Second State of the Carbon Cycle Report

USGCRP

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

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.

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.]

SHRINK

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:

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

Box ES.1 Main Findings of SOCCR2

Global Atmospheric Carbon Levels. Globally, atmospheric carbon dioxide (CO₂) has risen over 40%, from a preindustrial level of about 280 parts per million (ppm) to the current concentration of more than 400 ppm. Over the same time period, atmospheric methane (CH₄) has increased from about 700 parts per billion (ppb) to more than 1,850 ppb, an increase of over 160%. Current understanding of atmospheric carbon sources and sinks confirms the overwhelming role of human activities, especially fossil fuel combustion, in driving these rapid atmospheric changes.
Emissions from Fossil Fuel Combustion. North American emissions from fossil fuel combustion have declined on average by 1% per year over the last decade, largely because of reduced reliance on coal, greater use of natural gas (a more efficient fossil fuel), and increased vehicle fuel efficiency standards. As a result, North America’s share of global emissions decreased from 24% in 2004 to 17% in 2013. Continued growth in economic activity demonstrates that CO₂ emissions can be decoupled, at least partly, from economic activity. Projections suggest that by 2040, total North American absolute² fossil fuel carbon emissions could range from a 12.8% decrease to a 3% increase compared to 2015 levels (see Ch. 19: Future of the North American Carbon Cycle).
Atmospheric Carbon Removal by Land. Evidence suggests that North American lands have persisted as a net carbon sink over the last decade, taking up about 600 to 700 teragrams of carbon (Tg C) per year, which is 11% to 13% of global carbon removal by terrestrial ecosystems (see Figure ES.2; Ch. 2: The North American Carbon Budget; and Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane). Previously conflicting atmospheric measurements and land inventories now converge on this range. Although uncertainties remain in estimates derived from both approaches, the weight of the evidence leaves little doubt about the direction and overall magnitude of the land sink. Future impacts from climate change, land-use change, and disturbances (both natural and human induced) may diminish this sink.
Inland and Coastal Waters as Both Sources and Sinks. Inland waters emit about 247 Tg C per year to the atmosphere but also bury about 155 Tg C per year in sediments. Tidal wetlands and estuaries represent a combined net sink of 17 Tg C per year from the atmosphere, and 14 Tg C per year are buried in sediments. The coastal ocean directly absorbs about 160 Tg C per year from the atmosphere and buries about 65 Tg C per year in sediments. These detailed findings and their uncertainties (see Figure ES.3) represent marked improvements in the understanding of the carbon cycle in North America’s aqueous environments and highlight the size of carbon transfers in water and across land-water interfaces. However, uncertainties for many of the fluxes remain large.
Methane Concentration and Emissions. Observations indicate that the globally averaged atmospheric CH₄ concentration increased at a rate of 3.8 ± 0.5 ppb per year from 2004 to 2013. Although this increase represents a significant rise in global emissions, the picture for North America is less clear. Most analyses of atmospheric data suggest relatively stable North American CH₄ emissions despite increases in natural gas extraction and use.
Carbon Management Opportunities. Analyses of social systems and their reliance on carbon demonstrate the relevance of carbon cycle changes to people’s everyday lives and reveal feasible pathways to reduce greenhouse gas (GHG) emissions or increase carbon removals from the atmosphere. Such changes could include, for example, decreasing fossil fuel use (which has the largest reduction potential), expanding renewable energy use, and reducing CH₄ emissions from livestock. Increased afforestation and improved agricultural practices also could remove emitted CO₂ from the atmosphere. Although activities in North America cannot alone reduce emissions enough to limit global temperature rise to 2°C, the estimated cumulative cost from 2015 to 2050 for the United States to reduce emissions by 80% relative to 2005 levels (an amount considered to be in line with the 2°C goal), by using a variety of technological options, is in the range of $1 trillion to $4 trillion (US$2005). The total annual cost in 2050 alone for climate change damages across health, infrastructure, electricity, water resource, agriculture, and ecosystems in the United States is conservatively estimated to range from $170 billion to $206 billion (US$2015; see Ch. 3: Energy Systems).
Carbon Accounting and Urban Environments. Because urban environments in North America are the primary sources of anthropogenic carbon emissions, carbon monitoring and budgeting in these areas are extremely important. In addition to direct emissions, urban areas are responsible for indirect sources of carbon associated with goods and services produced outside city boundaries for consumption by urban dwellers. Careful accounting of direct and indirect emissions is necessary to avoid double counting of CO₂ fluxes measured in other sectors and to identify sources to inform management and policy. (For more details on alternatives for carbon accounting and emissions attribution, see Frameworks for Carbon Accounting in the Preface and Appendix D: Carbon Measurement Approaches and Accounting Frameworks.)
Projections of the Carbon Cycle. Projections suggest that energy production, land-use change (especially urbanization), climatic changes such as warming and droughts, wildfires, and pest outbreaks will increase GHG emissions in the future. Carbon stored in soil pools in the circumpolar permafrost zone is at particular risk. With the current trajectory of global and Arctic warming, 5% to 15% of this carbon is vulnerable for release to the atmosphere by 2100.
Ocean Acidification. Rising CO₂ has decreased seawater pH at long-term observing stations around the world, including in the open ocean north of Oahu, Hawai‘i; near Alaska’s Aleutian Islands and the Gulf of Maine shore; and on Gray’s Reef in the southeastern United States. This ocean acidification already has affected some marine species and altered fundamental ecosystem processes, with further effects likely.
User-Inspired Science. Demand for carbon cycle science from diverse institutions, including carbon registries, major corporations, municipal governments, utilities, and non-governmental organizations, has remained strong over the past decade. Social science research could map the capacity of these different organizations to use carbon cycle science to help identify relevant research questions and to produce information in formats that align with standard organizational practices and stakeholder needs.
Research and Monitoring Gaps. This report documents an improving ability to attribute observed changes in the North American carbon budget to specific causes. Additional research is needed to better understand the impacts of human activities on the carbon cycle, feedbacks between increasing CO₂ concentrations and terrestrial ecosystems, natural disturbance alterations caused by climate change, and societal responses to these changes. Understanding these processes and their interactions is essential for improving projections of future changes in the carbon cycle and addressing adaptation needs and management options. Advancing the understanding of carbon cycling and resource management on public, private, and tribal lands requires further research, as does improving the integration of social science with natural science related to the carbon cycle. Additional focused monitoring would benefit carbon accounting and management, particularly in Arctic and boreal regions, grasslands, wetlands, inland and coastal waters, and tropical ecosystems.

See Full Chapter & References

Front Matter

Synthesis

Human Dimensions of the Carbon Cycle

State of Air, Land and Water

Consequences and Ways Forward

Appendices

Executive Summary

Authors

Acknowledgments

Recommended Citation

Executive Summary

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