What’s at Stake
You might want to live in a “hot neighborhood,” but you probably don’t mean it literally. Urban heat islands are areas that are significantly hotter than the surrounding countryside.
According to NOAA, heat islands can have peak summer temperatures “15°F to 20°F hotter than nearby areas with more trees and less pavement.”
Heat islands occur because asphalt, concrete, bricks, and other building materials absorb and radiate heat. And city life generates a great deal of heat: Car, truck, and bus engines are just part of the problem. Air conditioning from urban buildings can add 20% more heat to the outside air during a heatwave compared with typical summer weather. In contrast, trees and other plants absorb heat energy and help cool the air through evaporation.
As we saw during the July 2021 heatwave in the Pacific Northwest, extreme heat is dangerous to public health. It makes it unsafe to work outside or live in un-airconditioned buildings. It also creates increased demand for energy use, contributing to greenhouse gas emissions, brownouts, and a financial burden for consumers.
Urban heat islands also highlight inequities. Low-income communities and communities of color are often located in heat islands because there has been less historic investment in green infrastructure in those neighborhoods.
Find your local Tree Equity Score using this interactive tool.
According to a recent report by Climate Central, the cities with the worst heat islands are New Orleans, Newark, N.J., New York City, Houston, and San Francisco.
What are the solutions?
1. Planting trees. Shaded surfaces may be 20–45°F (11–25°C) cooler than unshaded surfaces. Trees also absorb carbon dioxide (a greenhouse gas), improve air quality, and slow down stormwater runoff. And trees raise property values. Urban forestry studies found that large trees in yards or along streets could add 3-15% to neighborhood home values; houses next to parks and open green spaces were valued 8-20% higher than similar properties.
2. Green roofs. These rooftop gardens provide shade and lower temperatures. Green roof temperatures can be 30–40°F lower than on conventional roofs, and green roofs can reduce city-wide ambient temperatures by up to 5°F.
3. Cool roofs consist of advanced materials that reflect heat rather than absorbing it, reducing energy use. These roofs absorb less heat and stay up to 50–60°F (28–33°C) cooler than conventional materials during the peak summer heat.
Homeowners and developers can choose to incorporate such features into their urban neighborhoods, and cities can pass ordinances encouraging and subsidizing them. The result would be a more pleasant and livable urban environment.