Climate Models Underestimate Pre-Industrial Fire Emissions

Abstract. An improved understanding of how fire regimes changed from the pre-industrial to the present-day is required to reduce uncertainty in anthropogenic aerosol radiative forcing. Direct observations of pre-industrial fires are scarce so here we evaluate assumptions about historical fire regime change using black carbon data from polar and alpine ice core records and global climate model simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the Large Ensemble Community Project (LENS). Black carbon is a tracer of both natural and anthropogenic combustion, enabling comparison of fire and industrial combustion emission changes from the pre-industrial (1850 CE) to the present day (1980 CE). Polar ice core records indicate relatively modest increases in mean and median 1980 CE/1850 CE black carbon ratios of 1.27 and 1.15, compared with 2.89 and 2.64 in the CMIP6 ensemble, respectively. Alpine ice cores record larger increases (mean 2.31; median 1.93), compared with even larger increases simulated in CMIP6 in alpine regions (mean 3.59; median 2.50). We tested three possibilities to explain the difference between black carbon simulated by models and recorded in observations: (1) pre-industrial fire emissions are too low in models; (2) changes in aerosol transport over the Industrial Era using data from LENS; and (3) differences in modelled aerosol properties and deposition parameterizations using CESM2 data. Analysis of these different drivers revealed that the most effective method to reconcile the model-observation disparity is to increase pre-industrial fire emissions in the models and that transport or aerosol properties are of secondary consideration.