In recent years, the Los Angeles area – home to 9.8 million people – has experienced unprecedented heat waves and a steady increase in average temperatures due to climate change. While climate change is a global problem, its effects often affect disadvantaged communities more strongly than others. A new study shows that in Los Angeles County, lower-income neighborhoods have higher surface temperatures than higher-income neighborhoods. These variations can reach 36 degrees Fahrenheit at noon on a summer day.
The study showed that the disparities are primarily due to higher levels of vegetation, which helps dissipate heat in higher-income regions. Meanwhile, planting more trees and using sustainable conservation and irrigation practices in low-income areas can help lower surface temperatures. In dense urban areas where planting trees is less feasible, the authors suggest that increasing the reflectivity of surfaces—roofs, streets, etc.—could help lower temperatures.
The study is described in a paper titled “Uneven Exposure to Heatwaves in Los Angeles: The Effect of Uneven Greenery” and published in the journal Nature. Science advances On April 28, it took place in the laboratories of Christian Frankenberg, professor of environmental science and engineering and research scientist at the Jet Propulsion Laboratory, which is operated by Caltech for NASA; and Paul Weinberg, R. Stanton Avery Professor in Atmospheric Chemistry and Environmental Science and Engineering.
“Increasing awareness of the dangers of heat waves, and recognizing unequal exposure across different socioeconomic groups, is increasingly important,” Frankenberg says. “Studies like ours can inform urban planning choices to promote environmental justice and increase the resilience of our cities as summer heats up.”
Led by Caltech research scientist Yi Yin, the team looked at high-resolution surface temperature measurements collected over the past four years. This data was taken by ECOSTRESS, a satellite instrument developed at JPL and installed on the International Space Station that accurately measures the surface temperature of individual city blocks, allowing researchers to examine interior spatial patterns. The researchers found that average household income had a strong negative relationship with surface temperature. In other words, a higher median household income is very likely to indicate cooler surface temperatures.
“Initially, this started as a personal interest—I was looking for housing and noticed that green, lush areas were more expensive to live in,” says Yin. “This inspired me to conduct the study with detailed temperature data from the ECOSTRESS tool and compare it to data on median household income. The association was shockingly strong.”
What causes these disparities? After all, the sun shines evenly over the entire Los Angeles area, so why do roofs in some areas get hotter than others?
While geography, such as distance from a cold ocean, plays a role, the researchers found that the controlling factor in surface temperature variation is the amount of water evaporating into the atmosphere. When water evaporates – turning from a liquid into a gas – it carries heat away from the surface. This is similar to someone getting a cold when getting out of a pool – the water evaporating from your skin carries the heat away from you. The study found that more water evaporates in affluent neighborhoods as a result of the dense tree canopy, which leads to a cooling effect.
In Los Angeles’ semi-arid climate, vegetation is supported primarily by irrigation rather than rainfall during the dry seasons. In the study, the researchers recommended planting more trees in lower-income neighborhoods and prioritizing tree cover while staying away from turfgrass. Although lawn grass increases the amount of water evaporation, it does not provide shade. “Although it can be difficult to plant and maintain trees in dense urban environments, especially those with limited open space and water availability, trees produce shade and improve the quality of life for residents,” says Yin.
“While irrigation in dry urban areas is often described as wasteful, this study demonstrates the enormous benefits that come from using water in reducing exposure to heat,” Weinberg adds.
Another physical factor that can modify temperatures is the amount of sunlight absorbed by surfaces. For example, asphalt parking lots efficiently absorb sunlight and thus heat up quickly. In contrast, it is possible to use paints and other materials that reflect more sunlight and thus reduce heating. Current Los Angeles City and County policies, for example, require reflective “cold roofs.”
Yi Yin et al, Unequal exposure to heatwaves in Los Angeles: the effect of uneven green spaces, Science advances (2023). DOI: 10.1126/sciadv.ade8501
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