National energy transitions under decarbonization goals. We investigate the environmental impacts (i.e., total air pollution emissions) and equality (i.e., regional distribution of emissions) of different electricity generation investment strategies under eight decarbonization strategies over 40 years (see Table 1 in Methods). Our decarbonization scenarios include the base case with no additional carbon constraints (Scenario A), two carbon cap scenarios which meet either the US nationally determined contributions (NDC) from the Paris Agreement or a pathway to stay under 1.5°C warming (Scenarios B and C respectively), and five technology specific portfolios which deploy either renewable energy (Scenarios D – F) or low carbon (Scenarios G and H) generation.
Figure 1 shows the annual generation by technology for the decarbonization scenarios. For Scenario A (the Base Case), which implements no additional carbon constraints or policies, coal, natural gas and nuclear mainly supply generation. By 2050 we see coal generation decrease to 7.5% of total generation (0.41 PWh), natural gas generation slightly increases to 20.0% (1.08 PWh), onshore wind generation increases to 33.8% (1.83 PWh), and solar PV generation increases to 20.9% (1.14 PWh) of total generation. The carbon cap scenarios (B and C), which place a strict limit on CO2eq. emissions from the electricity sector, achieve their carbon caps primarily through deploying solar PV and onshore wind. In both scenarios, wind and solar represented less than 3% of the generation in 2010. Still, by 2050 we see solar PV and onshore wind generation supplying 20–30% or 37–50% of total generation in 2050 respectively. Scenario C specifically sees the complete retirement of coal by 2035 and almost complete retirement of natural gas by 2050 (0.2% of generation). However, contrasting to Scenario C, the carbon cap defined in Scenario B allows an increase of CO2eq. emissions, resulting in an increase in coal generation from 2040 to 2050 (1.67% of generation in 2040 to 4.67% of generation in 2050).
Scenarios with an implemented renewable portfolio standard (RPS) (D, E, and F) invest in onshore wind generation to meet their renewable energy mandates. By 2050, onshore wind represents approximately 50% of generation in all three scenarios. Scenarios D, E, and F also see large solar deployment due to the implemented RPS. Scenario E deploys the highest generation of solar PV, CSP, biopower, and battery storage to meet the 100% renewable requirement by 2035, with solar PV technology representing 35.0% of total generation by 2040. Solar PV is still a significant contributor to generation in the other seven scenarios, with solar PV supplying 15–20% of US generation in 2040.
Natural gas in the low carbon scenarios (G and H) is relied on until their low carbon requirement year, when natural gas is retired due to the mandate. Upon reaching the technology mandate, natural gas CCS replaces natural gas. Thus, this technology would most likely continue to provide 10–20% of the total generation needs without a mandate. See Table S-4 in SI for a summary of generation by technology and scenario.
National environmental sustainability. Figure 2 shows the emissions impacts of the changing power plant profiles, which depicts the national operating emissions over the model timeline 2010–2050. Operating emissions are emissions produced directly from the power plant creating electricity. These results discuss the national operating emissions trends from CO2eq., NOx, SO2, and PM2.5. From Fig. 2, we see all pollutants have similar trends, with emissions decreasing through 2050 but at varying magnitudes across scenarios. Scenario A (base case) is an upper bound for national emissions across all pollutants in our analysis, indicating that implementing carbon or technology mandate policies will decrease emissions compared to no policies.
By 2035, operating emissions from Scenarios C, E, and G are under 100 Mt CO2eq. emissions. Coal has entirely retired by 2035 in these scenarios, so it does not contribute to emissions, and natural gas or natural gas CCS contributes under 10% of generation. By phasing out coal and natural gas plants, emissions from co-pollutants like NOx, SO2, and PM2.5 also fall significantly, with NOx levels at or below 0.02 Mt, SO2 levels below 0.003 Mt, and PM levels below 0.002 Mt. PM2.5 emissions (d) in Scenario E rise from 2035 to 2050 due to investments in biopower to maintain the 100% renewable energy mandate. Because of this, Scenarios C and F have lower levels of PM2.5 emissions in 2050.
Figure 2: National operating emissions across scenarios 2010–2050 (megatonnes, Mt). The emissions shown here are: a) CO2eq. operating emissions, b) NOx operating emissions, c) SO2 operating emissions, and d) PM2.5 operating emissions. Note that the y-axes are not consistent. We see the base case (black line) as an upper bound on all emissions types, and Scenarios C (solid yellow line) and E (dotted blue line) as a lower bound across all emissions types. Scenario E emissions reach close to zero by 2035, its mandate year, and remains close to zero 2035 to 2050 for CO2eq., NOx, and SO2 emissions. However, PM2.5 emissions in Scenario E rise because of investments in biopower, which help maintain the 100% renewable grid but still have co-pollutants that will be emitted.
Air pollution distribution. While national-level emissions analyses are important for measuring progress across the energy system as a whole, regional inequalities resulting from energy transitions can manifest in the unequal distribution of air pollution emissions. The operational co-pollutants will broadly impact people's health within the region, so operating emissions reductions of these pollutants will result in regional health benefits32–35. Therefore, we present an analysis of operating co-pollutant emissions (NOx, SO2, PM2.5) to illuminate how different decarbonization scenarios could impact emissions globally and regionally.
Once emitted from the power plant, air pollution travels through the air before depositing into communities and causing health impacts36. We use a reduced complexity model, InMAP, to model air pollution travel and understand where emissions are landing after being emitted from power plants for different decarbonization scenarios. Figure 3 displays the annual average total PM2.5 emissions across census tracts for 2020, 2035, and 2050. The NOx and SO2 emissions distribution reported similar trends to PM2.5 (see SI Figures S-7 and S-8). PM2.5 emissions in 2020 across scenarios have exposures over 1.0 µg/m3 in the Midwest and Eastern US. In 2035, Scenarios A, D, and H have concentrations over 1.0 µg/m3 located in the Eastern US and from Ohio to Iowa. Meanwhile, PM2.5 concentrations in Scenarios C, E, and G are under 0.25 µg/m3 across all regions by 2035 because of an aggressive carbon cap (C) or clean technology mandates (E and G). Similarly, when Scenarios F and H reach their 2050 mandate year, PM2.5 exposure is under 0.25 µg/m3 across all regions, indicating that total equality of emissions is achieved when the technology mandate year is met (2035 or 2050).
Emissions across vulnerable groups. Beyond regional analyses that measure the magnitude of air pollution, it is useful to understand the distribution of operating emissions across different demographic and socioeconomic indicators (race, ethnicity, income, poverty, urban or rural, etc.) across regions. This investigation shows the impact of different energy transitions on vulnerable regions. We focus on operating NOx, SO2, and PM2.5 emissions, due to the local health impacts of these co-pollutants.
Figure 4 compares the average concentration of PM2.5 to the percentage of a given race or ethnicity in a census tract. Census tracts were grouped by percentages of a race or ethnicity (< 10%, 10–20%, 20–30%, 30–50%, 50–70%, and > 70%. See SI Table S-3 for groups and sample sizes). Figure 5 shows five scenarios since Scenarios C, E, and G and Scenarios F and H have almost identical trends (see SI Figure S-9 for all scenarios). When census tracts have over a 30% Black population, the average PM2.5 concentrations are higher than those under 30% Black population. This trend continues in 2035 and 2050 for decarbonization scenarios that do not have technology mandates or a carbon cap that cause emissions to become minimal. Based on these results, Black populations are at risk for higher PM2.5 concentrations and its associated health impacts in energy transitions until renewable or low carbon mandates reach 100% renewable or low carbon technologies. The group with the second highest PM2.5 concentrations are non-Latinx white populations. NOx and SO2 concentrations showed almost identical trends and can be seen in SI Figures S-10 and S-11.
The distribution of operating emissions across different median incomes (Fig. 5) illustrates the disparities between high- and low-income regions (see SI for group definitions and the aggregation of median income census tracts to the ReEDS regions). SO2 and PM2.5 emissions have higher exposures in low-income communities over time, so these regions will continue to bear the brunt of emissions (and associated health impacts)26 unless a technology mandate or carbon cap requires fossil fuels to be retired (as seen in Scenarios C and F). NOx emissions are higher in higher-income areas in 2020 and plateaus over time to very little difference in concentration across income groups. However, higher-income people have more access to the health care facilities and insurance, so low-income groups with the same concentrations may be left worse off since they cannot access healthcare for health impacts from air pollution as easily37. SI Figures S-13 and S-14 show the results for high poverty populations and rural populations respectively. We see regions with high poverty have higher concentrations of all co-pollutants and rural populations have higher concentrations of PM2.5 until technology mandates are met in 2035 or 2050.