Birdsey, R., M. A. Mayes, P. Romero-Lankao, R. G. Najjar, S. C. Reed, N. Cavallaro, G. Shrestha, D. J. Hayes, L. Lorenzoni, A. Marsh, K. Tedesco, T. Wirth, and Z. Zhu, 2018: Executive summary. 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. 21-40, https://doi.org/10.7930/SOCCR2.2018.ES.
Future changes to the carbon cycle are projected using different kinds of models based on past trends, current data and knowledge, and assumptions about future conditions. Model projections reported in SOCCR2 seek to understand the potential of different components of North American ecosystems to serve as carbon sources or sinks, even though such projections have uncertainties (see Box ES.2, Projection Uncertainties).
The best available projections suggest that emissions from fossil fuel combustion in the energy sector will continue into the future. These projections also indicate that by 2040, total North American fossil fuel emissions could range from 1.5 to 1.8 Pg C per year, a range representing a 12.8% decrease to 3% increase in emissions compared to 2015 levels (see Ch. 19: Future of the North American Carbon Cycle). Projections include the combined effects of policies, technologies, prices, economic growth, demand, and other variables. Human activities, including energy and land management, will continue to be key drivers of carbon cycle changes into the future. A wide range of plausible futures exists for the North American energy system in regard to carbon emissions. For the United States, backcasting scenarios suggest that a significant reduction in emissions is plausible.
The persistence of the overall North American land carbon sink is highly uncertain, with models projecting that terrestrial ecosystems could continue as net sinks of carbon (up to 1.5 Pg C per year) or switch to net sources of carbon to the atmosphere (up to 0.6 Pg C per year) by the end of the century. Low confidence in these projections results from uncertainties about the complex interactions among several factors, ranging from emissions scenarios, climate change, rising atmospheric CO2, and human-driven changes to land cover and land use (see Ch. 19).
Soils store a majority of land carbon, particularly the permafrost soils of northern high-latitude regions, which are experiencing the most rapid rates of warming caused by climate change. Increased temperatures very likely will lead to accelerated rates of permafrost thaw, releasing previously frozen soil carbon to the atmosphere. Globally, rising temperatures could cause the soil pool of 1,500 to 2,400 Pg C to release 55 ± 50 Pg C by 2050. However, the magnitude and timing of these carbon losses are not well understood, partly because of poor coverage and distribution of measurements, as well as inadequate model representation of permafrost feedbacks (see Ch. 11: Arctic and Boreal Carbon; Ch. 12: Soils; and Ch. 19: Future of the North American Carbon Cycle).
The Exclusive Economic Zone of North American coastal areas has taken up 2.6 to 3.4 Pg C since 1870 and is projected to take up another 10 to 12 Pg C by 2050 under business-as-usual, human-driven emissions scenarios. However, coastal ecosystems such as mangroves, wetlands, and seagrass beds that historically have removed carbon from the atmosphere are particularly vulnerable to loss of stored carbon caused by the combination of sea level rise, warming, storms, and human activity; the extent and impact of these vulnerabilities are highly uncertain (see Ch. 19). Taken together, these projections portray significant but uncertain future potential changes in the carbon cycle and associated consequences.
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