Fire is an important process at the global scale, with far-reaching effects on vegetation dynamics, biogeochemical cycling, and atmospheric chemistry. At least 8 Gt/yr of dry biomass is burned globally, equivalent to the release of over 4 Gt of carbon, or about 70% of anthropogenic fossil fuel emissions per year. Burning of tropical forests contributes about 2.4 Gt C per year. Fires of this magnitude can lead to substantial changes in ecosystem functioning and vegetation distribution, which in turn affect vegetation's susceptibility to fire. Fires are also a major source of several chemical species—including CO, N2O, NOx, and CH4—which play key roles in atmospheric chemistry and of carbonaceous aerosol particles, which affect the formation and physical properties of clouds.