The climate benefit of forests is mostly recognized in their removal of carbon dioxide from the atmosphere. Over the course of a tree’s lifetime, the accumulated carbon in biomass is carbon removed from the atmosphere. Species differ in growth rate, size at maturity, and longevity, but the basic principle of biomass accumulation over the lifetime of a tree is the basis for using forests to remove carbon dioxide from the atmosphere. Decomposition of organic material in the soil emits carbon dioxide and reduces the net carbon gain by forests. Wildfires, insect outbreaks, logging, and other disturbances also release carbon dioxide to the atmosphere. The combination of these processes – carbon gain from biomass growth; carbon loss from the soil and from disturbances – makes some forests a sink for atmospheric carbon; the forests have a net gain of carbon annually. Other forests are a source of carbon, in which there is a net loss of carbon to the atmosphere. Forests are, at a global scale, an annual carbon sink, which reduces the accumulation of carbon dioxide in the atmosphere. Nature-based solutions to mitigate climate change aim to enhance the carbon sink.