PNNL

Abstract

This paper documents the biogeochemistry configuration of the Energy Exascale Earth System Model (E3SM), i.e., model version E3SMv1.1-BGC and related congurations. The model is capable of simulating the coupled carbon cycle-climate system, and introduces several innovations in the treatment of soil nutrient limitation mechanisms, including enabling explicit dependence on phosphorus availability. Here we describe the initial suite of simulations performed for the coupled biogeochemistry simulation campaign, which comprises the E3SM contributions to the Coupled Climate-Carbon Cycle Model Intercomparison Project and several other projects as well as additional simulations to explore structural uncertainty. We describe the model spinup and evaluation procedures, provide an overview of results from the simulation campaign, and highlight key features of the simulations. A major goal of the analyses is to evaluate the effects of nitrogen and phosphorous limitation on climate-biogeochemistry interactions, and how sensitive model predictions are to structural uncertainty in the representation of these interactions. Cumulative warming over the twentieth century in E3SMv1.1-BGC is similar to observations, with a mid-century cold bias offset by stronger than observed warming in recent decades. The model simulates historical terrestrial carbon cycle dynamics, including the loss in carbon associated with land use and land cover change. The inclusion of nutrient limitations results in weaker carbon fertilization and carbon-climate feedbacks than exhibited by other Earth System Models that exclude those limitations. Ocean biomass production and carbon uptake are underpredicted, likely due to biases in ocean transport that lead to widespread anoxia and undersupply of nutrients to surface waters. Finally, we compare two alternative representations of terrestrial carbon and nutrient cycling. While both configurations agree well with observational benchmarks, they differ significantly in their distribution of carbon among different pools and in the strength of nutrient limitations.