The changing radiative forcing of fires: global model estimates for past, present and future
1Earth and Atmospheric Science, Cornell University, Ithaca, New York
2Land in the Earth System, Max Planck Institute for Meteorology, Hamburg, Germany
3Earth System Science, University of California, Irvine, California
4Biological and Environmental Engineering, Cornell University, Ithaca, New York
Abstract. Fires are a global phenomenon that impact climate and biogeochemical cycles, and mediate numerous interactions between the biosphere, atmosphere and cryosphere. These impacts occur on a range of temporal and spatial scales and are difficult to quantify on a global scale based solely on observations. Here we assess the role of fires in the climate system using model estimates of radiative forcing (RF) from global fires in the preindustrial, present day, and future time periods. Fire emissions of trace gases and aerosols were derived from transient simulations with the Community Land Model and then used in a series of Community Atmosphere Model simulations with representative emissions from the years 1850, 2000, and 2100. Additional simulations were carried out with fire emissions from the Global Fire Emission Database for a present-day comparison. Reduced land carbon storage due to fires suggests a large preindustrial positive RF from atmospheric CO2. This effect of fires also limits the amount of carbon that can be released during the large-scale conversion of forests to agricultural land that took place during the 19th and 20th centuries, resulting in a negative change in RF from fire-emitted CO2 from the year 1850 to 2000. The remaining greenhouse gas forcings from fire emissions (methane, nitrous oxide and ozone) were smaller in magnitude. The indirect radiative effects of fire aerosols on clouds are dominant in the present and future time periods with a negative RF (cooling) of 1.0 W m−2 or greater for all time periods. We also consider the impacts of fire on the aerosol direct effect, land and snow surface albedo, and indirect aerosol effects on biogeochemistry, which lead to small RFs. Overall, we conclude that fires are responsible for an RF of about −1.2 W m−2 in the preindustrial climate (with respect to a preindustrial climate without fires), and human activities have increased the RF of fires by about 0.7 W m−2 from 1850 to 2000 and potentially 0.4 W m−2 from 1850 to 2100 in the model representation by a combination of effects on fire activity and on the background environment in which fires occur. Thus, fires play an important role in both the natural equilibrium climate and the climate perturbed by anthropogenic activity and need to be considered in future climate projections.