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      -  			ABOUT		    Report in BriefTable of ContentsHighlightsPrefaceRecommended CitationsReport CreditsContact UsU.S. Carbon Cycle Science Program OfficeUS. Global Change Research Program 
      -  CHAPTERS Front Matter- Highlights
- Preface
- Executive Summary
Synthesis**- Overview of the Global Carbon Cycle
- The North American Carbon Budget
Human Dimensions of the Carbon Cycle **- Energy Systems
- Understanding Urban Carbon Fluxes
- Agriculture
- Social Science Perspectives on Carbon
- Tribal Lands
State of Air, Land and Water**- Observations of Atmospheric CO2 and Methane
- Forests
- Grasslands
- Arctic and Boreal Carbon
- Soils
- Terrestial Wetlands
- Inland Waters
- Tidal Wetlands and Estuaries
- Coastal Ocean and Continental Shelves
Consequences and Ways Forward**- Biogeochemical Effects of Rising Atmospheric CO2
- Carbon Cycle Science in Support of Decision Making
- Future of the North American Carbon Cycle
Appendices- Report Development Process
- Information Quality in the Assessment
- Selected Carbon Cycle Research Observations and Measurement Programs
- Carbon Measurement Approaches and Accounting Frameworks
- Fossil Fuel Emissions Estimates for North America
- Acronyms, Abbreviations, and Units
- Glossary
 
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# Second State of the Carbon Cycle Report

SOCCR2 is an authoritative decadal assessment of carbon cycle science across North America, developed by over 200 experts from the U.S., Canadian and Mexican governments, national laboratories, universities, private sector, and research institutions.SOCCR2 is a Sustained Assessment Product of the U.S. Global Change Research Program.		    

 Recommended Report Citation

## HIGHLIGHTS

## REPORT IN BRIEF

## PREFACE

## EXECUTIVE SUMMARY

## SECTIONS

## DOWNLOADS

## SOCCR2 by Sections

##### 			Front Matter		    

Highlights

Executive Summary

Preface                  - About this Report
                  - Report Development, Review, and Approval Process
                  - Guide to this Report
                  - Frameworks for Carbon Accounting
                  - Treatment of Uncertainty in SOCCR2
                  - Interagency Context of U.S. Carbon Cycle Science

##### 			Section I: Synthesis		    

These chapters introduce the carbon cycle—what it is and why it is important. They assess the present state, trends, and potential future directions of the North American carbon budget—the balance of carbon fluxes, stocks, and transformations—and how this budget fits into the carbon cycle at a global scale.

Ch. 1:  Overview of the Global Carbon Cycle

Ch. 2:  The North American Carbon Budget

##### 			Section II: Human Dimensions of the Carbon Cycle		    

These chapters highlight fluxes and processes in socialecological systems, including urban areas, energy systems, agricultural enterprises, societal institutions, and lands belonging to Indigenous communities. The carbon cycle in these sectors is inextricably linked to human needs and actions as well as to societal decisionmaking outcomes.

Ch. 3:  Energy Systems

Ch. 4:  Understanding Urban Carbon Fluxes

Ch. 5:  Agriculture

Ch. 6:  Social Science Perspectives on Carbon

Ch. 7:  Tribal Lands

##### 			Section III: State of Air, Land, and Water		    

These chapters present carbon cycle fluxes and processes in different physical and ecological domains, including the atmosphere, soils, inland and coastal waters, and the coastal ocean, as well as in terrestrial ecosystems such as forests, grasslands, and those in Arctic regions. Understanding these ecosystems is fundamental to assessing and predicting net carbon sources and sinks, including feedbacks to and from the climate system. These ecosystems also represent key carbon reservoirs with sensitivity to changes in climate and atmospheric composition.

Ch. 8:  Observations of Atmospheric Carbon Dioxide and Methane

Ch. 9:  Forests

Ch. 10:  Grasslands

Ch. 11:  Arctic and Boreal Carbon

Ch. 12:  Soils

Ch. 13:  Terrestrial Wetlands

Ch. 14:  Inland Waters

Ch. 15:  Tidal Wetlands and Estuaries

Ch. 16:  Coastal Ocean and Continental Shelves

##### 			Section IV: Consequences and Ways Forward		    

These chapters outline future projections of rising atmospheric carbon and its associated consequences. They detail how science can inform decision making at the federal, provincial, state, tribal, and local levels across North America, and how those decisions could affect the carbon cycle in the future.

Ch. 17:  Biogeochemical Effects of Rising Atmospheric Carbon Dioxide

Ch. 18:  Carbon Cycle Science in Support of Decision Making

Ch. 19:  Future of the North American Carbon Cycle

##### 			Appendices		    

Appendix A:  Report Development Process

Appendix B:  Information Quality in the Assessment

Appendix C:  Selected Carbon Cycle Research Observations and Measurement Programs

Appendix D:  Carbon Measurement Approaches and Accounting Frameworks

Appendix E:  Fossil Fuel Emissions Estimates for North America

Appendix F:  Acronyms, Abbreviations, and Units

Appendix G:  Glossary

# SECOND STATE OF THE CARBON CYCLE

## Highlights

##### 						Authors					

                    - Authors:
                    -  Gyami Shrestha , U.S. Carbon Cycle Science Program and University Corporation for Atmospheric Research								
                    -  Nancy Cavallaro , USDA National Institute of Food and Agriculture								
                    -  Laura Lorenzoni , NASA Earth Science Division								
                    -  Abigail Seadler , NASA Earth Science Division								
                    -  Zhiliang Zhu , U.S. Geological Survey								
                    -  Noel P. Gurwick , U.S. Agency for International Development								
                    -  Elisabeth Larson , North American Carbon Program and NASA Goddard Space Flight Center, Science Systems and Applications Inc.								
                    -  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)								

##### 					Translations				

                    - English
                    - Chinese
                    - Spanish
                    - French

##### 						Recommended Citation					

Shrestha, G., N. Cavallaro, L. Lorenzoni, A. Seadler, Z. Zhu, N. P. Gurwick, E. Larson, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, and P. Romero-Lankao, 2018: Highlights. 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. 1-4, doi: 10.7930/SOCCR2.2018.Highlights

The *Second State of the Carbon Cycle Report* (SOCCR2) provides a current state-of-the-science assessment of the carbon cycle in North America (i.e., the United States, Canada, and Mexico) and its connection to climate and society (see Box 1, What Is SOCCR2?). Information from the report is relevant to climate and carbon research as well as to management practices in North America and around the world. This general overview provides abbreviated highlights of some of the many significant findings from the 19 chapters in SOCCR2.

*North American carbon cycling illustration, courtesy Ron Oden, University of Nevada, Reno.*

This graphic represents the dynamic nature of carbon stocks and fluxes in the United States, Canada, and Mexico described inthe *Second State of the Carbon Cycle Report*.

- The center sketch of researchers taking soil samples pays tribute to the hundreds of scientists who served asauthors for this report and the thousands of researchers whose data were used throughout the document. Arrows depict carbon emissions to the atmosphere (red) and carbon uptake by different land types and aquaticenvironments (teal), processes described in Ch. 1: Overview of the Global Carbon Cycle and Ch. 2: The NorthAmerican Carbon Budget.  Plotted data—collected by the National Oceanic and Atmospheric Administration’s Earth System ResearchLaboratory—show monthly means of atmospheric carbon dioxide (CO2) concentrations (red curve in parts permillion) taken at the Mauna Loa Observatory and monthly means of methane (CH4) concentrations (green curvein parts per billion) from globally averaged marine surface sites. Deseasonalized data are depicted by the black lines(Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane).  Coral reefs, fish, and beaches represent carbon processes in coastal waters (Ch. 15: Tidal Wetlands and Estuariesand Ch. 16: Coastal Ocean and Continental Shelves). These are key areas experiencing carbon cycle changes due todirect effects of increasing CO2 (Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide).Forests (first inset, lower left) and their soils represent the largest terrestrial carbon sink in North America. Factorsinfluencing the strength of this sink and trends in disturbances such as wildfire, insects, and land-use change aredescribed in 
Ch. 9: Forests
.
Mountains with melting glacier (second inset, lower left) illustrate the effects of greenhouse gas–induced warmingon carbon cycling, particularly in high-latitude and boreal areas (
Ch. 11: Arctic and Boreal Carbon
).
Pastoral scene (center inset, bottom) captures the interdependent carbon cycling processes among differentterrestrial and aquatic systems (
Ch. 5: Agriculture
, 
Ch. 7: Tribal Lands
,
Ch. 10: Grasslands
, 
Ch. 12: Soils, Ch. 13:Terrestrial Wetlands, and Ch. 14: Inland Waters).Power plant (second inset, lower right) illustrates carbon fluxes from the energy sector and other human systemsand their potential impact on future carbon cycling (
Ch. 3: Energy Systems
 and 
Ch. 19: Future of the North American Carbon Cycle
).
Coastal city and port (first inset, lower right) represent the many ways carbon is embedded in social systems andthe different levels of information and governance involved in carbon decision making (
Ch. 4: UnderstandingUrban Carbon Fluxes
, 
Ch. 6: Social Science Perspectives on Carbon
, and 
Ch. 18: Carbon Cycle Science in Supportof Decision Making
).
Box 1. What Is SOCCR2?
Authored by more than 200 scientists from the United States, Canada, and Mexico, the *Second State of the Carbon Cycle Report* (SOCCR2) provides an up-to-date assessment of scientific knowledge of the North American carbon cycle. This comprehensive report addresses North American carbon fluxes, sources, and sinks across atmospheric, aquatic, and terrestrial systems, as well as relevant perspectives from scientific observations and modeling, decision support, carbon management, and social sciences. The report presents Key Findings and actionable information on the observed status and trends within the North American carbon cycle, as influenced by natural and human-induced factors.

These findings are based on multidisciplinary research that includes experimental, observational, and modeling studies from the last decade. 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, SOCCR2 provides information to inform mitigation and adaptation policies and management decisions related to the carbon cycle and climate change. It also will help support improved coordination for pertinent research, monitoring, and management activities necessary to respond to global change. SOCCR2 informs policies but does not prescribe or recommend them.
Carbon Dynamics in North America and the United States in a Global Context
Land ecosystems and the ocean play a major role in the removal and sequestration of carbon dioxide (CO2) from the atmosphere. From 2007 to 2016, these reservoirs annually removed and stored an average of about 5.4 billion metric tons of carbon that otherwise would have remained in the atmosphere—about half the amount emitted during that period. About 11% to 13% of global ecosystem carbon removal can be attributed to North American ecosystems. Whether the land and ocean will continue to absorb similar amounts of carbon in future years is unclear, since changes in climate, human activities, and ecosystem responses may alter future long-term removals of carbon from the atmosphere. Although North America contributed substantially to global atmospheric carbon emissions over the past decade, its total carbon emissions due to fossil fuel use (referred to in this document as “fossil fuel emissions”) decreased by about 23 million metric tons of carbon per year. Meanwhile, global emissions continued to increase, thus reducing the relative contribution of North America to total fossil fuel emissions from 24% in 2004 to less than 17% in 2013.

In addition to reducing the use of fossil fuels, mitigation and management activities in North America and around the world include afforestation and reduced deforestation, restoration of coastal1 and terrestrial wetlands, and improved land-management practices in forests, grasslands, and croplands. These activities can maintain or increase ecosystem carbon sinks (i.e., carbon storage or removal) while decreasing the sources or emissions of carbon to the atmosphere. However, Arctic warming and disturbances such as pest outbreaks, wildfires, and destruction of wetlands may disrupt and decrease carbon removal, thereby releasing previously removed carbon back to the atmosphere (see Box 2, Why Is the Carbon Cycle Important?).
Box 2. Why Is the Carbon Cycle Important?
The carbon cycle encompasses the flow, storage, and transformation of carbon compounds that are central to life and to the production of food, fiber, and energy. Carbon also helps regulate Earth’s climate, including temperature, weather events, and more. This report assesses the complex, interconnected ecological and societal aspects of the carbon cycle, illustrating the importance of the carbon cycle to ecosystems, regions, and communities and projecting possible future changes to the carbon cycle and impacts on humans and ecosystems, while also presenting relevant issues for decision makers.
Fossil Fuels and Economic Impacts
Over the past decade, fossil fuel emissions continued to be by far the largest North American carbon source. The United States is currently responsible for about 80% to 85% of fossil fuel emissions from North America. The financial crisis around 2008 contributed to a reduction in North American fossil fuel emissions as economic and industrial growth slowed. Yet, as the economy has recovered, increased energy efficiency and economic structural changes have enabled economic growth while continuing the trend of lowering CO2 emissions. Over the last decade, North America has reduced its CO2 emissions from fossil fuels by about 1% per year, as the result of various market, technology, and policy drivers.
A Changing Landscape
At the global level, land-use change due to social, demographic, and economic trends is projected to contribute between 11 and 110 billion metric tons of carbon to the atmosphere by 2050. However, the trend in the United States is the opposite: current assessments suggest that better forest management practices, as well as reforestation and other improvements in ecosystem and resource management, are helping the nation decrease its carbon emissions.
Ocean Acidification
Ocean acidification, or the decrease in seawater pH due to increased oceanic CO2 absorption, can adversely affect many marine populations and ecosystem processes, including organisms that people rely on for food and ecosystem services that sustain economies and cultures throughout North America. Acidification is occurring faster in circumpolar regions and some coastal areas than in the open ocean. For example, over the past decade, Arctic and Pacific Northwest coastal waters have experienced longer, more frequent periods of lower pH, putting livelihoods reliant on these areas at increased risk. Maintaining and expanding existing ocean observing programs, as well as continuing coordinated work with stakeholders, will be critical to ensure a healthier ocean, resilient communities, and strong economies.
Arctic Changes
The environment of high-latitude regions, such as the Arctic, is changing at a faster pace than the rest of North America. For example, Arctic surface air temperatures are rising about 2.5 times faster than the global average. This increase can destabilize permafrost soils (i.e., soil that remains permanently frozen at some depth) and surrounding landscapes, which exist throughout the Arctic and store almost twice the amount of carbon currently contained in the atmosphere. Warming temperatures can release this stored carbon into the atmosphere. In addition, accelerated warming increases the frequency and intensity of fires, which also release large amounts of carbon stored in Arctic permafrost, surface soils, and vegetation.
Carbon in Crops
Most carbon in croplands is stored in the soil and is sensitive to increasing temperatures, land-use changes, and agricultural development and practices, all of which can result in the loss of carbon from the soil to the atmosphere. Soil carbon stocks can be increased or stabilized by incorporating practices that 1) keep the land covered with plants, especially deep-rooted perennials and cover crops, 2) protect the soil from erosion (e.g., by decreasing tillage), and 3) improve nutrient management. Additionally, optimizing nitrogen fertilizer management to sustain crop yields and reduce nitrogen losses to air and water can help reduce greenhouse gas (GHG) emissions and increase food availability for growing populations.
Indigenous Communities
North American non-Indigenous, fossil fuel–based societies can benefit from understanding how Indigenous communities manage carbon in day-to-day living. These communities offer potentially valuable lessons on how to address emissions reduction and carbon capture through people-focused approaches that couple technological and ecological systems with their traditional practices of agrarian-based infrastructure and tribal community values. While quantitative analysis of these practices is only beginning, many Indigenous communities across the United States, Canada, and Mexico are managing carbon stocks and fluxes to reduce GHG emissions through sustainable management of forests, agriculture, and natural resources.
Cities and Carbon
Urban areas in North America are the primary source of anthropogenic carbon emissions. Emissions from the urban built environment are directly shaped by societal factors, including regulations and policies governing land use, technologies such as transportation, and indirect factors such as demands for goods and services produced outside city boundaries. Such societal drivers can lock in dependence on fossil fuels in the absence of major technological, institutional, and behavioral change. In urban areas many pivotal decisions and policies are made that shape carbon fluxes and mitigation (see Box 3, How Can SOCCR2 Inform Decision Making?).
Box 3. How Can SOCCR2 Inform Decision Making?
The information in the *Second State of the Carbon Cycle Report* (SOCCR2) reflects the current peer-reviewed, scientific consensus of the multidisciplinary carbon cycle research community. This decadal assessment responds to the needs of multiple stakeholder groups that rely on the science it encompasses to manage ecosystem services and prioritize actions for reducing carbon emissions, as these groups aim to mitigate the effects of climate change on their communities and environments. Stakeholders in governments and institutions at the federal, provincial, state, and local levels, as well as carbon registries, utilities, and corporations, can use SOCCR2 information to better inform management strategies and options for transportation systems, critical infrastructure, land and ecosystem management, and other decisions that are sensitive to carbon cycle changes.
Knowledge Gaps and Science Informing Investments in the Future
Future research will facilitate improvements in knowledge, practices, and technologies for managing carbon emissions, removing carbon from the atmosphere, and accumulating and storing it in Earth systems over the long term. Expansions in monitoring, advanced syntheses of available observations, improvements in assessment tools and models, and extension of existing modeling capabilities can help provide more reliable measurements and future estimates of carbon stocks and flows at the local, regional, and global level. Co-benefits, such as improvements in air quality, crop productivity, energy efficiency, economic savings to taxpayers, and enhanced quality of life, often result from reduction in carbon emissions. Research identifying and responding to such opportunities—as well as addressing needs for research in carbon management and emissions mitigation across decision-making stakeholders, sectors, and governance at multiple levels—is an investment in the sustainable well-being of Earth, society, and future generations.
- Coasts and coastal ecosystems in SOCCR2 include mangroves, tidal marshes, and seagrass meadows.↩
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