Fire affects ecosystems by altering both their structure and the cycling of carbon and nutrients. The emissions from fires represent an important biogeochemical pathway by which the biosphere affects climate. For climate change studies it is important to model fire as a mechanistic climate-dependent process in dynamic global vegetation models (DGVMs) and the terrestrial ecosystem components of climate models. We expand on those current approaches which neglect disturbance by fire, which use constant specified loss rates, or which depend on simple empirical relationships, and develop a process-based fire parameterization for use in the terrestrial ecosystem components of climate and Earth system models. The approach is straightforward and general enough to apply globally and for current and future climates. All three aspects of the fire triangle, fuel availability, the readiness of fuel to burn depending on conditions, and the presence of an ignition source, are taken into account. The approach also represents some anthropogenic effects on natural fire regimes, albeit in a simple manner. The fire parameterization is incorporated in the Canadian Terrestrial Ecosystem Model (CTEM) which simulates net primary productivity, leaf area index, and vegetation biomass. The fire parameterization generates burned area, alters vegetation biomass, and generates CO2 emissions. The parameterization is tested by comparing simulated fire return intervals and CO2 emissions with observation-based estimates for tropical savanna, tropical humid forests, mediterranean, and boreal forest locations.