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G. J. Boer and V. K. Arora

1. Introduction The distribution of carbon in the atmosphere, land, and ocean is changing as a consequence of the anthropogenic emission of CO 2 . Biogeochemical processes in the carbon cycle are directly affected by an increase in atmospheric CO 2 , which alters the flux of carbon between the atmosphere and the underlying surface. An increase in atmospheric CO 2 also affects the energy budget, resulting in warmer temperatures and other changes in climate that, in turn, affect the carbon

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Vivek K. Arora, George J. Boer, Pierre Friedlingstein, Michael Eby, Chris D. Jones, James R. Christian, Gordon Bonan, Laurent Bopp, Victor Brovkin, Patricia Cadule, Tomohiro Hajima, Tatiana Ilyina, Keith Lindsay, Jerry F. Tjiputra, and Tongwen Wu

1. Introduction Earth system models (ESMs) incorporate terrestrial and ocean carbon cycle processes into coupled atmosphere–ocean general circulation models (AOGCMs) in order to represent the interactions between the carbon cycle and the physical climate system. Changes in the physical climate affect the exchange of CO 2 between the atmosphere and the underlying land and ocean, and the resulting changes in atmospheric concentration of CO 2 in turn affect the physical climate. Aspects of the

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J. M. Gregory, C. D. Jones, P. Cadule, and P. Friedlingstein

1. Introduction During coming decades and centuries, climate change is expected in response to anthropogenic emissions into the atmosphere, especially of carbon dioxide. Projections of global climate change, for instance as assessed by Meehl et al. (2007) for the Intergovernmental Panel on Climate Change, are based principally on the results of three-dimensional atmosphere–ocean general circulation models (AOGCMs), which simulate relevant dynamical and physical processes at a horizontal

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Kirsten Zickfeld, Michael Eby, H. Damon Matthews, Andreas Schmittner, and Andrew J. Weaver

1. Introduction Since preindustrial times, human activities have emitted large amounts of carbon dioxide (CO 2 ) into the atmosphere (490 PgC from 1850 to 2006; Canadell et al. 2007 ). About half of these emissions have been taken up by sinks in the ocean and the terrestrial biosphere ( Denman et al. 2007 ). On land, uptake of anthropogenic CO 2 is driven by the stimulation of photosynthesis through elevated atmospheric CO 2 (CO 2 fertilization effect; Norby et al. 2005 ), warmer

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Peng Zhu, Qianlai Zhuang, Lisa Welp, Philippe Ciais, Martin Heimann, Bin Peng, Wenyu Li, Carl Bernacchi, Christian Roedenbeck, and Trevor F. Keenan

inversions suggest that the net carbon uptake has increased ( Hayes et al. 2011 ; Welp et al. 2016 ). However, correlations between spring temperature and changes in atmospheric CO 2 concentrations suggest the positive effect of warming on net carbon uptake has weakened in recent years ( Piao et al. 2017 ). Satellite observations of vegetation “greenness” suggest that there is a shift from widespread greening to browning trends since 2000 across the boreal zone possibly due to fire disturbance and

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Lori T. Sentman, Elena Shevliakova, Ronald J. Stouffer, and Sergey Malyshev

1. Introduction The terrestrial biosphere is an important component of the global carbon cycle and actively exchanges carbon with the atmosphere on varying time scales. Conversion of natural lands for agriculture and wood harvesting has shaped the land surface for centuries. Hurtt et al. ( Hurtt et al. 2006 ) found that 42%–68% of the global land surface was altered by anthropogenic land-use activities between 1700 and 2000. Pongratz et al. ( Pongratz et al. 2009 ) showed that anthropogenic

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G. J. Boer and V. Arora

1. Introduction Anthropogenic emissions of CO 2 affect both the energy and carbon balance of the climate system. Boer and Arora (2009 , hereafter BA) analyze the response of the global mean carbon budget to anthropogenic emission of CO 2 in terms of “carbon–temperature” and “carbon–concentration” feedbacks. Increasing surface CO 2 concentration in the atmosphere promotes carbon uptake by the underlying land and ocean and this acts to counteract the atmospheric CO 2 increase and so acts

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Pierre Friedlingstein, Malte Meinshausen, Vivek K. Arora, Chris D. Jones, Alessandro Anav, Spencer K. Liddicoat, and Reto Knutti

-estimate projections and uncertainty ranges for emission scenarios, there are two major sources of uncertainty that need to be taken into account. The first relates to physical processes and feedbacks, and the uncertainty they induce on climate response for a given greenhouse gas (GHG) concentration and aerosol forcing in terms of the global-mean temperature response, and regional climate change; while the second relates to carbon cycle processes and feedbacks, with the associated uncertainty on the relationship

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Andrei P. Sokolov, David W. Kicklighter, Jerry M. Melillo, Benjamin S. Felzer, C. Adam Schlosser, and Timothy W. Cronin

1. Introduction Carbon uptake by terrestrial ecosystems plays an important role in defining changes in the atmospheric CO 2 concentration and changes in climate. In turn, carbon uptake is influenced by these changes. It has long been recognized that nitrogen limitations often constrain carbon accumulations in mid- and high-latitude ecosystems, such as temperate and boreal forests (e.g., Mitchell and Chandler 1939 ; Tamm et al. 1982 ). Recent research on plant responses to elevated CO 2

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Jingfeng Xiao, Qianlai Zhuang, Eryuan Liang, Xuemei Shao, A. David McGuire, Aaron Moody, David W. Kicklighter, and Jerry M. Melillo

Introduction Midlatitude regions experienced frequent droughts during the twentieth century ( Dai et al. 1998 ), including severe extended droughts that spanned multiple years and affected a large swath of land (e.g., Schubert et al. 2004 ). However, the impacts of these severe extended droughts on the terrestrial carbon cycling are unclear ( Ciais et al. 2005 ). Moreover, global climate models project widespread summer drying in midlatitude regions during the twenty-first century ( Solomon et

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