-fold: ecosystems without trees are not necessarily degraded and can store carbon already, as savannas and tundra do underground, and such estimates often leave out soil carbon and slowing tree growth. Trees can also darken the surface, lowering its albedo, so it absorbs more heat, particularly where white snow would otherwise cover the ground.

The planted forest matters too. Fast-growing monocultures of eucalyptus, casuarina or pine are often more profitable to plant, but their biodiversity benefits are not comparable to native forest, and offset projects are frequently controversial. A meta-analysis found that mixed-species plantations would increase carbon storage alongside other benefits.

And carbon in a forest is not locked away for good. Wildfires, disease and political change can release it again; in 2019 forests took up a third less carbon than in the 1990s. Many scientists therefore put protection first: researchers have found it better to avoid deforestation than to let forest be cleared and later reforest, and the benefits of new planting come further in the future than keeping existing forest intact.