Climate Change Threatens Already Poor Air Quality in California’s Central Valley

July 26, 2022 | 3:57 pm
California farmland seen under a hazy skyLevi Meir Clancy/Unsplash
Carly Phillips
Research Scientist

California’s Central Valley consistently experiences the country’s worst air quality, and climate change is poised to make air quality even worse.

In a region known for its exceptional agricultural productivity, climate change is quickly amplifying a dangerous type of climate risk in California’s Central Valley: air pollution.

The Central Valley produces about 25% of our nation’s food, despite occupying just 1% of nationwide farmland. Amidst this abundance and more than 15% of California’s population, are some of the highest rates of poverty, housing insecurity, and at-risk workers in California.

While for many of us, the Central Valley remains out of sight and out of mind, the region is a cornerstone of our nation’s food supply. As my colleagues’ blogs have illustrated, extreme heat, record breaking drought, and staggering inequities affect the region and its communities, but also cause ripples around the world. The Central Valley is exceptional in another way as well: it records some of the worst air quality in the country, and the health of this region affects us all.

Air pollution in the Central Valley

Even without the effects of human-caused climate change, the Central Valley experiences some of California’s worst air quality. Bakersfield, Fresno, and Visalia are consistently ranked as cities with the worst particulate and ozone pollution, according to the American Lung Association. Poor air quality is of particular concern for residents in disadvantaged communities that tend to be located closer to pollution sources and have fewer resources to adapt. For example, they may need to choose between cooling down with their windows open at night or breathing polluted air, as residents often don’t have air conditioners or the economic capacity to pay for higher electricity bills.

Annual average PM2.5 and ozone concentrations highlighting air quality issues in the Central Valley. Graphics from CalEnviroScreen 4.0 report.

The Valley’s specific topography contributes by trapping polluted air between the Coast Range and Sierra Nevada mountains that border the region, but the drivers of pollutants themselves follow seasonal patterns: wildfires and agricultural burning in the summer and autumn, residential fireplaces in the winter, and pollution from vehicle traffic and agricultural production year-round. 

Each of these sources emits a range of pollutants, including ozone and fine particulate matter, often referred to as PM2.5. This type of pollutant is particularly dangerous, since it can pass into the bloodstream and has been linked to health conditions from asthma to stroke to premature death. The effects of persistently poor air quality can be seen throughout the Valley, where residents are at greater risk for asthma and heart attacks. Outdoor workers, who help to produce a quarter of our country’s food, are at particular risk.

Impacts of poor air quality in the Central Valley shown as emergency department visits for asthma and cardiovascular disease. Graphics from CalEnviroScreen 4.0 report.

For years, state and local air quality districts have implemented polices and regulations aimed at curbing these different sources of pollution, like working to phase out burning as a method of disposing of agricultural waste, incentivizing transitions to gas-powered heat, and creating regulations to limit vehicle pollution.

Climate change amplifies air quality problems

Despite some improvements in air quality, climate change is complicating these efforts and exacerbating many drivers of pollution, threatening to worsen the Central Valley’s already poor air quality. While agricultural burning declined between 2003 and 2011, reducing particulate pollution by more than 50%, California’s record-breaking drought has led to a tripling of the amount of waste burned, with some fearing further escalation of burning as drought persists.

Extreme heat, due to climate change, can also worsen air quality by increasing energy production that generates pollutants, and accelerating chemical reactions that create pollutants like ozone. And like drought, more extreme heat is in our future as the climate continues to warm. Historically, Fresno experienced 3 days per year on average with temperatures over 105°F. By the end of the century, those same extreme temperatures will occur 59 days per year. This pattern of future extreme heat is consistent across many cities in the Central Valley, including Bakersfield, Modesto, Stockton, and Sacramento.

Further, climate-fueled wildfires threaten air quality and human health, as wildfire smoke can lead to worse respiratory impacts than pollution from other sources and was recently shown to have contributed to COVID-19 deaths in the Western US. Smoke exposure has been linked to a lower birth weight and higher risk of pre-term birth in California, and poses a risk for more of the year, as wildfire season has become a year-round phenomenon. The Central Valley is particularly vulnerable, due to the Valley’s geography, which traps wildfire smoke, and the 850,000 people who work outdoors in the region—nearly 22% of California’s outdoor workers. This was particularly pronounced during the wildfire seasons of 2020 and 2021, when 4 of the 5 largest wildfires in California’s history burned.  During the large fire years of 2018-2020, Fresno experienced 51 high particle pollution days, up from 34 days for the period of 2017-2019. 

When it comes to smoke impacts, communities in the Central Valley are especially vulnerable due to the number of people who work outdoors and a series of structural inequities that, through decades, have limited resources and enforcement of regulations that would reduce air pollution. Lower income communities have fewer choices about being outdoors during the smokiest days, and are also less likely to have air filtration equipment at home. Despite long-term air quality issues and intensifying wildfire seasons, Cal/OSHA did not update regulations to protect outdoor workers from wildfire smoke until 2019. Even still, many workers report not receiving the required N95 masks or other personal protective equipment from their employer on smoky days.

Actions like prescribed burns—planned burns under specific weather conditions—are one way to mitigate the harm of wildfire smoke exposure. The consequences of catastrophic wildfires can last for weeks to months and reduce air quality across the country. In comparison, the air quality impacts of prescribed burning are more localized and occur over shorter time periods.   

Climate change futures in the Central Valley

As my colleague Pablo Ortiz writes, climate change impacts across the Central Valley should serve as a warning of what’s to come. The combination of extreme heat, extreme drought, and poor air quality impact the region’s communities, workers, and economic backbone with consequences for the entire country.  

Climate action, like enhancing clean energy, adopting EVs, and reducing wildfire risk via prescribed burning, can contribute to improved air quality. But without meaningful climate action, poor air quality throughout California’s Central Valley is likely to persist.