U.S. West Coast droughts and heat waves exacerbate pollution inequality and can evade emission control policies

Air pollution in the United States remains a significant public health concern, contributing to a substantial number of premature deaths annually. A considerable portion of this pollution stems from fossil fuel combustion, with electric power plants accounting for a notable percentage. However, power sector air pollution and its societal cost are not static; they are significantly influenced by hydrometeorological factors, particularly extreme events such as droughts and heat waves. In California, these events have a substantial impact due to the state's reliance on hydropower. Droughts decrease hydropower availability, while heat waves boost electricity demand for cooling, forcing a greater reliance on fossil fuel-based electricity generation and consequently increasing emissions. This study investigates the complex interplay between droughts, heat waves, and the spatial distribution of air pollution from power plants, focusing on how these factors influence the distribution of health damages across different demographic groups in California. A key question addressed is whether these extreme weather events exacerbate existing pollution inequalities. Furthermore, the research explores the resilience of emission control policies, specifically financial penalties on emissions, under such stressful conditions. Existing estimates of the cost-benefit ratio of pollution control policies may be inaccurate if these policies prove less effective during droughts and heat waves.

Decades of air quality improvement efforts in the United States have yielded progress, yet air pollution remains a major public health issue. Studies have established a strong link between air pollution from fossil fuel combustion and premature mortality. The power sector contributes significantly to this pollution. Previous research has highlighted the influence of hydrometeorological factors, particularly droughts and heat waves, on power plant emissions. For instance, studies have demonstrated how droughts in California increase the carbon footprint of energy production by reducing hydropower generation. Similarly, research has shown how warmer weather increases electricity sector emissions due to increased building energy use for cooling. In addition, existing literature has highlighted the disproportionate impact of air pollution from power plants on marginalized socioeconomic groups and people of color, even under normal weather conditions. However, a crucial gap remains in understanding how droughts and heat waves, by exacerbating power plant emissions, specifically impact different communities. The effectiveness of current emission control policies under extreme weather conditions also requires further investigation.

This research employs a combined approach using the open-source California and West Coast Power System (CAPOW) model, a stochastic power system simulation tool, and the Air Pollution Emission Experiments and Policy (AP3) integrated assessment model. The CAPOW model simulates hourly power plant emissions of SO2, NOx, and PM2.5 in California under a 500-year synthetic weather ensemble encompassing daily streamflow, air temperatures, wind speeds, and solar irradiance data. This ensemble was generated using a Gaussian copula approach and historical data, capturing the multi-scale uncertainties in these variables. The model incorporates the complex interactions within the West Coast power grid, allowing for the examination of how hydrometeorological anomalies in one region (e.g., the Pacific Northwest) impact emissions in another (e.g., California). The AP3 model then translates these emissions into monetary estimates of associated human health damages at the county level, considering the spatial distribution of pollution and population demographics. The research investigates multiple emissions penalty scenarios, including a base case with no taxes, a local tax on local pollutants, and scenarios incorporating CO2 emissions penalties. The analysis tracks the spatial distribution of health damages across different demographic groups, focusing on the correlation between damages and both the percentage of people of color in a county and the average CalEnviroScreen Score (measuring existing pollution burdens). The study further examines the effectiveness of the emissions tax under various hydrometeorological conditions, specifically during droughts and heat waves, to assess the policy's robustness under extreme events. Correlations between annual and daily system states and performance metrics were analyzed to identify key drivers of pollution damages and the effectiveness of emission control policies.

The simulations reveal that human health damages from power plant emissions are significantly higher in hot, dry years. Counties with a higher percentage of people of color and existing high pollution burdens experience a disproportionate share of these increased damages. Analysis of a 500-year weather ensemble shows that the average annual air quality damages are $22.46 per capita for people of color and $19.56 per capita for White residents in the no-tax scenario. A local air pollution tax effectively reduces overall damages by approximately 69%, with more significant reductions for people of color (71%) compared to White residents (66%). However, the effectiveness of this tax is significantly diminished during severe heat waves, even in years with average hydrologic conditions. During these extreme heat events, the system operator is forced to utilize nearly all available power plants to meet the high demand, rendering the emissions tax ineffective in preventing damages on the worst polluting days. This occurs because the value of preventing blackouts exceeds the cost of the pollution damages. This study finds that late summer heat waves pose the most significant acute operational risks, coinciding with seasonally low hydropower production. In contrast, droughts' impact on the system is generally more chronic, affecting hydropower availability and increasing pollution damages over several months. Correlations between annual data indicate that hydrologic conditions, especially drought, are the primary driver of yearly human exposure to power plant emissions, while daily electricity demand is the most important driver of daily emission damages.

The findings highlight the significant influence of droughts and heat waves on exacerbating existing pollution inequalities and challenging the effectiveness of conventional emission control policies. The disproportionate impact on communities of color and those already burdened by pollution underscores the need for more equitable policies. The ineffectiveness of the emissions tax during extreme heat events highlights the critical need for incorporating grid reliability and supply scarcity constraints into emission control policy design. The results suggest that strategies for managing grid scarcity should consider the economic cost and inequalities of air pollution exposure. The study's findings have major implications for grid operators and policymakers in planning for and mitigating the risks posed by climate change on air quality. Adaptive strategies are necessary to protect vulnerable populations from the combined effects of heat and air pollution. This research underscores the need to develop more resilient and equitable approaches to both managing grid operations under extreme events and mitigating air pollution.

This study demonstrates the significant role of hydrometeorological extremes, particularly heat waves and droughts, in driving human exposure to power plant emissions and exacerbating existing pollution inequalities. While emissions taxes effectively reduce average pollution damages, they are ineffective on days of extreme heat and high electricity demand due to supply scarcity. Future research should incorporate dynamic estimates of health damages that consider weather conditions' influence on air chemistry reactions and utilize higher-resolution power grid models. This research is crucial for informing effective strategies to protect human health and build more resilient and equitable power systems under climate change.

This study has some limitations. Computational constraints necessitated simplification of the grid operations model, notably the network topology. The model does not consider planned or forced outages of generators or transmission lines, potentially underestimating extreme scarcity probabilities and the likelihood of emissions control policy failure. Damage rate calculations assume constant rates, neglecting the influence of weather on air chemistry reactions. The study examines only the 2018 version of the CAISO and West Coast power grids under stationary hydrometeorological uncertainty. Future work should incorporate dynamic damage estimates, more detailed grid models, and the effects of planned decarbonization and climate change.