List of Report Key Findings

• Chapter 1: Overview of the Global Carbon Cycle
  Key Finding 1
  

  Atmospheric carbon dioxide (CO₂) has increased from a preindustrial abundance of 280 parts per mil- lion (ppm) of dry air to over 400 ppm in recent years—an increase of over 40%. As of July 2017, global average CO₂ was 406 ppm. Methane (CH₄ has increased from a preindustrial abundance of about 700 parts per billion (ppb) of dry air to more than 1,850 ppb as of 2017—an increase of over 160%. The current understanding of the sources and sinks of atmospheric carbon supports the dominant role of human activities, especially fossil fuel combustion, in the rapid rise of atmospheric carbon (very high confidence).

• Chapter 1: Overview of the Global Carbon Cycle
  Key Finding 2
  

  In 2011, the total global anthropogenic radiative forcing resulting from major anthropogenic green- house gases (GHGs, not including anthropogenic aerosols) relative to the year 1750 was higher by 2.8 watts per meter squared (W/m²). As of 2017, the National Oceanic and Atmospheric Administra- tion’s Annual Greenhouse Gas Index estimates anthropogenic radiative forcing at 3.1 W/m², an increase of about 11% since 2011. In 2017, CO₂ accounted for 2.0 W/m² and CH₄ for 0.5 W/m² of the rise since 1750. The global temperature increase in 2016 relative to the 1880 to 1920 average was over +1.25°C, although this warming was partially boosted by the 2015–2016 El Niño. Global temperature, excluding short-term variability, now exceeds +1°C relative to the 1880–1920 mean in response to this increased radiative forcing (Hansen et al., 2017; very high confidence)

• Chapter 1: Overview of the Global Carbon Cycle
  Key Finding 3
  

  Global fossil fuel emissions of CO₂ increased at a rate of about 4% per year from 2000 to 2013, when the rate of increase declined to about 2% per year. In 2014, the growth in global fossil fuel emissions further declined to only 1% per year (Olivier et al., 2016). During 2014, the global economy grew by 3%, implying that global emissions became slightly more uncoupled from economic growth, likely a result of greater efficiency and more reliance on less carbon intensive natural gas and renewable energy sources. Emissions were flat in 2015 and 2016 but increased again in 2017 by an estimated 2.0% (high confidence).

• Chapter 1: Overview of the Global Carbon Cycle
  Key Finding 4
  

  Net CO₂ uptake by land and ocean removes about half of annually emitted CO₂ from the atmo- sphere, helping to keep concentrations much lower than would be expected if all emitted CO₂ remained in the atmosphere. The most recent estimates of net removal by the land, which accounts for inland water emissions of about 1 petagram of carbon (Pg C) per year, indicate that an average of 3.0 ± 0.8 Pg C per year were removed from the atmosphere between 2007 and 2016. Removal by the ocean for the same period was 2.4 ± 0.5 Pg C per year. Unlike CO₂, CH₄ an atmospheric chemical sink that nearly balances total global emissions and gives it an atmospheric lifetime of about 9 to 10 years. The magnitude of future land and ocean carbon sinks is uncertain because the responses of the carbon cycle to future changes in climate are uncertain. The sinks may be increased by mitigation activities such as afforestation or improved cropping practices, or they may be decreased by natural and anthropogenic disturbances (high confidence).

• Chapter 1: Overview of the Global Carbon Cycle
  Key Finding 5
  

  Estimates of the global average temperature response to emissions range from +0.7 to +2.4°C per 1,000 Pg C using an ensemble of climate models, temperature observations, and cumulative emissions (Gillett et al., 2013). The Intergovernmental Panel on Climate Change (IPCC 2013) estimated that to have a 67% chance of limiting the warming to less than 2°C since 1861 to 1880 will require cumulative emissions from all anthropogenic sources to stay below about 1,000 Pg C since that period, meaning that only 221 Pg C equivalent can be emitted from 2017 forward. Current annual global CO₂ emis- sions from fossil fuel combustion and cement production are 10.7 Pg C per year, so this limit could be reached in less than 20 years. This simple estimate, however, has many uncertainties and does not include carbon cycle–climate feedbacks (medium confidence). These conclusions are consistent with the findings of the recent Climate Science Special Report (USGCRP 2017).

• Chapter 10: Grasslands
  Key Finding 1
  

  Total grassland carbon stocks in the conterminous United States, estimated to be about 7.4 petagrams of carbon (Pg C) in 2005, are projected to increase to about 8.2 Pg C by 2050. Although U.S. grasslands are expected to remain carbon sinks over this period, the uptake rate is projected to decline by about half. In the U.S. Great Plains, land-use and land-cover changes are expected to cause much of the change in carbon cycling as grasslands are converted to agricultural lands or to woody biomes (medium confidence).

• Chapter 10: Grasslands
  Key Finding 2
  

  Increasing temperatures and rising atmospheric carbon dioxide (CO₂) concentrations interact to increase productivity in northern North American grasslands, but this productivity response will be mediated by variable precipitation, soil moisture, and nutrient availability (high confidence, very likely).

• Chapter 10: Grasslands
  Key Finding 3
  

  Soil carbon in grasslands is likely to be moderately responsive to changes in climate over the next several decades. Field experiments in grasslands suggest that altered precipitation can increase soil carbon, while warming and elevated CO₂ may have only minimal effects despite altered productivity (medium confidence, likely).

• Chapter 10: Grasslands
  Key Finding 4
  

  Carbon stocks and net carbon uptake in grasslands can be maintained with appropriate land management including moderate levels of grazing. Fire suppression can lead to encroachment of woody vegetation and increasing carbon storage in mesic regions, at the expense of grassland vegetation (high confidence, likely).

• Chapter 11: Arctic and Boreal Carbon
  Key Finding 1
  

  Factors that control terrestrial carbon storage are changing. Surface air temperature change is amplified in high-latitude regions, as seen in the Arctic where temperature rise is about 2.5 times faster than that for the whole Earth. Permafrost temperatures have been increasing over the last 40 years. Disturbance by fire (particularly fire frequency and extreme fire years) is higher now than in the middle of the last century (very high confidence).

• Chapter 11: Arctic and Boreal Carbon
  Key Finding 2
  

  Soils in the northern circumpolar permafrost zone store 1,460 to 1,600 petagrams of organic carbon (Pg C), almost twice the amount contained in the atmosphere and about an order of magnitude more carbon than contained in plant biomass (55 Pg C), woody debris (16 Pg C), and litter (29 Pg C) in the boreal and tundra biomes combined. This large permafrost zone soil carbon pool has accumulated over hundreds to thousands of years. There are additional reservoirs in subsea permafrost and regions of deep sediments that are not added to this estimate because of data scarcity (very high confidence).

• Chapter 11: Arctic and Boreal Carbon
  Key Finding 3
  

  Following the current trajectory of global and Arctic warming, 5% to 15% of the soil organic carbon stored in the northern circumpolar permafrost zone (mean 10% value equal to 146 to 160 Pg C) is considered vulnerable to release to the atmosphere by the year 2100. The potential carbon loss is likely to be up to an order of magnitude larger than the potential increase in carbon stored in plant biomass regionally under the same changing conditions (high confidence, very likely).

• Chapter 11: Arctic and Boreal Carbon
  Key Finding 4
  

  Some Earth System Models project that high-latitude carbon releases will be offset largely by increased plant uptake. However, these findings are not always supported by empirical measurements or other assessments, suggesting that structural features of many models are still limited in representing Arctic and boreal zone processes (very high confidence, very likely).

• Chapter 12: Soils
  Key Finding 1
  

  Estimates for soil carbon stocks in the conterminous United States plus Alaska range from 142 to 154 petagrams of carbon (Pg C) to 1 m in depth. Estimates for Canada average about 262 Pg C, but sampling is less extensive. Soil carbon for Mexico is calculated as 18 Pg C (1 m in depth), but there is some uncertainty in this value (medium confidence).

• Chapter 12: Soils
  Key Finding 2
  

  Most Earth System Models (ESMs) are highly variable in projecting the direction and magnitude of soil carbon change under future scenarios. Predictions of global soil carbon change through this century range from a loss of 72 Pg C to a gain of 253 Pg C with a multimodel mean gain of 65 Pg C. ESMs projecting large gains do so largely by projecting increases in high-latitude soil organic carbon (SOC) that are inconsistent with empirical studies that indicate significant losses of soil carbon with predicted climate change (high confidence).

• Chapter 12: Soils
  Key Finding 3
  

  Soil carbon stocks are sensitive to agricultural and forestry practices and loss of carbon-rich soils such as wetlands. Soils in North America have lost, on average, 20% to 75% of their original top soil carbon (0 to 30 cm) with historical conversion to agriculture, with a mean estimate for Canada of 24% ± 6%. Current agricultural management practices can increase soil organic matter in many systems through reduced summer fallow, cover cropping, effective fertilization to increase plant production, and reduced tillage. Forest soil carbon loss with harvest is small under standard management practices and mostly reversible at the century scale. Afforestation of land in agriculture, industry, or wild grasslands in the United States and Canadian border provinces could increase SOC by 21% ± 9% (high confidence).

• Chapter 12: Soils
  Key Finding 4
  

  Large uncertainties remain regarding soil carbon budgets, particularly the impact of lateral movement and transport of carbon (via erosion and management) across the landscape and into waterways. By 2015, cumulative regeneration of soil carbon at eroded agricultural sites and the preservation of buried, eroded soil carbon may have represented an offset of 37 ± 10% of carbon returned to the atmosphere by human-caused land-use change (medium confidence).

• Chapter 12: Soils
  Key Finding 5
  

  Evidence is strong for direct effects of increased temperature on loss of soil carbon, but warming and atmospheric carbon dioxide increases also may enhance plant production in many ecosystems, resulting in greater carbon inputs to soil. Globally, projected warming could cause the release of 55 ± 50 Pg C over the next 35 years from a soil pool of 1,400 ± 150 Pg C. In particular, an estimated 5% to 15% of the peatland carbon pool could become a significant carbon flux to the atmosphere under future anthropogenic disturbances (e.g., harvest, development, and peatland drainage) and change in disturbance regimes (e.g., wildfires and permafrost thaw) >(medium confidence).

• Chapter 13: Terrestrial Wetlands
  Key Finding 1
  

  The assessment of terrestrial wetland carbon stocks has improved greatly since the First State of the Carbon Cycle Report (CCSP 2007) because of recent national inventories and the development of a U.S. soils database. Terrestrial wetlands in North America encompass an estimated 2.2 million km², which constitutes about 37% of the global wetland area, with a soil and vegetation carbon pool of about 161 petagrams of carbon that represents approximately 36% of global wetland carbon stock. Forested wetlands compose 55% of the total terrestrial wetland area, with the vast majority occurring in Canada. Organic soil wetlands or peatlands contain 58% of the total terrestrial wetland area and 80% of the carbon (high confidence, likely).

• Chapter 13: Terrestrial Wetlands
  Key Finding 2
  

  North American terrestrial wetlands currently are a carbon dioxide sink of about 123 teragrams of carbon (Tg C) per year, with approximately 53% occurring in forested systems. However, North American terrestrial wetlands are a natural source of methane (CH₄), with mineral soil wetlands emitting 56% of the estimated total of 45 Tg C as CH₄ (CH₄ –C) per year (medium confidence, likely).

• Chapter 13: Terrestrial Wetlands
  Key Finding 3
  

  The current rate of terrestrial wetland loss is much less than historical rates (about 0.06% of the wetland area from 2004 to 2009), with restoration and creation nearly offsetting losses of natural wetlands. Although area losses are nearly offset, there is considerable uncertainty about the functional equivalence of disturbed, created, and restored wetlands when comparing them to undisturbed natural wetlands. Correspondingly, there remains considerable uncertainty about the effects of disturbance regimes on carbon stocks and greenhouse gas (GHG) fluxes. For this reason, studies and monitoring systems are needed that compare carbon pools, rates of carbon accumulation, and GHG fluxes across disturbance gradients, including restored and created wetlands. Those studies will produce data that are needed for model applications (high confidence, likely).

• Chapter 14: Inland Waters
  Key Finding 1
  

  The total flux of carbon—which includes gaseous emissions, lateral flux, and burial—from inland waters across the conterminous United States (CONUS) and Alaska is 193 teragrams of carbon (Tg C) per year. The dominant pathway for carbon movement out of inland waters is the emission of carbon dioxide gas across water surfaces of streams, rivers, and lakes (110.1 Tg C per year), a flux not identified in the First State of the Carbon Cycle Report (SOCCR1; CCSP 2007). Second to gaseous emissions are the lateral fluxes of carbon through rivers to coastal environments (59.8 Tg C per year). Total carbon burial in lakes and reservoirs represents the smallest flux for CONUS and Alaska (22.5 Tg C per year) (medium confidence).

• Chapter 14: Inland Waters
  Key Finding 2
  

  Based on estimates presented herein, the carbon flux from inland waters is now understood to be four times larger than estimates presented in SOCCR1. The total flux of carbon from inland waters across North America is estimated to be 507 Tg C per year based on a modeling approach that integrates high-resolution U.S. data and continental-scale estimates of water area, discharge, and carbon emissions. This estimate represents a weighted average of 24 grams of carbon per m² per year of continental area exported and removed through inland waters in North America (low confidence).

• Chapter 14: Inland Waters
  Key Finding 3
  

  Future research can address critical knowledge gaps and uncertainties related to inland water carbon fluxes. This chapter, for example, does not include methane emissions, which cannot be calculated as precisely as other carbon fluxes because of significant data gaps. Key to reducing uncertainties in estimated carbon fluxes is increased temporal resolution of carbon concentration and discharge sampling to provide better representations of storms and other extreme events for estimates of total inland water carbon fluxes. Improved spatial resolution of sampling also could potentially highlight anthropogenic influences on the quantity and quality of carbon fluxes in inland waters and provide information for land-use planning and management of water resources. Finally, uncertainties could likely be reduced if the community of scientists working in inland waters establishes and adopts standard measurement techniques and protocols similar to those maintained through collaborative efforts of the International Ocean Carbon Coordination Project and relevant governmental agencies from participating nations.

• Chapter 15: Tidal Wetlands and Estuaries
  Key Finding 1
  

  The top 1 m of tidal wetland soils and estuarine sediments of North America contains 1,886 ± 1,046 teragrams of carbon (Tg C) (high confidence, very likely).

• Chapter 15: Tidal Wetlands and Estuaries
  Key Finding 2
  

  Soil carbon accumulation rate (i.e., sediment burial) in North American tidal wetlands is currently 9 ± 5 Tg C per year (high confidence, likely), and estuarine carbon burial is 5 ± 3 Tg C per year (low confidence, likely).

• Chapter 15: Tidal Wetlands and Estuaries
  Key Finding 3
  

  The lateral flux of carbon from tidal wetlands to estuaries is 16 ± 10 Tg C per year for North America (low confidence, likely).

• Chapter 15: Tidal Wetlands and Estuaries
  Key Finding 4
  

  In North America, tidal wetlands remove 27 ± 13 Tg C per year from the atmosphere, estuaries outgas 10 ± 10 Tg C per year to the atmosphere, and the net uptake by the combined wetland-estuary system is 17 ± 16 Tg C per year (low confidence, likely).

• Chapter 15: Tidal Wetlands and Estuaries
  Key Finding 5
  

  Research and modeling needs are greatest for understanding responses to accelerated sea level rise; mapping tidal wetland and estuarine extent; and quantifying carbon dioxide and methane exchange with the atmosphere, especially in large, undersampled, and rapidly changing regions (high confidence, likely).

• Chapter 16: Coastal Ocean and Continental Shelves
  Key Finding 1
  

  Observing networks and high-resolution models are now available to construct coastal carbon budgets. Efforts have focused primarily on quantifying the net air-sea exchange of carbon dioxide (CO₂), but some studies have estimated other key fluxes, such as the exchange between shelves and the open ocean.

• Chapter 16: Coastal Ocean and Continental Shelves
  Key Finding 2
  

  Available estimates of air-sea carbon fluxes, based on more than a decade of observations, indicate that the North American margins act as a net sink for atmospheric CO₂. This net uptake is driven primarily by fluxes in the high-latitude regions. The estimated magnitude of the net flux is 160 ± 80 teragrams of carbon per year ( medium confidence) for the North American Exclusive Economic Zone, a number that is not well constrained.

• Chapter 16: Coastal Ocean and Continental Shelves
  Key Finding 3
  

  The increasing concentration of CO₂ in coastal and open-ocean waters leads to ocean acidification. Corrosive conditions in the subsurface occur regularly in Arctic coastal waters, which are naturally prone to low pH, and North Pacific coastal waters, where upwelling of deep, carbon-rich waters has intensified and, in combination with the uptake of anthropogenic carbon, leads to low seawater pH and aragonite saturation states in spring, summer, and early fall (very high confidence, very likely).

• Chapter 16: Coastal Ocean and Continental Shelves
  Key Finding 4
  

  Expanded monitoring, more complete syntheses of available observations, and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.

• Chapter 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide
  Key Finding 1
  

  Rising carbon dioxide (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; on the Gulf of Maine shore; and on Gray’s Reef in the southeastern United States. This ocean acidification process has already affected some marine species and altered fundamental ecosystem processes, and further effects are likely (high confidence, likely).

• Chapter 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide
  Key Finding 2
  

  While atmospheric CO₂ rises at approximately the same rate all over the globe, its non-climate effects on land vary depending on climate and dominant species. In terrestrial ecosystems, rising atmospheric CO₂ concentrations are expected to increase plant photosynthesis, growth, and water-use efficiency, though these effects are reduced when nutrients, drought, or other factors limit plant growth ( very high confidence, very likely). Rising CO₂ would likely change carbon storage and influence terrestrial hydrology and biogeochemical cycling, but concomitant effects on vegetation composition and nutrient feedbacks are challenging to predict, making decadal forecasts uncertain.

• Chapter 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide
  Key Finding 3
  

  Consequences of rising atmospheric CO₂ are expected to include difficult-to-predict changes in the ecosystem services that terrestrial and oceanic systems provide to humans. For instance, ocean acidification resulting from rising CO₂ has decreased the supply of larvae that sustains commercial shellfish production in the northwestern United States. In addition, CO₂ fertilization (increases) plus warming (decreases) are changing terrestrial crop yields (high confidence, likely).

• Chapter 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide
  Key Finding 4
  

  Continued persistence of uptake of carbon by the land and ocean is uncertain. Climate and environmental changes create complex feedbacks to the carbon cycle; how these feedbacks modulate future effects of rising CO₂ on carbon sinks is unclear. There are several mechanisms that would reduce the ability of land and ocean sinks to continue taking up a large proportion of rising CO₂ (very high confidence).

• Chapter 18: Carbon Cycle Science in Support of Decision Making
  Key Finding 1
  

  Co-production of knowledge via engagement and collaboration between stakeholder communities and scientific communities can improve the usefulness of scientific results by decision makers (high confidence).

• Chapter 18: Carbon Cycle Science in Support of Decision Making
  Key Finding 2
  

  Integrating data on human drivers of the carbon cycle into Earth system and ecosystem models improves representation of carbon-climate feedbacks and increases the usefulness of model output to decision makers (high confidence).

• Chapter 18: Carbon Cycle Science in Support of Decision Making
  Key Finding 3
  

  Attribution, accounting, and projections of carbon cycle fluxes increase the usefulness of carbon cycle science for decision-making purposes (very high confidence).

• Chapter 18: Carbon Cycle Science in Support of Decision Making
  Key Finding 4
  

  Developing stronger linkages among research disciplines for Earth system processes, carbon management, and carbon prediction, with a focus on consistent and scalable datasets as model inputs, will improve joint representation of natural and managed systems needed for decision making (high confidence).

• Chapter 19: Future of the North American Carbon Cycle
  Key Finding 1
  

  Emissions from fossil fuel combustion in the North American energy sector are a source of carbon to the atmosphere. Projections suggest that by 2040, total North American fossil fuel emissions will range from 1,504 to 1,777 teragrams of carbon (Tg C) per year, with most coming from the United States (~80%, or 1,259 to 1,445 Tg C per year). Compared to 2015 levels, these projections represent either a 12.8% decrease or a 3% increase in absolute emissions (high confidence).

• Chapter 19: Future of the North American Carbon Cycle
  Key Finding 2
  

  Land, ocean, coastal, and freshwater systems are currently net sinks of carbon from the atmosphere, taking up more carbon annually than they release. However, emerging understanding suggests that the future carbon uptake capacity of these systems may decline, depending on different emissions scenarios, with some reservoirs switching from a net sink to a net source of carbon to the atmosphere high confidence).

• Chapter 19: Future of the North American Carbon Cycle
  Key Finding 3
  

  Human-driven changes in land cover and land use will continue to be key contributors to carbon cycle changes into the future, both globally and in North America. Globally, land-use change is projected to contribute 10 to 100 petagrams of carbon (Pg C) to the atmosphere by 2050 and between 19 and 205 Pg C by 2100. Conversely, in the United States, land use and land-use change activities are projected to increase carbon stocks in terrestrial ecosystems by about 4 Pg C from 2015 to 2030. This projected increase is primarily driven by the growth of existing forests and management activities that promote ecosystem carbon uptake, often in response to changes in market, policy, and climate (high confidence).

• Chapter 19: Future of the North American Carbon Cycle
  Key Finding 4
  

  The enhanced carbon uptake capacity of ocean and terrestrial systems in response to rising atmospheric carbon dioxide (CO₂) will likely diminish in the future. In the ocean, warmer and more CO₂-enriched waters are expected to take up less additional CO₂. On land, forest maturation, nutrient limitations, and decreased carbon residence time in soils will likely constrain terrestrial ecosystem response to rising CO₂ (high confidence).

• Chapter 19: Future of the North American Carbon Cycle
  Key Finding 5
  

  Soil carbon losses in a warming climate will be a key determinant of the future North American carbon cycle. An important region of change will be the Arctic, where thawing permafrost and the release of previously frozen carbon will likely shift this region from a net sink to a net source of carbon to the atmosphere by the end of the century (very high confidence).

• Chapter 19: Future of the North American Carbon Cycle
  Key Finding 6
  

  Carbon storage in both terrestrial and aquatic systems is vulnerable to natural and human-driven disturbances. This vulnerability is likely to increase as disturbance regimes shift and disturbance severity increases with changing climatic conditions (high confidence).

• Chapter 2: The North American Carbon Budget
  Key Finding 1
  

  North America—including its energy systems, land base, and coastal ocean—was a net source of carbon dioxide to the atmosphere from 2004 to 2013, contributing on average about 1,008 teragrams of carbon (Tg C) annually (±50%) (very high confidence).

• Chapter 2: The North American Carbon Budget
  Key Finding 2
  

  Fossil fuel emissions were the largest carbon source from North America from 2004 to 2013, averaging 1,774 Tg C per year (±5.5%). Emissions during this time showed a decreasing trend of 23 Tg C per year, a notable shift from the increasing trend over the previous decade. The continental proportion of the global total fossil fuel emissions decreased from 24% in 2004 to 17% in 2013 (very high confidence)>.

• Chapter 2: The North American Carbon Budget
  Key Finding 3
  

  Approximately 43% of the continent’s total fossil fuel emissions from 2004 to 2013 were offset by natural carbon sinks on North American land and the adjacent coastal ocean (medium confidence).

• Chapter 2: The North American Carbon Budget
  Key Finding 4
  

  Using bottom-up, inventory-based calculations, the Second State of the Carbon Cycle Report (SOCCR2) estimates that the average annual strength of the land-based carbon sink in North America was 606 Tg C per year (±75%) during the 2004 to 2013 time period, compared with the estimated 505 Tg C per year (±50%) in ca. 2003, as reported in the First State of the Carbon Cycle Report (CCSP 2007). There is apparent consistency in the two estimates, given their ranges of uncertainty, with SOCCR2 calculations including additional information on the continental carbon budget. However, large uncertainties remain in some components (very high confidence).

• Chapter 2: The North American Carbon Budget
  Key Finding 5
  

  The magnitude of the continental carbon sink over the last decade is estimated at 699 Tg C per year (±12%) using a top-down approach and 606 Tg C per year (±75%) using a bottom-up approach, indicating an apparent agreement between the two estimates considering their uncertainty ranges.*
  
  

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  *Note: Confidence level excluded due to Key Finding’s emphasis on methodological comparisons.

• Chapter 3: Energy Systems
  Key Finding 1
  

  In 2013, primary energy use in North America exceeded 125 exajoules,¹ of which Canada was responsible for 11.9%, Mexico 6.5%, and the United States 81.6%. Of total primary energy sources, approximately 81% was from fossil fuels, which contributed to carbon dioxide equivalent (CO₂e)² emissions levels, exceeding 1.76 petagrams of carbon, or about 20% of the global total for energy-related activities. Of these emissions, coal accounted for 28%, oil 44%, and natural gas 28% (very high confidence, likely).
  
  

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  ¹ One exajoule is equal to one quintillion (1018) joules, a derived unit of energy in the International System of Units.
  ² Carbon dioxide equivalent (CO₂e): Amount of CO₂ that would produce the same effect on the radiative balance of Earth’s climate system as another greenhouse gas, such as methane (CH₄) or nitrous oxide (N₂O), on a 100-year timescale. For comparison to units of carbon, each kg CO₂e is equivalent to 0.273 kg C (0.273 = ¹⁄₃.67). See Box P.2 in the Preface for more details.

• Chapter 3: Energy Systems
  Key Finding 2
  

  North American energy-related CO₂e emissions have declined at an average rate of about 1% per year, or about 19.4 teragrams CO₂e, from 2003 to 2014 (very high confidence)

• Chapter 3: Energy Systems
  Key Finding 3
  

  The shifts in North American energy use and CO₂e emissions have been driven by factors such as 1) lower energy use, initially as a response to the global financial crisis of 2007 to 2008 (high confidence, very likely); but increasingly due to 2) greater energy efficiency, which has reduced the regional energy intensity of economic production by about 1.5% annually from 2004 to 2013, enabling economic growth while lowering energy CO₂e emissions. Energy intensity has fallen annually by 1.6% in the United States and 1.5% in Canada (very high confidence, very likely). Further factors driving lower carbon intensities include 3) increased renewable energy production (up 220 petajoules annually from 2004 to 2013, translating to an 11% annual average increase in renewables) (high confidence, very likely); 4) a shift to natural gas from coal sources for industrial and electricity production (high confidence, likely); and 5) a wide range of new technologies, including, for example, alternative fuel vehicles (high confidence, likely).

• Chapter 3: Energy Systems
  Key Finding 4
  

  A wide range of plausible futures exists for the North American energy system in regard to carbon emissions. Forecasts to 2040, based on current policies and technologies, suggest a range of carbon emissions levels from an increase of over 10% to a decrease of over 14% (from 2015 carbon emissions levels). Exploratory and backcasting approaches suggest that the North American energy system emissions will not decrease by more than 13% (compared with 2015 levels) without both technological advances and changes in policy. For the United States, however, decreases in emissions could plausibly meet a national contribution to a global pathway consistent with a target of warming to 2°C at a cumulative cost of $1 trillion to $4 trillion (US$ 2005).

• Chapter 4: Understanding Urban Carbon Fluxes
  Key Finding 1
  

  Urban areas in North America are the primary source of anthropogenic carbon emissions, with cities responsible for a large proportion of direct emissions. These areas are also indirect sources of carbon through the emissions embedded in goods and services produced outside city boundaries for consumption by urban dwellers (medium confidence, likely).

• Chapter 4: Understanding Urban Carbon Fluxes
  Key Finding 2
  

  Many societal factors drive urban carbon emissions, but the urban built environment and the regulations and policies shaping urban form (e.g., land use) and technology (e.g., modes of transportation) play crucial roles. Such societal drivers can lock in dependence on fossil fuels in the absence of major technological, institutional, and behavioral change. Some fossil fuel–related infrastructure can have lifetimes of up to 50 years (high confidence).

• Chapter 4: Understanding Urban Carbon Fluxes
  Key Finding 3
  

  Key challenges for urban carbon flux studies are observational design, integration, uncertainty quantification, and reconciliation of the multiple carbon flux approaches to detect trends and inform emissions mitigation efforts (medium confidence, likely).

• Chapter 4: Understanding Urban Carbon Fluxes
  Key Finding 4
  

  Improvements in air quality and human health and the reduction of the urban heat island are important co-benefits of urban carbon emissions mitigation (high confidence, very likely).

• Chapter 4: Understanding Urban Carbon Fluxes
  Key Finding 5
  

  Urban methane (CH₄) emissions have been poorly characterized, but the combination of improved instrumentation, modeling tools, and heightened interest in the problem is defining the range of emissions rates and source composition as well as highlighting infrastructure characteristics that affect CH₄ emissions (high confidence).

• Chapter 4: Understanding Urban Carbon Fluxes
  Key Finding 6
  

  Urban areas are important sites for policymaking and decision making that shape carbon fluxes and mitigation. However, cities also are constrained by other levels of government, variations in their sources of authority and autonomy, capacity, competing local priorities, and available fiscal resources (high confidence).

• Chapter 5: Agriculture
  Key Finding 1
  

  Agricultural greenhouse gas (GHG) emissions in 2015 totaled 567 teragrams (Tg)¹ of carbon dioxide equivalent (CO₂e)² in the United States and 60 Tg CO₂e in Canada, not including land-use change; for Mexico, total agricultural GHG emissions were 80 Tg CO₂e in 2014 (not including land-use change) (high confidence). The major agricultural non-CO₂ emission sources were nitrous oxide (N₂O) from cropped and grazed soils and enteric methane (CH₄) from livestock (very high confidence, very likely).³
  
  

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  ¹ Excludes emissions related to land use, land-use change, and forestry activities.
  ² Carbon dioxide equivalent (CO;₂e): Amount of CO;₂ that would produce the same effect on the radiative balance of Earth’s climate system as another greenhouse gas, such as methane (CH₄) or nitrous oxide (N;₂O), on a 100-year timescale. For comparison to units of carbon, each kg CO;₂e is equivalent to 0.273 kg C (0.273 = ¹⁄₃.67). See Box P.2 in the Preface for more details.
  ³ Estimated 95% confidence interval lower and upper uncertainty bounds for agricultural greenhouse gas emissions: –11% and +18% (CH₄ emissions from enteric fermentation) and –18% and +20% and –16% and +24% (CH₄ and N;₂O emissions from manure management, respectively; U.S. EPA 2018).

• Chapter 5: Agriculture
  Key Finding 2
  

  Agricultural regional carbon budgets and net emissions are directly affected by human decision making. Trends in food production and agricultural management, and thus carbon budgets, can fluctuate significantly with changes in global markets, diets, consumer demand, regional policies, and incentives (very high confidence).

• Chapter 5: Agriculture
  Key Finding 3
  

  Most cropland carbon stocks are in the soil, and cropland management practices can increase or decrease soil carbon stocks. Integration of practices that can increase soil carbon stocks include maintaining land cover with vegetation (especially deep-rooted perennials and cover crops), protecting the soil from erosion (using reduced or no tillage), and improving nutrient management. The magnitude and longevity of management-related carbon stock changes have strong environmental and regional differences, and they are subject to subsequent changes in management practices (high confidence, likely).

• Chapter 5: Agriculture
  Key Finding 4
  

  North America’s growing population can achieve benefits such as reduced GHG emissions, lowered net global warming potential, increased water and air quality, reduced CH₄ flux in flooded or relatively anoxic systems, and increased food availability by optimizing nitrogen fertilizer management to sustain crop yields and reduce nitrogen losses to air and water (high confidence, likely).

• Chapter 5: Agriculture
  Key Finding 5
  

  Various strategies are available to mitigate livestock enteric and manure CH₄ emissions. Promising and readily applicable technologies can reduce enteric CH₄ emissions from ruminants by 20% to 30%. Other mitigation technologies can reduce manure CH₄ emissions by 30% to 50%, on average, and in some cases as much as 80%. Methane mitigation strategies have to be evaluated on a production-system scale to account for emission tradeoffs and co-benefits such as improved feed efficiency or productivity in livestock (high confidence, likely).

• Chapter 5: Agriculture
  Key Finding 6
  

  Projected climate change likely will increase CH₄ emissions from livestock manure management locations, but it will have a lesser impact on enteric CH₄ emissions (high confidence). Potential effects of climate change on agricultural soil carbon stocks are difficult to assess because they will vary according to the nature of the change, onsite ecosystem characteristics, production system, and management type (high confidence).

• Chapter 6: Social Science Perspectives on Carbon
  Key Finding 1
  

  Broadened Approaches — A range of social scientific research approaches, including people-centered analyses of energy use, governance, vulnerability, scenarios, social-ecological systems, sociotechnical transitions, social networks, and social practices, complements physical science research and informs decision making. Approaches that are people centered and multidisciplinary emphasize that carbon-relevant decisions are often not about energy, transportation, infrastructure, or agriculture, as such, but rather about style, daily living, comfort, convenience, health, and other priorities (very high confidence).

• Chapter 6: Social Science Perspectives on Carbon
  Key Finding 2
  

  Assumed versus Actual Choices — Planners have assumed economically rational energy-use and consumption behaviors and thus have failed to predict actual choices, behaviors, and intervening developments, leading to large gaps between predicted rates of economically attractive purchases of technologies with lower carbon footprints and actual realized purchase rates (high confidence).

• Chapter 6: Social Science Perspectives on Carbon
  Key Finding 3
  

  Social Nature of Energy Use — Opportunities to go beyond a narrow focus on the energy-efficiency industry to recognize and account for the social nature of energy use include 1) engaging in market transformation activities aimed at upstream actors and organizations in supply chains, 2) implementing efficiency codes and standards for buildings and technologies, 3) conducting research to understand how people’s behaviors socially vary and place different loads on even the most efficient energy-using equipment, and 4) adding consideration of what people actually do with energy-using equipment to plans for technology and efficiency improvements (high confidence).

• Chapter 6: Social Science Perspectives on Carbon
  Key Finding 4
  

  Governance Systems — Research that examines governance at multiple formal levels (international, national, state/province, cities, other communities) as well as informal processes will identify overlaps and gaps and deepen understanding of effective processes and opportunities involved in carbon management, including a focus on benefits such as health, traffic management, agricultural sustainability, and reduced inequality (medium confidence).

• Chapter 7: Tribal Lands
  Key Finding 1
  

  Many Indigenous peoples in North America follow traditional agricultural and land-use practices that govern carbon cycling on tribal lands. These practices include no-till farming; moving domesticated animals seasonally in accordance with forage availability; growing legumes and cover crops; raising crops and livestock native to ancestral landscapes; and managing forests sustainably with fire, harvest, and multispecies protection.

• Chapter 7: Tribal Lands
  Key Finding 2
  

  Scientific data and peer-reviewed publications pertaining to carbon stocks and fluxes on Indigenous (native) lands in North America are virtually nonexistent, which makes establishing accurate baselines for carbon cycle processes problematic. The extent to which traditional practices have been maintained or reintroduced on native lands can serve as a guide for estimating carbon cycle impacts on tribal lands by comparisons with practices on similar non-tribal lands.

• Chapter 7: Tribal Lands
  Key Finding 3
  

  Fossil fuel and uranium energy resources beneath tribal lands in the United States and Canada are substantial, comprising, in the United States, 30% of coal reserves west of the Mississippi River, 50% of potential uranium reserves, and 20% of known oil and gas reserves, together worth nearly $1.5 trillion. Fossil fuel extraction and uranium mining on native lands have resulted in emissions of carbon dioxide and methane during extraction and fuel burning. Energy resource extraction on tribal lands also has resulted in substantial ecosystem degradation and deforestation, further contributing to carbon emissions.

• Chapter 7: Tribal Lands
  Key Finding 4
  

  Renewable energy development on tribal lands is increasing but is limited by federal regulations, tribal land tenure, lack of energy transmission infrastructure on reservations, and economic challenges.

• Chapter 7: Tribal Lands
  Key Finding 5
  

  Colonial practices of relocation, termination, assimilation, and natural resource exploitation on native lands have historically hindered the ability of Indigenous communities to manage or influence landuse and carbon management both on and off tribal lands. These factors combined with contemporary socioeconomic challenges continue to impact Indigenous carbon management decision making.

• Chapter 7: Tribal Lands
  Key Finding 6
  

  The importance placed on youth education by Indigenous communities creates opportunities for future generations to sustain and pass on traditional knowledge important to managing carbon stocks and fluxes on native lands.

• Chapter 8: Observations of Atmospheric Carbon Dioxide and Methane
  Key Finding 1
  

  Global concentrations of carbon dioxide (CO₂) and methane (CH₄) have increased almost linearly since the First State of the Carbon Cycle Report (CCSP 2007; see Figure 8.1). Over the period 2004 to 2013, global growth rates estimated from the National Oceanic and Atmospheric Administration’s marine boundary layer network average 2.0 ± 0.1 parts per million (ppm) per year for CO₂ and 3.8 ± 0.5 parts per billion (ppb) per year for CH₄. Global mean CO₂ abundance as of 2013 was 395 ppm (compared to preindustrial levels of about 280 ppm), and CH₄ stands at more than 1,810 ppb (compared to preindustrial levels of about 720 ppb) (very high confidence).

• Chapter 8: Observations of Atmospheric Carbon Dioxide and Methane
  Key Finding 2
  

  Inverse model analyses of atmospheric CO₂ data suggest substantial interannual variability in net carbon uptake over North America. Over the period 2004 to 2013, North American fossil fuel emissions from inventories average 1,774 ± 24 teragrams of carbon (Tg C) per year, partially offset by the land carbon sink of 699 ± 82 Tg C per year. Additionally, inversion models suggest a trend toward an increasing sink during the period 2004 to 2013. These results contrast with the U.S. land sink estimates reported to the United Nations Framework Convention on Climate Change, which are smaller and show very little trend or interannual variability.

• Chapter 8: Observations of Atmospheric Carbon Dioxide and Methane
  Key Finding 3
  

  During most of the study period covered by the Second State of the Carbon Cycle Report> (2004 to 2012), inverse model analyses of atmospheric CH₄ data show minimal interannual variability in emissions and no robust evidence of trends in either temperate or boreal regions. The absence of a trend in North American CH₄ emissions contrasts starkly with global emissions, which show significant growth since 2007. Methane emissions for North America over the period 2004 to 2009 estimated from six inverse models average 66 ± 2 Tg CH₄ per year. Over the same period, CH₄ emissions reported by the U.S. Environmental Protection Agency equate to a climate impact of 13% of CO₂ emissions, given a 100-year time horizon.

• Chapter 9: Forests
  Key Finding 1
  

  Net uptake of 217 teragrams of carbon (Tg C) per year by the forest sector in North America is well documented and has persisted at about this level over the last decade. The strength of net carbon uptake varies regionally, with about 80% of the North American forest carbon sink occurring within the United States (high confidence, very likely).

• Chapter 9: Forests
  Key Finding 2
  

  Forest regrowth following historical clearing plays a substantial role in determining the size of the forest carbon sink, but studies also suggest sizeable contributions from growth enhancements such as carbon dioxide fertilization, nitrogen deposition, or climate trends supporting accelerated growth (medium confidence). Resolving each factor’s contribution is a major challenge and critical for developing reliable predictions.

• Chapter 9: Forests
  Key Finding 3
  

  Annual harvest removals from forestry operations in select regions decrease forest carbon stocks, but this decline in stocks is balanced by post-harvest recovery and regrowth in forestlands that were harvested in prior years. Removal, processing, and use of harvested biomass causes carbon emissions outside of forests, offsetting a substantial portion (about half ) of the net carbon sink in North American forests (high confidence).

• Chapter 9: Forests
  Key Finding 4
  

  Recent trends in some disturbance rates (e.g., wildfires and insects) have diminished the strength of net forest carbon uptake across much of North America. Net loss of forest carbon stocks from land conversions reduced sink strength across the continent by 11 Tg C per year, with carbon losses from forest conversion exceeding carbon gains from afforestation and reforestation (medium confidence).

• Chapter 9: Forests
  Key Finding 5
  

  Several factors driving the carbon sink in North American forests are expected to decline over coming decades, and an increasing rate of natural disturbance could further diminish current net carbon uptake (medium confidence).