Spatial distribution and magnitude of pre-industrial fire emissions contribute high sensitivity to aerosol radiative forcing

Abstract:

Uncertainty in pre-industrial aerosol emissions, including fires, is one of the largest sources of uncertainty in estimating anthropogenic radiative forcing since the Industrial Revolution. The current literature suggests aerosol radiative forcing is more sensitive to changes in the magnitude of fire emission than uncertainties due to other properties, including fire episodicity and plume height, within their current uncertainty ranges. Here, we explore a previously untested aspect of fire emission uncertainty: the importance of altering the fire location for a fixed emission magnitude. We use the Community Earth System Model version 2 (CESM2) and four plausible pre-industrial fire emission estimates to conduct two sets of simulations: a) base simulations for each pre-industrial estimate and b) normalized simulations where the global annual magnitude of fire aerosol emissions are equal to the emission dataset which has previously been shown to align most robustly with paleofire proxy evidence.

We find the change in aerosol forcing from present-day relative to pre-industrial is between -0.4 and 0.3 W/m² for the direct aerosol radiative forcing and between -1.9 and 0.4 W/m² for the cloud albedo forcing. Altering the spatial distribution of pre-industrial fires alone adds a previously unaccounted 25% uncertainty to the total aerosol radiative forcing range (17% to direct forcing and 27% to cloud albedo forcing). Additionally, we find a higher cloud albedo forcing sensitivity to changes in the aerosol state compared to previous studies, which appears consistent with the emerging CESM2 literature, likely owing to the improved cloud processes.

As climate models increase forcing sensitivity to changes in aerosol loading, quantifying the pre-industrial atmospheric baseline becomes increasingly important. Future studies must therefore account for, and aim to constrain, the uncertainty in the spatial distribution of historical fires and changes to other aerosol emissions as regional differences contribute substantial additional uncertainty to anthropogenic radiative forcing estimates.