Carbon is the basis of life on Earth, forming bonds with oxygen, hydrogen, and nutrients to create the organic compounds that make up all living things. Essential for fundamental human activities and assets, carbon is a vital component of the fossil fuels used for energy production, cooking, agriculture, manufacturing, and transportation. The carbon cycle encompasses the physical, chemical, and biological processes that store or transfer carbon between different stocks or reservoirs (see Figure ES.2, p. 26). Examples of such reservoirs include the carbon stored as CO₂ and CH₄ gas in the atmosphere; as coal, petroleum, and natural gas (the primary energy sources for modern societies); and as organic and inorganic carbon in Earth’s ocean, freshwaters, forests, grasslands, and soils. Carbon transfer among these reservoirs occurs via a range of different processes, such as plant uptake of atmospheric CO₂ for growth (photosynthesis), release of CO₂ to the atmosphere from organic matter decomposition and combustion, and “lateral” transfers of carbon and burial within aqueous systems (see Figure ES.3 and Ch. 1: Overview of the Global Carbon Cycle).

Carbon is also critical in regulating climate because carbon-containing GHGs³ absorb radiant energy emitted from Earth’s surface, thereby warming the planet. This warming creates a climate within the narrow range of conditions suitable for life. Changes in atmospheric concentrations of GHGs influence Earth’s ecosystems and society in many ways, both positive and negative. Consequences of increasing GHGs include impacts on air quality, human health, water quality and availability, ecosystem productivity, species distributions, biological diversity, ocean chemistry, sea level rise, and many other processes that determine human well-being. Thus, the carbon cycle is tightly coupled to the environment, society, and the global climate system.

The carbon cycle is changing at a much faster pace than observed at any time in geological history (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide). These changes primarily are attributed to current energy and transportation dependencies on the burning of fossil fuels, which releases previously stable or sequestered carbon. Also contributing to rapid changes in the carbon cycle are cement production and gas flaring, as well as net emissions from forestry, agriculture, and other land uses. The associated rise in atmospheric GHGs is largely responsible for Earth’s increased temperature over the past 100 years. The global mean temperature in 2017 relative to the 1880 to 1920 average has increased by more than 1.25°C in response, as documented in the Climate Science Special Report (USGCRP 2017). Human-induced warming is having significant—usually negative—impacts including more frequent heatwaves, heavy precipitation, and coastal flooding, all of which lead to lost lives, damaged communities, and disrupted ecosystems.

Since SOCCR1, concentrations of atmospheric CO₂ and CH₄ have been on the rise (see Figure ES.4, this page). From 2007 to 2015, the global rate of increase averaged 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 (see Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane). Current understanding of the sources and sinks of atmospheric carbon confirms the overwhelming role of human activities, especially fossil fuel combustion, in driving the atmospheric changes in CO₂ concentrations (see Ch. 1: Overview of the Global Carbon). In North America, projections suggest that by 2040, total fossil fuel emissions, in terms of total carbon, will range from 1.5 petagrams of carbon (Pg C) to 1.8 Pg C per year, with the United States contributing 80% of this total. Compared to 2015 levels, these projections represent a range from a 12.8% decrease to a 3% increase in absolute emissions of carbon (see Ch. 19: Future of the North American Carbon Cycle).

Globally, land and ocean ecosystems are net sinks of atmospheric carbon, taking up more carbon annually than they release. The most recent estimates suggest that from 2006 to 2015, land ecosystems removed about 3.1 ± 0.9 Pg C per year while the ocean removed 2.3 ± 0.5 Pg C per year. Combined, these removals equal about half the amount of CO₂ emitted from fossil fuel combustion and land-use change (see Ch. 1: Overview of the Global Carbon Cycle). However, a range of research suggests the carbon uptake capacity of all these systems may decline in the future, with some reservoirs switching from a net sink to a net source of carbon to the atmosphere.

In North America, GHGs are emitted primarily from fossil fuel burning; cement production; organic matter decomposition in inland lakes and rivers; land-use changes; and agricultural activities, particularly on drained peatland soils. Conversion of carbon gases (mainly CO₂) to organic matter through photosynthesis occurs in forests, grasslands, other land ecosystems, and coastal waters. Just under one-half of CO₂ emissions (43%) are offset by carbon sinks in the land and coastal waters. Compared to SOCCR1, this report defines more land and aquatic ecosystem components, providing an improved understanding of their respective roles in carbon cycling. Selected highlights about the North American carbon cycle follow.

Fossil Fuels Are Still the Largest Source

Carbon dioxide emissions from fossil fuels in North America averaged 1,774 teragrams of carbon (Tg C) per year (±6%) from 2004 to 2013 (see Figure ES.2). This estimate is similar to the 1,856 Tg C per year (±10%) reported for the decade prior to 2003 (CCSP 2007). From 2004 to 2013, CO₂ fossil fuel emissions decreased about 1% per year because of various market, technology, and policy drivers, as well as the financial crisis (see Ch. 3: Energy Systems). During this same time period, North America likely acted as a net source of CH₄ to the atmosphere, contributing on average about 66 Tg CH₄ per year. Currently, the United States is responsible for about 85% of total fossil fuel emissions from North America. As of 2013, the continent contributes about 17% of total global emissions from fossil fuels, a decline from about 24% in 2004 because of increasing emissions elsewhere and reduced emissions in the United States (see Figure ES.5; Ch. 2: The North American Carbon Budget; Ch. 3: Energy Systems; and Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane).

Aquatic Ecosystems Are Both Sources and Sinks

Although SOCCR1 did not directly quantify net CO₂ emissions from inland waters to the atmosphere, this report estimates those emissions at about 247 Tg C per year (±100%; see Figure ES.2; Figure ES.3; and Ch. 14: Inland Waters). Burial in lakes and reservoirs, which is part of the terrestrial carbon sink, is about 155 Tg C per year (±100%), a level much higher than a similar estimate made for SOCCR1 (25 Tg C per year ± 120%) but still within the uncertainty bounds of each estimate, making the identification of a trend impossible (see Figure ES.3 and Ch. 14). Lateral transfers from inland waters to estuaries total about 105 Tg C per year and from estuaries to the coastal ocean about 106 Tg C per year (±30%; see Ch. 14 and Ch. 15: Tidal Wetlands and Estuaries). The transfer from the coastal ocean to the open ocean is estimated to be 151 Tg C per year (±70%; see Ch. 16: Coastal Ocean and Continental Shelves). These estimates were not included in SOCCR1, except for transfers from rivers to coastal waters, which were estimated at 35 Tg C per year (±100%).

Carbon losses from inland waters in North America total about 507 Tg C per year (see Figure ES.3). Although there is a reasonably good basis for this estimate, knowledge of carbon sources to inland waters is extremely poor. The only source that has been estimated is the lateral transport of dissolved organic carbon from terrestrial wetlands, which equals only 16 Tg C per year. Other sources include different types of carbon from terrestrial wetlands (e.g., dissolved inorganic carbon and particulate carbon) and carbon from surface runoff, groundwater flow, and erosion. Assuming no accumulation of carbon in inland waters, these sources should total 491 Tg C per year (see Figure ES.3).

Three types of wetlands constitute small net sinks of CO₂: 1) terrestrial nonforested wetlands, estimated at 60 Tg C per year; 2) forested wetlands, estimated at 67 Tg C per year (also included in the forestland category); and 3) tidal wetlands, estimated at 27 Tg C per year (see Figure ES.3; Ch. 13: Terrestrial Wetlands; and Ch. 15). Terrestrial wetlands are a natural source of CH₄ (see Ch. 13), annually emitting an estimated 45 Tg of carbon as CH₄ (±75%). Carbon moving in and out of terrestrial wetlands cannot be fully traced. The carbon budget (see Figure ES.3) does not balance because the net uptake from the atmosphere (82 Tg C per year equals CO₂ uptake minus CH₄ release) exceeds by 26 Tg C per year the sum of accumulation in vegetation (44 Tg C per year) and soils (48 Tg C per year) and the loss of dissolved organic carbon (16 Tg per year; see Figure ES.3).

Land and Coastal Waters Are a Net Sink

Natural sinks on North American land and adjacent coastal waters offset approximately 43% of the total fossil fuel emissions of CO₂ from 2004 to 2013 (see Ch. 2: The North American Carbon Budget). The magnitude of the North American terrestrial sink estimated from “bottom-up” methods (i.e., inventory and biosphere-based approaches such as field measurements and ecosystem process models) is about 606 Tg C per year (±50%). This value is derived from estimates of net uptake by ecosystems and tidal wetlands minus emissions from harvested wood, inland waters, and estuaries (see Figure ES.2). The bottom-up estimate is about the same as the estimated 699 Tg C per year (±12%) inferred by “top-down” (atmospheric-based) observations but with larger uncertainties (see Ch. 2 and Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane.).

The coastal ocean of North America (the Exclusive Economic Zone, not including tidal wetlands and estuaries) is an estimated sink of 160 Tg C (±50%) annually, based on estimates of air-sea carbon fluxes and a numerical model (see Figure ES.3). This net uptake from the atmosphere is driven primarily by fluxes in high-latitude regions (see Ch. 16: Coastal Ocean and Continental Shelves).

Soil Stocks

Carbon stocks in North American soils are estimated as 627 Pg C, representing more than 90% of the continent’s total carbon stocks including biomass (see Table 2.1 in Ch. 2: The North American Carbon Budget). Because soil carbon concentrations vary by depth, estimates of soil carbon depend on the soil depth considered in surveys, which often do not account for deeper soil carbon. Summing the estimates of organic carbon contained in soils to a depth of 1 m from Canada, the United States, and Mexico yields about 400 Pg C (see Ch. 12: Soils). Globally, stocks in the circumpolar Arctic and boreal regions are estimated as 1,400 to 1,600 Pg C based on inventories of soils and sediments to a 3-m or more depth. About one-third of this carbon is in North America (see Ch. 11: Arctic and Boreal Carbon).

Forests

Forests, including their soils, constitute the largest component of the land sink, taking up a net 217 Tg C per year (±25%) from 2004 to 2013 (see Ch. 9: Forests). Across the continent, afforestation added 27 Tg C per year and deforestation led to a loss of 38 Tg C per year (see Ch. 9). Woody encroachment, which refers to increasing density of woody vegetation on grasslands and shrublands, is part of the carbon sink, and it is included within the terrestrial categories of forests and grasslands as appropriate.

Agriculture

Agricultural GHG emissions totaled 567 Tg CO₂ equivalent (CO₂e)⁴ for the United States in 2015, 60 Tg CO₂e for Canada in 2015, and 80 Tg CO₂e for Mexico in 2014. These estimates do not include emissions from land-use change involving agriculture, as reported in each country’s GHG inventory submission to the United Nations Framework Convention on Climate Change. The major non-CO₂ emissions from agricultural sources are N₂O from cropped and grazed soils and manure and enteric CH₄ emissions from livestock production (see Ch. 5: Agriculture). Because management plays a large role in determining the carbon cycle of agricultural systems, there are significant opportunities to reduce emissions and increase the magnitude of carbon sinks in these areas.

Arctic and Boreal Ecosystems

Arctic and boreal ecosystems are estimated to be a small sink of 14 Tg C annually (see Ch. 2: The North American Carbon Budget and Ch. 11: Arctic and Boreal Carbon). Confidence in this estimate is low because the extent to which these results overlap or leave gaps with other terrestrial categories, particularly boreal forests and terrestrial wetlands, is not clear due to the relatively limited data coverage for these northern ecosystems.

Changes to the carbon cycle can affect North Americans in a wide variety of ways. For example, the ocean provides multiple benefits or “services,” including the provision of fish, carbon storage, coastal protection by reefs, and climate modulation. These services face significant risks from the combined effects of ocean acidification, warming ocean waters, and sea level rise (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide). Rising atmospheric CO₂ has decreased seawater pH, leading to ocean acidification as evidenced from measurements at long-term observing stations around North America (see Ch. 16: Coastal Ocean and Continental Shelves and Ch. 17). This decrease in pH, mainly due to oceanic uptake of CO₂, also is affected by other factors including circulation and eutrophication (i.e., nutrient enrichment of water that can lead to increased primary production and, subsequently, poorer water quality). Ocean acidification also enhances corrosive conditions and can inhibit the formation of calcium carbonate shells essential to marine life. Compared to many other coastal waters, Arctic and North Pacific coastal waters are already more acidic, and therefore small changes in pH due to CO₂ uptake have affected marine life in these waters more significantly (see Ch. 16). In addition to impacts on marine species, ocean acidification has altered fundamental ecosystem processes, with further effects likely in the future.

In terrestrial ecosystems, rising atmospheric CO₂ enhances photosynthesis and growth and increases water-use efficiency (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide). These carbon cycle–induced increases in plant growth and efficiency are referred to as “CO₂ fertilization.” For example, crops exposed to higher atmospheric CO₂ often show increased yield. However, the CO₂ fertilization effect is not observed consistently in all ecosystems because of nutrient limitations or other factors. Furthermore, CO₂ fertilization typically is associated with increased leaf fall and root production, which can enhance microbial decomposition of organic materials in soils, thereby increasing net CO₂ emissions to the atmosphere (see Ch. 12: Soils). All these changes have altered and will continue to alter vegetation composition (e.g., species distribution, biodiversity, and invasive species), carbon distribution and storage, terrestrial hydrology, and other ecosystem properties. Current and future changes to climate that are driven by altered carbon cycling also will affect ecosystems and their services, as well as interact with effects such as ocean acidification and CO₂ fertilization.

Overall, alterations to the North American carbon cycle will continue to affect the benefits that terrestrial and ocean systems provide to humans. The effects of rising atmospheric CO₂ concentrations interact with climate, sea level rise, and other global changes as described in SOCCR2 companion reports such as the Third National Climate Assessment (Melillo et al., 2014) and Climate Science Special Report (USGCRP 2017). For example, the frequency and intensity of disturbances such as fire, insect and pathogen outbreaks, storms, and heatwaves are expected to increase with higher temperatures and climate variability. Moreover, ecosystem responses to and interactions with such effects are often unpredictable and depend on ecosystem type, disturbance frequency, and magnitude of events (see Ch. 17).

Carbon is a key element in multiple social, ecological, physical, and infrastructural realms including croplands, grasslands, forests, industry, transportation, buildings, and other structures (see Ch. 3–10). As described in this report, North American social and economic activities, practices, and infrastructures significantly affect the carbon cycle. Energy use predominantly involves burning carbon-based fuels (see Ch. 3: Energy Systems), but society also uses carbon in other less obvious ways such as food and buildings. Carbon is thus embedded in social life (see Ch. 6: Social Science Perspectives on Carbon), and widespread variations in everyday activities result in carbon emissions that cause ripples of intended and unintended social and biophysical effects.

Not only are all parts of the carbon cycle tightly interlinked, they also interact with climate and society in complex ways that are not fully understood (see Figure ES.6 and Ch. 18: Carbon Cycle Science in Support of Decision Making). Given this complexity, a systems approach can provide valuable assistance in identifying mechanisms to reduce carbon emissions to the atmosphere. Such an approach examines carbon comprehensively, holistically, and from an interdisciplinary viewpoint and considers social, economic, and environmental factors as highlighted in examples that follow.

Energy Systems

System drivers and interactions within the energy sector are particularly complex. Differences in social practices, technical and infrastructural efficiency, market dynamics, policies, waste management, and environmental conditions explain variations in observed levels of energy use and land use, which are two key drivers of carbon emissions across North American households, organizations, firms, and socioecological systems (see Figure ES.6 and Ch. 18). Carbon emissions from burning fossil fuels have decreased because of growth in renewables, new technologies (such as alternative fuel vehicles), rapid increases in natural gas production, the 2007 to 2008 global financial crisis, and more efficient energy production and use (see Figure ES.5; Ch. 2: The North American Carbon Budget; and Ch. 3: Energy Systems). Social mechanisms have influenced carbon emissions through acceptance of rooftop solar energy and wind farms, the dynamics of routines in provision (i.e., attempts by suppliers to encourage and increase demand through marketing), and demand patterns related to the locus of work and the cultural definition of approved practices (see Ch. 6: Social Science Perspectives on Carbon). Although social drivers can lock in dependencies for particular energy systems, North American energy systems are poised for significant infrastructure investment, given the age and condition of transportation infrastructure and existing components for energy generation, transmission, and storage (see Ch. 3: Energy Systems).

Urban Areas

Urban areas occupy only 1% to 5% of the North American land surface but are important sources of both direct anthropogenic carbon emissions and spatially concentrated indirect emissions embedded in goods and services produced outside city boundaries for consumption by urban users (see Ch. 4: Understanding Urban Carbon Fluxes). The built environment (i.e., large infrastructural systems such as buildings, roads, and factories) and the regulations and policies shaping urban form, structure, and technology (such as land-use decisions and modes of transportation) are particularly important in determining urban carbon emissions. Such societal drivers can lock in dependence on fossil fuels in the absence of major technological, institutional, and behavioral change. Moreover, some fossil fuel–burning infrastructures can have lifetimes of up to 50 years. Urban areas also are important sites for policy- and decision-making activities that affect carbon fluxes and emissions mitigation. Co-benefits of urban mitigation efforts can be considerable, particularly in terms of improvements in air quality and human health, as well as reductions in the heat island effect (i.e., elevated ambient air temperatures in urban areas).

Agricultural Practices

Factors driving GHG emissions from agricultural activities include the creation of new croplands from forests or grasslands, nitrogen fertilizer use, and decisions about tillage practices and livestock management. Trends in global commodity markets, consumer demands, and diet choices also have large impacts on carbon emissions through land-use and land-management changes, livestock systems, inputs, and the amount of food wasted (see Ch. 5: Agriculture). Policy incentives and local regulations affect some of these decisions.

Tribal Lands

Carbon cycling and societal interactions on tribal lands have important similarities to and differences from those on surrounding public or private lands. Managing tribal lands and resources poses unique challenges to Indigenous communities because of government land tenure, agricultural and water policies, relocation of communities to reservations in remote areas, high levels of poverty, and poor nutrition. Nevertheless, multiple tribal efforts involve understanding and benefitting from the carbon cycle. For example, there are several case studies examining traditional practices of farming and land management for sequestering carbon on tribal lands (see Ch. 7: Tribal Lands).

Land-Use Change

Land-use change has long been a driver of net reductions in atmospheric CO₂ emissions in the United States and Canada. Over the past decade, Canada and Mexico have lost carbon from land-use changes involving forests, but in the United States carbon losses from deforestation have balanced carbon gains from new forestland. Recent increases in natural disturbance rates, likely influenced by climate change and land-management practices, have diminished the strength of net forest uptake across much of North America. In addition, carbon emissions from the removal, processing, and use of harvested forest products offset about half of the net carbon sink in North American forests (see Ch. 9: Forests).

Future changes to the carbon cycle are projected using different kinds of models based on past trends, current data and knowledge, and assumptions about future conditions. Model projections reported in SOCCR2 seek to understand the potential of different components of North American ecosystems to serve as carbon sources or sinks, even though such projections have uncertainties (see Box ES.2, Projection Uncertainties).

The best available projections suggest that emissions from fossil fuel combustion in the energy sector will continue into the future. These projections also indicate that by 2040, total North American fossil fuel emissions could range from 1.5 to 1.8 Pg C per year, a range representing a 12.8% decrease to 3% increase in emissions compared to 2015 levels (see Ch. 19: Future of the North American Carbon Cycle). Projections include the combined effects of policies, technologies, prices, economic growth, demand, and other variables. Human activities, including energy and land management, will continue to be key drivers of carbon cycle changes into the future. A wide range of plausible futures exists for the North American energy system in regard to carbon emissions. For the United States, backcasting scenarios suggest that a significant reduction in emissions is plausible.

The persistence of the overall North American land carbon sink is highly uncertain, with models projecting that terrestrial ecosystems could continue as net sinks of carbon (up to 1.5 Pg C per year) or switch to net sources of carbon to the atmosphere (up to 0.6 Pg C per year) by the end of the century. Low confidence in these projections results from uncertainties about the complex interactions among several factors, ranging from emissions scenarios, climate change, rising atmospheric CO₂, and human-driven changes to land cover and land use (see Ch. 19).

Soils store a majority of land carbon, particularly the permafrost soils of northern high-latitude regions, which are experiencing the most rapid rates of warming caused by climate change. Increased temperatures very likely will lead to accelerated rates of permafrost thaw, releasing previously frozen soil carbon to the atmosphere. Globally, rising temperatures could cause the soil pool of 1,500 to 2,400 Pg C to release 55 ± 50 Pg C by 2050. However, the magnitude and timing of these carbon losses are not well understood, partly because of poor coverage and distribution of measurements, as well as inadequate model representation of permafrost feedbacks (see Ch. 11: Arctic and Boreal Carbon; Ch. 12: Soils; and Ch. 19: Future of the North American Carbon Cycle).

The Exclusive Economic Zone of North American coastal areas has taken up 2.6 to 3.4 Pg C since 1870 and is projected to take up another 10 to 12 Pg C by 2050 under business-as-usual, human-driven emissions scenarios. However, coastal ecosystems such as mangroves, wetlands, and seagrass beds that historically have removed carbon from the atmosphere are particularly vulnerable to loss of stored carbon caused by the combination of sea level rise, warming, storms, and human activity; the extent and impact of these vulnerabilities are highly uncertain (see Ch. 19). Taken together, these projections portray significant but uncertain future potential changes in the carbon cycle and associated consequences.

The anthropogenic effects on the carbon cycle as synthesized in this report clearly show there is ample capacity to affect carbon pools and cycles. In the past, such effects have mostly been unintentional, but they underscore contemporary policy and management opportunities for managing the North American carbon cycle and mitigating carbon emissions. There is global scientific consensus for the need to limit carbon emissions and resultant projected global warming in this century to less than 2°C above preindustrial levels (and preferably to less than 1.5°C) while also reducing net anthropogenic GHG emissions to zero via “negative emissions” technologies, carbon management, and mitigation. Based on current rates of global fossil fuel use and land-use change, emissions could be sufficient in about 20 years to cause global temperature to increase 2°C, assuming the land and ocean sinks remain at current levels (see Ch. 1: Overview of the Global Carbon Cycle). According to global climate simulations, cumulative carbon emissions since preindustrial times cannot exceed about 800 Pg C for a 67% chance that the global average temperature increase would be less than 2°C. As of 2015, total cumulative emissions were about 570 Pg C. Therefore, to keep warming below 2°C, probably no more than an additional 230 Pg C may be released globally.⁵ National, international, and local initiatives provide mechanisms for Mexico, Canada, and the United States to decrease carbon emissions (see Box ES.3, Multiscale Efforts to Reduce Carbon Emissions). To help reduce emissions, subnational entities in North America have implemented activities such as green building codes and efforts related to regional energy systems (see Ch. 3: Energy Systems).

Carbon Management Tools and Options

There are multiple options to decrease GHG emissions or increase carbon sinks. One is to reduce the use of fossil fuels, replacing them with renewable energy sources (e.g., solar, wind, biofuels, and water) that often release less carbon into the atmosphere. Other strategies involve capturing CO₂ at point sources, compressing and transporting it (usually in pipelines), and safely and securely storing it deep underground. Negative emissions activities represent a third option that leverages approaches to remove previously emitted CO₂ by increasing its capture from the atmosphere and its subsequent long-term storage, mainly in terrestrial, geological, and oceanic reservoirs (see Ch. 1: Overview of the Global Carbon Cycle). Each option has benefits but also tradeoffs that are important to evaluate.

Multiple lines of evidence throughout SOCCR2 demonstrate that humans have the capacity to significantly affect the carbon cycle. Understanding the mechanisms and consequences of these effects offers opportunities to use knowledge of the carbon cycle to make informed and potentially innovative carbon management and policy decisions. In the past, planners have assumed economically rational energy use and consumption behaviors and thus were unable to predict actual choices, behaviors, and intervening developments, leading to large gaps between predicted versus actual purchase rates of economically attractive technologies with lower carbon footprints (see Ch. 6: Social Science Perspectives on Carbon). Approaches that are people-centered and multidisciplinary emphasize that carbon-relevant decisions often are not about energy, transportation, infrastructure, or agriculture, but rather style, daily living, comfort, convenience, health, and other priorities (see Ch. 6). With this consideration, some technical and science-based tools and carbon management options are highlighted here. These options aim to reduce the likelihood of rapid climate change in the future and increase the benefits of a well-managed carbon cycle (see Ch. 3: Energy Systems; Ch. 6; and Ch. 18: Carbon Cycle Science in Support of Decision Making).

Energy Sector. Mitigation options include reduced use of carbon-intensive energy sources, such as oil and coal, and increased use of natural gas and renewables. Replacement of aging infrastructure with modern and more efficient facilities can also reduce emissions. Equally important are market mechanisms and technological improvements that increase energy-use efficiency and renewable energy production from wind, solar, biofuel, and geothermal technologies (see Ch. 3: Energy Systems).

Urban Areas. Emissions reductions in these areas mostly focus on transportation, buildings, and energy systems. Transportation options include facilitating the transition to lower-emission vehicles and expanding the availability and use of public transit. Green building design and the energy embodied in building construction are metrics incorporated into green building codes (see Ch. 4: Understanding Urban Carbon Fluxes). Replacing aging pipelines can also reduce leakage of natural gas.

Carbon Capture and Storage. Capturing carbon released from the burning of fossil fuels directly prevents CO₂ from entering the atmosphere. However, the technology remains costly and would benefit from additional research (see Ch. 3).

Land-Use and Land-Management Changes. Carbon management options include 1) avoiding deforestation; 2) sequestering carbon (i.e., accumulating and storing it long term) through afforestation, agroforestry, or grassland restoration; 3) improving forest management to increase and maintain higher levels of carbon stocks or to increase CO₂ uptake from the atmosphere; and 4) directing harvest removals toward either biomass energy as a substitute for fossil fuels or long-lived wood products as substitutes for more fossil fuel–intensive building materials. Conversion of grasslands to croplands, however, is likely to reduce carbon stocks (see Ch. 5: Agriculture; Ch. 9: Forests; Ch. 10: Grasslands; and Ch. 12: Soils). Accumulating carbon into vegetation and soils could remove 1.6 to 4.4 Pg C per year globally from the atmosphere, but the availability of land area, nutrients, and water could constrain such efforts (see Ch. 12).

Grazing and Livestock Management. These management activities affect grassland carbon stocks and their net carbon uptake by tens of teragrams per year (see Ch. 10). Although various management strategies can reduce CH₄ emissions from ruminants (i.e., enteric) by 20% to 30% and from manure by 30% to 80%, they need to be evaluated over appropriate scales to account for emissions co-effects, such as improved land productivity (see Ch. 5).

Agriculture Cropland and Waste Management. Mitigation strategies include covering the land year-round with deeply rooted crops, perennials, or cover crops; protecting the carbon in agricultural soils via residue management and improved nutrient management; and reducing food waste and inefficiencies. In addition, optimizing nitrogen fertilizer to sustain crop yield and reduce nitrogen losses to air and water reduces GHG emissions, protects water and air quality, decreases CH₄ fluxes in flooded or relatively anoxic systems, and provides food for a growing population (see Ch. 5 and Ch. 12).

Wetland Restoration or Creation. These efforts will affect wetland CO₂ and CH₄ fluxes, which vary widely among wetland sites, type, and time since restoration (see Ch. 13: Terrestrial Wetlands and Ch. 15 Tidal Wetlands and Estuaries). In the long term, restored wetlands are considered carbon sinks because of plant uptake and subsequent organic matter accumulation.

Tribal Lands. Indigenous communities in the United States, Canada, and Mexico are applying traditional knowledge through sustainable management of forests, agriculture, and natural resources on tribal lands. Emerging carbon trading markets provide opportunities for these communities to benefit economically from such initiatives (see Ch. 7: Tribal Lands). Successful efforts on tribal lands provide examples that could be followed on non-tribal lands.

Costs, Co-Benefits, and Tradeoffs

Estimates suggest that the cumulative cost over 35 years of reducing GHG emissions to meet a 2°C trajectory by 2050 ranges from $1 trillion to $4 trillion (US$2005) in the United States. Alternatively, the annual cost of not reducing emissions is conservatively estimated at $170 billion to $206 billion (US$2015) in the United States in 2050 (see Ch. 3: Energy Systems).

Strategies for reducing carbon emissions often result in co-benefits such as improvements in air quality and energy-use efficiency, increased revenues, economic savings to taxpayers, greater crop productivity, and enhanced quality of life (see Ch. 4: Understanding Urban Carbon Fluxes). Changes in land carbon stocks (either increases or decreases) can occur as co-effects of management for other products and values. For example, sound carbon cycle science could inform management options that might produce sustained co-benefits by considering the vulnerability of forests to disturbances (e.g., wildfires) and consequently focusing development of carbon sequestration activities in low-disturbance environments. An example trade-off in science-informed decision making is a management strategy to reduce the risk of severe wildfires in fire-prone areas that results in intentional, short-term reductions in ecosystem carbon stocks to reduce the probability of much larger reductions over the long term (see Ch. 9: Forests). Likewise, management of wildfire regimes in vegetated landscapes can influence soil carbon storage via management effects on productivity and inputs of recalcitrant, pyrogenic (i.e., fire-produced) organic matter or black carbon in soils (see Ch. 12: Soils). Protection of grasslands from conversion to croplands (e.g., in the Dakotas) can reduce emissions significantly. However, with high market prices for corn, carbon offsets alone cannot provide enough economic incentive to retain grasslands (see Ch. 10: Grasslands).

Leveraging Integrated Carbon Cycle Science

Local, state, provincial, and national governments in North America can benefit from scientific knowledge of the carbon cycle. When context and stakeholder involvement are considered, changes in technologies, infrastructure, organization, social practices, and human behavior are more effective. For example, the National Indian Carbon Coalition was established in the United States to encourage community participation in carbon cycle programs with the goal of enhancing both land stewardship and economic development on tribal lands. With the emergence of carbon markets as an option for addressing climate change, First Nations in Canada formed the “First Nations Carbon Collaborative” dedicated to enabling Indigenous communities to access and benefit from emerging carbon markets (see Ch. 7: Tribal Lands).

Integrating data on societal drivers of the carbon cycle into Earth system and carbon cycle models improves representation of carbon-climate feedbacks and increases the usefulness of model output to decision makers. Better integrating research on Earth system processes, carbon management, and carbon prediction improves model accuracy, thereby refining shared representations of natural and managed systems needed for decision making (see Figure ES.6 and Ch. 18: Carbon Cycle Science in Support of Decision Making). Consequently, both carbon cycle science and carbon-informed decision making can be improved by increased interaction among scientists, policymakers, land managers, and stakeholders.

The conclusions from this report underscore the significant advances made in the understanding of the North American carbon cycle in the decade since SOCCR1 (CCSP 2007). Results show that emissions from the burning of fossil fuels for energy and other technological systems still represent the largest single source of the North American carbon budget. About 43% of these emissions are offset by terrestrial and coastal ocean sinks of atmospheric CO₂. A better understanding of inland waters is among the major scientific advances since SOCCR1 that are highlighted in this report. In contrast to SOCCR1, SOCCR2 clearly identifies a significant source of CO₂ from inland waters, as well as a similarly sized sink in the coastal ocean. This report also describes progress in documenting key elements of the CH₄ budget, which were largely absent in SOCCR1. Improved consistency between bottom-up inventories and top-down atmospheric measurements is encouraging for the design of future monitoring, reporting, and verification systems. Such systems will be enhanced greatly if uncertainties in the two approaches continue to decline as new measurement systems are deployed and as integrated analysis methods are developed. Importantly, understanding of the main causes of observed changes in the carbon budget has improved over the last decade, helping to establish a strong foundation for assessing options for reducing atmospheric carbon concentrations and for developing and using carbon management choices. Reducing carbon emissions from existing and future sources and increasing carbon sinks will need to involve science-informed decision-making processes at all levels: international, national, regional, local, industrial, household, and individual.

Despite improvements in calculating the carbon budget since SOCCR1, some regions and ecosystems still have highly uncertain estimates compared with others and thus need significant improvements in research and monitoring. Among these areas are Arctic and boreal regions, grasslands, tropical ecosystems, and urban areas. Also needed is a better overall understanding of the CH4 cycle. The continued advancement of cross-disciplinary and cross-sectoral carbon cycle science to fill these gaps and to address the research challenges and opportunities identified in this report will be important for the third SOCCR to assess a decade from now.