Birdsey, R., M. A. Mayes, P. Romero-Lankao, R. G. Najjar, S. C. Reed, N. Cavallaro, G. Shrestha, D. J. Hayes, L. Lorenzoni, A. Marsh, K. Tedesco, T. Wirth, and Z. Zhu, 2018: Executive summary. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M.A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 21-40, https://doi.org/10.7930/SOCCR2.2018.ES.
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.
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