The Decarb America Research Initiative analyzes policy and technology pathways for the United States to reach net-zero greenhouse gas emissions by 2050. Our work aims to advance our understanding of the tradeoffs between different proposed strategies for achieving net-zero and to identify the national, regional, and state-level economic opportunities that a new clean energy economy will generate. Our analytical results are intended to inform policymakers as they consider options for addressing climate change and modernizing America’s energy systems.
To develop these results, Decarb America commissioned Evolved Energy Research and Industrial Economics, Inc. to undertake a rigorous, multi-part modeling analysis (more information is available at About the Initiative). The analysis explores five main research topics: (1) Pathways to Net-Zero Emissions, (2) Energy Infrastructure Needs for a Net-Zero Economy, (3) Power Sector Deep Dive, (4) Clean Energy Innovation Breakthroughs, and (5) Impacts on Jobs and the Economy.
This report presents key takeaways on topic (1) from the modeling analysis and responds to three critical, policy-relevant questions:
- What do pathways to net-zero look like under various technology and deployment constraints?
- How far will a package of sector-specific decarbonization policies reduce greenhouse gas emissions?
- What additional strategies might be needed to achieve net-zero by 2050?
- Reaching net-zero won’t be free, but it is affordable. Major clean energy investments over the next three decades will result in an energy system that is less costly in 2050, relative to the size of the U.S. economy, than it is today.
- Though different pathways have different cost implications, the fact that costs remain small as a percent of GDP across all scenarios suggests that deployment constraints, rather than cost differences, will be more important in determining which mix of technologies is used to achieve net-zero.
- Rapid decarbonization of the U.S. economy will require a diverse set of existing and new clean energy technologies.
- Inclusive policies that allow for a wide range of low- and zero-carbon solutions will also help ensure that all regions of the U.S. can leverage their different resource endowments to develop new clean energy industries.
- Achieving net-zero requires planning for more than double today’s electricity demand.
- Widespread consumer adoption of zero-carbon technologies is essential.
- A package of sector-specific decarbonization policies can reduce U.S. emissions by 70% relative to current emissions.
- A combination of fuel switching, carbon capture, additional efficiency, and the use of zero-carbon fuels can help eliminate emissions from hard-to-decarbonize sectors.
- In addition to aggressive emissions mitigation strategies, all pathways to net-zero require the deployment of carbon removal strategies (both natural and technology-based).
Evolved Energy Research modeled nine scenarios that make different assumptions about the policy and technology landscape for achieving net-zero U.S. greenhouse gas emissions over the next three decades. Key assumptions for each scenario are summarized in Table 1.
Table 1. Scenario descriptions
|Reference||Baseline scenario that assumes no additional policy changes. Uses the Energy Information Administration’s Annual Energy Outlook (AEO) 2019 with updated fuel prices and clean energy policies from AEO 2020.|
|Sectoral Policies||Analyzes a package of frequently discussed low-carbon or clean energy policies in the transportation, electricity, buildings, and other sectors. Together, these policies are estimated to cut emissions by approximately 70% below current levels – a substantial reduction but not enough to fully decarbonize the U.S. economy. This scenario combines a zero-emission vehicle standard, zero-carbon fuel standard (for diesel, gasoline, jet fuel, and hydrogen), electrification and efficiency standards for buildings, clean energy standard for the power sector (100% clean electricity by 2050), and policies to reduce emissions of methane and ozone-depleting substances.|
|High Renewables/High Electrification||Achieves net-zero greenhouse gas emissions across the U.S. economy by 2050. This scenario applies the sectoral policies analyzed above and then allows the model to choose the optimal path to net-zero. This scenario includes assumptions common to other net-zero analyses in terms of achieving high levels of electrification and renewable energy deployment.|
|Constrained Renewables||Achieves net-zero emissions by 2050 with constraints on deployment of renewable electricity technologies to reflect siting challenges. Reduces available renewable energy to just 5% of the National Renewable Energy Laboratory’s estimate of the technical potential for onshore wind, compared to 25% in the “Net-Zero by 2050” scenario. Solar deployment is limited by availability of land, with no more than 0.5% of available land area in any region allowed to be used for utility-scale solar. This scenario also constrains offshore wind deployment to 25% of technical potential to reflect potential hurdles in terms of siting of supporting transmission infrastructure and avoiding encroachment on existing ocean uses.|
|Slow Consumer Adoption||Assumes that fuel-switching in the transportation, industrial, and buildings sectors is delayed by 20 years, reflecting slower consumer adoption of efficiency equipment, hydrogen end-use technologies, and electrification technologies. Zero-carbon fuels replace electricity and direct use of hydrogen to meet a large share of energy demands and still achieve net-zero.|
|Constrained Renewables + Slow Consumer Adoption||This scenario pairs the demand-side assumptions from the “Slow Consumer Adoption” scenario with the renewable constraints used in the “Constrained Renewables” scenario. Given these constraints, this scenario relies heavily on zero-carbon fuels, electricity generation from non-renewables (e.g., nuclear), and carbon capture technologies to meet energy demands and still achieve net-zero.|
|High Conservation||Achieves net-zero emissions by 2050 with constraints on the overall footprint of the energy system. Assumes reduced energy demands in buildings, transportation, and industry. To reflect potential hurdles in terms of siting utility-scale energy and transmission infrastructure, this scenario deploys distributed solar and energy storage technologies at 75% of technical potential to meet a significant share of electricity demand.|
|Low Biomass||Achieves net-zero emissions by 2050 with reduced availability of biomass feedstocks to produce hydrogen, other synthetic gases, liquid biofuels, and on-site heat and electricity. Assumes a maximum available supply of 460 million metric tons (MMT), compared to 710 MMT in the Net Zero by 2050 scenario. Assumes that land currently used for corn ethanol will not be converted into land supplying other herbaceous energy crops, reducing available biomass supply by 34%.|
|No Fossil||Achieves net-zero emissions by 2050 by requiring the complete phase out of fossil-derived energy by 2050. This is achieved by the use of a zero carbon fuel standard and the elimination of all fossil fuel combustion, resulting in a substantial increase in the use of hydrogen, synthetic hydrocarbons, and biofuels.|
What do pathways to net-zero look like under various technology and deployment constraints?
The intent of developing multiple decarbonization pathways was to make a robust case for the achievability of the net-zero-by-2050 goal despite the breadth and magnitude of the economic, socio-economic, political, and technical challenges that lie ahead. Our modeling shows that net-zero can be achieved through a coherent set of technology choices and policies without necessitating a one-size fits all approach. Different scenarios and assumptions produce substantially different outcomes based on regional resource endowments and the differing needs of the energy system. We designed our scenarios to explore a wide range of pathways while also bounding potential outcomes to reflect a variety of constraints that could materially affect the scale and mix of technologies used to produce, convert, deliver, and consume energy in a net-zero economy.
Key Takeaway 1
Reaching net-zero won’t be free, but it is affordable. Major clean energy investments over the next three decades will result in an energy system that is less costly in 2050, relative to the size of the U.S. economy, than it is today.
In our modeling analysis, estimated costs for the dramatic scale-up of low- and zero-carbon energy needed to reach net-zero range from $151 billion per year by 2050 in our High Conservation scenario to as much as $797 billion per year by 2050 for our Zero Fossil scenario. These sums are not insubstantial but they are manageable in the context of an overall economy that is projected to grow to $62 trillion (annual GDP) over the same timeframe. Even at the higher end of our cost range, $797 billion per year translates to 2.2% of projected economic output in 2050. This is still well below historic levels of spending on the U.S. energy system, which have ranged between 5% and 10% of GDP over the last two decades, with volatile fossil fuel prices playing a large role in year-to-year fluctuations and total cost. By contrast, spending on energy as a share of GDP is projected to decline in all our net-zero scenarios, as a result of reduced demand and lower prices for fossil fuels together with improvements in energy efficiency, notably from the electrification of the light-duty vehicle fleet. A more detailed discussion of infrastructure investment, economic impacts, and costs is available in a companion report by Decarb America, available at:
Figure 1. Overall U.S. energy system costs as a percent of GDP, actual from 2000-2020 and projected in our modeling scenarios for 2020-2050.
Key Takeaway 2
Though different pathways have different cost implications, the fact that costs remain small as a percent of GDP across all scenarios suggests that deployment constraints, rather than cost differences, will be more important in determining which mix of technologies is used to achieve net-zero.
All pathways to net-zero show a significant shift in the mix of primary energy sources used to power the U.S. economy as high-carbon fuels such as coal and oil are entirely or largely displaced by low- and zero-carbon sources. In 2050, the four principal sources of primary energy in all our modeling scenarios are renewables, nuclear, natural gas, and biomass. Each of these sources, however, is subject to a range of real-world deployment challenges, ranging from limits on resource availability and siting considerations to transmission or other infrastructure requirements and socio-political acceptance. These constraints, more than relative technology cost, are likely to shape the mix of primary energy sources used to achieve net-zero by 2050.
Table 2 highlights the range of different outcomes, by energy source, that emerge in our modeling scenarios when various deployment constraints are imposed on particular low- and zero-carbon options. To facilitate comparison, the primary energy contribution from each source is given in common units of quadrillion British thermal units (‘quads’) and in more familiar sector-specific units.
Overall, the table helps to illustrate the point that no single technology or resource holds the key to reaching net-zero. Rather it will be important to ensure that a variety of climate-friendly energy technologies are commercially ready and positioned to play a substantial role—precisely because of the large uncertainties and significant deployment constraints that apply. For this reason, a portfolio approach that emphasizes diversity and inclusivity across multiple technology options is critical to help increase the chances of successful decarbonization.
Table 2. Potential constraints and range of buildout requirements for different primary energy sources in Decarb America’s net-zero by 2050 modeling scenarios.
|Primary Energy Source||Key Deployment Challenges||2050 Build-out Across the Range of Modeling Scenarios|
|Renewables||Resource availability, siting, social license, and transmission requirements||1,700 – 5,500 gigawatts|
|Nuclear||Commercial status of new technology, ability to rapidly scale deployment in light of siting challenges and complex regulatory requirements, socio-political acceptance, and need for resolution of waste disposal issue||11 – 113 gigawatts|
|Gas||Need to limit methane emissions from extraction, address local environmental impact, social license, infrastructure and other constraints on CO2 injection rate for geologic sequestration||0 – 30 trillion cubic feet|
|Biomass||Limits on feedstock types and volumes that can be considered carbon-neutral||350 – 700 million metric dry tons|
While Table 2 shows how widely outcomes can range for individual primary energy sources, Figure 2 shows the comparative contribution from different sources, relative to each other, in the different modeling scenarios. For example, the figure shows that wind and solar scale-up in the “No Fossil” scenario is considerably greater than in the other scenarios. Generally speaking, deployment constraints of the kind noted in Table 2 are likely to create more significant challenges in any scenario that dramatically increases the role of a particular technology (or technologies) relative to other scenarios.
Figure 2. Primary energy contribution from solar, wind, nuclear, natural gas, biomass, and other sources for seven pathways to net-zero emissions by 2050.
Figure 3 compares total primary energy use in each scenario. It shows that overall energy demand in 2050 depends on assumptions about how energy is consumed and about the energy efficiency of end-use technologies—both of which will be influenced by consumer choices. The Slow Consumer Adoption scenarios assume lower levels of electrification—for example, because of slower adoption of technologies such as electric vehicles. Lower levels of electrification reduce efficiency and ultimately result in higher primary energy consumption. Conversely, the High Conservation scenario assumes more aggressive end-use efficiency improvements, which substantially reduces primary energy consumption (though it is worth noting that this scenario still requires a tremendous scale-up of zero-carbon energy relative to today).
Figure 3. Primary energy consumption for seven modeled pathways to net-zero by 2050.
Table 3 summarizes the key primary energy features or impacts of each scenario relative to our High Renewables/High Electrification scenario.
Table 3. Summary of key features/impacts of different net-zero scenarios relative to the High Renewables/High Electrification scenario.
|Scenario||Impact on Primary Energy Requirements Relative to the High Renewables/High Electrification Scenario|
|Constrained Renewables||Constraining renewables reduces hydrogen production through electrolysis and necessitates a larger contribution from natural gas—principally to produce hydrogen, but also to produce electricity. This scenario also requires more than 100 gigawatts of new nuclear capacity.|
|Slow Consumer Adoption||Limited electrification reduces the overall efficiency of the energy system, resulting in higher primary energy requirements. These are satisfied by additional deployment of wind and solar, and by the use of natural gas to produce hydrogen (“blue” hydrogen).|
|Constrained Renewables + Slow Consumer Adoption||Because this scenario limits the deployment of additional renewables, the increase in primary energy demand that results from slow consumer adoption of electrification technologies is met by using natural gas to produce “blue” hydrogen.|
|High Conservation||Reduced energy service demand in buildings reduces overall primary energy requirements and requires lower levels of deployment for all zero-carbon energy sources.|
|Low Biomass||Constraining the biomass contribution means that more zero-carbon fuels have to be produced using hydrogen (either “green”’ hydrogen from solar- or wind-powered electrolysis or “blue” hydrogen from natural gas). This translates to larger-scale deployment of wind and solar and increased consumption of natural gas.|
|No Fossil||Restricting the use of fossil energy by 2050 rules out the use of natural gas to produce hydrogen and increases the need for fuel substitutes (to replace oil as a chemical feedstock). This necessitates significant additional deployment of wind and solar to produce “green” hydrogen via electrolysis.|
Key Takeaway 3
Rapid decarbonization of the U.S. economy will require a diverse set of existing and new clean energy technologies.
In our modeling results, a variety of technologies play a role in achieving the net-zero goal. Thus, an inclusive approach will be critical, not only to reduce overall costs and increase the resilience of the energy system, but as an insurance policy against the possibility that deployment constraints—whether those constraints involve socio-political factors (such as siting hurdles or public acceptance), technical hurdles, or regulatory challenges (such as the difficulty of carbon accounting for marginal biomass feedstocks) —limit the contribution of any particular zero-carbon technology.
Put simply, it would be premature to foreclose options at this time, given the magnitude of the net-zero challenge, the substantial deployment hurdles that must be overcome, and large uncertainties about future technological and socio-political developments. From a policy standpoint, this means that technologies should not be pre-emptively disqualified (for example, by making non-renewable, zero-carbon sources ineligible for clean energy standards) or effectively excluded through policy neglect (for example, by failing to provide sufficient R&D funding).
Our modeling results indicate that an inclusive approach will also have significant economic benefits because technology and fuel diversity reduce the overall cost of decarbonizing the energy system. For example, the annual energy system cost of our High Renewables/High Electrification scenario is $405 billion in 2050. Reaching the same net-zero goal without the use of any fossil fuel resources—our No Fossil scenario—nearly doubles the annual cost in 2050 (to $797 billion per year).
Reaching net-zero will require rapid scale-up of already commercial clean energy technologies, such as wind and solar. But it also requires expanded investment in innovation to ensure that additional zero-carbon technologies are commercially ready to play a role by 2030. Our modeling results point to several critical areas for further clean technology development and innovation:
- Carbon Capture
- Combined-cycle generation natural gas turbines with carbon capture
- Cement production with carbon capture
- Hydrogen production from natural gas with carbon capture
- Bioenergy with carbon capture
- Direct air capture (to remove C02) from the ambient air)
- Geological CO2 storage (for use with technology-based carbon capture)
- Land-based carbon capture
- Zero-Carbon Electricity Generation
- Floating offshore wind
- Generation turbines that can run on all or nearly all hydrogen
- Advanced nuclear to provide firm, dispatchable electric power
- Energy Storage
- Battery technology (for use in the grid and electric vehicles)
- Long duration storage
- Zero-Carbon Fuels
- Synthetic methane reformation for hydrogen production (with carbon capture and storage)
- Electrolysis for hydrogen production
- Biogasification with carbon capture for electricity production
- Synthetic zero-carbon fuels for use in gas turbines and in industrial heat and transportation application (important because these “drop-in” fuels can use existing fuel infrastructure)
- Bioenergy with carbon capture and storage (BECCS) for zero-carbon fuel production
- Electric vehicles
- Zero-emission trucks, whether electric or powered by zero-carbon fuels
- Engines capable of running on ammonia, especially for shipping, and for other transportation modes that are difficult to electrify
- Advanced (more efficient) aviation technology
- Industrial Materials
- Hydrogen-direct reduced Iron
Key Takeaway 4
Inclusive policies for technology innovation and deployment will allow all regions of the U.S. to leverage their different resource endowments and develop new clean energy industries.
Our modeling scenarios illustrate the energy opportunities that exist in all regions of the United States based on their different zero-carbon resource endowments. Figure 4 shows modeled 2050 primary energy production for four states in different net-zero scenarios: Florida, Iowa, Pennsylvania, and Texas. The very different resource breakdowns for these states point to the need to ensure that a wide range of zero-carbon technologies reach commercial readiness over the next several decades, both to achieve the net-zero goal and to give all parts of the country an opportunity to develop new clean energy industries.
Figure 4. Primary energy production in 2050 by resource for Florida, Iowa, Pennsylvania, and Texas in different net-zero modeling scenarios.
In the High Renewables/High Electrification scenario, for example, Florida becomes a major producer of nuclear and solar energy; Iowa is dominated by wind and biomass, for in-state use and export; Pennsylvania remains a major producer of natural gas, much of which is used to make hydrogen; and Texas remains the largest energy-producing state, with a significant deployment of renewables as well as carbon sequestration.
Key Takeaway 5
Achieving net-zero requires planning for more than double today’s electricity demand.
Decarbonizing electricity production while simultaneously electrifying other sectors of the economy, including buildings and transportation in particular, is foundational for any strategy to reach net-zero by 2050. This means that electricity demand at least doubles in all our modeled pathways, including in the Sectoral Policies case, which represents our most conservative scenario other than the Reference case. (As we discuss in later sections, Sectoral Policies alone falls short of achieving net-zero because it does not consider additional measures to address hard-to-decarbonize sources of emissions.) Electricity demand may nearly triple in some scenarios, such as our High Renewables/High Electrification scenario.
Figure 5 shows the increase in modeled electricity load, by end use, for the Sectoral Policies scenario. The figure points to a large increase in load from electrifying the principal thermal end-uses in buildings (space heating, water heating, and cooking) and an even more significant increase from electrifying on-road transportation, with the bulk of the added load coming from light- and medium-duty electric vehicles.
Figure 5. Modeled increases in electricity load by end-use for the buildings and transportation sectors for the Sectoral Policies scenario. Load is shown in terawatt-hours (TWh).
Accommodating a large increase in electricity demand will require a significant build-out of new generating capacity in the electric power sector. Table 4 shows average annual load growth over the next three decades, again for our Sectoral Policies scenario compared to our Reference scenario. In the 2020s, energy efficiency improvements offset much of the load growth from electrification, but in the 2030s and 2040s demand growth accelerates as more end-uses electrify and as electricity requirements for hydrogen production (via electrolysis) and direct air capture increase. These trends combine to double or even triple total U.S. electricity demand by 2050, depending on the specific scenario modeled. These results underscore the scale of the infrastructure challenge that confronts the electric sector, which will have to add clean generating capacity at a rate that is historically unprecedented on a sustained basis, while also rapidly expanding and modernizing the grid to continue delivering reliable, high-quality service with significantly larger loads.
Table 4. Annual load growth associated with the Sectoral Policies and Reference scenarios.
While electrification on this scale clearly presents formidable challenges, it also represents a key cost containment strategy for decarbonizing many sectors. For the foreseeable future, the power sector will remain the easiest and least expensive sector to decarbonize. Electrifying other sectors is a way to leverage the technology progress that has been achieved in clean electricity generation over the last several decades. In this way, increased reliance on relatively established low-carbon electricity generating technologies, such as wind and solar, can be used to effectively reduce the need for major breakthroughs in sectors that have not seen comparable progress in developing low-carbon alternatives. Overall, electrification has high potential to reduce the cost and complexity of decarbonization.
Key Takeaway 6
Widespread consumer adoption of zero-carbon technologies is essential.
Consumer choice and the speed of consumer uptake of new clean energy technologies have a significant influence on pathways to net-zero. This influence is often overlooked when designing policies, which frequently fail to address the need for institutional and programmatic support that would incentivize behavioral change and technology adoption. Zero-emission vehicles (ZEVs) are a good example. In our modeling, sales of these vehicles begin to ramp up beginning around 2023–2025, just at a time when many market analysts predict ZEVs will reach price parity with gasoline vehicles (Figure 6). However, price parity by itself does not guarantee that individuals and companies will buy and use ZEVs at the pace and scale indicated by our modeling scenarios. Policymakers will need to consider incentives and other policy mechanisms to ensure widespread consumer adoption of ZEVs.
Figure 6. Modeled trajectory of U.S. ZEV sales in the Sectoral Policies scenario
If consumer adoption of ZEVs is slower than the trajectory shown in Figure 6, a significant increase in the production and use of zero-carbon fuels becomes necessary to keep the net-zero target in reach. To explore these implications, our Slow Consumer Adoption scenario assumes a 20-year delay in the uptake of key end-use technologies, including electric vehicles. The results show an increased reliance on other zero-carbon fuels for vehicles and industry, such as hydrogen and ammonia. Thus, it will be important to implement policies that promote innovation in these areas, with the aim of advancing the commercial viability and reducing the cost of producing zero-carbon fuel alternatives. This should include both electrolysis- and natural-gas-based methods for producing hydrogen (sometimes called “green” and “blue” hydrogen, respectively) to provide flexibility in case constraints on the rapid deployment of new renewable generating capacity mean that “blue” hydrogen and ammonia need to play a larger role.
How far will a package of sector-specific decarbonization policies reduce greenhouse gas emissions?
Key Takeaway 7
A package of sector-specific decarbonization policies can reduce U.S. emissions by 70% relative to current emissions.
The power, transportation, building, and industrial sectors face different decarbonization challenges. This is because of significant variation in the near-term availability of low-carbon solutions for each sector, and because of differences in the pace of investment cycles and capital turnover. Targested decarbonization policies can help address the unique needs of each sector. To see how far a package of commonly discussed, sector-based decarbonization policies would get to achieving net-zero U.S. greenhouse gas emissions by 2050, we modeled a “Sectoral Policies” case.
Our results for this scenario show a 70% reduction in economy-wide emissions by 2050 (relative to current emissions levels), with an 80% reduction in energy and industrial CO2 emissions (Figure 7). This is a substantial reduction, but clearly additional policies would be needed to achieve the net-zero-by-2050 goal. Specifically, our Sectoral Policies scenario shows a shortfall or gap of 1.6 gigatons CO2-equivalent (Gt CO2e) in emissions reductions to reach full decarbonization at the mid-century mark.
Figure 7. Modeled emissions for the Sectoral Policies and High Renewables/High Electrification scenarios.
What additional strategies might be needed to achieve net-zero by 2050?
As noted above, our Sectoral Policies scenario results in a 70% reduction in U.S. greenhouse gas emissions relative to the High Renewables/High Electrification scenario. This translates to an emissions reduction shortfall of 1.6 million metric tons, primarily due to remaining emissions from a number of “hard-to-decarbonize” sources, which are highly heterogeneous. Reaching the net-zero goal in this scenario thus requires further reductions in non-CO2 greenhouse gases as well as additional CO2 reductions from carbon removal (including land-based sequestration, direct air capture, and carbon capture and storage) and from further emissions mitigation in the areas of bioenergy, industrial heat, and off-road transportation. Comparing model results from the Sectoral Policies and High Renewables/High Efficiency scenarios provides an indication of how these additional reductions might be achieved (Table 5).
Table 5. Change in 2050 emissions between the Sectoral Policies and High Renewables/High Electrification scenarios, along with strategies to close the gap.
|Sector||2050 emissions in Sectoral Policies scenario||2050 emissions in High Renewables/High Electrification scenario||Change in emissions between the two scenarios|
|Industry & Transportation||1.29 Gt CO2e, primarily from industrial manufacturing,and off-road and heavy-duty vehicles||0.83 Gt CO2e through a combination of: •Electrification of some process heating, •Fuel switching to hydrogen (400 TBtu by 2050), •Carbon capture and storage (including 0.125 Gt CO2e from cement & lime and 0.025 Gt CO2e from iron & steel production), and •Fuel switching to hydrogen in the transportation sector (400 TBtu by 2050)||Additional reduction of 0.46 Gt CO2e from the industrial and transportation sectors in the High Renewables/High Efficiency scenario|
|Agriculture & Other Remaining Sources of Non-CO2 Emissions||0.99 Gt CO2e, primarily from livestock, fertilizer use, and use of fluorinated gases (e.g., ozone- depleting substances used as refrigerants and industrial gases) -0.65 Gt CO2e from geologic and land-based sequestration (soils & vegetation)||0.85 Gt CO2e in non-energy, non-CO2 emissions with reductions achieved through improved and more efficient management of fertilizers, manure, and ozone-depleting substances -0.83 Gt CO2e from additional carbon capture at industrial and biofuels production facilities, plus direct air capture; and -0.85 Gt CO2e from land-based sequestration||Additional reduction of 0.14 Gt CO2e in non-CO2 greenhouse gases in the High Renewables/High Efficiency scenario|
|Net Emissions||1.63 Gt CO2e||0 Gt CO2e|
Key Takeaway 8
A combination of fuel switching, carbon capture, additional efficiency, and zero-carbon fuels can help eliminate emissions from hard-to-decarbonize sectors.
Large emissions reductions can be achieved using technologies that are already commercially available. Getting all the way to net-zero by 2050, however, will require continued innovation to develop and commercialize new zero- and low-carbon technologies. This section outlines key areas that warrant increased R&D support and targeted demonstration and early deployment policies.
Industry contributes close to a quarter of current U.S. greenhouse gas emissions, with commodity materials production such as refining, chemicals, pulp and paper, cement and lime, and iron and steel alone contributing close to 10% of the total emissions inventory. Table 5 points to the need for additional emission reduction strategies for this sector, including additional electrification, hydrogen fuel switching, and carbon capture. These strategies are needed because some industrial processes have specific heat requirements that are difficult to meet with electrification or involve fundamental chemical reactions that release greenhouse gases independent of any energy requirements.
Examples of such difficult-to-mitigate process emissions include the liberation of CO2 from limestone during calcining (a key step in the manufacture of cement and lime), the rejection of carbon during some hydrocarbon conversions (including synthesis gas production), and the production of CO2 when carbon is used to chemically reduce iron oxide ores to metallic iron in blast furnaces. Switching to a zero-carbon energy source doesn’t eliminate these emissions. Carbon capture is needed for that. Costs for carbon capture in industry can be higher than in other sectors, however, and policy interventions beyond those currently included in our Sectoral Policies scenario will likely be required (for example, an increase in the dollar value of the existing 45Q tax credit for carbon capture and storage).
Most industrial heat applications use carbon-based fuels as their energy source either directly (e.g., to fire heaters) or in industrial boilers. To decarbonize process heating will require a combination of increased efficiency, electrification, fuel switching to hydrogen, and carbon capture (especially where the same source also generates process emissions). The balance of these approaches over time will depend on evolving technical feasibility and economics and on the economics of new versus incumbent fuels. Our modeling includes a role for all of these strategies and highlights the need for significant additional policy action beyond what is contemplated in the Sectoral Policies scenario. Our High Renewables/High Electrification scenario, for example, assumes significant electrification of industrial heating—as a consequence, it also leads to significantly higher electricity demand in 2050, relative to the Sectoral Policies scenario (Figure 8).
Figure 8. Change in industrial energy demand in the High Renewables/High Electrification net-zero scenario relative to the Sectoral Policies scenario.
Even with aggressive assumptions about the electrification of light- and medium-duty vehicles, deep decarbonization of the transportation sector will require zero-carbon fuels such as hydrogen or ammonia for hard-to-electrify transportation modes, such as heavy-duty vehicles, air, rail, and marine vessels. Our Sectoral Policies scenario eliminates emissions from all on-road transportation (e.g., light-duty, medium-duty, heavy-duty vehicles and buses) with a combination of the ZEV mandate and zero-carbon fuels standard. However, additional policies and innovation in zero-carbon fuels, such as hydrogen, are needed to decarbonize off-road transportation (air, rail, and marine vessels). Figure 9 shows the increased use of hydrogen (along with slightly more electrification) for off-road transportation in our High Renewables/High Electrification net-zero scenario compared to the Sectoral Policies scenario. If consumers are slower to adopt electric vehicles than is assumed in our High Renewables/High Electrification scenario, then additional quantities of zero-carbon fuels such as hydrogen and ammonia will be needed for use by light- and medium-duty vehicles also. In our modeling, hydrogen plays an important role in closing the emissions gap for transportation as well as for industry.
Figure 9. Change in energy sources for off-road transportation (air, rail, and marine vessels) in the High Renewables/High Electrification scenario relative to the Sectoral Policies scenario.
Agriculture and Non-CO2 Emissions
Achieving net-zero will require additional mitigation of non-energy emissions from agriculture and the use of fluorinated gases (e.g., ozone-depleting substances used as refrigerants and industrial gases). In 2018, the agriculture sector accounted for 10% of U.S. greenhouse gas emissions, and fluorinated gases made up 3% of total emissions. Non-energy emissions from agriculture and livestock primarily include methane and nitrous oxide, both of which have significantly greater global warming potentials than CO2. Enteric fermentation by livestock, manure management, and rice cultivation are the main sources of agricultural methane emissions whereas soil management, including fertilizer application, is the primary source of agricultural nitrous oxide emissions. Our Sectoral Policies scenario does not include policies to address non-energy, non-CO2 emissions from agriculture or fluorinated gases. In our High Renewables/High Electrification net-zero scenario, we model a modest reduction of 0.14 GT CO2e in non-CO2 emissions assuming improved and more efficient management of fertilizers, manure, and ozone depleting substances. Carbon removal strategies make up for the remaining 0.85 GT CO2e in order to reach net-zero in 2050.
Opportunities to reduce non-energy emissions in agriculture and livestock could be scaled up with policy and programmatic support. According to a study by the U.S. Environmental Protection Agency, mitigation practices for non-energy emissions of greenhouse gases from non-rice croplands include no-till management and reduced application of nitrogen fertilizer. For rice cultivation, periodically draining flooded fields during the growing season can significantly reduce methane emissions. Livestock emissions can be addressed through manure management practices and technologies, such as anaerobic digesters, which convert animal waste to usable methane.
Key Takeaway 9
Cost-effective pathways to net-zero deploy carbon removal strategies (both natural and technology-based) along with additional emissions reduction strategies
Because it won’t be practically and economically feasible to eliminate 100% of human-caused emissions of greenhouse gases over the next few decades, technologies for pulling CO2 out of the atmosphere and permanently storing it will be needed to achieve net-zero emissions by 2050. Key carbon removal technologies include direct air capture (DAC), appropriately scaled bioenergy with carbon capture and storage (BECCS), and strategies for increasing land-based carbon sequestration (in soils and vegetation). In our High Renewables/High Electrification scenario, geologic sequestration from both DAC and BECCS removes 830 million metric tons (MMT) of carbon from the atmosphere and land-based removals reach 850 MMT by 2050. These sources of negative emissions will be critical to achieving the net-zero goal in a cost-effective manner.
Direct Air Capture
While DAC is one of the most expensive CO2 reduction options in our pathways analysis, several of our modeling scenarios see economic deployment of this technology on the path to net-zero. In our Sectoral Policies case, DAC provides a carbon feedstock that is utilized to synthesize non-fossil hydrocarbons into zero-carbon fuels. In the scenarios that achieve net-zero, DAC is deployed with carbon sequestration as a negative emissions technology. The carbon capture tax credit included in our Sectoral Policies scenario, 45Q, is not enough, by itself, to result in the application of this technology at scale. Increased incentives or domestic offset policies would be needed to spur DAC deployment at the level modeled in the High Renewables/High Electrification scenario.
Bioenergy with Carbon Capture and Storage (BECCS)
BECCS refers to technologies that convert biomass into an energy source (electricity, liquid fuels, and/or heat) and capture and store any CO2 produced in the process. BECCS has the potential to be a negative-emissions strategy if the amount of CO2 stored is greater than the CO2 emitted during biomass production, transport, conversion, and utilization. In practice, achieving negative emissions through BECCS will likely require extensive use of climate-neutral waste biomass that has been responsibly sourced. Supplies of biomass that can meet this criterion are likely to be limited for ecological, environmental, logistical, and economic reasons. The net climate impact of using other types of biomass will vary significantly from case to case, and this is an active area of research. Accordingly, our modeling applies some limitations to the potential contribution from BECCS. Where BECCS does appear in our net-zero scenarios, it is primarily deployed for hydrogen production as a negative-emissions strategy.
Land-based Carbon Sequestration
The land can act as a carbon sink in areas where the vegetation and soil take up more carbon from the atmosphere than they release. Forest-based carbon sequestration strategies can include reforestation, afforestation, and changes in forest management techniques (such as thinning the forest understory to reduce wildfire risk). On agricultural lands, cover crops, practices that improve soil health, rotational grazing, and grassland restoration can increase carbon uptake. According to a recent report by the U.S. Environmental Protection Agency, although there remains uncertainty about the current magnitude of the U.S. land sink, it is estimated to have provided nearly 800 MMT of CO2 sequestration in 2018, enough to offset approximately 12% of total U.S. greenhouse gas emissions that year.
By 2050, our Sectoral Policies scenario assumes that with no land management interventions, there will be a decline in land-based carbon sequestration to 300 MMT CO2e per year, primarily due to aging forest stands. Our High Renewables/High Electrification scenario, by contrast, models an increase in total land-based sequestration to 850 MMT CO2e in 2050, assuming implementation of land-based carbon sequestration strategies. Maintaining and increasing carbon uptake by forest and agricultural lands over the next three decades, including through improved land management practices, will be critical to achieving the net-zero goal in a cost-effective manner.
Our modeling analysis shows that there are a variety of ways to get to net-zero by 2050. It also shows that all plausible pathways require a diverse set of low- and zero-carbon technologies—and create substantial clean energy opportunities in every region of the United States.
The scale and pace of low- and zero-carbon technology deployment that will be required, however, is daunting—as our modeling results also show. It is unlikely to be achievable without a significant commitment of federal support, together with robust private sector investment and policy and market drivers. Experience suggests that a combination of well-designed push and pull policies can be very effective in creating demand for clean energy technologies and incentivizing businesses and consumers to invest in them. Performance standards drive the majority of emissions reductions in our Sectoral Policies case, so our analysis does not explicitly quantify the impact of other incentive- or investment-based policies such as funding for infrastructure and technology innovation or programs to provide tax credits, grants, direct subsidies and loans, and other incentives.
Such policies, which rely on public spending, can play a crucial role in jumpstarting the transition to a net-zero economy while also managing costs and building markets for clean energy solutions. They work well with performance standards by reducing the cost of compliance over time and supporting widespread consumer adoption of zero-carbon technologies. But while it is possible to build a strategy that is solely focused on incentives and public spending on technology and infrastructure, achieving net-zero will require much higher levels of investment and more ambitious investment- and incentive-based proposals than Congress is currently discussing. For example, the clean energy standard in our Sectoral Policies scenario drives more than $65 billion of annual investment in clean energy by 2030; $110 billion annually by 2040; and $160 billion annually by 2050. Thus, active private-sector participation, in combination with policy and market drivers, will also be critical to deliver and sustain investment at the level needed to achieve the net-zero goal.