Fifth National Climate Assessment
32. Mitigation

Mitigation Goals

To achieve the Paris Agreement (an international treaty on climate change) goal of limiting global warming to well below 2°C above preindustrial levels and pursuing efforts to limit global warming to 1.5°C above preindustrial levels, global CO₂ emissions need to reach net zero around 2050 and remain net zero or net negative afterward.² Thus, US CO₂ emissions reaching net zero around midcentury would be consistent with Paris goals, although a wide range of trajectories is possible based on considerations of international equity, burden-sharing, costs, and policy assumptions.^3,4,5 This chapter addresses pathways and options for mitigation of US emissions from all sectors consistent with national and international climate goals.

As part of the Paris Agreement, countries communicate nationally determined contributions (NDCs)—emissions-reduction targets that they intend to achieve. The latest NDC communicated by the United States to the United Nations Framework Convention on Climate Change (UNFCCC) Secretariat sets an economy-wide target of reducing all its net GHG emissions (not only CO₂) by 50%–52% below 2005 levels in 2030, or roughly –6% per year beginning in 2022, putting the country on a path to achieve the goal of reaching net-zero GHG emissions by no later than 2050 (Figure 32.1).⁷ In addition, 24 states and Washington, DC, have their own reduction targets (KM 32.5).

Major Trends

Between 1990 and 2019, US CO₂ and nitrous oxide (N₂O) emissions increased by approximately 3% in each case. Emissions of fluorinated gases increased by 86%, and methane (CH₄) emissions decreased by 15%.⁸ Although the latest EPA inventory reports emissions through 2021,⁹ this chapter focuses on trends in emissions to 2019 because the COVID-19 pandemic caused substantial but largely temporary changes in energy-related emissions worldwide (see, e.g., Davis et al. 2022; Liu et al. 2020^10,11). The EPA estimates that US carbon dioxide equivalent (CO₂-eq) GHG emissions were about 6.6 billion metric tons (or gigatons; Gt) in 2019, 2% more than in 1990.^8,12 The sources of these emissions are primarily electricity generation, transportation, and combustion of fuels in other sectors (i.e., commercial, residential, and industrial), with smaller contributions from agriculture, industrial processes, and waste (Figure 32.1). Major sinks were land-use change and especially forests, which resulted in net uptake of 0.7 Gt of CO₂ in 2019. Net GHG emissions from all sources and sinks were thus 5.8 Gt of CO₂-eq in 2019.^8,9,12

Between 2005 and 2019, US GHG emissions decreased by 12%, mainly because of reductions in electricity generation emissions. Indeed, since 2017, the largest share of GHG emissions has come from the transportation sector (Figure 32.1). Estimates include emissions occurring within all US territories, as annually reported to the UNFCCC Secretariat by the EPA. Independent estimates by other scientific bodies and researchers are similar but not identical.^13,14,15,16

Sector-Specific Trends and Drivers

Between 1990 and 2019, economic and population growth have acted to increase US energy-related emissions but have been counterbalanced by reductions in both the energy used per dollar of GDP (or “energy intensity of economic activity”) and the CO₂ emissions per unit of energy used (or “emissions intensity of energy”).¹⁷ In particular, decreases in energy emissions since 2007 were driven by a steady and substantial fall in CO₂ emissions per unit of energy consumed from a maximum of 59 million metric tons (megatons; Mt) of CO₂ per exajoule (10¹⁸ joules) of energy consumed in 2007 to 51 Mt per exajoule in 2019 (Figure 32.2).

Electricity Sector Emissions

GHG emissions from the electricity sector in 2019 were 1,629 Mt of CO₂-eq, or 30% of energy-related emissions.⁸ Decreases in US energy-related emissions since 2007 mostly reflect changes in the electricity sector, especially the retirement and reduced use of coal-fired power plants and corresponding increases in lower-cost electricity from natural gas–fired power plants (and to a lesser extent renewable technologies; Figure 32.3b). US emissions from electricity generation in 2019 were roughly 40% below 2005 levels (Figure 32.3a).

Transportation Sector Emissions

GHG emissions from the transportation sector in 2019 were 1,874 Mt of CO₂-eq.⁸ Most transportation emissions are CO₂ emissions from combustion of gasoline (59.6%, mostly for light trucks and cars), diesel (26.4%, mostly for heavy trucks, buses, and trains), and jet fuel (9.8%; Figure 32.4). In contrast to electricity sector emissions, transportation emissions increased by 23% between 1990 and 2018, largely reflecting 49% growth in demand for passenger vehicle transport over the period (measured in passenger-kilometers, or the distance traveled in km multiplied by the number of passengers), which was partially offset by a 22% decrease in energy required per passenger-kilometer. Over the same 1990–2018 time period, demand for heavy trucks (measured in vehicle-kilometer, or the total distance traveled by the truck fleet) more than doubled, and improvements in energy per vehicle-kilometer were more modest (an 8.6% decrease in energy required per vehicle-kilometer).

Residential- and Commercial-Building Sector Emissions

Direct GHG emissions from residential and commercial buildings were 699 Mt CO₂-eq in 2019. Since 1990, direct emissions from US residential and commercial buildings (i.e., excluding electricity) have risen by roughly 14% (Figure 32.5). The increase is primarily related to steady growth of fugitive emissions of fluorinated gases from building cooling systems.⁹ Over the same period, energy efficiency improvements and increasing electrification have kept flat direct CO₂ emissions from onsite fuel combustion despite 50% increases in both residential and commercial building floor area (Figure 32.5).^19,20

Industrial Sector Emissions

GHG emissions from the industrial sector were 1,568 Mt of CO₂-eq in 2019.⁹ Direct emissions from the industrial sector, including onsite fuel combustion as well as all process and fugitive GHG emissions (e.g., emissions from calcination of limestone in cement production and methane leakage from oil and gas infrastructure), decreased by 14% between 1990 and 2020, primarily due to decreases in total fossil fuel combustion, fluorinated gas production and use, and metals-related process emissions (Figure 32.6). The manufacturing sector (i.e., production of goods and materials) is the largest source of direct emissions within the overall industrial sector and accounts for substantial electricity sector emissions related to purchases of power and heat (Figure 32.6b). Six key manufacturing subsectors (petroleum refining, chemicals, cement, iron and steel, aluminum, and forest products) account for around 70% of all emissions attributable to the manufacturing sector (Figure 32.6b).

Between 1994 and 2018, electricity-related emissions from US manufacturing have decreased by about 32% due to improved process efficiencies, deployment of combined heat and power systems, and decarbonization of purchased electricity. However, direct emissions from onsite fossil fuel combustion have decreased by only 10% over the same period and now account for about three-quarters of direct manufacturing emissions (Figure 32.6b).

Land-Related Emissions

Annual US GHG emissions related to land use in 2019 can be split into emissions of 615 Mt of CO₂-eq from agriculture and uptake of 704 Mt of CO₂-eq by other land use and land-use change (including forests; Figure 32.7). Thus in 2019, there was a net land-related uptake (i.e., negative emissions) of 90 Mt CO₂-eq. Forests take up carbon, but the amount of carbon sequestered by US forest land has decreased from 816 Mt CO₂ in 1990 to 638 Mt CO₂ in 2019⁹ due to a combination of drought, wildfire, and disturbances by insects and disease (Box 7.2; KM 6.1).^21,22,23 Agricultural emissions (excluding fuel combustion) increased slightly from 548 Mt CO₂-eq in 1990 to 615 Mt CO₂-eq in 2019. The net uptake of 90 Mt CO₂-eq from US lands in 2019 represents a 73% decrease from the uptake of 333 Mt CO₂-eq in 1990.⁹

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Established Opportunities to Reduce Energy-Related Emissions

In modeling studies, deeply decarbonized and net-zero-emissions energy systems share several common characteristics, but regional approaches may depend on differences in resources,^24,25 industrial bases,²⁶ existing infrastructure,^27,28 geography,²⁹ governance and politics,³⁰ public acceptance,³¹ and broader policy priorities.³²

Improve Energy Efficiency

Improving energy efficiency means supplying the same level of end-use services or output while using less energy. Efficiency of buildings and appliances can be improved by design or retrofits (e.g., better insulation),³³ as well as by optimizing control and management of devices (e.g., HVAC and lighting; KM 12.3; Figure 5.5).³⁴ Further efficiency gains are available in the transportation sector: urban design can reduce travel demands;^35,36 public and active transportation modes can greatly reduce energy use per passenger-mile;^37,38 and advanced engines, electrification, reducing the weight of vehicles, and aerodynamic improvements can reduce energy use per passenger-mile (KM 13.3).^39,40,41 In model scenarios of energy systems that successfully reach net-zero CO₂ emissions, total US energy use often decreases relative to current levels, despite economic and population growth.^42,43

One of the concerns with energy efficiency is whether it can induce rebound effects of different types. Studies have shown that the direct rebound effect (i.e., use of more of a good or service as it becomes more affordable) is low in the context of energy goods and services, but there is more uncertainty regarding indirect rebound effects (i.e., how an increase in energy efficiency of a good or service may lead to a change in the use of other goods and services or changes in the overall economy).^44,45

Decarbonize the Electricity Sector

Options for reducing electricity system emissions include variable renewables (e.g., solar and wind resources, which are not available on demand; KM 5.3), dispatchable or “firm” renewables (e.g., biomass, hydropower, and geothermal, which can be available on demand), and other low-emitting dispatchable resources (e.g., nuclear and carbon capture and storage [CCS]–equipped fossil-fired generators); energy storage technologies; improved transmission (both upgrading conductors and new rights-of-way); and demand management. The rate and scale at which these technologies may be deployed in the future depend on the uncertain trajectories of their costs and energy markets, as well as a host of non-economic factors (KM 32.4).^{46,47,48,49,50,51,52,53,54}

However, given their plummeting costs (Figure 32.8a, b) and growing policy support (e.g., the Inflation Reduction Act of 2022⁵⁵), variable renewable-energy resources—especially wind and photovoltaic solar generation—are expected to play central roles in decarbonizing electricity systems across the United States. Energy system models project that the capacity of wind and solar would need to increase 2 to 10 times faster each year than maximum historical rates (Figure 32.9b) to reach the 2030 target of halving economy-wide GHG emissions and midcentury net-zero targets (Figure 32.1).^42,43 In such scenarios, expansion of energy storage generally supports greater reliance on wind and solar (Figure 32.1).^42,43

The same model scenarios consistently project the rapid decline of coal-fired electricity generation in decarbonized systems to near zero by 2030 (Figure 32.10).^42,43 In contrast, natural gas–fired electricity generation declines more slowly in most of these net-zero emissions scenarios, facilitating penetration of variable renewables but operating less frequently over time unless equipped with CCS.^53,56,57

Finally, net-zero CO₂ emissions scenarios often maintain—but do not greatly expand—existing nuclear and hydropower capacity in the absence of significant cost declines (such as improved economics from small modular designs; KM 5.3) and/or constraints on the deployment of other technologies.^42,43,58 In contrast, both transmission infrastructure (i.e., power lines) and international and interregional transfers of electricity often increase in decarbonization scenarios, although the scale of such increases varies.^42,43,59

Electrify Energy End Uses

As electricity systems are decarbonized, energy model scenarios consistently project widespread electrification of energy end uses such as on-road transportation and heat for buildings and industry (KM 5.1).^{42,43,50,52,53,54,62} Electricity may also be used to produce low-carbon fuels, such as hydrogen and e-fuels (liquid fuels produced by combining carbon captured from the atmosphere with hydrogen produced by electrolysis), for difficult-to-electrify applications (Box 32.1).⁶³ The share of US final energy demands (i.e., energy used) met by electricity in net-zero-emissions energy systems will depend on the costs of low-carbon alternatives such as biofuels and hydrogen, but estimates range from 30%–60% in 2050, up from about 20% today (Figure 32.11).^42,43

In transportation, light-duty electric vehicles (EVs) have had policy support (e.g., tax refunds) at both the state and federal level for a long time, and new EV sales have increased in recent years (Figure 32.8).^64,65,66,67 The EV share of new light-duty vehicle sales in the US is expected to grow quickly,^67,68 which is the case in model scenarios that reach net-zero emissions by midcentury.^42,43 Many medium- and heavy-duty vehicles can also be electrified,⁶⁹ although some applications (e.g., long-distance trips) may present special challenges.^{70,71,72,73,74} Decarbonization of the most difficult-to-electrify transportation sectors (e.g., aviation, international shipping) may require liquid biofuels or fuels synthesized using electrolytic hydrogen and carbon captured from the atmosphere.^75,76

Insofar as electricity is generated from non-emitting sources, electrification of space and water heating would drastically reduce direct emissions from residential and commercial buildings in the United States (where these end uses account for the bulk of natural gas and oil consumption).^77,78,79 Similarly, most industrial energy demand could be electrified using existing technologies,⁸⁰ although achieving net-zero emissions in some industries may present special challenges^81,82,83,84—particularly related to the costs of supplying high-temperature heat with electricity⁸⁵ and/or fundamental changes in processes such as switching to direct reduction of iron ore with electrolytic hydrogen or installing carbon capture and storage on thousands of cement kilns worldwide.⁸⁶

Established Opportunities to Reduce Land-Related Emissions

Despite increasing demand for food and the headwinds of climate change impacts on agriculture, there are multiple options for decreasing land-use emissions and protecting and enhancing terrestrial carbon sinks (Ch. 11).

Use Most-Productive Land for Agriculture

Agriculture requires more land, by far, than any other human activity.⁸⁷ One way to reduce the land required to grow food is to continue farming the most productive lands (those that grow more crops per land area). Removing the most productive areas from cultivation would lead to an increase in the overall land area required for agriculture.⁸⁸ Loss of productive US farmland to sprawl or even restoration could thus lead to substantial land-use change and related GHG emissions elsewhere (e.g., if demanded agricultural goods are imported from other regions).^89,90 For these reasons, studies have suggested mitigation efforts should prioritize restoration of marginal (i.e., not the most productive) lands.⁹¹

Reduce Food Waste

More than a third of all food in the US is currently wasted, more than 40% of which is food discarded by retailers and consumers (Box 11.2; Figure 11.12).^92,93 Multiyear campaigns (from 4 to 11 years long) in five other developed countries successfully reduced food waste per person by 8%–29% through public education and public and private initiatives.⁹² Assuming similar reductions could be achieved, US agricultural land and land-related GHG emissions could be reduced by 4%–13%.

Shift Diets

GHG emissions produced during food production, distribution, transportation, and sale at retail or restaurants vary across different foods, so that different diets will entail different levels of life-cycle GHG emissions.^94,95,96,97 In particular, although meat is a good source of protein and micronutrients, it generally produces more emissions per calorie than plant-based foods because energy is lost at each trophic level.⁹⁴ Emissions related to meat production also vary: for example, ruminant animals usually produce much more GHG emissions per calorie of meat and per gram of protein than poultry (Figure 11.8).⁹⁸ By reducing demand for emissions-intensive food, shifts to pescatarian, vegetarian, vegan, Mediterranean, or “flexitarian” (less meat consumption but not strictly vegetarian) diets can reduce land-related GHG emissions while providing direct health benefits (Figure 32.12),^{95,99,100,101} although analyses and models differ as to the level of future food demand related to such diets and other socioeconomic changes.¹⁰² Shifting diets and associated changes in agricultural practices have implications for land-use change as well as supply chains, air pollution, and human health.¹⁰³ Consideration of energy and other inputs per unit of production and the resulting impacts on net GHG emissions is important for comparison of different dietary choices.

However, 10.4% of American households are food insecure (Box 11.1),¹⁰⁴ so any approach to reduce the consumption of higher-emissions foods that results in higher food prices could disproportionately harm these households. Instead, policies might encourage less emissions-intensive diets while also reducing food costs and increasing consumer choice by making a diversity of plant-based and other lower-emissions, nutritious, and affordable options more widely available.

Improve Management of Croplands and Pasture

There are numerous opportunities to decrease the intensity of emissions (and/or increase sequestration; see Box 32.2) of croplands and pasture, including 1) improving soil health, 2) improving nitrogen fertilizer management, 3) increasing the number of trees and other perennials on the landscape (e.g., by agroforestry; see Ch. 11),^105,106,107 and 4) avoiding methane emissions. Soil health and carbon sequestration can also be improved by amendments (including biochar; Figure 11.5), cover crops, reduced tillage,¹⁰⁸ and diversification of crop rotations.¹⁰⁹ Careful and sustained implementation of these practices can increase not only soil carbon but also yields, resilience, and profitability.

Better aligning the timing and amount of fertilization with plants’ needs can reduce fertilizer use^110,111 and thereby also reduce both nitrous oxide (N₂O) emissions from the soil and fossil fuel emissions from fertilizer production. Fertilizers with synthetic nitrification inhibitors can further reduce N₂O emissions.¹¹² Increased fertilizer efficiency and inhibition of nitrification processes in soil together can reduce N₂O emissions by roughly 50%.^113,114

There are also feasible options for reducing agricultural (livestock and rice) and waste (landfill and wastewater) sources of methane emissions.¹¹⁵ Methane is a relatively short-lived GHG that has contributed to at least 25% of climate warming to date.^116,117 Consequently, technically feasible near-term methane emissions reductions could slow global decadal warming by 30%, avoiding a quarter degree Celsius of warming by midcentury.¹¹⁵ In addition to land-related sources of methane, there are large reductions possible from the oil and gas sector,¹¹⁸ primarily by repairing leaks at little or no net cost¹¹⁵ and ideally prioritizing disproportionately large sources (i.e., super-emitters).^119,120,121

Avoid Conversion and Monitor Carbon Fluxes on Unmanaged Land

Between 50 and 150 Mt of annual CO₂ emissions could be avoided by stopping conversions of unmanaged land in the United States (i.e., natural forests, grasslands, wetlands, or other ecosystems where there has been no substantial human influence or intervention).¹²² Strategies for stopping such conversions include densification of already-developed areas, zoning, and property tax incentives, as well as land protection such as conservation easements and public parks.^{123,124,125,126} Related to this opportunity, the recent decrease in carbon sequestration by US forests (KM 32.1) is a concern. Further weakening of this carbon sink would make reaching net zero proportionally that much more difficult. Improved monitoring of forest carbon fluxes and their drivers is therefore important, including those on unmanaged land and in boreal Alaska (KM 7.2).^127,128,129

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Potential Opportunities to Reduce Energy-Related Emissions

There are many uncertainties and outstanding questions related to mitigation of energy-related emissions. These uncertainties are reflected by the large differences in the scale and mix of energy sources and use as well as carbon management across modeled net-zero-emissions energy systems, which highlight potential mitigation opportunities.

The Mix of Electricity Sources in Net-Zero-Emissions Energy Systems

In recently modeled net-zero-emissions US energy systems, the share of electricity demand met by variable renewables—as opposed to firm sources—varied from 45%–89% depending on the availability of energy storage, transmission, and the mix of solar and wind.^42,43 Although grid managers are gaining experience planning and operating electricity systems with large amounts of solar and wind generation, questions persist as to the maximum share of these resources that should be included in reliable and resilient decarbonized systems¹³⁰ and the best approaches for dealing with their natural variability.¹³¹ Large shares of variable renewables can be incorporated in electricity grids through the use of 1) batteries, hydrogen, and other types of energy storage; 2) transmission and interregional transfers of electricity; 3) firm low-carbon electricity sources; and 4) greater demand-side responses. The costs and effectiveness of these approaches for managing variability differ and are related to the spatial and temporal variability of solar and wind resources,^{25,76,132,133,134,135,136,137} in addition to a host of non-cost factors (KM 32.4). Moreover, energy sources and technologies will interact in complex ways to fulfill the different functions in electricity systems (e.g., providing energy, capacity, and ancillary services over different timescales), depending on their relative costs and system benefits, policy stringency and design, geophysical resources and infrastructure, environmental co-benefits, and societal preferences.^56,57,138 Further research, development, and demonstration of technologies and approaches are needed to resolve uncertainties, identify key sensitivities, and clarify the most attractive options for providing reliable, resilient, and affordable electricity in net-zero-emissions energy systems (Figure 5.6).

Alternative Fuels for Difficult-to-Electrify Sectors

As with electricity, there is considerable uncertainty about the scale and mix of other energy carriers (e.g., hydrogen, bioenergy, e-fuels) that may be needed by difficult-to-electrify sectors such as long-distance transportation of freight, long-haul aviation, high-temperature industrial heating, and space heating in very cold climates.^{76,85,139,140,141} Hydrogen, ammonia, alcohols, and carbon-based fuels (e.g., methane, petroleum, methanol) can all be produced with low and eventually net-zero CO₂ emissions (Box 32.1). However, it is not clear whether producing and burning these fuels would be lower in cost and more sustainable than continuing to use fossil fuels and managing the related emissions through CCS or carbon dioxide removal (CDR; removal of CO₂ from the atmosphere).⁶³ Here again, further research, development, and demonstration of technologies will help reveal critical dependencies and trade-offs and clarify the most sustainable and cost-effective pathways to net-zero-emissions fuels.

Carbon Management

Most model scenarios that reach net-zero emissions in the United States entail substantial use of CDR technologies, not as a replacement for emissions reductions but instead to offset continuing emissions from the most difficult-to-decarbonize sectors and processes, such as aviation and cement making (sources of emissions that may be much more expensive to eliminate), to offset non-energy GHG emissions, and to reduce GHG concentrations in the atmosphere. The degree and form of CDR deployment, including the balance between industrial carbon capture and intentional enhancement of natural carbon sinks, remain highly uncertain, however, and depend on technological readiness, economics, public acceptance, and institutional and political considerations (Box 32.2).

Changes in Transportation Modes and Behavior

Uncertain changes in mobility and travel behavior could facilitate or hinder mitigation. For example, autonomous vehicles are rapidly evolving but still need to overcome challenges of consistent safety measures, standardization of technology liability, and security and privacy concerns.^210,211 Studies have shown that autonomous vehicles could increase or decrease energy use and GHG emissions depending on the conditions of adoption and use.^212,213,214 New mobility services (e.g., ride-hailing or transit services with a monthly subscription) are becoming widespread and have the potential to transform current patterns of travel behavior, but they still face challenges of cost-competitiveness and consumer acceptance.^215,216,217 And as with automation, these mobility services may reduce emissions under a limited set of conditions (e.g., electrification and shared use cases).^{214,218,219,220}

Sector Coupling

The integration of different parts of energy systems, sometimes referred to as sector coupling, involves coordinated planning, operations, and markets for electricity, fuels, and thermal resources to meet end-use service demands. Linking energy industries, processes, and geographies could lower costs, reduce environmental impacts, and increase the reliability of low-carbon energy systems.^76,221,222

Potential Opportunities to Reduce Land-Related Emissions

Modern Foods

Recent innovations aim to increase food choices with plant-based and cultured meat^223,224,225 and foods synthesized chemically without photosynthetic inputs^226,227 that may be able to displace demand for foods with substantially higher emissions per calorie. However, the potential benefits will depend on the scalability and public demand for such products.

Interventions to Reduce Methane and Nitrous Oxide Emissions

There are a number of options for reducing non-CO₂ GHG emissions from agriculture whose potential remains uncertain. Ruminant feed supplements may suppress methane emissions (although some such supplements have not yet been approved for use in the United States).^228,229 Methane from manure lagoons can be captured and used for bioenergy or reduced through flaring.²³⁰ Seasonally flooded rice paddies can undergo temporary drainage to reduce emissions by about 40%.²³¹ And crops may be bred to produce root exudates that inhibit nitrification and thereby reduce N₂O emissions from croplands.²³²

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A number of important dimensions rarely represented in mitigation scenarios may nonetheless determine the pace, feasibility, likelihood, efficacy, and cost-effectiveness of mitigation opportunities.

Air Pollution

Air pollutants that impact human health are often co-emitted with greenhouse gases. Exposure to ambient fine particulate matter (PM_2.5) and ozone, which are among the largest risk factors for disease, causes 60,000–300,000 excess deaths per year in the United States (KM 14.5),^{233,234,235,236,237,238,239} with health effects observed at concentrations below the current national standard.^240,241,242 Racial, ethnic, and socioeconomic disparities in air-pollution exposure are well documented^{235,243,244,245,246,247,248} and have persisted despite overall decreases in air pollution.^235,241

Transitioning to a net-zero-emissions energy system has the potential to generate substantial air pollution benefits. Estimates of cumulative net benefits by 2050 range from roughly 200,000 to 2,000,000 avoided deaths,^{234,249,250,251} the monetized value (i.e., statistical value) of which could exceed the total expected costs of the transition to net zero.^50,252 However, the distribution and magnitude of air pollution benefits over the transition period depend on the pace of electrification, technology selection, and siting decisions,^{50,253,254,255} especially regarding retirement of fossil fuel power plants (Figure 32.15)^{255,256,257,258,259,260,261} and vehicle electrification.^262,263 Electrification of heating,^257,264 reduction in fossil fuel production, electrification of the industrial sector, and shifting diets¹⁰³ can also all generate meaningful air pollution benefits. Carbon capture and hydrogen technologies may also reduce air pollutant emissions, although it is not yet clear by how much.

It is also possible that mitigation efforts could increase air pollution at local and regional scales, for example, due to increases in bioenergy, residential wood heating, and domestic manufacturing to meet demands for materials and products (e.g., Gallagher and Holloway 2020; Commane and Schiferl 2022^253,265).

Siting and Land Use

Net-zero-emissions energy systems may require large land areas, with land requirements in rough proportion to the share of wind and solar energy. Cumulative US land use for solar and wind energy in recent net-zero scenarios ranges from about 250,000 to more than 1 million square kilometers (including the entire area of solar and wind farms),^42,43,53 with solar concentrated in the Northeast and Southeast and wind in the Midwest, Great Plains, and Texas (KM 6.3).⁵⁰ Even at the low end of this range, the projected scale of land use is massive, and may face public opposition. For example, the visual impact and competition for land of such extensive systems would need to gain and maintain the support of many communities; this recently has been a challenge in the siting of solar and wind projects.^266,267 Similar challenges may apply to siting and demonstration of other infrastructure regardless of its land footprint, such as new electricity transmission,²⁶⁶ CCS,²⁶⁸ and CDR.²⁶⁹ Others express concern over the potential environmental impacts of solar and wind farms, including land-cover change, loss of plant and animal habitats, barriers to migration and collision deaths of birds and bats,^270,271,272 and competition for land between agriculture and renewables.²⁷³ Notably, competition with agriculture has also long been a concern about bioenergy, which may be alleviated if demand for corn ethanol decreases due to electrification of transport.^274,275 Researchers have thus begun developing pathways that take some of these concerns and constraints into account,^50,276 as well as identifying changes in governance and administrative law that may help streamline siting processes;²⁷⁷ however, siting may prove a key obstacle for renewables-based net-zero-emissions systems.²⁷⁸ Engagement with community groups and stakeholders early in the planning process has the potential to reduce project delays and cancellations.²⁷⁹

Water Use

The water requirements of net-zero-emissions energy systems could be lower than current consumption,²⁸⁰ largely because wind and solar require little water (Figure 32.16; KM 5.1).^{281,282,283,284} However, some processes for energy conversion and carbon management, such as electrolysis for hydrogen production, chemical synthesis of hydrocarbons (e.g., by the Fischer-Tropsch process), and CCS, are water intensive and could offset water savings from fuel switching. Ultimately, water use (and related quality), temporal, and locational needs, depend heavily on the mix of resources and processes used to achieve net-zero emissions.^285,286,287

Labor

The productivity, supply, and disposition of labor, in addition to national discourse and community-level support and concern regarding labor, has the potential to accelerate or constrain mitigation efforts. Nearly 8 million Americans were directly employed in energy-related jobs in 2021, comprising roughly 5% of the total labor force.^289,290 Of those 8 million energy-related jobs, approximately 41% were in net-zero-emissions-aligned areas in 2022.²⁹¹ Energy-related jobs tend to be geographically concentrated in certain states and communities (Figure 32.17). More than 10% of the labor force in 150 counties (of 3,142) is directly employed in energy-related jobs^289,292,293—often the production of coal, oil, and gas—but employment in mitigation-related activities is growing and is already high in many counties (e.g., energy efficiency in Vermont, wind installations in the Southern Great Plains; KM 26.2).

Reaching net-zero emissions in the United States by 2050 would generate jobs related to manufacturing and deployment of new infrastructure but reduce fossil fuel–related jobs.²⁹⁴ Many analyses find that employment and wage losses in fossil fuel sectors would be entirely offset (in aggregate) by increases in low-carbon resource industries.^{294,295,296,297,298} The number and local distribution of mitigation-related jobs will depend on the ultimate mix of energy sources, siting and investment decisions, labor supply constraints, the extent of domestic manufacturing, and political bargaining; however, decarbonization could lead to long-term expansion in the energy workforce in most states, even when accounting for increased worker productivity (which is often an underlying assumption in technology cost projections). Large-scale and sustained workforce development programs, high-road labor practices and policies, and corresponding federal support could accelerate a transition to net-zero emissions.^294,299

However, there is already evidence of hiring difficulties in energy labor markets,²⁹² portending labor supply bottlenecks in the absence of counteracting policies. Although there is public support for employment benefits related to climate mitigation,²⁹⁵ there is also evidence of mistrust associated with historical energy-related job creation narratives.³⁰⁰ Moreover, there are existing racial and gender disparities in the energy workforces.²⁹²

Meanwhile, despite policy and political discourse regarding just transitions for fossil fuel workers,^295,301,302 many fossil fuel–dependent communities have experienced large declines in employment.^27,303,304 Moreover, former fossil fuel workers often relocate because their skills are not always transferable to other local industries, and nearby communities lose tax revenues that support public infrastructure and social services (KM 26.2).^305,306 Going forward, domestic policies that consider when and where workforces in declining energy industries could fill new jobs in emerging energy sectors (e.g., natural gas and carbon capture supply chains; coal mining; and solar manufacturing) have the potential to moderate labor supply bottlenecks, concentrated unemployment, and low-carbon boom-and-bust cycles. Where there is flexibility in siting of infrastructure and allocation of funding, such funds might also be leveraged to build political support and more equitably distribute costs and benefits. For example, provisions in the Inflation Reduction Act of 2022 offer enhanced tax credits to clean energy projects that pay prevailing wages to workers and use registered apprentices,²⁹¹ that manufacture and source materials domestically,³⁰⁷ and/or that are located in “energy communities” defined by thresholds in the share of fossil fuel–related jobs.³⁰⁸

Energy Equity and Environmental Justice

Social inequities in the United States are rooted in systemic discriminatory practices, such as redlining, that marginalize communities based on race or socioeconomics. Social equity involves several energy- and climate-related aspects of recognition, procedural, and distributional justice (KMs 23.4, 27.3).^310,311 In the context of energy and climate decision-making, recognition justice refers to an understanding that certain individuals and groups are presently bearing, and have historically borne, disparate burdens related to our collective energy systems and may therefore require extra resources or mitigation efforts. Procedural justice considers who is involved and has influence in energy and climate decision-making processes, with the goal of ensuring that those who want to be included in decision-making processes—and especially those who will be affected by the outcomes—are meaningfully engaged through fair and inclusive procedures (see, e.g., KM 30.3 regarding mitigation informed by Indigenous Knowledge). Distributional justice refers to the allocation of benefits and burdens based on geography and sociodemographics, with the objective that no single population receives a disproportionate share of energy or climate harms (e.g., energy-related air pollution; KM 14.3) or benefits (e.g., access to low-carbon and efficient energy technologies or clean-tech jobs).

The disproportionate public health burdens of energy systems on communities of color and/or low-income communities, such as from vehicle emissions and power plants, have been extensively documented (Figure 32.18).^{312,313,314,315} Energy insecurity (e.g., regularly struggling to pay energy bills) also disproportionately affects low-income households, communities of color, rural and Indigenous communities, families with children, and older adults (Ch. 16).^{314,316,317,318,319,320,321} This disproportionate burden of energy insecurity reflects that Black Americans, for example, are more likely to live in older homes that are less energy-efficient.^318,319,322 Moreover, redlined areas often lack trees and green spaces to mitigate the urban heat island effect and thus experience higher summer temperatures than surrounding urban areas,^323,324,325 which in turn increases energy demands and burdens³²⁶ and makes residents more susceptible to the adverse health effects of extreme heat (KM 15.3).^326,327

Although environmental impacts and energy insecurity are not borne proportionately across social groups, it is possible to pursue mitigation options that also redress current and historical injustices. For example, low-income communities and communities of color could experience disproportionate improvements in air pollution.^252,260 Energy equity considerations also include access to sufficient energy services,^328,329 as well as reductions in energy burden or energy poverty,^322,330,331 and the upfront costs of energy efficiency and low-carbon technologies.³³² Mitigation efforts that increase the availability and affordability of energy services (including safe and comfortable temperatures) could improve energy equity outcomes. For example, improving thermal efficiency of buildings would both reduce energy costs and help to maintain safe indoor thermal temperatures in the absence of functional air-conditioning.³³³

Studies have found that low-carbon and efficient technologies (e.g., electric vehicles, solar panels, battery storage, and LED lightbulbs) tend to be disproportionately owned by—and the financial incentives for such are received by—higher-income, more educated, and White households.^{312,313,314,334} Job opportunities in clean energy have also tended to exclude women and people of color.³³⁵ Moreover, insofar as mitigation increases energy costs, more households will experience energy poverty, and energy inequities may get worse.^336,337 In addition, changes in the type, timing, and cost of energy needed to provide safe and comfortable temperatures under climate change and anticipated electrification patterns may exacerbate health risks, financial energy burdens, and other measures of energy equity.^328,336

Supply Chains, Energy Security, and Geopolitics

Climate mitigation efforts may drastically increase domestic and global demand for products (e.g., solar photovoltaics, batteries, electric motors, wind turbines) and metal and mineral resources (e.g., lithium, nickel, cobalt, copper), which may have implications for supply security, markets, advanced manufacturing (e.g., robotics and EVs), geopolitics, and mining (Focus on Risks to Supply Chains).^{338,339,340,341} Moreover, in the United States there are currently 50 listed critical minerals (up from 35 in 2018),^342,343 defined as those essential to economic or national security and whose supply chains are vulnerable to disruption (Figure 32.19). With increased demand as the system decarbonizes, there could be near-term shortages in several minerals and metals. Note that a series of executive orders anticipates this challenge and calls for monitoring and reduction in US dependence on imported critical materials, for example, by increased recycling (e.g., Executive Order 13817, “A Federal Strategy to Ensure Secure and Reliable Supplies of Critical Minerals,”³⁴⁴ and Executive Order 13953, “Addressing the Threat to the Domestic Supply Chain from Reliance on Critical Minerals from Foreign Adversaries and Supporting the Domestic Mining and Processing Industries”³⁴⁵) and more resilient supply chains generally (Executive Order 14017, “America’s Supply Chains”; ⁶ see also Focus on Risks to Supply Chains; KMs 17.2, 18.1).

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A wide range of actors across the US have been involved in efforts to accelerate clean energy transition and mitigate GHG emissions, including new legislation; rules, regulations, and executive orders; and voluntary actions. For example,

• the US has committed under the Paris Agreement to reduce GHG emissions by 50%–52% in 2030 relative to 2005;

• through the Bipartisan Infrastructure Law and the Inflation Reduction Act and relevant programs, there are federal subsidies to clean energy businesses and for household purchases of EVs and heat pumps;³⁴⁷

• 25 states,³⁴⁸ 675 cities, 300 universities, and hundreds of companies have announced net-zero-emissions targets; and

• bottom-up coalitions such as the America Is All In initiative have support from subnational leaders who represent a constituency of more than half the US population (see, e.g., KMs 21.4, 30.3).

Since 2018, the total number of state-level mitigation activities has increased by 83%, and 169 more cities have introduced emissions reduction targets since then (Figure 32.20; see also Ch. 12).³⁴⁹

The pathways toward achieving these goals often include a broad collection of measures and policies, including investments in infrastructure and clean technologies that will require substantial capital, financial backing, and resource allocation. The feasibility and impact of these measures are dependent on local and regional factors, which are often reflected in more granular sector- or economy-specific mitigation targets and actions (see Figure 32.21).³⁵⁰

To this end, nearly 40 states have introduced renewable portfolio standards or voluntary renewable energy goals, which further guide and codify decarbonization efforts within the energy sector and induce incremental shifts toward increased penetration of renewable electricity (KM 32.1). Similarly, more than 30 local governments have enacted requirements for energy efficiency, ranging from building codes and benchmarking ordinances to establishing performance standards (see, e.g., KM 12.3). With federal corporate average fuel economy (CAFE) standards in place for vehicles, local transportation-sector efforts are often focused on behavioral mode-shift goals, such as promoting clean and public transport options and reducing vehicle-miles traveled. The proposed federal Agriculture Resilience Act is designed to address the adaptive needs of US farmers and consumers as a result of a changing climate, as well as to reduce the emissions associated with agricultural production.³⁵¹ In addition, the Securities and Exchange Commission is in the process of finalizing new rules that would require public companies to disclose greenhouse gas emissions related to their operations and supply chains, as well as climate risks to their business.³⁵² Such rules would build on the voluntary reporting and reduction efforts of corporations under the Carbon Disclosure Project; Science Based Targets initiative; and Environmental, Social, and Corporate Governance frameworks and will need to be supported by improved accounting protocols and focused scientific research.^{175,353,354,355,356}

Beyond goal-setting and implementing regulatory measures, the enabling of financial mechanisms is often a core element of mitigation strategy. Regional cap-and-trade programs utilize a system of accountability and performance to incentivize emissions reductions at the electricity-generation level. Meanwhile, federal subsidies, such as those provided to clean energy businesses and tax credits for electric vehicle purchases, can bolster behavior change.⁵⁵ By enabling access to financial capital—whether within the government, commercial, or residential sectors—investments in infrastructure and the built environment, as well as research and development, may further drive these advances.

Available mitigation strategies vary in terms of emissions-reduction potential and costs (Figure 32.22), as well as in environmental, technical, and social implications (Figure 32.23). However, with the advancement of measurement technologies and insights gained from the deployment of various actions taken in vastly different environments, there is now more empirical evidence to inform strategy design for a given community (see regional chapters for examples of state, city, community, and Tribal mitigation actions; e.g., Box 21.1; KM 30.3). Additionally, more jurisdictions are adopting community-driven and holistic approaches to climate action planning, incorporating practices that address equitable access to information (including considerations for languages used and internet access) and events (including transportation vouchers, food and childcare provisions, and payment for subject-matter expertise to community members with lived experience), with a goal of improving and increasing capacity and ability to influence decision-making and, ultimately, assisting elected leaders in making the best-informed and most-impactful decisions for their unique communities.^357,358,359

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