Preindustrial-control and twentieth-century carbon cycle experiments with the Earth system model CESM1(BGC)

Version 1 of the Community Earth SystemModel, in the configurationwhere its full carbon cycle is enabled, is introduced and documented. In this configuration, the terrestrial biogeochemical model, which includes carbon-nitrogen dynamics and is present in earlier model versions, is coupled to an ocean biogeochemical model and atmospheric CO₂ tracers. The authors provide a description of the model, detail how preindustrial-control and twentieth-century experiments were initialized and forced, and examine the behavior of the carbon cycle in those experiments. They examine how sea- and land-to-air CO₂ fluxes contribute to the increase of atmospheric CO₂ in the twentieth century, analyze how atmospheric CO₂ and its surface fluxes vary on interannual time scales, including how they respond to ENSO, and describe the seasonal cycle of atmospheric CO₂ and its surface fluxes. While the model broadly reproduces observed aspects of the carbon cycle, there are several notable biases, including having too large of an increase in atmospheric CO₂ over the twentieth century and too small of a seasonal cycle of atmospheric CO₂ in the Northern Hemisphere. The biases are related to a weak response of the carbon cycle to climatic variations on interannual and seasonal time scales and to twentieth-century anthropogenic forcings, including rising CO₂, land-use change, and atmospheric deposition of nitrogen.