Lead Authors:
Richard Birdsey, Woods Hole Research Center
Melanie A. Mayes, Oak Ridge National Laboratory
Paty Romero-Lankao, National Center for Atmospheric Research (currently at National Renewable Energy Laboratory)
Raymond G. Najjar, The Pennsylvania State University
Sasha C. Reed, U.S. Geological Survey
Nancy Cavallaro, USDA National Institute of Food and Agriculture
Gyami Shrestha, U.S. Carbon Cycle Science Program and University Corporation for Atmospheric Research
Daniel J. Hayes, University of Maine
Laura Lorenzoni, NASA Earth Science Division
Anne Marsh, USDA Forest Service
Kathy Tedesco, NOAA Ocean Observing and Monitoring Division and University Corporation for Atmospheric Research
Tom Wirth, U.S. Environmental Protection Agency
Zhiliang Zhu, U.S. Geological Survey
Review Editor:
Rachel Melnick, USDA National Institute of Food and Agriculture
All Chapter Leads:
Vanessa L. Bailey, Pacific Northwest National Laboratory
Lori Bruhwiler, NOAA Earth System Research Laboratory
David Butman, University of Washington
Wei-Jun Cai, University of Delaware
Abhishek Chatterjee, Universities Space Research Association and NASA Global Modeling and Assimilation Office
Sarah R. Cooley, Ocean Conservancy
Grant Domke, USDA Forest Service
Katja Fennel, Dalhousie University
Kevin Robert Gurney, Northern Arizona University
Alexander N. Hristov, The Pennsylvania State University
Deborah N. Huntzinger, Northern Arizona University
Andrew R. Jacobson, University of Colorado, Boulder, and NOAA Earth System Research Laboratory
Jane M. F. Johnson, USDA Agricultural Research Service
Randall Kolka, USDA Forest Service
Kate Lajtha, Oregon State University
Elizabeth L. Malone, Independent Researcher
Peter J. Marcotullio, Hunter College, City University of New York
Maureen I. McCarthy, University of Nevada, Carnegie Institution for Science and Stanford University
John B. Miller, NOAA Earth System Research Laboratory
David J. P. Moore, University of Arizona
Elise Pendall, Western Sydney University
Stephanie Pincetl, University of California, Los Angeles
Vladimir Romanovsky, University of Alaska, Fairbanks
Edward A. G. Schuur, Northern Arizona University
Carl Trettin, USDA Forest Service
Rodrigo Vargas, University of Delaware
Tristram O. West, DOE Office of Science
Christopher A. Williams, Clark University
Lisamarie Windham-Myers, U.S. Geological Survey

Executive Summary

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.

See Full Chapter & References