<b>Huntzinger</b>, D. N., A. <b>Chatterjee</b>, D. J. P. Moore, S. Ohrel, T. O. West, B. Poulter, A. P. Walker, J. Dunne, S. R. Cooley, A. M. Michalak, M. Tzortziou, L. Bruhwiler, A. Rosenblatt, Y. Luo, P. J. Marcotullio, and J. Russell, 2018: Chapter 19: Future of the North American carbon cycle. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 760- 809, https://doi.org/10.7930/SOCCR2.2018.Ch19.
Future of the North American Carbon Cycle
The physical climate system and the carbon cycle are tightly coupled. Each is sensitive to changes in the other, leading to complex feedbacks between the two (Ciais et al., 2013). A core goal of carbon cycle research is to understand how the carbon cycle will interact with and influence future climate (Michalak et al., 2011). In addition to changing climate (e.g., changing temperature and precipitation patterns), the carbon cycle is sensitive to changing atmospheric composition (e.g., ozone and nutrient deposition), extreme events such as droughts and floods, disturbances including fire and insects, and human activities such as fossil fuel emissions and land-management decisions. Land, ocean, coastal, and freshwater systems currently are net “sinks” of carbon from the atmosphere (e.g., Le Quéré et al., 2016), meaning that they annually take up more atmospheric carbon than they release, but emerging understanding of these systems (e.g., Raupach et al., 2014) suggests the possibility of a decline in their future carbon uptake capacity. Furthermore, some reservoirs could switch from a net sink to a net “source” of carbon to the atmosphere (e.g., Canadell et al., 2010; Schimel et al., 2015). Projecting future carbon cycle changes thus requires the ability to estimate the response of land and aquatic systems to numerous, often competing, drivers. Equally important to identifying the vulnerability of specific carbon reservoirs is understanding the processes controlling their behavior to better inform management and policy decisions (Canadell et al., 2010).
This chapter reviews current understanding of potential changes in the carbon budget of major global and North American carbon reservoirs. Also examined are the drivers of future carbon cycle changes including carbon-climate feedbacks, atmospheric composition, nutrient availability, human activity, and resource management decisions. Not all carbon reservoirs are equally vulnerable or resilient to changing climate, nor will they have the same response to these drivers. The majority of work examining future carbon cycle changes and potential feedbacks with climate has been conducted at the global scale as part of coupled carbon-climate model intercomparison efforts, including the Coupled Model Intercomparison Project Phase 5 (CMIP5; Friedlingstein 2015; Friedlingstein et al., 2014). These global projections are summarized in Sections 19.3–19.6. However, projections of future carbon cycle changes specific to North America remain limited. Where possible, this chapter includes projected changes in net carbon uptake and release by the North American land surface out to 2100 (see Section 19.4). Also examined are the likely drivers of future changes in the North American carbon cycle as they relate to terrestrial, ocean and coastal, and freshwater systems (see Sections 19.4–19.6). Finally, this chapter highlights ongoing knowledge gaps and research needs critical for improving understanding of future carbon cycle changes (see Section 19.7).
Such a discussion of future carbon cycle changes is new in the Second State of the Carbon Cycle Report (SOCCR2). Since the First State of the Carbon Cycle Report (SOCCR1; CCSP 2007), progress has been made at identifying the vulnerability of key carbon pools, including high-latitude permafrost (see Ch. 11: Arctic and Boreal Carbon), soils and peatlands (see Ch. 12: Soils), temperate forests (see Ch. 9: Forests), and freshwater wetlands (see Ch. 13: Terrestrial Wetlands). Other progress includes greater understanding of potential carbon losses in terrestrial ecosystems subject to disturbance events, such as insects, fire, and drought (see Ch. 9: Forests), as well as the impact of increasing atmospheric carbon dioxide (CO2) on terrestrial and aquatic systems (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide). Synthesizing and building on this previous information, this chapter focuses on potential future changes to the North American carbon cycle while putting it in a global context. Finally, this chapter covers multiple carbon stocks and flows, each with different standard conventions in terms of units and metrics. Any change in unit from mass of carbon (e.g., teragrams of carbon [Tg C] or petagrams of carbon [Pg C]) to mass of CO2 or methane (CH4) or CO2 equivalent (CO2e) has been clearly marked.
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