The central role of the rainforest in the Congo Basin for climate action and biodiversity is at risk: Its function as a CO2 sink is dramatically declining according to a study. Even with a halt to deforestation, the second-largest rainforest on the planet could soon emit more CO2 than it absorbs, warn researchers. Although this has been known for years, models in climate science have systematically underestimated this effect. A debate has now begun about correcting these models.