Lead Authors:
Gyami Shrestha, U.S. Carbon Cycle Science Program and University Corporation for Atmospheric Research
Nancy Cavallaro, USDA National Institute of Food and Agriculture
Richard Birdsey, Woods Hole Research Center
Melanie A. Mayes, Oak Ridge National Laboratory
Raymond G. Najjar, The Pennsylvania State University
Sasha C. Reed, U.S. Geological Survey
Paty Romero-Lankao, National Center for Atmospheric Research (currently at National Renewable Energy Laboratory)
Noel P. Gurwick, U.S. Agency for International Development
Peter J. Marcotullio, Hunter College, City University of New York
John Field, Colorado State University
Review Editor:
Rachel Melnick, USDA National Institute of Food and Agriculture


The Second State of the Carbon Cycle Report (SOCCR2), a special interagency “highly influential scientific assessment,” is led and developed by the Carbon Cycle Interagency Working Group (CCIWG) under the auspices of the U.S. Global Change Research Program (USGCRP).1 Contributing to the congressionally mandated Fourth National Climate Assessment (NCA4), SOCCR2 is a USGCRP Sustained Assessment Product focused on advances in the science and understanding of the carbon cycle across North America since the First State of the Carbon Cycle Report (SOCCR1; CCSP 2007). Specifically, SOCCR2 focuses on U.S. and North American carbon cycle processes, stocks, fluxes, and interactions with global-scale carbon budgets and climate change impacts in managed and unmanaged systems (see Box P.1, Carbon Cycle Terminology and Reporting Units). The report includes an assessment of carbon stocks and fluxes in urban areas, agriculture, human settlements, the atmosphere, forests, grasslands, Arctic ecosystems, soils, and aquatic systems (wetlands, estuaries, and the coastal ocean). It considers relevant carbon management science perspectives and science-based tools for supporting and informing decisions, as addressed in and related to the publication titled A U.S. Carbon Cycle Science Plan (Michalak et al., 2011). SOCCR2 also is aligned with 1) the USGCRP Strategic Plan 2012–2021 (USGCRP 2012); 2) the 2017 USGCRP Triennial Update to the Strategic Plan (USGCRP 2017a), including the “Goal 3: Conduct Sustained Assessments” content therein; and 3) the Global Change Research Act (1990). SOCCR2 provides a status of measurements, observations, and projections of carbon stocks and fluxes, identifying their uncertainties and emerging opportunities for improvements.

Intended Audience

SOCCR2 is intended for a diverse audience that includes scientists; decision makers in the public and private sectors; and communities across the United States, North America, and the world. Overall, this is a scientific, technical report written to inform both expert and nonexpert users. It includes an Executive Summary that is also technical but designed for a somewhat broader, more general audience. This report provides updated information on the observed status and trends in the carbon cycle as influenced by natural and anthropogenic changes. It also informs policies but does not prescribe or recommend them. In this respect, SOCCR2 helps inform mitigation and adaptation policies and management decisions related to the carbon cycle, supporting improved coordination for pertinent research, monitoring, and management activities for responding to global change.

USGCRP’s Sustained Assessment Process and the National Climate Assessment

SOCCR2 has been developed as part of the U.S. Global Change Research Program’s Sustained Assessment2 process. This process facilitates continuous and transparent participation of scientists and stakeholders across regions and sectors, enabling the synthesis of new information and insights as they emerge. As a Sustained Assessment process report, SOCCR2 provides a comprehensive assessment of the science and associated human dimensions of carbon cycling in land, air, and water, with a focus on the United States and North America in a global context. SOCCR2 contributes to and informs the congressionally mandated National Climate Assessment (NCA) process of the Global Change Research Act (1990). The report also updates the carbon cycle science presented in the Third National Climate Assessment (NCA3; Melillo et al., 2014) and provides the authors of the forthcoming NCA4 Vol. II with additional consensus-based carbon cycle knowledge to bolster their own assessment of the impacts and risks posed by climate change across regions and sectors of the United States. The USGCRP assessment reports together cover sectors and topics (see Table P.1) mandated by the Global Change Research Act (1990), responding to Section 106 on Scientific Assessments by:

  1. Integrating, evaluating, and interpreting USGCRP findings and discussing the scientific uncertainties associated with such findings;

  2. Analyzing the effects of global change on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity; and

  3. Analyzing current trends in global change, both human induced and natural, and projecting major trends for the next 25 to 100 years.

Table P.1. Examples of SOCCR2 Chapters with Topics Related to NCA4 Vol. II Chaptersa

SOCCR2 Sections No. SOCCR2 Chapters Examples of Pertinent NCA4 Vol. II Chapters
Preface: About This Report
Preface: Guide to Report
Preface: Interagency Context of U.S. Carbon Cycle Science
Executive Summary
I: Synthesis 1 Overview of the Global Carbon Cycle Our Changing Climate, Complex Systems, Adaptation, Mitigation
2 The North American Carbon Budget Adaptation, Mitigation, Land
II: Human Dimensions of the Carbon Cycle 3 Energy Systems Mitigation, Energy, Transportation, Regions (including Southwest)
4 Understanding Urban Carbon Fluxes Built Environment
5 Agriculture Agriculture and Rural
6 Social Science Perspectives on Carbon Ecosystems, Land, International
7 Tribal Lands Tribal and Indigenous, Land
III: State of Air, Land, and Water 8 Observations of Atmospheric Carbon Dioxide and Methane Our Changing Climate, Air Quality
9 Forests Forests, Regions (including Southwest)
10 Grasslands Ecosystems, Land
11 Arctic and Boreal Carbon International, Alaska
12 Soils Ecosystems, Land
13 Terrestrial Wetlands Ecosystems, Water
14 Inland Waters Ecosystems, Water
15 Tidal Wetlands and Estuaries Ecosystems, Oceans, Coastal
16 Coastal Ocean and Continental Shelves Coastal Effects, Oceans, International, Regions
IV: Consequences and Ways Forward 17 Biogeochemical Effects of Rising Atmospheric Carbon Dioxide Mitigation, Air Quality, Oceans
18 Carbon Cycle Science in Support of Decision Making Adaptation, International
19 Future of the North American Carbon Cycle Our Changing Climate, International

a SOCCR2, Second State of the Carbon Cycle Report; NCA4, Fourth National Climate Assessment.

Sources Used in This Report

The findings in SOCCR2 are based on a large body of scientific, peer-reviewed research, as well as a number of other publicly available sources, including well-established and carefully evaluated observational and modeling datasets. The team of authors carefully reviewed approximately 3,000 such sources to ensure a reliable assessment of the state of scientific understanding. Each source of information was determined to meet the four parts of the Information Quality Act (OMB 2002): 1) utility, 2) transparency and traceability, 3) objectivity, and 4) integrity and security. Report authors assessed and synthesized information from peer-reviewed journal articles, technical reports produced by governmental and non-governmental agencies, scientific assessments (e.g., CCSP 2007; IPCC 2013; Melillo et al., 2014), reports of the National Academies of Sciences, Engineering, and Medicine (NASEM) and its associated National Research Council, various conference proceedings, and governmental statistics from North American and global sources.

Report Development, Review, and Approval Process

SOCCR2 is a U.S. government interagency product of the U.S. Global Change Research Program. This assessment is organized, led, and overseen by the following member agencies of the Carbon Cycle Interagency Working Group, which leads the U.S. Carbon Cycle Science Program:

  • National Aeronautics and Space Administration (NASA)

  • National Science Foundation (NSF)

  • U.S. Agency for International Development (USAID)

  • U.S. Department of Agriculture (USDA), including the Forest Service, National Institute of Food and Agriculture, Agricultural Research Service, Economic Research Service, and Natural Resources Conservation Service

  • U.S. Department of Commerce, including the National Institute of Standards and Technology (NIST) and the National Oceanic and Atmospheric Administration (NOAA)

  • U.S. Department of Energy (DOE)

  • U.S. Department of the Interior, including the U.S. Geological Survey (USGS)

  • U.S. Environmental Protection Agency (EPA)

A Federal Steering Committee, composed of a subset of the CCIWG and its member departments and agencies, was established in early 2015 to develop a Prospectus3 to guide SOCCR2 and provide regular guidance to authors. USDA served as the federal administrative lead for this report (see Appendix A: Report Development Process).

The process for preparing SOCCR2 is consistent with the guidelines for preparing USGCRP products, with referenced materials derived primarily from the existing, peer-reviewed scientific literature and consistent with USGCRP guidance regarding use of grey literature (see Appendix B: Information Quality in the Assessment). Because SOCCR2 is a USGCRP Sustained Assessment report and contributes to NCA4, many of its author guidelines are consistent with or directly derived from those for NCA3 (Melillo et al., 2014) and two other Sustained Assessment reports: The Impacts of Climate Change on Human Health in the United States (USGCRP 2016) and Climate Science Special Report: Fourth National Climate Assessment, Volume I (USGCRP 2017b). The guidance documents for NCA3 and the Climate Science Special Report were made available to the U.S. Carbon Cycle Science Program Office at the beginning of SOCCR2 development in early 2015, were adapted to the specific context of this effort, and used to develop the SOCCR2 Prospectus, which was approved by the Subcommittee on Global Change Research (SGCR) in May 2015. Following a Federal Register Notice for author nominations, technical input, and comments on the SOCCR2 Prospectus in February 2016, the CCIWG selected lead authors for 19 chapters and more than 100 additional contributing authors. This writing team comprises scientists and technical experts representing national laboratories; government agencies; universities; and the private sector across the United States, Canada, and Mexico. Additional contributing authors were chosen later to provide special input on select areas of the assessment. Also selected was a team of five Science Leads from U.S. agencies, national laboratories, and academia to provide high-level scientific expertise and assistance and to ensure consistency in scientific information throughout the report. Drawing from the CCIWG members, one to two Federal Liaisons were assigned to each chapter to review and provide guidance within their area of expertise and pertinent federal research or programmatic portfolio. Further details on the SOCCR2 development processes, timeline, and team roles and responsibilities are provided in Appendix A: Report Development Process.

Multiple formal and internal reviews of consecutive SOCCR2 drafts have taken place (see Figure P.1), including the following six reviews.

  1. Interagency review of the “Second Order Draft” by the SGCR (November 8–23, 2016).

  2. Interagency review of the “Third Order Draft” by the SGCR (June 23 to July 21, 2017).

  3. NASEM committee review of the “Fourth Order Draft” (November 3, 2017, to March 12, 2018).

  4. Public comment period for the “Fourth Order Draft” (November 3, 2017, to January 12, 2018).

  5. Iterative internal reviews of multiple drafts by the CCIWG, SOCCR2 Federal Steering Committee members, five Science Leads, SOCCR2 Chapter Leads, Expert Reviewers, Oak Ridge National Laboratory (ORNL) technical editors, and federal experts from different agencies (September 2016 to July 2018). For example, prior to the “Third Order Draft” review by the SGCR, several additional layers of input, reviews, and revisions (February to May 2017) were provided by 1) USDA (i.e., the administrative agency lead for SOCCR2), 2) SOCCR2 Federal Liaisons, 3) external Expert Reviewers, 4) USGCRP leadership, and 5) SOCCR2 writing teams.

  6. Following the public comment period and a formal review by NASEM experts, the writing team further revised the report in coordination with Review Editors who were selected via an open call to ensure appropriate responses to comments. The draft was subsequently reviewed and approved for final publication by USGCRP member agencies as part of the interagency clearance process: Final Interagency Clearance of the “Fifth Order Draft” by the SGCR (July 31 to August 20, 2018).


Figure P.1: Major SOCCR2 Process Highlights, Reviews, and High-Level Timeline

Figure P.1: Brown boxes denote foundational, developmental stages in the process. Dark blue boxes denote drafting, review, and revision stages.


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