<b>Bruhwiler</b>, L., A. M. <b>Michalak</b>, R. Birdsey, J. B. Fisher, R. A. Houghton, D. N. Huntzinger, and J. B. Miller, 2018: Chapter 1: Overview of the global 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. 42-70, https://doi.org/10.7930/SOCCR2.2018.Ch1.
Overview of the Global Carbon Cycle
Coupled carbon cycle–climate models forced with future “business as usual” emissions scenarios suggest that the changing carbon cycle will be a net positive feedback on climate, reinforcing warming, but the size of the projected feedback is highly uncertain (Friedlingstein et al., 2014). Besides the uncertain trajectories of human factors such as fossil fuel emissions, land use, or significant mitigation efforts, various natural processes can lead to the carbon cycle being a positive feedback. For example, a warming climate can lead to increased fires and droughts and less storage of carbon in the terrestrial biosphere. In particular, warming is expected to decrease carbon uptake in the tropics and midlatitudes. In the high latitudes, a warmer climate is expected to lead to a more productive biosphere and more uptake but also may result in increased respiration and release of stored CO2 and CH4 in soils and lakes. Negative feedbacks also are possible, such as increased atmospheric CO2, leading to increased carbon storage in the terrestrial biosphere (e.g., Schimel et al., 2015), although the relative roles of this effect relative to land-use change, nitrogen deposition, and temperature increases on the cumulative land carbon sink over the last century are not fully understood (Huntzinger et al., 2017).
Human impacts on land use can directly impact climate. Deforestation and agriculture can affect carbon storage in soil and biomass. Fertilizer use also affects the global nitrogen budget and can increase carbon storage. Large-scale drainage of wetlands and conversion to agricultural land can reduce CH4 emissions from anaerobic respiration while potentially increasing faster soil carbon loss through aerobic respiration.
The ocean carbon sink is driven primarily by the partial pressure difference of CO2 between the atmosphere and the ocean surface (∆pCO2). Although this mechanism would imply that increasing atmospheric CO2 concentrations would, therefore, lead to increased uptake of CO2 in the ocean, there actually is substantial uncertainty in future uptake due to uncertainty in future changes to ocean circulation, warming, and chemical changes, all of which would impact the ocean sink (Lovenduski et al., 2016; Randerson et al., 2015). In addition, the sequestration of CO2 in ocean water also can lead to undesirable impacts as the ocean becomes more acidic. For example, ocean acidification disrupts the ability of organisms to build and maintain calcium carbonate (CaCO3) shells, substantially perturbing ocean ecosystems.
Frozen Arctic soils compose another potential carbon cycle–climate feedback (see Ch. 11: Arctic and Boreal Carbon and Ch. 19: Future of the North American Carbon Cycle). An estimated 1,460 to 1,600 Pg C are frozen in Arctic soils, and warming has proceeded in the Arctic faster than in any other region. Current understanding suggests that approximately 146 to 160 Pg C, primarily as CO2, could be vulnerable to thaw and release to the atmosphere over the next century (Schuur et al., 2015; see Ch. 11: Arctic and Boreal Carbon). This release of carbon from permafrost is likely to be gradual and occur on century timescales (Schuur et al., 2015). If the amount of carbon estimated to enter the atmosphere by Schuur et al. (2015) were released annually at a constant rate, emissions would be far lower than annual fossil fuel emissions (about 9 Pg C per year) but comparable to land-use change (0.9 Pg C per year).
Factors that will affect the carbon cycle are explored in much more depth in respective chapters of this report, and Ch. 19 describes future projections and the results of different IPCC scenarios on the North American carbon cycle in a global context.
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