Satellite-based burned area reconstructions in land surface models: impacts on the land carbon cycle

Abstract. Fire is a key Earth system process influencing vegetation distribution and the land carbon cycle, yet its representation within dynamic global vegetation (DGVM) models is generally poor, adding to the high level of inter-model uncertainty around simulated burned area (BA), fire emissions, and hence land carbon sink estimates. Here, we address this uncertainty by applying a satellite-based long-term reconstruction of burned area from 1901 to 2020 to five DGVM models, in order to assess the impacts of diagnostic BA on global carbon cycle estimates.

By diagnosing BA, we reduced the inter-model standard deviation in global fire emissions by 59 % compared to the prognostic BA simulations, and significantly increased estimate agreement with observations spatially and regionally. We also moderately improved net land carbon sink estimates, reducing bias by 0.3 PgC year^-1 against top-down constraints on average over 2001–2020, however the inter-model spread persisted. Additionally, regional simulated fire combustion rate (fire carbon emissions per unit BA) magnitudes and trends did not align with observations under diagnostic BA, influencing simulated land sink trends. This indicates poor representation of fire dynamics, specifically around the representation of fuel loads and combustion completeness, fire-induced mortality, fire intensity, and vegetation recovery processes.

We also explore land sink uncertainty associated with various diagnostic BA reconstructions using the JULES DGVM. By performing two JULES simulations using two reconstructed BA datasets derived from distinct satellite products (FireCCI5.1 and GFED5), we found a substantial difference in the land carbon sink (0.8 PgC year^-1) between the two simulations over 2001–2020. This shows that the underlying uncertainty in BA mapping and reconstructed BA trends can add a large source of uncertainty to modern day land sink estimates. Using a counterfactual simulation, we show that this uncertainty likely stems from differences in reconstructed pre-satellite era burned area dynamics leaving legacy impacts on both fire emission and ecosystem productivity estimates.

Our results show that although diagnostic burned area can greatly improve fire emission realism and constrain DGVMs, an enhanced representation of post-fire ecosystem impacts, fire intensity and fuel loads is needed in order to capture regional impacts of fire on the carbon cycle.