The changing radiative forcing of fires: global model estimates for past, present and future

Fires are a global phenomenon that impact climate and biogeochemical cycles, and interact with the biosphere, atmosphere and cryosphere. These impacts occur on a range of temporal and spatial scales and are difficult to quantify globally 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 pre-industrial, present day, and future time periods. Fire emissions of trace gases and aerosols are derived from Community Land Model simulations and then used in a series of Community Atmosphere Model simulations with representative emissions from the years 1850, 2000, and 2100. Additional simulations are carried out with fire emissions from the Global Fire Emission Database for a present-day comparison. These results are compared against the results of simulations with no fire emissions to compute the contribution from fires. We consider the impacts of fire on greenhouse gas concentrations, aerosol effects (including aerosol effects on biogeochemical cycles), and land and snow surface albedo. Overall, we estimate that pre-industrial fires were responsible for a RF of -1 Wm(-2) with respect to a pre-industrial climate without fires. The largest magnitude pre-industrial forcing from fires was the indirect aerosol effect on clouds (-1.6 Wm(-2)). This was balanced in part by an increase in carbon dioxide concentrations due to fires (+0.83 Wm(-2)). The RF of fires increases by 0.5 Wm(-2) from 1850 to 2000 and 0.2 Wm(-2) from 1850 to 2100 in the model representation from a combination of changes in fire activity and changes in the background environment in which fires occur, especially increases and decreases in the anthropogenic aerosol burden. 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.