Figure 5 (from Ch 11). The difference between mean annua l Arc t i c surface air temperatures (deg C) from the period between 2001 and 2105 a n d t h o s e f r o m a b a s e l i n e p e r i o d o f 1971-2000.

1Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, 2Woods Hole Research Center, Falmouth, MA, 3Department of Meteorology and Atmospheric Science, Pennsylvania State University, University Park, PA, 4Southwest Biological Science Center, United States Geological Survey, Moab, UT, and 5National Center for Atmospheric Research, Boulder, CO

State  of  the  Carbon  Cycle  of  North  America:  DRAFT  Overarching  Findings    

Report  Editorial  Leads  (Melanie  Mayes1,  Richard  Birdsey2,  Raymond  Najjar3,  Sasha  Reed4,  Patricia  Romero-­‐Lankao5),  and  all  SOCCR-­‐2  Chapter  Authors  

Human Dimensions of the Carbon Cycle

The State of Air and Land

The State of Water

Lori Bruhwiler, Anna Michalak, Daniel Hayes, Rodrigo Vargas, Debbie Huntzinger •  The North American continent — including its energy systems,

land-base and coastal oceans — is very likely to have been a net source of carbon to the atmosphere over the 2004-2013 time period, having contributed on average approximately 1332 million tons (+/- 25%) annually (Fig. 1).

•  About ¼ of the total fossil fuel emissions over the 2004 – 2013 time period were likely offset by natural sinks on North American land and adjacent coastal ocean.

•  The strength of the natural sink was similar to SOCCR-1, maintained primarily by carbon uptake with forest growth and storage in wood products offsetting carbon losses from natural disturbance and land-use change.

•  There is low confidence in the future evolution of the land carbon cycle, with models projecting that terrestrial ecosystems could continue to be a net sink of carbon (of up to 1.19 PgC per year) or switch to a net source of carbon to the atmosphere (of up to 0.60 PgC per year) by the end of the century under business-as-usual human-driven emission scenarios.

Energy and Transportation – Chapter 3 – Peter Marcotullio, Emily McGlynn • North American energy related CO2 emissions have declined from 2003 to 2014, at an average

rate of approximately 1% y-1 over that time period, or approximately 71 million metric tons CO2-eq (Fig. 7).

•  North American economies have reduced the energy intensity of economic production by ~ 1.5% y-1 over the past 10 years due to greater efficiency and economic structural changes, allowing for economic growth while energy CO2-eq emissions have fallen.

Urban Carbon – Chapter 4 – Kevin Gurney, Stephanie Pincetl •  North American cities generally have the highest per capita carbon emissions of global cities but

significant variation remains between and within North American countries. Increased urbanization and urban emissions are expected, likely magnified by climate change and the heat island effect.

Consequences (Chapter 17) Sarah Cooley, David Moore •  Rising CO2 increases the carbon sink in the oceans, while decreasing

seawater pH and causing acidification (Fig. 2). Commercial fish stocks and the efficacy of carbonate-fixing organisms, e.g., shellfish, are impacted by ocean acidification.

•  In terrestrial ecosystems, increased atmospheric CO2 causes enhanced photosynthesis, increased net primary production, and increased water-use efficiency. Key ecosystem services including crop production yields, carbon storage in soils and forests, water use efficiency, and vegetation composition in natural and managed ecosystems are altered as a result of rising CO2.

Atmosphere – Chapter 8 – Andrew Jacobson, John Miller •  Since SOCCR-1, global CO2 concentrations increased by ~ 2.1 ± 0.1 ppm

yr-1 and CH4 by~ 7.0 ± 0.6 ppb yr-1. CO2 concentrations are now over 400 ppm and CH4 stands at about 1830 ppb (Fig. 3). SOCCR-2 will address both CO2 and CH4.

Forests – Chapter 9 – Grant Domke, Chris Williams •  Net carbon uptake by North American forests is well documented. The

strength of uptake varies regionally, with about 70% of the NA forest sink for atmospheric carbon occurring within the US.

•  Forest regrowth following historical clearing continues to make a significant contribution to the terrestrial carbon sink, but net uptake is expected to decline as regrowing forests get older and grow slower, deforestation continues, and rates of natural disturbance increase (Fig. 4).

Agriculture – Chapter 5 – Alexander Hristov, Charles Rice •  Promising and readily applicable technologies can reduce enteric methane emissions from ruminants by 20-30%. Other mitigation

technologies can reduce manure methane emissions by 30-50% and up to 80%. Methane mitigation strategies have to be evaluated on a production system scale, accounting for emission trade-offs and co-benefits such as improved feed efficiency or productivity in livestock.

Social Science Perspectives on Carbon – Chapter 6 – Elizabeth Malone •  Carbon is embedded in almost all societal activities, from industrial, energy, transportation, urban and rural configurations, and agricultural

systems to buildings, governance policies and functions, and business and social practices. •  Opportunities to reduce emissions and capture carbon can be identified by systems approaches that are centered on people and include

coupled technological and ecological systems.

Tribal Lands – Chapter 7 – Maureen McCarthy •  From an ecosystem perspective, carbon cycle issues on tribal lands are not dissimilar to those on surrounding public or private lands;

however, managing carbon programs poses unique challenges to indigenous communities in the United States, Canada and Mexico due to land tenure, water rights, and socioeconomic factors.

•  Indigenous communities in North America are applying traditional knowledge to enhance carbon stocks and reduce greenhouse gas emissions through sustainable management of forests, agriculture, and natural resources and engagement in emerging carbon trading markets.

Figure 7 (from Ch 2). Annual fossil fuel emissions (in billion tons of C) from each country along with the continental total (left vertical axis) and global proportion (right vertical axis) for the period 1960 to 2014.

Figure 1 (from Ch 2). Draft estimates of carbon sinks (-) and sources (+) in SOCCR-2.

Figure 8 (from Ch 18). Primary drivers of carbon stocks and emissions, with some examples provided. Carbon estimates can be generated using observations, models of differing complexity, or both. To understand and estimate future carbon stocks and emissions, drivers of carbon stock changes and carbon emissions must be considered and represented. This schematic illustrates components needed to represent carbon stock changes prior to addressing policy drivers.

Terrestrial wetlands – Chapter 13 – Randy Kolka, Carl Trettin • North American wetlands are 42% of the global wetland area and store 174 Pg of C, of

which 72% is in organic soils. •  North American wetlands are a CO2 sink of 91 Tg C yr-1 and a source of CH4, releasing

~20 Tg CH4 yr-1. Inland waters – Chapter 14 – David Butman • The flux of carbon from inland waters of North America is 507 Tg C yr-1. • Carbon flux from inland waters in CONUS and AK is 192 Tg C yr-1, with 57% as CO2

and CH4 emissions, 31% as lateral fluxes to coastal waters, and 12% as burial in lakes and reservoirs (Fig. 9). Estimates do not yet exist for Mexico and Canada.

Tidal wetlands and estuaries – Chapter 15 – Lisamarie Windham-Myers, Wei-Jun Cai • In North America, tidal wetlands remove 20 ± 10 Tg C y-1 from the atmosphere,

estuaries outgas 10 ± 10 Tg C y-1 to the atmosphere, and the exchange with the atmosphere of the combined wetland-estuary system (an uptake of 10 ± 14 Tg C y-1) is not significantly different from zero.

•  Tidal wetland soils and estuarine sediments of North America contain roughly 1323 Tg C as organic C, in the top 1 m of soil and sediment.

Figure 3 (from Ch 8). Global monthly mean CO2 (blue) and CH4 (red) concentrations from the NOAA marine boundary layer product. Dotted vertical lines in 2007 and 2016 represent approximate reference times for publication of the first and second SOCCR reports.

Figure 2 (from Ch 17). Ocean time-series station evidence for ocean acidification, i.e., decreasing pH, from monitoring stations ALOHA in the Pacific Ocean, BATS in the central Atlantic Ocean, and ESTOC off the west coast of Africa. Courtesy Richard Feely, NOAA.

Grasslands – Chapter 10 – Elise Pendall •  Total grassland carbon stocks in the U.S. are projected to increase to about

8.2 Pg by 2050, while the rate of storage is projected to decline due to land-use and land-cover change with continued agricultural incursions into grasslands.

•  Warming temperatures and rising atmospheric CO2 concentrations interact to increase productivity in grasslands, partly due to longer growing seasons.

Arctic and Boreal Carbon – Chapter 11 – Ted Schuur, Dave McGruire, Vladimir Romanovsk

•  Carbon stored in permafrost zone circumpolar soils are equal to 1330-1580 Pg carbon, almost twice that contained in the atmosphere and an order of magnitude greater than carbon contained in vegetation.

•  Five to 15% of the carbon stored in permafrost zone circumpolar soil pools is considered vulnerable to release to the atmosphere by the year 2100 following the current trajectory of global and Arctic warming (Fig. 5).

Figure 4 (from Ch 9). Major disturbance types mapped at a 30-m resolution across the conterminous United States based on NAFD, MTBS, and ADS datasets, the records from which span 1984-2010, 1984-2014, and 1997-2014, respectively.

Coastal oceans and continental shelves – Chapter 16 – Katja Fennel

• Available estimates of air-sea carbon fluxes indicate that the North American margins act as a net sink for atmospheric CO2. This net uptake is primarily driven by fluxes in high-latitude regions. The magnitude of the net flux is not well constrained.

Figure 9 (from Ch 14). Primary drivers of disturbance for carbon fluxes from inland wetland environments. Dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), particulate organic carbon (POC) are the primary forms of mobilized aqueous carbon, and arrows refer to expected changes in release rates in different environments. The blue arrows in the center of the figure refer to gaseous releases of CO2 and CH4, while the arrows at the bottom refer to lateral transfers from inland wetlands to discharge points such as tidal estuaries or coastal oceans.

ACKNOWLEDGEMENTS The Editorial Leads like to thank the numerous authors contributing to the SOCCR-2 report. We also thank the Carbon Cycle Interagency Working Group and the US Global Change Research Program, Gyami Shrestha, director of the US Carbon Cycle Science Program Office, and the ORNL Editorial Team including Betty Mansfield, Holly Haun, Kris Christen, Marissa Mills, and Sheryl Martin. Oak Ridge National Laboratory is managed by the University of Tennessee-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy.

The next State of the Carbon Cycle Report (SOCCR-2) is being developed by the Carbon Cycle Interagency Working Group under the auspices of the US Global Change Research Program to inform the Sustained National Climate Assessment

Syntheses (Chapters 1 & 2) and Projections (Chapter 19)

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Soils – Chapter 12 – Kate Lajtha, Vanessa Bailey • Soil carbon stocks (Fig. 6) are sensitive to agricultural management, land

use and land cover change, changing climate, and development and loss of C-rich soils. Management practices can mitigate agricultural soil C loss. Forest soil C appears relatively resistant to standard management practices. Afforestation of formerly agricultural lands is a significant driver of increased soil C stocks across North America.

Figure 6 (from Ch 12). Updated soil organic carbon stocks. https://www.nrcs.usda.gov/wps/p o r t a l / n r c s / d e t a i l / s o i l s / s u r v e y / ?cid=nrcs142p2_054164

https://www.carboncyclescience.us/state-carbon-cycle-report-soccr

Carbon Cycle Science in Support of Decision Making – Chapter 18 – Tristram West, Molly Brown, Keith Paustian •  Strong, long-term links among research on climate, carbon, carbon-related management,

and carbon prediction will improve model accuracy and coordination among research disciplines, thereby improving joint representation of natural and managed systems needed for decision making (Fig. 8).