PNNL

Abstract

Emissions-driven simulations, which prognostically simulate atmospheric CO2 concentrations, are crucial for capturing the dynamic carbon-climate interactions. We implement an emissions-driven land–atmosphere coupled biogeochemistry (BGC) configuration (BGCv2LNDATM) in version 2.1 of the Energy Exascale Earth System Model (E3SMv2.1). Here, we document the implementation, evaluate the model's performance against observations and other models, and establish a structured evaluation protocol for emissions-driven simulations. We conduct transient historical simulations (1850 - 2014) with BGCv2LNDATM and compare them to reference simulations - a land-atmosphere coupled simulation without BGC and a standalone land simulation with BGC; both using prescribed CO2 concentrations - and to observations. We employ evaluation tools including the E3SM Diagnostics Package (E3SM Diags), the International Land Model Benchmarking package (ILAMB), and the Greenhouse Gas Diagnostics for Earth System Simulations (GDESS). Our findings show that BGCv2LNDATM overestimates atmospheric CO2 concentrations by 11–23?ppm compared to observations but remains within the 40?ppm range seen across CMIP models and maintains similar physical climate properties to the reference simulations. The CO2 biases are partly attributed to underrepresented oceanic CO2 uptake and inadequate representations of some terrestrial processes. We established a structured evaluation protocol including spin-up assessment, atmospheric CO2 benchmarking, physical climate evaluation, and land biogeochemical analysis. This protocol ensures scientific rigor and facilitates inter-model comparisons. The successful implementation of BGCv2LNDATM lays the groundwork for future enhancements, including improved terrestrial biogeochemical processes, integrated marine biogeochemistry, and additional human–Earth system interactions. These developments advance E3SM toward fully coupled emissions-driven simulations, enabling more accurate carbon–climate feedback projections and supporting effective climate policy.