At a Glance
A growing number of companies and governments worldwide, including the United States, are pledging to reach net-zero greenhouse gas emissions by 2050, but there is a dearth of understanding of how to execute on these ambitions. The Net-Zero America study provides unprecedented sectoral, spatial, and temporal detail in describing five diverse net-zero pathways for the United States. The analysis quantifies challenges and opportunities at state and sub-state levels, including those relating to land use, employment, air pollution-related health, capital mobilization, incumbent fossil fuel businesses, and new clean-energy industries. The unique granularity of the analysis is drawing significant media interest and informing federal and state policy makers, private industry, non-governmental organizations, and future research initiatives around the world.
The research team put together five diverse cost-minimized pathways to reach net-zero by 2050. These are described below. A key high-level finding is that cost of energy services is not the major challenge to reaching net-zero emissions economy wide: across all pathways total annualized energy system cost as a percentage of gross domestic product (GDP) ranged from 4.2% to 5.8%. U.S. energy spending has ranged between 5% and 7% of GDP during the past 20 years, except during times of financial crisis or oil price shocks, when energy spending was as high as 10% to 14% of GDP. More significant hurdles than cost to achieving net-zero by 2050 are execution challenges. These vary across the modelled pathways, but often involve historically unprecedented rates of plant and infrastructure deployment and mobilization of related capital investment and labor, as described below.
The five modeled pathways all rest on a common set of six decarbonization pillars: (1) End-use energy efficiency and electrification; (2) Clean electricity; (3) Clean fuels, especially bioenergy and hydrogen; (4) Capture, utilization, and storage of CO2 ; (5) Reduced emissions of non-CO2 greenhouse gases; and (6) Enhanced land carbon sinks. The five pathways explore execution challenges and impacts relating to the first four of these pillars. The same exogenous assumptions regarding contributions to emissions reductions from pillars 5 and 6 were used across all five pathways.
Two of the pathways (E+ and E-) highlight the impact of the rate of electrifying vehicles and building space heat. Electrification is very rapid in E+ and less-rapid, but still ambitious, in E-. For example, new vehicles in 2050 with electric drivetrains represent 100% of sales in E+ and 88% in E-. The slower electrification rate for E- leaves more petroleum and natural gas in the primary energy mix in 2050 than for E+, necessitating more CO2 capture and storage in E- (Figure 1.1).
Two other pathways (E+ RE- and E+ RE+) adopt rapid electrification on the demand side and explore the impact of different levels of renewable solar and wind electricity on the supply side. In E+ RE-, the annual expansion of solar and wind generating capacity is limited to what has been achieved historically in the U.S. (about 35 GW/y combined). The E+ RE+ pathway removes this constraint and requires 100% of primary energy to be renewable by 2050 and prohibits underground storage of CO2 . The constraint on solar and wind capacity in E+ RE- leads to nearly a tripling by 2050 in the amount of nuclear power capacity relative to today. This is the largest use of natural gas power generation with CO2 capture and storage across the five pathways, and the highest amount of geologic CO2 storage (Figure 1.1). In E+ RE+, solar and wind deployment grows very substantially, providing over 98% of all electricity by 2050. Moreover, total electricity demand in 2050 grows to 4 times today’s level. In other pathways, electricity demand grows by “only” a factor of 2 to 2.5. The demand growth is especially high in E+ RE+ primarily because electricity is used to make hydrogen, which is used with captured CO2 to synthesize liquid hydrocarbon fuels for use in difficult to electrify applications (such as aviation).
The fifth pathway (E- B+), adopting the less-rapid demand-side electrification rate, explores the impact of relaxing the constraint placed on bioenergy in the other four pathways. In the latter, the potential biomass available for energy was restricted to biomass that could be provided without any change in land use for energy. Notably, since about 40% of corn produced in the U.S. today is converted to ethanol fuel, the constrained bioenergy supply does allow up to 40% of today’s corn-growing land to continue to be used for energy production, for example, by converting it to perennial energy grasses as ethanol use is phased out. In the E- B+ scenario, the full biomass supply projected in the U.S. Department of Energy’s “Billion Ton Study,” including some conversion of food-agriculture lands to energy crops, was made available to the modeled energy system. By 2050, all five pathways use essentially all biomass available for energy, with E- B+ using nearly twice as much as the other four pathways (Figure 1.1). Moreover, most of the biomass is converted to hydrogen with the byproduct CO2 captured and stored. This mode of biomass use provides a fuel (hydrogen) with net negative emissions, thus enabling some continued emissions from difficult or very-costly-to-reduce sources.
Figure 1.2 illustrates some of the high spatial resolution mapping completed in the work for the E+ pathway. The left panel shows 220 GW of combined utility-scale solar and wind farms in 2020 growing to 730 GW in 2030 and reaching 3 TW in 2050. High-voltage transmission capacity grows 60% by 2030 and triples by 2050 relative to today. The right panel shows the build-out by 2030 of an interstate CO2 transportation network needed to connect the CO2 capture plants that are rapidly deployed in the 2030s and 2040s to geologic storage reserves. By 2050, over 1000 capture plants are feeding CO2 into the pipeline network. The majority of capture occurs at biomass conversion facilities in the Midwest. About ¾ of the stored CO2 goes to geologic formations in the Gulf Coast region.
The researchers carried out high-resolution analysis across a variety of other features of the net-zero pathways. This included the large increase in capital that must be mobilized for the energy-supply system, the significant growth in aggregate energy-related employment, air-pollution related health impacts, and phasing down of fossil fuel industries and associated infrastructure. The granular analysis quantifies execution challenges and opportunities across multiple dimensions of the net-zero pathways. Figure 1.3 translates aggregate national level quantitative findings for nine dimensions of the energy system in 2050 into an ordinal ranking of the pathways. This is intended to help visualize the trade-offs with both execution challenges (left panel) and impacts (right panel) suggested by the modeling. For example, the right panel shows that the RE+ pathway would see the greatest extent of land use impact (for solar and wind projects) and the most extensive additions of high voltage transmission lines. But the RE+ pathway also would provide the highest level of health benefits and entail the largest increase in energy-related jobs. In contrast, the RE- pathway involves the least land use change and added high voltage transmission while still providing health benefits and costing somewhat less than RE+. However, RE- requires a massive build out of nuclear power, the largest amount of CO2 storage, and adds the least number of energy-related jobs.
The Net-Zero America study does not offer specific policy recommendations. Rather, the research is intended to help inform policy and private-sector decision makers in pursuit of a net-zero emissions economy by 2050.
Future work includes: further deep dives into some of the questions about feasibility that emerge from these findings; evaluating proposed decarbonization policies and scoring them against the transition pathways in the study; helping launch similarly granular net-zero studies by colleagues in other countries; and building open-access modeling tools that facilitate the type of high-resolution modeling demonstrated in the Net-Zero America study.