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

Carbon is the basis of life on Earth, forming bonds with oxygen, hydrogen, and nutrients to create the organic compounds that make up all living things. Essential for fundamental human activities and assets, carbon is a vital component of the fossil fuels used for energy production, cooking, agriculture, manufacturing, and transportation. The carbon cycle encompasses the physical, chemical, and biological processes that store or transfer carbon between different stocks or reservoirs (see Figure ES.2, p. 26). Examples of such reservoirs include the carbon stored as CO2 and CH4 gas in the atmosphere; as coal, petroleum, and natural gas (the primary energy sources for modern societies); and as organic and inorganic carbon in Earth’s ocean, freshwaters, forests, grasslands, and soils. Carbon transfer among these reservoirs occurs via a range of different processes, such as plant uptake of atmospheric CO2 for growth (photosynthesis), release of CO2 to the atmosphere from organic matter decomposition and combustion, and “lateral” transfers of carbon and burial within aqueous systems (see Figure ES.3 and Ch. 1: Overview of the Global Carbon Cycle).


Figure ES.2: Major Carbon Fluxes of North America

Figure ES.2: Net fluxes and transfers of carbon among the atmosphere, land, and water are depicted in this simplified representation of the North American carbon cycle. The diagram includes fluxes of carbon dioxide but not methane or other carbon-containing greenhouse gases. These carbon flows include 1) emissions (red arrows); 2) uptake (black arrows); 3) lateral transfers (blue arrows); and 4) burial (blue arrows), which involves transfers of carbon from water to sediments and soils. Estimates—derived from Figure ES.3 and Figure 2.3 in Ch. 2: The North American Carbon Budget—are in teragrams of carbon (Tg C) per year. The increase in atmospheric carbon, denoted by a positive value, represents the net annual change resulting from the addition of carbon emissions minus net uptake of atmospheric carbon by ecosystems and coastal waters. The estimated increase in atmospheric carbon of +1,009 Tg C per year is from Figure 2.3 and that value is slightly different from the +1,008 Tg C per year value used elsewhere in Ch. 2 because of mathematical rounding. Net ecosystem carbon uptake represents the balance of carbon fluxes between the atmosphere and land (i.e., soils, grasslands, forests, permafrost, and boreal and Arctic ecosystems). Coastal waters include tidal wetlands, estuaries, and the coastal ocean (see Figure ES.3 for details). The net land sink, denoted by a positive value, is the net uptake by ecosystems and tidal wetlands (Figure ES.3) minus emissions from harvested wood and inland waters and estuar- ies (Figure ES.3). For consistency, the land sink estimate of 606 Tg C per year is adopted from Ch. 2. Because of rounding of the numbers in that chapter, this value differs slightly from the combined estimate from Figures ES.2 and ES.3 (605 Tg C per year). Asterisks indicate that there is 95% confidence that the actual value is within 10% (☆☆☆☆☆), 25% (☆☆☆☆), 50% (☆☆☆), 100% (☆☆), or >100% (☆) of the reported value. [Figure source: Adapted from Ciais et al., 2013, Figures 6.1 and 6.2; Copyright IPCC, used with permission.]



Figure ES.3: Total Carbon Budget of North American Aquatic Ecosystems

Figure ES.3: Flux estimates, in teragrams of carbon (Tg C) per year, are derived from Ch. 13: Terrestrial Wetlands; Ch. 14: Inland Waters; Ch. 15: Tidal Wetlands and Estuaries; and Ch. 16: Coastal Ocean and Continental Shelves. Carbon exchanges with the atmosphere are limited to carbon dioxide (CO2) except for terrestrial wetlands, which include CO2 and methane. Arrows leading from the atmosphere to different aquatic ecosystem compartments imply a loss of atmospheric carbon from the atmosphere to the ecosystem (a carbon sink). Arrows leading from the ecosystem to the atmosphere imply a loss of carbon from the ecosystem to the atmosphere (a carbon source). Horizontal arrows refer to transfer of carbon between ecosystems. Changes in some reservoir sizes are provided inside the boxes with deltas (Δ). Asterisks indicate that there is 95% confidence that the actual value is within 10% (☆☆☆☆☆), 25% (☆☆☆☆), 50% (☆☆☆), 100% (☆☆), or >100% (☆) of the reported value.


Carbon is also critical in regulating climate because carbon-containing GHGs3 absorb radiant energy emitted from Earth’s surface, thereby warming the planet. This warming creates a climate within the narrow range of conditions suitable for life. Changes in atmospheric concentrations of GHGs influence Earth’s ecosystems and society in many ways, both positive and negative. Consequences of increasing GHGs include impacts on air quality, human health, water quality and availability, ecosystem productivity, species distributions, biological diversity, ocean chemistry, sea level rise, and many other processes that determine human well-being. Thus, the carbon cycle is tightly coupled to the environment, society, and the global climate system.

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