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

Central to life on Earth, carbon is essential to the molecular makeup of all living things and plays a key role in regulating global climate. To understand carbon’s role in these processes, researchers measure and evaluate carbon stocks and fluxes. A stock is the quantity of carbon contained in a pool or reservoir in the Earth system (e.g., carbon in forest trees), and a flux is the direction and rate of carbon’s transfer between pools (e.g., the movement of carbon from the atmosphere into forest trees during photosynthesis). This document, the Second State of the Carbon Cycle Report (SOCCR2), examines the patterns of carbon stocks and fluxes—collectively called the “carbon cycle.” Emphasis is given to these patterns in specific sectors (e.g., agriculture and energy) and ecosystems (e.g., forests and coastal waters) and to the response of the carbon cycle to human activity. The purpose of SOCCR2 is to assess the current state of the North American carbon cycle and to present recent advances in understanding the factors that influence it. Concentrating on North America—Canada, the United States, and Mexico—the report describes carbon cycling for air, land, inland waters (streams, rivers, lakes, and reservoirs), and coastal waters (see Figure ES.1).


Figure ES.1: Domain of the Second State of the Carbon Cycle Report

Figure ES.1: In addition to the land masses and inland waters of Canada, Mexico, and the United States (divided into U.S. National Climate Assessment regions), this report covers carbon dynamics in coastal waters, defined as tidal wetlands, estuaries, and the coastal ocean, the latter being defined by the Exclusive Economic Zone (EEZ). The seaward boundary of the EEZ is typically 200 nautical miles from the coast. The geographical scope of the U.S. analysis includes the conterminous United States, Alaska, Hawai‘i, Puerto Rico, and the U.S. Virgin Islands. [Figure source: Christopher DeRolph, Oak Ridge National Laboratory.]


The questions framing the publication A U.S. Carbon Cycle Science Plan (Michalak et al., 2011) inspired development of three slightly modified questions that guide SOCCR2’s content and focus on North America in a global context:

  1. How have natural processes and human actions affected the global carbon cycle on land, in the atmosphere, in the ocean and other aquatic systems, and at ecosystem interfaces (e.g., coastal, wetland, and urban-rural)?

  2. How have socioeconomic trends affected atmospheric levels of the primary carbon-containing gases, carbon dioxide (CO2) and methane (CH4)?

  3. How have species, ecosystems, natural resources, and human systems been impacted by increasing greenhouse gas (GHG) concentrations, associated changes in climate, and carbon management decisions and practices?

SOCCR2 synthesizes the most recent understanding of carbon cycling in North America, assessing new carbon cycle findings and information, the state of knowledge regarding core methods used to study the carbon cycle, and future research needed to best inform carbon management and policy options. Focusing on scientific developments in the decade since the First State of the Carbon Cycle Report (SOCCR1; CCSP 2007), SOCCR2 summarizes the past, current, and projected state of carbon sources, sinks, and natural processes, as well as contributions by human activities. In addition to CO2 and CH4, the report sometimes discusses nitrous oxide (N2O), a GHG associated with activities and processes that affect fluxes of carbon gases.1 SOCCR2 also describes improvements in analysis tools; developments in decision support; and new insights into ecosystem carbon cycling, human causes of changes in the carbon cycle, and social science perspectives on carbon. Since publication of SOCCR1, coordinated research from agencies in the three North American countries has enabled innovative observational, analytical, and modeling capabilities to further advance understanding of the North American carbon cycle (see Appendix D: Carbon Measurement Approaches and Accounting Frameworks). Some of the report’s main conclusions, based on the Key Findings of each chapter, are highlighted in Box ES.1, Main Findings of SOCCR2 .

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