Preindustrial-Control and Twentieth-Century Carbon Cycle Experiments with the Earth System Model CESM1(BGC)

David M Lawrence,Forrest M Hoffman,Gordon B Bonan,J Keith Moore,James T Randerson,Keith Lindsay,Matthew C Long,Natalie M Mahowald,Peter E Thornton,Scott C Doney

doi:10.1175/jcli-d-12-00565.1

Abstract

Abstract Version 1 of the Community Earth System Model, in the configuration where 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.

Highlights

Climate models based on atmospheric and ocean general circulation models have been used as tools in recent decades to aid our understanding of Earth’s climate
In the remainder of the manuscript, we refer to computations of radiative transfer, terrestrial photosynthesis, and sea-to-air gas flux collectively as CO2related computations. We describe below these new features of CESM1(BGC) and aspects of CCSM4 that are relevant to biogeochemical parameterizations
The core atmosphere, ocean, and land models are very similar between CCSM4 and CESM1(BGC), differing only with the introduction of prognostic chlorophyll in the ocean in the PRES experiments, which affects the vertical distribution of shortwave absorption, and the additional introduction of prognostic CO2 in the PROG experiments, which affects radiative transfer and terrestrial photosynthesis computations

Summary

Introduction

Climate models based on atmospheric and ocean general circulation models have been used as tools in recent decades to aid our understanding of Earth’s climate. While this term does not have a uniformly accepted definition, models that couple a prognostic carbon cycle model to a climate model are generally agreed to qualify as Earth system models. These models can predict atmospheric CO2, allowing for internally consistent feedbacks between the varying model climate and atmospheric CO2. This is in contrast to traditional climate models that use prescribed atmospheric CO2 trajectories that are produced by an independent, and typically reduced-complexity, model. Usage of such earth system models is becoming more widespread (Friedlingstein et al 2006; Arora et al 2013), and efforts to establish benchmarks quantifying model reliability are ongoing (Randerson et al 2009; Cadule et al 2010; Anav et al 2013; Hoffman et al 2014)

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Results

Conclusion