Carbon‐Neutral Pathways for the United States
James H. Williams, Ryan A. Jones, et.al, 14 January 2021 (Advancing Earth and Space
The Intergovernmental Panel on Climate Change (IPCC) Special Report on Global Warming of 1.5°C points to the need for carbon neutrality by mid‐century. Achieving this in the United States in only 30 years will be challenging, and practical pathways detailing the technologies, infrastructure, costs, and tradeoffs involved are needed. Modeling the entire U.S. energy and industrial system with new analysis tools that capture synergies not represented in sector‐specific or integrated assessment models, we created multiple pathways to net zero and net negative CO2 emissions by 2050. They met all forecast U.S. energy needs at a net cost of 0.2–1.2% of GDP in 2050, using only commercial or near‐commercial technologies, and requiring no early retirement of existing infrastructure. Pathways with constraints on consumer behavior, land use, biomass use, and technology choices (e.g., no nuclear) met the target but at higher cost. All pathways employed four basic strategies: energy efficiency, decarbonized electricity, electrification, and carbon capture. Least‐cost pathways were based on >80% wind and solar electricity plus thermal generation for reliability. A 100% renewable primary energy system was feasible but had higher cost and land use. We found multiple feasible options for supplying low‐carbon fuels for non‐electrifiable end uses in industry, freight, and aviation, which were not required in bulk until after 2035. In the next decade, the actions required in all pathways were similar: expand renewable capacity 3.5 fold, retire coal, maintain existing gas generating capacity, and increase electric vehicle and heat pump sales to >50% of market share. This study provides a playbook for carbon neutrality policy with concrete near‐term priorities.
Plain Language Summary
We created multiple blueprints for the United States to reach zero or negative CO2 emissions from the energy system by 2050 to avoid the most damaging impacts of climate change. By methodically increasing energy efficiency, switching to electric technologies, utilizing clean electricity (especially wind and solar power), and deploying a small amount of carbon capture technology, the United States can reach zero emissions without requiring changes to behavior. Cost is about $1 per person per day, not counting climate benefits; this is significantly less than estimates from a few years ago because of recent technology progress. Models with more detail than used in the past revealed unexpected synergies, counterintuitive results, and tradeoffs. The lowest‐cost electricity systems get >80% of energy from wind and solar power but need other resources to provide reliable service. Eliminating fossil fuel use altogether is possible but higher cost. Restricting biomass use and land for renewables is possible but could require nuclear power to compensate. All blueprints for the United States agree on the key tasks for the 2020s: increasing the capacity of wind and solar power by 3.5 times, retiring coal plants, and increasing electric vehicle and electric heat pump sales to >50% of market share.
The Paris Agreement calls for “holding the increase in the global average temperature to well below 2°C above pre‐industrial levels and pursuing efforts to limit the temperature increase to 1.5°C (UNFCCC, 2015).” Moreover, avoiding the worst impacts of climate change may require not only staying below 1.5°C but a return to 1°C by 2100 (Hansen et al., 2013). Climate outcomes of 2°C, 1.5°C, and 1°C are associated with end of century atmospheric CO2 concentrations of roughly 450, 400, and 350 ppm, respectively, entailing global net CO2 emissions trajectories that reach zero by roughly 2070, 2055, and 2040 and are negative thereafter (Hansen et al., 2017; IPCC, 2018).
This paper examines specific pathways by which emissions reductions consistent with these trajectories can be achieved in the United States. We focus on reductions in energy and industrial (E&I) CO2, which constitutes more than 80% of current gross U.S. greenhouse gas (GHG) emissions (U.S. EPA, 2019a). We combined our modeled results for E&I with published values for non‐CO2 GHG emissions and the land CO2 sink to obtain a range of economy‐wide CO2e values for comparison to global trajectories and policy targets adopted by United States and other jurisdictions, including “80% by 2050,” “net zero by 2050,” and “350 ppm by 2100” (Le Quéré et al., 2018; U.S. Climate Alliance, 2020).
Our objective in this paper was to develop realistic deep decarbonization scenarios that reach net zero or net negative E&I CO2 emissions by 2050 while meeting all forecast demand for energy services at the lowest possible cost, using only technologies that are commercial or have been demonstrated at large pilot scale. The scope of the analysis includes all energy flows through the U.S. economy, from primary energy inputs, such as petroleum and natural gas, to energy conversion processes, such as oil refining and power generation, to end uses in buildings, transportation, and industry that consume final energy in the form of electricity and solid, liquid, and gaseous fuels. We modeled the transition pathways in all these areas in detail to answer high‐level questions of interest to policy makers—technical feasibility, infrastructure requirements, cost, the implications of different assumptions and tradeoffs, and the required types and scale of policy interventions—as well as technical questions of interest to specialists, for example, how to optimally integrate high levels of variable renewable energy (VRE), produce low‐carbon fuels from biomass and electricity, decarbonize challenging end uses in industry and freight transport, and incorporate carbon capture, utilization, and storage (CCUS) into the overall E&I system (Bataille, 2020; Davis et al., 2018; Dessens et al., 2016; Rogelj et al., 2015)…
The Low‐Carbon Transition…Four Pillars of Deep Decarbonization
The emissions objectives were reached in all scenarios, while meeting all energy needs. As in previous deep decarbonization pathways studies, the transition from a high‐carbon to a low‐carbon energy system was based on the strategies of (1) using energy more efficiently, (2) decarbonizing electricity, and (3) switching from fuel combustion in end uses to electricity (Bataille et al., 2016; White House, 2016; Williams et al., 2012, 2015). Since the emissions reduction impacts of these strategies are multiplicative, they must be simultaneously applied to achieve their full potential. This study further shows that reaching net zero E&I emissions, including non‐energy CO2 from industrial processes, requires an additional strategy: (4) capturing carbon, which can either be sequestered geologically or utilized in making carbon‐neutral fuels and feedstocks (section 7.3) (Haley et al., 2018). Benchmark values for the four strategies are shown in Figure 2 (Figure S11). Per capita energy use declined 40% in 2050 compared to 2020, and energy intensity of GDP declined by two thirds. The carbon intensity of electricity was reduced 95%, while electricity's share of end use energy tripled, from 20% to 60%, including electrically derived fuels. Carbon capture reached almost 800 Mt CO2/year, up from negligible levels today; of this, about 60% was utilized and about 40% was geologically sequestered.
Metrics for the four main strategies of deep decarbonization, 2050 central case compared to current levels.
The energy system transformation resulting from applying the four strategies is shown for two bookend cases in Figure 3. The 100% renewable primary energy case has no fossil fuels remaining in 2050, while the central case with low fossil fuel prices has the highest residual fossil fuel use. In both scenarios, both primary and final energy uses are lower in 2050 than in today's system, despite meeting higher energy service demand due to rising population and GDP. The shares of coal, oil, and natural gas in primary energy supply decrease dramatically from today's level, replaced primarily by wind, solar, and biomass.
Low‐carbon electricity and fuels replace fossil fuels in most final energy uses.
Conversion processes that currently play a minimal role—biomass refining and production of hydrogen and synthetic fuels from electricity—become important in the decarbonized energy system, replacing most or all petroleum refining (Figures S1–S4). Contrasts between the decarbonized cases are discussed in section 5.3 (Table 2).
Sankey diagrams for (top) the current U.S. energy system, (middle) the central carbon‐neutral case with low fossil fuel prices, and (bottom) the 100% renewable primary energy case. Primary energy supplies are on the left, conversion processes in the middle, and final energy consumption on the right. Line widths are proportional to magnitude of energy flows.
Deep decarbonization entails an infrastructure transition over the next three decades in which high‐emitting, low‐efficiency, and fuel‐consuming technologies are replaced by low‐emitting, high‐efficiency, and electricity‐consuming technologies, at the scale and pace necessary to reach the emission targets (Davis et al., 2010, 2018; Davis & Socolow, 2014; Shearer et al., 2020). The required scale and pace are illustrated in Figure 4 for three sectors that together comprise two thirds of current E&I CO2 emissions: electric power generation, vehicles, and space and water heating in buildings (Figures S12–S14 and S22) (U.S. EPA, 2019a). By 2050, electric generation capacity increased by 3,200 GW; virtually all of the net increase was wind and solar (section 6.4). Coal was fully retired. Out of 296 million cars and light trucks, more than 280 million were battery electric vehicles. In residential buildings, electric heat pumps constituted 119 million out of 147 million space heating units and 88 million out of 153 million water heating units, with electric resistance heaters comprising most of the remainder. This transition was accomplished over a period of 30 years by replacement of equipment at the end of its normal lifetime, without early retirement.
The constrained scenarios demonstrate that feasible alternate pathways to the same carbon target exist even in the face of limits on technology choices and resource availability. However, these scenarios required compensating changes in other areas, resulting in higher net cost and greater use of other resources (Table 2):
Low land…Delayed electrification…Low demand…100% renewable primary energy…Net negative…
The levelized net energy system cost of this transformation for the central case was $145 billion in 2050, equivalent to 0.4% of GDP in that year (Figures 5 and S7–S10). This is the difference in the annualized capital and operating costs of supplying and using energy in the central case compared to the reference case, plus the net cost of reducing or offsetting non‐energy industrial process emissions. Except where noted, cost inputs were the reference values of DOE long‐term fossil fuel price and technology cost forecasts (NREL, 2019; U.S. EIA, 2019). The net present value of net system cost was $1.7 trillion over the 2020–2050 period, using a 2% societal discount rate. In the central case, increased spending on incremental capital costs for low‐carbon, efficient, and electrified technologies ($980 billion in 2050) was offset by reduced spending on fossil fuels and incumbent technologies (−$835 billion in 2050). A sensitivity case using the DOE low fossil fuel price forecast raised the central case net cost to 1.2% of GDP in 2050 (net cost is higher because the counterfactual reference case cost is lower); using the low technology cost forecasts for renewables lowered it to 0.2% of GDP. The net costs of all other scenarios ranged from about 0.45% in the low land case up to 0.9% in the 100% renewable primary energy case. The net negative case consistent with a 1°C/350 ppm trajectory had a net cost of less than 0.6% of GDP in 2050….
Until recently, it was unclear whether VRE, nuclear, or fossil fuel with CCS would become the main form of generation in a decarbonized electricity system. Analyses of U.S. economy‐wide deep decarbonization (~80% GHG reductions) have generally shown roughly equal shares of generation from each of these sources, with the proportions changing depending on policy and cost assumptions (Bistline et al., 2018; Clarke et al., 2014; White House, 2016; Williams et al., 2012, 2015). The cost decline of VRE over the last few years, however, has definitively changed the situation.
Our analysis shows that electricity from VRE is the least‐cost form, not only of power generation but of primary energy economy wide, even when that requires investment in complementary technologies and new operational strategies to maintain reliability. All cost‐minimizing pathways to deep decarbonization are organized around using VRE to the maximum feasible extent, to supply both traditional loads and new loads such as EVs, heat pumps, and hydrogen production. As a result, electricity demand increases dramatically, to roughly three times the current level by 2050 (230% to 360% across cases; Figure 6b and Table 2). This demand is met primarily by VRE in all cases. In the central case, the generation mix was 90% wind and solar (Figure 6a); the minimum level was 81% in the limited‐land case (Figures S23–S25 and S27). It is possible that dramatic cost breakthroughs in new generating technologies such as Allam Cycle CCS and Gen IV nuclear could result in a reduced VRE share, but the breakthroughs would need to happen soon in order to deploy them at the pace and scale required in these scenarios.
There has been a vigorous debate over the feasibility of electricity systems with very high levels of VRE generation…The provision of reliable capacity (MW) in a decarbonized electricity system is fundamentally separate from the provision of energy (MWh)…Transmission…Batteries…
Carbon Neutrality Is Affordable
We have shown that achieving net zero and net negative CO2 emissions from energy and industry in the U.S. by mid‐century can be done at low net cost. Recent declines in solar, wind, and vehicle battery prices have made decarbonizing the U.S. economy increasingly affordable on its own terms, without counting the economic benefits of avoided climate change and air pollution (Garcia‐Menendez et al., 2015; Hsiang et al., 2017; Nemet et al., 2010; West et al., 2013; Risky Business Project, 2016). The net cost of deep decarbonization, even to meet a 1°C/350 ppm trajectory, is substantially lower than estimates for less ambitious 80% by 2050 scenarios a few years ago (Clarke et al., 2014; Williams et al., 2015); even with decarbonization, future energy costs as a share of GDP are expected to be lower than today's.
Renewable Electricity Is the Foundation of an Affordable Transition
The least‐cost decarbonized electricity system combines high VRE generation (>80% share) with low‐cost reliable capacity such as natural gas without carbon capture operating infrequently. If renewables and transmission cannot be built at the scale required, for example, due to difficulty in siting, nuclear and fossil CCS generation become important. Implementing high VRE systems may require changes in wholesale electricity markets to allow cost recovery for thermal generation needed for reliability but operated <15% of the time and to provide incentives for industrial loads such as electrolysis and electric boilers to operate flexibly on renewable over‐generation (Jones et al., 2018).
The Social Effects of Changes in the Energy Economy Need to Be Managed
Deep decarbonization entails a major shift in the U.S. energy economy. The variable costs of fossil fuels will be replaced by the capital cost of low‐carbon technologies. Incremental capital investment averaging $600B per year represents about 10% of current U.S. annual capital investment of $6 T in all sectors, indicating that finance per se is not a barrier if policies that limit risk and allow cost recovery are in place (Federal Reserve Bank of St. Louis, 2019). A greater challenge is likely to be the political economy of effectively redirecting >$800B/year from fossil fuels into low‐carbon technologies. The distributional impacts of such a transition could be ameliorated through policies that support communities and sectors dependent on fossil fuel extraction, while new jobs emerge under policies that ensure a significant domestic share of the manufacturing‐based low‐carbon economy (Busch et al., 2018).
Consumer Incentives Are Needed to Support Timely Electrification
Carbon neutrality is aided by complete consumer adoption of electric end use technologies in light‐duty transportation and buildings. Slow adoption that leads to delayed or incomplete electrification will result in greater cost and resource use. Direct mandates and/or carbon prices can drive decarbonization of electricity and fuels production, since utilities and industrial enterprises are responsive to such signals. Different policies may be required to influence consumers who are sensitive only to upfront cost. As demonstrated historically with solar PV, one option is customer incentives such as rebates that effectively lower the purchase price of EVs and heat pumps. These have the potential to dramatically increase sales, drive innovation, reduce manufacturing costs, and lower purchase prices in a self‐sustaining market transformation (Nemet, 2019).
Recognizing Tradeoffs Between Decarbonization Strategies Is Essential
The scale and pace of infrastructure buildout and demands on the land in a low‐carbon transition imply competition among social, environmental, and economic priorities. Our scenarios illustrate the kinds of tradeoffs that can be anticipated and their impacts. The use of biomass and of land for renewable siting are indispensable for all net zero pathways, but the amount required can differ by a factor of 2 or more. It needs to be understood that reducing biomass and land for siting implies increasing fossil fuels, nuclear power, and negative emissions. In addition to siting and biomass, increasing the land carbon sink is another element of the competing priorities among climate mitigation, food production, and other land uses (Griscom et al., 2017).
Given the regional character of energy use and resources and the U.S. system of government, many of the tradeoffs faced will need be resolved at the state and local level (Betsill & Rabe, 2009; Williams et al., 2015). Rigid positions on tradeoffs will not be helpful for informed decision‐making as they may lead to over‐constrained problems and policy paralysis; better public participation, analysis, and data are more likely to improve outcomes. Recent work in California, where conflicts between renewables siting, biodiversity conservation, and agriculture have emerged, points to the potential of incorporating geospatial analysis into energy planning to help reconcile competing land uses in large‐scale wind, solar, and transmission buildouts (Wu et al., 2016, 2020).
The Actions Required in the Next 10 Years Are Known With High Confidence
Carbon‐neutral pathways diverge in energy strategy, resource use, and cost primarily after 2035. The highest‐priority near‐term actions are similar across pathways and have clear quantitative benchmarks for policy: renewables build‐out (>500 GW total wind and solar capacity by 2030); coal retirement (<1% of total generation by 2030); maintaining current nuclear and natural gas capacity; and electrification of light‐duty vehicles (EVs > 50% of LDV sales by 2030) and buildings (heat pumps >50% of residential HVAC sales by 2030). Longer‐term uncertainties are related mainly to fuels and CCUS, areas in which technical potential, costs, and environmental impacts at large scale need to be better known before specific strategies are adopted. There is time for society to explore different approaches to these questions and learn from the results before solutions are needed in bulk in the 2030s, but the solutions will only be ready if the preparatory work—R&D, demonstrations, early commercial subsidies—is begun now. In other words, taking decisive near‐term action in the areas that are well understood, combined with laying the necessary groundwork in the areas of uncertainty, puts the United States on a carbon‐neutral pathway right away while allowing the most difficult decisions and tradeoffs to be made with better information in the future.