High-ambition climate action in all sectors can achieve 65% greenhouse gas emissions reduction in the United States by 2035

doi:10.21203/rs.3.rs-3796621/v1

Download PDF

Article

High-ambition climate action in all sectors can achieve 65% greenhouse gas emissions reduction in the United States by 2035

https://doi.org/10.21203/rs.3.rs-3796621/v1

This work is licensed under a CC BY 4.0 License

Journal Publication

published 24 Jul, 2024

Read the published version in npj Climate Action →

You are reading this latest preprint version

Under the next cycle of target setting under the Paris Agreement, countries will be updating and submitting new nationally determined contributions (NDCs) over the next two years. To this end, there is a growing need for the United States to assess potential pathways toward a new, maximally ambitious 2035 NDC. In this study, we use an integrated assessment model with state-level detail to model existing policies from both federal and non-federal actors, including the Inflation Reduction Act, Bipartisan Infrastructure Law, and key state policies, across all sectors and gases. Additionally, we develop a high-ambition scenario, which includes feasible, new and enhanced policies from these actors. We find that existing policies can reduce net greenhouse gas (GHG) emissions by 44% (with a range of 37% to 52%) by 2035, relative to 2005 levels, and the high-ambition scenario can deliver net GHG reductions up to 65% (with a range of 59% to 71%) by 2035. This level of reductions would provide a basis for continued progress toward the country’s 2050 net-zero emissions goal.

Scientific community and society/Scientific community/Policy

Scientific community and society/Social sciences/Climate change/Climate-change mitigation

Scientific community and society/Social sciences/Carbon and energy/Energy modelling

The Paris Agreement sets out global goals to reduce greenhouse gas (GHG) emissions in the pursuit of limiting global warming to 1.5°C above pre-industrial levels. To support this goal, each country has committed to communicating its near-term climate actions and targets through regularly updated Nationally Determined Contributions (NDCs). Many countries have also articulated long-term strategies through 2050 towards net-zero emissions to guide and direct near-term planning. Yet, the current 2030 NDCs are insufficient for achieving the long-term Paris Agreement goals, and therefore ratcheting up and accelerating near-term climate policy ambition is necessary to bridge this gap.^1–3 Countries are now undertaking the planning and target-setting phase for the next round of NDCs, which are due in 2025. As national governments assess and formulate new pathways to 2035 and beyond, setting ambitious and achievable 2035 targets will be critical for putting the world on a path toward 1.5°C.

As the world’s largest economy and second-largest emitter, the United States will be a keystone of global success to deliver sufficiently high levels of ambition. Through its current NDC, the country has pledged to reduce emissions by 50–52% below 2005 levels by 2030, with a long-term strategy to reach net-zero by 2050.^4,5 Recent U.S. actions have made substantial contributions toward its climate targets. At the federal level, the historic Inflation Reduction Act (IRA) of 2022 and the Bipartisan Infrastructure Law (BIL) of 2021 are estimated to provide well over $1 trillion for clean energy and carbon storage incentives, transportation infrastructure, methane reduction measures, and more—a collective investment estimated to reduce emissions by 33–40% by 2030.^6–10

At the same time, U.S. states, cities, and other non-federal actors also play an important role in enhancing climate ambition.^11–15 Sub-national governments in particular have been an important force behind the evolution of increasing U.S. climate action. They have done so through, for example, establishing renewable electricity targets, electric vehicle (EV) sales targets, building codes to increase efficiency and electrification, and more.^16–18 Other non-federal actors, such as businesses and non-governmental organizations, have established innovative partnerships and collaboration through coalitions that have also supported climate action (). Even beyond the significant actions by the federal government, the policy authorities of these subnational governments and other non-federal actors collectively are contributing substantially to emissions reductions for 2030.^14,15

While combined actions from federal and non-federal actors have brought the United States closer to meeting its 2030 NDC, a gap of 10–17% emissions reductions still remains.^6–10 Existing studies have assessed specific, additional actions needed to achieve the 2030 NDC, including stronger federal standards for fossil fuel power plants and clean electricity standards, accelerated permitting and siting of electricity transmission and renewables, enhanced incentives and sales targets for EVs, stronger building appliance standards that push the market towards all-electric appliances, and more stringent regulations on oil and gas sector methane.^7,8,19–21 Studies have also covered broader, sectoral trends needed to reach net-zero in 2050, underscoring the importance of decarbonizing the power grid, electrifying end-use sectors, and supporting the development of clean fuels and negative emissions technologies.^22–25 However, little assessment exists that details feasible actions needed between the 2030 NDC and the 2050 net-zero goal.

A critical next step is to determine a high-ambition 2035 NDC that would be in line with the U.S. goal of net-zero by 2050, but also rooted in a plausible and attainable set of policy actions that can be delivered through both federal and non-federal actions. In this study, we examine pathways toward an ambitious and plausible 2035 NDC through policies, measures, and actions from both federal and non-federal actors. We build upon previous work on federal and non-federal climate action to provide an analysis of options that would enable the United States to not only meet its 2030 NDC, but also be on a path by 2035 toward net-zero in 2050.¹⁴ Using an integrated assessment model, combined with analysis to account for state-level climate actions, we first quantify the implications of existing actions, including IRA and major climate policies across all sectors and greenhouse gases. Then, we develop a comprehensive suite of new and expanded policy actions, including extending IRA provisions and ratcheting up non-federal climate targets. We find that a comprehensive policy platform of federal, state, and other policies can deliver reductions in net GHGs of 65% below 2005 levels by 2035.

Assessing impacts of current policies, and additional policies to achieve an ambitious and achievable 2035 NDC

In this study, we model federal and state-level policies and measures that have the potential to reduce GHG emissions, with the assumption that other non-federal actions would be supportive of state-level actions. We project GHG emissions under two scenarios. The Current Policies scenario reflects existing, on-the-books climate actions, including IRA and non-federal actions like renewable portfolio standards (RPS) and electric vehicle (EV) sales targets.

The high-ambition scenario, called the Enhanced Ambition scenario, expands upon Current Policies and includes new potential policies from both federal and non-federal actors that are designed to meet the current 2030 NDC and a 2035 NDC that is on the path to net zero. To ensure feasibility, we based our modeling assumptions on already implemented or proposed climate actions. Enhanced non-federal action scales existing climate policies based on three levels of potential ambition. Tier 1 states follow the most ambitious policies from climate-leading states, such as California’s EV sales targets and zero-emission appliance standards. Tier 2 states lag behind Tier 1 states but still increase their current ambition, and Tier 3 states do not implement major changes.

Table 1 summarizes the modeled policies in both scenarios. A detailed description of these assumptions can be found in Supplementary Information Tables S2-S6.

Table 1

Summary of modeled policies under the *Current Policies* and *Enhanced Ambition* scenarios.
Sector	Current Policies Scenario	Enhanced Ambition Scenario
Electricity	Federal actions: Modeled IRA provisions include the production tax credit (PTC), investment tax credit (ITC) extension, new clean electricity PTC and ITC, residential clean energy credit, PTC for existing nuclear, energy infrastructure reinvestment financing, and the extension of the 45Q tax credits for captured CO₂. Non-federal actions: Current renewable portfolio standards (RPS) are modeled at the state level. The Regional Greenhouse Gas Initiative (RGGI) is modeled as a cap and trade on emissions from power generation in RGGI states.	Federal actions: All modeled IRA provisions from the Current Policies scenario are extended through 2035 and the 45Q tax credit is enhanced. The proposed federal standards under Clean Air Act section 111(b) and 111(d) are included in this scenario. Non-federal actions: The RGGI cap is modeled in the same way as in the Current Policies scenario. RPS targets are enhanced from the Current Policies scenario for Tier 1 and Tier 2 states. Federal & non-federal actions: All unabated coal-fired electricity generation is assumed to be phased out by 2030.
Transportation	Federal actions: Modeled IRA tax credits include the clean vehicle credit, alternative refueling property credit, commercial clean vehicle credit, and extension of incentives for biofuels. BIL funding for light duty vehicle (LDV) and freight truck EV charging infrastructure and funding for school and transit bus electrification are included. Existing CAFE and GHG emissions standards for LDV and freight trucks are modeled. Non-federal actions: Major existing incentives for LDV EVs are modeled at the state level. EV sales mandates for LDVs are modeled to be consistent with California’s Advanced Clean Cars I (ACC I) for five states and consistent with California’s Advanced Clean Cars II (ACC II) for California and 10 other states. For freight truck electrification, California and 12 other states are assumed to achieve sales targets consistent with California’s Advanced Clean Trucks (ACT) legislation.	Federal actions: All modeled IRA tax credits are extended through 2035, while BIL funding for electrification is modeled in the same way as in the Current Policies scenario. Fuel efficiency of LDVs and freight trucks with internal combustion engines are assumed to improve at an accelerated rate beyond what is achieved in the Current Policies scenario. In addition, a “cash-for-clunkers” style program is assumed to accelerate the retirement of older inefficient vehicles. Non-federal actions: Major existing incentives for LDV electrification in the Current Policies scenario are not changed in this scenario. Tier 1 states are assumed to achieve EV sales consistent with California’s ACC II and ACT legislation, with Tier 2 and Tier 3 states achieving ACC II and ACT sales targets on a delayed schedule. Federal & non-federal actions: A combination of federal and non-federal investments and fleet procurement targets lead to 100% electrification of new bus sales by 2030. Federal, state, and local planning are assumed to lead to annual average per capita passenger transportation demand reductions.
Buildings	Federal actions: Modeled IRA provisions include the energy efficient commercial building deduction, energy efficient home improvement credit, energy efficient home credit, home energy efficiency credit, and high efficiency home rebate program. Non-federal actions: Current state-level energy efficiency resource standards (EERS) are modeled.	Federal actions: All modeled IRA provisions from the Current Policies scenario are extended through 2035. Non-federal actions: EERS assumptions are strengthened from the Current Policies scenario. Zero-emissions appliance standards are modeled by driving space heating and water heating appliance sales to 100% electric by 2030 in Tier 1 states and 2035 in Tier 2 states.
Industry & others	Federal actions: Modeled RA provisions include the 45Q tax credits for captured CO₂, PTC for clean hydrogen, manufacturing investment tax credit for advanced energy projects, advanced industrial facilities deployment program, and methane emissions reduction program. Phasedown of HFC emissions is modeled to be consistent with the American Innovation and Manufacturing (AIM) Act. Non-federal actions: No explicitly modeled policies. Federal & non-federal actions: Implementation of state and federal agriculture and forestry policies, including relevant IRA and BIL provisions, leads to sequestration from land-use, land-use change, and forestry (LULUCF) of -858 MtCO₂.	Federal actions: The PTC for clean hydrogen and manufacturing investment tax credit for advanced energy projects are unchanged from the Current Policies scenario. Additional industrial CCS is achieved across cement, biofuels, and pulp and paper production. For the advanced industrial facilities deployment program, it is assumed that new coal is not used as a fuel across all industrial sectors, in addition to assumptions in the Current Policies scenario. The methane fee from the IRA’s methane emissions reduction program is assumed to become an economy-wide methane fee. Non-federal actions: In addition to HFC reductions from the AIM Act in the Current Policies scenario, Tier 1 states achieve additional reduction through measures such as the Significant New Alternatives Policy and Refrigerant Management Programs. Federal & non-federal actions: LULUCF sequestration reaches − 926 MtCO₂ from a combination of $160 billion in investments in climate-smart resulting from enhanced state-level action. Direct air capture (DAC) removes 31 MtCO₂ annually by 2035, consistent with the potential removals from announced DAC facilities in the United States.

Our results show that the Current Policies scenario delivers a 40% reduction in GHG emissions in 2030 and a 44% reduction in 2035, relative to 2005 levels (Fig. 1). This is in line with projections from other studies (SI Fig. 2). Between 2020 and 2030, the average rate of emissions reduction is 123 MtCO₂e/year. The rate between 2030 and 2035 is around half that at 60 MtCO₂e/year, as existing policies begin to expire or impacts otherwise taper off.

With expanded and new actions from both the federal government and non-federal actors, the Enhanced Ambition scenario delivers 53% (52.6%) in emissions reductions by 2030, at the upper end of the current U.S. NDC range. By 2035, reductions reach 65% (Fig. 1). The average rate of emissions reduction from 2020 to 2030 is 208 MtCO₂e/year, and 167 MtCO₂e/year between 2030 and 2035.

To better capture the potential range of emissions reductions, we also varied assumptions about future GDP and population pathways, technological change, fossil fuel prices, and the size of the land sink in each scenario. These sensitivities suggest that the Current Policies reductions could range from 37–52% by 2035, while the Enhanced Ambition reductions could be as low as 59% or as high as 71% (see “Methods” for further discussion).

The 2030 NDC of 50–52% GHG reduction (relative to 2005) represents a significant increase in the pace of emissions reduction relative to the previous target of 26–28% reduction by 2025. The United States needs to reduce its emissions at an average rate of 2.4% of 2005 emissions per year from 2021 to 2025 to meet its 2025 NDC. This rate doubles to 4.8% per year from 2026 to 2030 to meet its subsequent 2030 NDC. The 2030 NDC gets the country back on track for a more steady progression to net-zero; a linear pathway from 50% emissions reductions in 2030 to net zero in 2050 would require an average rate of decline of 2.5% of 2005 emissions per year (achieving 62.5% GHG reduction in 2035, relative to 2005). However, it’s broadly agreed that as economies close in on net zero, decarbonization becomes more challenging due to the remaining emissions from hard-to-decarbonize sectors, including high temperature heat applications in industry, aviation, shipping,^25,27,28 and non-CO₂ emissions such as nitrous oxide (N₂O) from agriculture and methane (CH₄) from livestock.^29–32 Thus, constant ambition may require faster rates of decarbonization in the near-term. Our Enhanced Ambition suite of policies results in emission reductions of 3.0% per year (relative to 2005) from 2030 to 2035, demonstrating that faster reductions are possible through the first half of next decade.

Figure 2 shows the breakdown of sectoral emissions reductions needed to achieve the 2035 reductions in the Enhanced Ambition scenario, starting from 2020 emissions levels. Formally, it is expected that any 2035 target in a new U.S. NDC would be based on 2005 emissions levels, similar to the 2025 and 2030 targets from previous rounds. However, to provide better context on the magnitude of change needed between today and 2035, we assess sectoral emissions reductions and other metrics relative to 2020 levels. The electricity sector has the largest emissions reductions at 1,370 MtCO₂e between 2020 and 2035, contributing to 47% of the overall reductions in this period. As the sector with the second largest reductions, the transportation sector contributes to 26% of the overall reductions at 766 CO₂e. Buildings, industry and methane sectors contribute to 20% of the overall reductions at 207 MtCO₂e, 132 MtCO₂e, and 237 MtCO₂e respectively, while the “other” sector, which includes direct air capture (DAC), land-use, land-use change, and forestry (LULUCF) CO₂, other CO₂, nitrous oxide (N₂O), and fluorinated gases, contribute the remaining 7%.

Importantly, we find that the Enhanced Ambition scenario requires expanded mitigation across all sectors and greenhouse gases, with expanded emphasis in sectors like buildings, industry and methane, which are not focal points of emissions reductions in the Current Policies scenario. Also, continued action in the electricity and transportation sectors will be needed as these will continue to deliver the bulk of overall reductions (Fig. 2).

Decarbonizing the electricity grid while addressing increased demand from end-use sectors

Notably, the electricity sector contributes the largest emissions reductions in 2035 under both scenarios. Under Enhanced Ambition, strengthened regulations on coal- and gas-fired power plants reduce unabated fossil fuel generation, while extended IRA tax credits and enhanced state and local renewable electricity targets drive up the share of electricity from clean technologies. These measures achieve a 76% emissions reduction from 2020 levels by 2030 (1,105 MtCO₂e), declining to a 94% reduction by 2035 (1,370 MtCO₂e) (Fig. 2). Even though coal power is phased out by 2030 in this scenario, the 265 MtCO₂e needed between 2030 and 2035 signifies the importance of reducing emissions from unabated natural gas-fired power plants.

In Current Policies, the power sector achieves a 54% emission reduction by 2030 (786 MtCO₂e). However, little progress is made between 2030 and 2035—only a 3 percentage point increase—as IRA tax credits expire and we assume no additional enhancement of renewable electricity targets and fossil fuel power plant regulations.

Total electricity demand increases by 41% from 3,739 TWh in 2020 to 5,301 TWh by 2035 under Enhanced Ambition, compared to 5,153 TWh in 2035 under Current Policies (Fig. 3). There is not a dramatic difference in electricity demand between scenarios due to additional efficiency measures in end-use sectors under Enhanced Ambition. Still, to match the rapid electrification in end-use sectors, very large expansion of renewables and the use of fossil with CCS will be necessary.

Under Enhanced Ambition, solar generation increases by seven fold and wind generation increases by four fold from 2020 to 2035, with wind making up a slight majority of renewable generation. Solar generation increases by 803 TWh from 2020 levels in 2030 and by 1,273 TWh in 2035, while wind generation increases by 1,098 TWh in 2030 and 1,625 TWh in 2035. For comparison, the highest decadal rates of coal and gas expansion are 575 TWh (1978–1998) and 639 TWh (2009–2019), respectively.³³

The share of gas with CCS is less than 1% of the generation mix in 2025 but rises to 18% by 2035. Meanwhile, unabated coal-fired power plants retire by 2030, and unabated gas-fired generation falls by 87% from 2020 levels in 2035, with a small number of peaker gas plants and gas back-up generators remaining to stabilize the power grid. Correspondingly, Enhanced Ambition achieves a generation mix that is 96% powered by clean technologies (Fig. 3). Clean sources include solar, wind, geothermal, nuclear, hydropower, and biomass and CCS technologies.

Rapid electrification of passenger and freight vehicles

The transportation sector is the second largest source of emissions reductions in both scenarios, with electrification of road vehicles being the primary driver of these reductions. Transportation emissions under Enhanced Ambition decline by 48% below 2020 levels in 2035 (766 MtCO₂e), compared to 33% under Current Policies (519 MtCO₂e) (Fig. 2).

A combination of EV tax credits from IRA, EV charging infrastructure investments from BIL, Corporate Average Fuel Economy (CAFE) standards, and state-level incentives and mandates accelerates road transport electrification in Enhanced Ambition. Electric light duty vehicle (LDV) sales come within a few percentage points of the current administration’s target of 50% EV sales by 2030, and reach 83% by 2035, in contrast to 45% by 2035 under Current Policies (Fig. 4). Freight trucks reach 42% EV sales by 2035 under Enhanced Ambition, which more than doubles the sales under Current Policies (Fig. 4).

However, electric sales will take time to penetrate into the existing fleet. Under Enhanced Ambition, national LDV stock is only 42% electrified by 2035, which is about half the share of EV sales in that year. Across states, the electric share of LDV stock ranges from 25–57% in 2035, with the majority of states achieving 44% or higher (Fig. 5). Under Current Policies, the range is 18–54%, though most of the states achieve less than 25% (Fig. 5).

Beyond electrification, reducing travel demand is another key lever to help meet decarbonization goals.³⁴ As a result of state-level policies that target vehicle miles traveled (VMT) reductions, which aim to reduce single-occupancy vehicle use and increase the use of sustainable modes of transportation, overall passenger demand is lowered under Enhanced Ambition. Road passenger service is assumed to increase only by 9% from 2020 levels by 2035 under Enhanced Ambition, compared to 28% under Current Policies.

The role of the building and industry sectors

In the buildings sector, IRA electrification and efficiency incentives paired with state-level appliance standards and efficiency standards result in emissions reductions of 38% between 2020 and 2035 (207 MtCO₂e) under Enhanced Ambition, compared to 16% (85 MtCO₂e) under Current Policies (Fig. 2). The final energy share of electricity increases from 54% in 2020 to 66% by 2035, compared to 59% under Current Policies. Electrification occurs even more rapidly for hot water and space heating appliances specifically: the electricity share for these appliances increases from 21% in 2020 to 40% by 2035, compared to 29% under Current Policies (Fig. 6).

The industrial sector has fewer near-term decarbonization opportunities compared to the other sectors, though it is still an important part of reaching net zero. IRA investments in this sector increase electrification while also accelerating the retirement of older, inefficient fossil fuels, resulting in electricity shares of around 20% in both scenarios by 2035. However, Enhanced Ambition assumes no new coal across all industrial sectors, as well as high-ambition industrial carbon capture policies that result in 77 MtCO₂ sequestered from cement, ethanol, and paper pulp CCS technologies. In comparison, Current Policies has only 5 MtCO₂ of sequestration from cement and ethanol CCS technologies (Fig. 6). Correspondingly, Enhanced Ambition sees emissions reduce by 12% between 2020 and 2035 (132 MtCO₂e), compared to a growth of 4% (43 MtCO₂e) under Current Policies (Fig. 2).

Setting the stage for 2050 with industrial decarbonization, carbon removal technologies, and land sector policies

Methane also plays a critical role in near-term emissions reductions, as a potent greenhouse gas with a short lifetime. Comprehensive methane mitigation can significantly slow near-term warming and ease the burden on CO₂ to achieve global climate targets.^29,30 With a methane fee that covers all sources, including agriculture and waste, to incentivize emission reduction measures, Enhanced Ambition’s methane emissions decrease by 29% (237 MtCO₂e) between 2020 and 2035, in contrast to 5% (41 MtCO₂e) under Current Policies.

Negative emissions through both natural and technological carbon removal also play a role in 2035, and will be increasingly important over the longer term for meeting net zero, as they help to balance the hard-to-abate emissions sources.³⁵ Historically, the LULUCF sink has remained stable or growing, but there is the possibility of it shrinking in the future if policies related to forest regrowth, forest management, and fire mitigation are not put into place, as well as uncertainties in the impact of climate change itself.^36,37. As a result of full implementation of BIL and IRA funds, and more ambitious state-level policies related to the lands sector under Enhanced Ambition, LULUCF sequestration increases by 73 MtCO₂ from 2020 to 2035, resulting in a sink of -926 MtCO₂ in 2035. In contrast, LULUCF sequestration increases by only 29 MtCO₂ under Current Policies, reaching a sink of -882 MtCO₂ in 2035.

Additionally, direct air capture (DAC) of CO₂ is introduced as a new technology after 2030 in the Enhanced Ambition scenario. As a result of IRA tax credits, which allow up to $180/tCO₂ captured for storage from DAC, and state-level initiatives, DAC sequesters 31 MtCO₂ in 2035.

Key actions and challenges for 2035

The period of 2030 to 2035 represents a transition period in the path to net zero in the United States. Broadly, the bulk of reductions in the 2020’s are expected to come from continued deployment of low-cost renewables and improved efficiency in the power sector, combined with increasing levels of reductions in transportation emissions through EV deployment. This analysis shows that these trends continue in the period from 2030 to 2035, with rapid and large decreases in the power and transportation sectors. However, there is a critical change from previous NDCs: high-ambition pathways will require the addition of substantial reductions across other sectors of the economy. Electrification and efficiency improvements in the buildings and industry sectors, sequestration from CCS and DAC technologies, additional methane mitigation – particularly for fossil fuels and agriculture – and enhancing the land sink will all be part of achieving a pathway on track to net zero in 2050. High levels of reductions in 2035 will therefore require acceleration of policy actions in these areas in the near term.

In the electricity sector, the Enhanced Ambition scenario includes continued, rapid reductions in unabated fossil power, with all coal-fired power plants either retired or retrofitted with CCS by 2030, and all large, frequently used natural gas-fired power plants retired or retrofitted with CCS by 2035.

Enhanced Ambition does not explicitly specify a clean electricity target; rather, our focus is on feasible, bottom-up actions, which result in some infrequently used gas peaker plants without CCS still in operation by 2035. It is widely agreed upon that costs increase dramatically as power systems approach 100% carbon-free due to the challenge of meeting peak demand, and achieving this will likely entail a combination of different strategies as well as a better understanding of trade-offs between technologies.²⁷ Nevertheless, despite growing demand from rapidly electrifying end-use sectors, electricity sector emissions under Enhanced Ambition achieve a 96% reduction in CO₂ intensity per unit of electricity generation.

Our results show that this level of fossil fuel phaseout is possible through the full implementation of IRA’s Energy Infrastructure Reinvestment provision, which provides up to $250 billion to transition away from fossil fuels, finalization and enforcement of proposed EPA regulations to equip new and existing coal- and natural gas-fired power plants with CCS technologies, and rapid expansion of non-federal renewable energy goals. However, updating and expanding existing transmission and distribution systems will also be needed to better support renewable and clean energy integration, which currently has 1 TW of zero-carbon generation capacity currently stuck in the interconnection queue.³⁸ Additionally, in response to growing demand and the increase in variable renewable energy sources, it will be critical to maintain grid reliability through enhanced grid management practices and investment in storage technologies.³⁹

The rapid electrification of end use sectors will also pose critical opportunities and challenges for 2035. In the transportation sector, key new policies include the widespread adoption and implementation of California’s regulations for passenger car and freight truck EV sales, the early retirement of old, inefficient vehicles, and expanded incentives that lower the costs of EVs. Though not explicitly modeled in our study, this level of EV adoption entails additional policies in the forms of commitments from automakers, municipal fleet targets, subsidies for low- and middle- income consumers, and additional investments in the buildout of sufficient EV charging infrastructure. Scaling the electrification of freight trucks will be especially challenging, given the limited supply of electric trucks at present, the charging needs across state borders, and high associated costs, especially for long-haul trucks.⁴⁰ Additionally, some uncertainties remain about the capacity of vehicle supply chains to meet these goals, as many materials necessary for EV production have been classified as critical or near-critical supply risks, such as lithium, cobalt, nickel, and platinum.⁴¹ Moreover, the extraction process for critical minerals is associated with substantial environmental justice concerns.⁴²

Across all sectors, the extension of IRA provisions beyond their legislated sunset dates will be another important element for 2035 reductions. In the year since its passage, IRA has accelerated renewable energy deployment, leading to the announcement of 272 new clean energy projects in EV, wind and solar, and battery manufacturing, totaling over $270 in new investments.⁴³

This level of clean energy investment in the United States is unprecedented, surpassing eight years’ worth of historical clean energy investments.⁴⁴ Beyond clean energy technologies, IRA also allocates over $20 billion in funds for various land sector initiatives and introduces a substantial fee for oil and gas methane, which would apply to facilities not covered by the EPA methane regulations. By lowering the costs of clean energy, IRA investments can motivate both federal and non-federal actors to seek more ambitious climate action. For instance, the recent proposed standards from EPA address technologies that receive subsidies from IRA, including CCS and hydrogen.⁴⁵ This level of momentum and innovation across all levels of society could stall if IRA’s provisions are left to expire as written.

This analysis assumes a full implementation of IRA climate and energy provisions. However, the effectiveness of some provisions are dependent on the uptake by state, local and individual actors. Lack of outreach and education may lead to missed opportunities in taking advantage of the full tax credit for provisions such as residential building rebates. Furthermore, it is also possible that some policies could be watered down upon finalization. IRA’s clean vehicle tax credit, for example, may not deliver the full scale of emissions reductions as assumed in this analysis if automakers face challenges meeting the eligibility requirements on mineral sourcing and battery assembly for this tax credit.

Modeling limitations and further considerations

The model used for our research, GCAM-USA-CGS, has many features that make it well-suited for this type of analysis. These include its ability to offer full coverage of all emitting sectors across the economy, beyond standard energy sector CO₂-only studies, as well as its ability to link the U.S. states with regions in the rest of the world. Additionally, the model includes dynamic feedbacks across sectors, a rich technological representation, and emissions, energy, and trade details for 50 states. Even so, our analysis has some limitations. The model has difficulty capturing the uncertainty in building out transmission and distribution infrastructure– which other studies have shown to be critical in power sector decarbonization.²¹ Additionally, though our analysis shows a key role for CCS technologies towards 2035, there is still considerable uncertainty as to the viability of these technologies at scale.⁴⁶

Charting a plausible, high-ambition emissions reduction pathway is the focus of this analysis. However, if the United States is to be successful in this endeavor, reducing emissions is only one piece of the puzzle. Continued research inclusive of a wide range of social perspectives, including a better understanding of distributional impacts, is vital to ensure that environmental justice concerns are addressed through supportive and complementary policies. Our results highlight the need for an accelerated buildout of renewable and low-carbon infrastructure, yet there are restrictive zoning and siting ordinances in many communities that may stall such efforts.⁴⁷ Furthermore, the rapid electrification required in end-use sectors would have implications for consumer cost, which may disproportionately impact marginalized communities and those communities dependent on GHG -intensive facilities. Thus, there is a need for research focused on identifying social issues associated with this kind of economy-wide transition, and working with local, state and federal governments to propose solutions so that certain communities do not bear the brunt of the burden.

Despite these limitations, our analysis provides valuable information on the magnitude and variety of actions needed to set a U.S. 2035 NDC target that will be both sufficiently ambitious and, though likely challenging, also plausible to achieve. There are many positive trends, particularly in the wake of IRA’s historic investment in climate mitigation, that point to an expanding potential for new action. Yet there are also risks in the broader economy and political situation. In this context, there is a key role for a broad suite of actors, including federal, state, and local governments, to continue to build new policies that are durable over the long term to ensure that the United States stays on a 1.5°C-compatible pathway. While no strategy can generate absolute certainty, embedding ambitious climate action at multiple levels of government can support a more effective and durable climate strategy, and can mitigate potential political backsliding if future elections return less climate-friendly governments. Moreover, a U.S. commitment to an appropriately ambitious 2035 NDC would also encourage other countries to ratchet up their own ambition in their NDCs, helping curb overall global emissions to keep the goals of the Paris Agreement within reach.

Overview of Modeling Approach

Policy representation in our modeled scenarios is built upon bottom-up aggregation tools and data analysis to evaluate and quantify the impacts of state-level policies and climate actions in isolation and within specific sectors. We then used this information with policy levers at the national level in GCAM-USA-CGS to estimate the economy-wide implications of these associated policies. The overall modeling approach used was consistent with previous analysis, including Accelerating America’s Pledge (2019), Hultman et al. (2020), An All-In Climate Strategy Can Cut U.S. Emissions by 50% by 2030 (2021), Blueprint 2030 (2021), and Beyond 50: An All-In Pathway to 2030 (2023).^7,48–51 GHG emissions in this work are reported in terms of CO₂ equivalents using IPCC AR5 100-year GWP values, consistent with current (2023) UNFCCC reporting guidance.⁵² See Supplementary Information for more detailed information on our approach.

GCAM-USA-CGS is based on the open-source release of GCAM-USA 6.0.¹ GCAM-USA-CGS has been updated for the purposes of this study, for example, to reflect the latest renewable energy costs.⁵³ It is also calibrated to the latest non-CO₂ marginal abatement cost curves from the U.S. Environmental Protection Agency.⁵⁴ GCAM-USA-CGS tracks emissions of a range of GHGs and air pollutants from energy, agriculture, land use, and other systems. The energy system formulation in GCAM-USA-CGS consists of detailed representations of depletable primary sources such as coal, gas, oil, and uranium, in addition to renewable resources such as bioenergy, solar, wind, and geothermal. GCAM-USA also includes representations of the processes that transform these resources into final energy carriers, such as oil refining and electric power. These energy carriers, in turn, are used to deliver services to end users in the buildings, transportation, and industrial sectors. The electric power sector includes representations of a range of power generation technologies, including those fueled by fossil fuels, renewables, bioenergy, and nuclear power.

All modeled policies in GCAM-USA-CGS are implemented at the state and/or national levels. Policies and actions from city governments, businesses, and institutions are assumed to be embedded within or supportive of the state and/or national level policy representation in the model, and are therefore not explicitly modeled. Regions in the rest of the world are assumed to roughly follow emissions pathways consistent with their announced NDCs.

Model parameters in GCAM-USA-CGS were varied according to information from our bottom-up aggregation analysis or changed directly for policy drivers where bottom-up aggregation was either not feasible or not necessary in the case of small-scale potential impacts. The purpose of this analysis is to assess the national emissions reduction potential in the United States for the policies modeled in our scenarios. Accordingly, non-federal policies and actions that are only modeled to the extent that doing so would have a meaningful impact on the national-level emissions outcome. In some policy areas, we did not specify state-level variation in policy implementation – for example, with bus electrification – as assessment of the national-level emissions impact did not require state-level precision.

Modeled scenarios

The Current Policies scenario includes existing federal policies – including several climate and energy provisions from IRA and BIL – and non-federal policies. Existing state-level RPS, CAFE standards, and several IRA tax credits for renewable electricity generation, EVs, and CCS are among the key policy drivers in this scenario.

The Enhanced Ambition scenario models GHG emissions reductions achievable under a comprehensive climate strategy that builds upon the policy framework in the Current Policies scenario with enhanced non-federal and federal action. Some of the additional policy actions modeled in this scenario include the proposed federal regulations for fossil fuel power plants, accelerated adoption of EVs in light-duty vehicle, bus, and freight truck markets, enhanced electric appliance standards, more stringent standards on oil and gas methane emissions, as well as extensions of the existing IRA tax credits beyond their legislated sunset dates.

A summary of modeled policies for each sector in both scenarios is included in Table 1, and detailed modeling assumptions underlying implementation of all policy representations in GCAM-USA-CGS are listed in Supplementary Information Tables S2-S6.

We also assessed emissions projections from the two scenarios by varying assumptions on a few important drivers, including GDP, population growth, oil and gas prices, solar and wind costs, and the land sink carbon sequestration potential. Although these drivers do not capture the full range of potential drivers, they do provide a reasonable range of emissions projections. Please see Supplementary Information Table S7 for a description of our assumptions in the sensitivity analysis.

Further details on the methodological approach and policy assumptions used in this study are available in the Supplementary Information accompanying this article.

Code availability

GCAM is an open source community model available at http://github.com/JGCRI/gcam-core/releases

(DOI: 10.5281/zenodo.5093192). Input files used for this study are available at DOI:

10.5281/zenodo.10425458, and detailed model results can be obtained running this input file set

using the release version of GCAM 6.0 available at the above web site. Results presented in this

paper may differ in some minor aspects from those obtained using the release version of the

model. Additionally, as noted in Supplementary Information Section 3.9, GHG emissions results were

post processed to harmonize with the EPA inventory and will differ from raw model results.

[Note to reviewers: The input data files will be posted to Zenodo upon publication, but has been

sent to the journal editor to provide to reviewers.]

Data availability

Comprehensive output data summaries are provided as a data supplement to this journal paper.

Acknowledgements

This work was supported by America Is All In and Bloomberg Philanthropies. K.T.V.O was partly supported by funding from the Pierre Elliott Trudeau Foundation and the Social Sciences and Humanities Research Council. The funders played no role in study design, data collection, analysis and interpretation of data, or the writing of this manuscript. The authors would like to thank Kevin Kennedy, Sujata Rajpurohit, John Feldmann, Kyle Clarke-Sutton, Chris Wade, Alice Favero, Xinyuan Huang, Alexandra Kreis, Shawn Edelstein, and Michael Westphal for their valuable inputs to the research. The computer simulations in this research were conducted using the University of Maryland high-performance computing cluster, Zaratan (http://hpcc.umd.edu).

Competing interests

All authors declare no financial or non-financial competing interests.

Contributions

A.Z., H.M., and N.H. conceptualized the study. A.Z., K.T.V.O., M.B., A.B., C.S., and M.Z. conducted the data collection and modeling for this project. A.Z., A.B., and M.Z. contributed to the visualizations. All authors contributed to the drafting and editing of the manuscript.

Calvin, K. et al. IPCC, 2023: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland. doi:10.59327/IPCC/AR6-9789291691647.
Elzen, M. den et al. Are the G20 economies making enough progress to meet their NDC targets? Energy Policy 126, 238–250 (2019).
Elzen, M. G. J. den et al. Updated nationally determined contributions collectively raise ambition levels but need strengthening further to keep Paris goals within reach. Mitig. Adapt. Strateg. Glob. Change 27, 33 (2022).
United States of America. Nationally Determined Contribution. (2021).
US Department of State & US Executive Office of the President. The Long-Term Strategy of the United States, Pathways to Net-Zero Greenhouse Gas Emissions by 2050. https://www.whitehouse.gov/wp-content/uploads/2021/10/US-Long-Term-Strategy.pdf (2021).
Bistline, J. et al. Emissions and energy impacts of the Inflation Reduction Act. Science 380, 1324–1327 (2023).
Zhao, A. et al. An All-In Pathway to 2030: The Beyond 50 Scenario. https://www.americaisallin.com/Beyond50 (2022).
King, B. et al. Pathways to Paris: Post-IRA Policy Action to Drive US Decarbonization. https://rhg.com/research/ira-us-climate-policy-2030/ (2023).
Mahajan, M., Ashmoore, O., Rissman, J., Orvis, R. & Gopal, A. Updated Inflation Reduction Act Modeling Using the Energy Policy Simulator. https://energyinnovation.org/publication/updated-inflation-reduction-act-modeling-using-the-energy-policy-simulator/ (2022).
Jenkins, J. D. et al. Climate Progress and the 117th Congress: The Impacts of the Inflation Reduction Act and Infrastructure Investment and Jobs Act. doi: 10.5281/zenodo.8087805 (2023).
Rayner, S. How to eat an elephant: a bottom-up approach to climate policy. Clim. Policy (2011) doi:10.3763/cpol.2010.0138.
Hale, T. “All Hands on Deck”: The Paris Agreement and Nonstate Climate Action. Glob. Environ. Polit. 16, 12–22 (2016).
Hsu, A., Brandt, J., Widerberg, O., Chan, S. & Weinfurter, A. Exploring links between national climate strategies and non-state and subnational climate action in nationally determined contributions (NDCs). Clim. Policy 20, 443–457 (2020).
Hultman, N. E. et al. Fusing subnational with national climate action is central to decarbonization: the case of the United States. Nat. Commun. 11, 5255 (2020).
Kuramochi, T. et al. Beyond national climate action: the impact of region, city, and business commitments on global greenhouse gas emissions. Clim. Policy 20, 275–291 (2020).
ZEV Transition Council. Phase-out targets: LDV. https://zevtc.org/tracking-progress/light-duty-vehicle-map/ (2023).
American Council for an Energy-Efficient Economy. Renewable Energy Efforts. ACEEE.org https://database.aceee.org/city/renewable-energy-efforts (2021).
American Council for an Energy-Efficient Economy. Energy Efficiency Resource Standards. https://database.aceee.org/state/energy-efficiency-resource-standards.
Orvis, R. et al. Closing The Emissions Gap Between The IRA And 2030 NDC: Policies To Meet The Moment. https://energyinnovation.org/publication/closing-the-emissions-gap-between-the-ira-and-ndc-policies-to-meet-the-moment/ (2022).
Bistline, J. et al. Actions for reducing US emissions at least 50% by 2030. Science 376, 922–924 (2022).
Jenkins, J. D., Farbes, J., Jones, R., Patankar, N. & Schivley, G. Electricity Transmission is Key to Unlock the Full Potential of the Inflation Reduction Act. https://zenodo.org/records/7106176 (2022) doi:10.5281/ZENODO.7106176.
Williams, J. H. et al. Carbon-Neutral Pathways for the United States. AGU Adv. 2, 25 (2021).
Horowitz, R. et al. The energy system transformation needed to achieve the US long-term strategy. Joule 6, 1357–1362 (2022).
Browning, M. et al. Net-zero CO2 by 2050 scenarios for the United States in the Energy Modeling Forum 37 study. Energy Clim. Change 4, (2023).
Davis, S. J. et al. Net-zero emissions energy systems. Science 360, (2018).
US Environmental Protection Agency. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2021. Greenhouse Gas Emissions https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks-1990-2021 (2023).
Mai, T. et al. Getting to 100%: Six strategies for the challenging last 10%. Joule 6, 1981–1994 (2022).
Cunliff, C. An Innovation Agenda for Hard-to-Decarbonize Energy Sectors. Issues Sci. Technol. 36, 74–79 (2019).
Ou, Y. et al. Deep mitigation of CO2 and non-CO2 greenhouse gases toward 1.5 °C and 2 °C futures. Nat. Commun. 12, 6245 (2021).
Harmsen, M. et al. The role of methane in future climate strategies: mitigation potentials and climate impacts. Clim. Change 163, 1409–1425 (2020).
Gernaat, D. E. H. J. et al. Understanding the contribution of non-carbon dioxide gases in deep mitigation scenarios. Glob. Environ. Change 33, 142–153 (2015).
Harmsen, M. et al. Uncertainty in non-CO2 greenhouse gas mitigation contributes to ambiguity in global climate policy feasibility. Nat. Commun. 14, 2949 (2023).
US Energy Information Administration. Table 7.2B Electricity Net Generation: Electric Power Sector. Total Energy https://www.eia.gov/totalenergy/data/browser/index.php?tbl=T07.02B#/?f=A&start=1949&end=2022&charted=1-2-3-5-8-14-13 (2023).
Alarfaj, A. F., Griffin, W. M. & Samaras, C. Decarbonizing US passenger vehicle transport under electrification and automation uncertainty has a travel budget. Environ. Res. Lett. 15, 0940c2 (2020).
Fuss, S. et al. Negative emissions—Part 2: Costs, potentials and side effects. Environ. Res. Lett. 13, 063002 (2018).
Anderegg, W. R. L. et al. Future climate risks from stress, insects and fire across US forests. Ecol. Lett. 25, 1510–1520 (2022).
Wang, J. A., Baccini, A., Farina, M., Randerson, J. T. & Friedl, M. A. Disturbance suppresses the aboveground carbon sink in North American boreal forests. Nat. Clim. Change 11, 435–441 (2021).
Rand, J. et al. Queued Up: Characteristics of Power Plants Seeking Transmission Interconnection. https://emp.lbl.gov/queues (2023).
Harker Steele, A. J., Burnett, J. W. & Bergstrom, J. C. The impact of variable renewable energy resources on power system reliability. Energy Policy 151, 111947 (2021).
Fleming, K. L., Brown, A. L., Fulton, L. & Miller, M. Electrification of Medium- and Heavy-Duty Ground Transportation: Status Report. Curr. Sustain. Energy Rep. 8, 180–188 (2021).
Office of Energy Efficiency & Renewable Energy. U.S. Department of Energy Issues Request for Information to Provide Feedback on Critical Materials Assessment. Energy.gov https://www.energy.gov/eere/articles/us-department-energy-issues-request-information-provide-feedback-critical-materials (2023).
Ballinger, B. et al. The vulnerability of electric vehicle deployment to critical mineral supply. Appl. Energy 255, 113844 (2019).
Climate Power. One Year of our Clean Energy Boom. https://climatepower.us/wp-content/uploads/sites/23/2023/07/Clean-Energy-Boom-Anniversary-Report-1.pdf (2023).
American Clean Power. Clean Energy Investing in America Report. https://cleanpower.org/resources/clean-energy-investing-in-america-report/ (2023).
US Environmental Protection Agency. EPA Proposes New Carbon Pollution Standards for Fossil Fuel-Fired Power Plants to Tackle the Climate Crisis and Protect Public Health. https://www.epa.gov/newsreleases/epa-proposes-new-carbon-pollution-standards-fossil-fuel-fired-power-plants-tackle (2023).
Budinis, S., Krevor, S., Dowell, N. M., Brandon, N. & Hawkes, A. An assessment of CCS costs, barriers and potential. Energy Strategy Rev. 22, 61–81 (2018).
Lopez, A. et al. Impact of siting ordinances on land availability for wind and solar development. Nat. Energy 8, 1034–1043 (2023).
Hultman, N. et al. Accelerating America’s Pledge: Technical Appendix. The America’s Pledge Initiative on Climate Change. 62 (2019).
Hultman, N. E. et al. Fusing subnational with national climate action is central to decarbonization: the case of the United States. Nat. Commun. 11, 5255 (2020).
Kennedy, K. et al. Blueprint 2030: An All-In Climate Strategy for Faster, More Durable Emissions Reductions. https://www.americaisallin.com/blueprint-2030 (2022).
Hultman, N. et al. An All-In climate strategy can cut U.S. emissions by 50% by 2030. https://cgs.umd.edu/research-impact/publications/all-climate-strategy-can-cut-us-emissions-50-2030 (2021).
Stocker, T. F. et al. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. 1535 https://www.ipcc.ch/report/ar5/wg1/ (2013).
National Renewable Energy Laboratory. 2022 Annual Technology Baseline. https://atb.nrel.gov/electricity/2022/index (2022).
US Environmental Protection Agency. Global Non-CO2 Greenhouse Gas Emission Projections & Mitigation Potential: 2015-2050. https://www.epa.gov/global-mitigation-non-co2-greenhouse-gases/global-non-co2-greenhouse-gas-emission-projections (2019).

(Not answered)

StateInputs.xlsx
Dataset 1: State-level inputs
Output.xlsx
Dataset 2: Model outputs
inputfiles.zip
Dataset 3: Model Input Files
SupplementaryInformationnpj12.22.23.docx

Download PDF

Journal Publication

published 24 Jul, 2024

Read the published version in npj Climate Action →

Editorial decision: revise
27 Feb, 2024
Review #2 received at journal
21 Feb, 2024
Review #1 received at journal
05 Feb, 2024
Reviewer #2 agreed at journal
02 Feb, 2024
Reviewer #1 agreed at journal
05 Jan, 2024
Reviewers invited by journal
04 Jan, 2024
Editor assigned by journal
28 Dec, 2023
Submission checks completed at journal
28 Dec, 2023
First submitted to journal
23 Dec, 2023

You are reading this latest preprint version

High-ambition climate action in all sectors can achieve 65% greenhouse gas emissions reduction in the United States by 2035

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results

Decarbonizing the electricity grid while addressing increased demand from end-use sectors

Rapid electrification of passenger and freight vehicles

The role of the building and industry sectors

Setting the stage for 2050 with industrial decarbonization, carbon removal technologies, and land sector policies

Discussion

Key actions and challenges for 2035

Modeling limitations and further considerations

Materials and Methods

Modeled scenarios

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1