On 17 July 2023, Fresno, Calif., experienced its hottest temperature of the year, peaking at about 111°F (44°C). Some residents who couldn’t stay indoors sheltered under shadows cast by sycamore, elm, and other tree canopies. But many in low-income neighborhoods were left exposed to the scorching sunshine.
In Fresno, high-income neighborhoods, such as Hyde Park, have high tree canopies and many shady spots. But low-income neighborhoods, like several in South Fresno, have little vegetation, many concrete parking lots, and industrial areas that border fallow, dirt fields.
“Extreme heat events have the most impact on people who have to spend time outside,” said Santa Clara University hydrologist and climate scientist Iris Stewart-Frey. In fact, jobs in the agricultural sector carry 35 times the risk of heat-related death compared with most other jobs in the United States. And many residents in the disadvantaged communities of California’s southern Central Valley are agricultural workers who put in long hours in hot environments.
A Haven from the Heat
Extreme heat causes more deaths in the United States than floods, lightning, tornadoes, hurricanes, or cold, according to 2022 U.S. Natural Hazard Statistics data. Heat waves can be especially harmful for people with such preexisting health conditions as arthritis, diabetes, and heart disease.
“It’s the most effective way of mitigating extreme heat.”
But for those who must be outdoors in sweltering conditions, sheltering under shade is an easy way to block solar radiation and cool down. “It’s the most effective way of mitigating extreme heat,” said Santa Clara University remote sensing scientist Jake Dialesandro.
Days like 17 July inspired Dialesandro, Stewart-Frey, and other colleagues to compare shade coverage in wealthier versus disadvantaged communities in the city of Fresno in California’s Central Valley.
For the first time, they documented the striking extent of the region’s shade inequity and accompanying health risk for low-income communities. The researchers presented the results of their research at AGU’s Annual Meeting 2023 in San Francisco.
Shedding Light on Shadows
The team’s main tool was lidar, which relies on an airplane beaming a laser over a region to measure the heights of buildings and treetops. By inputting those heights into an algorithm describing the positions of the Sun in the sky, they mapped where shade is cast each day. The researchers used a metric known as mean radiant temperature, which considers factors such as shade, humidity, and wind speed to quantify how much heat a person feels in a given environment.
Disadvantaged communities, defined as those having an income of no more than 80% of the state’s median income, had a mean radiant temperature of 122.7°F (50.4°C) on that date. In contrast, wealthier Fresno communities had a mean radiant temperature of 115.7°F (46.5°C)—a statistically significant difference for public health impacts.
The researchers then simulated increased tree canopy in both communities. They found greater temperature effects in disadvantaged communities because those areas had little tree cover to begin with. The team hopes to expand their study on shade inequity to more California cities soon, including Modesto in the Central Valley.
Seeking Environmental Justice
Other communities in the United States have taken action to mitigate shade-related inequities. Arizona’s Maricopa County, for example, requires buildings and pedestrian routes to meet certain shade standards: Walking routes considered safe during the summer are 62% covered by shade during the hottest times of the day, according to the Maricopa Association of Governments.
“We need to be able to tailor adaptation, equity, and reparations strategies so they do the most good for the communities that really need them.”
Dialesandro’s team partnered with California Rural Legal Assistance, a nonprofit advocacy organization that provides legal services to low-income residents of rural California counties. He hopes his team’s findings could help lead to legislation that would require heat-prone regions in California to set shade standards like those in Maricopa County.
“To catalyze action, we need to show the world hard evidence of who suffers most from the climate crisis,” wrote urban planner Stephen Wheeler of the University of California, Davis, who was not involved in the study, in an email. “And then we need to be able to tailor adaptation, equity, and reparations strategies so they do the most good for the communities that really need them.”
—Andrea Tamayo (@andreaxtamayo), Science Writer