- Scimyst

To mitigate the dangerous impacts of heat domes and the urban heat island effect, cities in the United States are actively planting trees, painting sidewalks white, and reducing concrete and cement surfaces.

In 2013, Los Angeles became the first major city to pass a regulation requiring all new homes to have heat-reflective roofs. Since then, the city has implemented numerous cooling initiatives, including white sidewalk paint and expanding its Green Building Standard to require heat-reflective roofs for non-residential buildings, according to BBC.

However, despite efforts to reduce heat, residents still face challenges associated with high temperatures. A study found that the number of emergency calls related to extreme heat in Los Angeles from 2018 to 2022 directly correlates with the number of days above 32 degrees Celsius.

In June, a prolonged heat dome brought dangerously high temperatures to the western United States. Residents experienced temperatures reaching 46.1 degrees Celsius in California, Nevada, Utah, and Arizona. The recent heat wave underscored the importance of being prepared for extreme weather in cities. Approximately 80% of the U.S. population lives in urban areas, where the urban heat island effect can worsen under extreme heat conditions. Urban heat islands are densely populated areas with few trees and extensive concrete and asphalt that absorb solar energy. As the sun sets, artificial materials release stored heat, keeping cities warm in the evening. Urban heat island temperatures can be up to 11 degrees Celsius higher than those in less populated areas. With the number of hot days continuing to rise, U.S. city governments are implementing various measures to cope with the sweltering weather.

Residents cool off under a water fountain in Houston, Texas. (Photo: Xinhua).

Tree Planting

Planting trees helps reduce ground and air temperatures by providing shade and cooling through the processes of evaporation and transpiration. Studies show that urban forests can have average temperatures 1.6 degrees Celsius lower than urban areas without trees. Many cities across the U.S. are launching greening initiatives. Austin, Texas, aims to green 50% of the city by 2050. In Phoenix, Arizona, the hottest city in the U.S., tree planting initiatives are providing shade for several neighborhoods. Authorities have approved over $1.4 million in investments to plant 1,800 trees throughout the city to create cooler corridors.

In Tucson, Arizona, drought is making tree planting more challenging in low-income neighborhoods, where residents lack the funds to plant and care for trees in their yards. The city has implemented a rebate program of up to $2,000 to assist residents in installing rainwater harvesting systems for watering their trees.

Trees are not only being planted on the ground but also on rooftops. In 2017, San Francisco mandated that at least 15% of the roof surface area of new buildings larger than 20,000 square feet must be covered with photovoltaic panels or greenery. A significant number of buildings in the city have green roofs, which not only remove heat from the air through evapotranspiration but also reduce the roof surface temperature. During hot summer days, green-roof temperatures can be cooler than the surrounding air, while traditional roofs can exceed 40 degrees Celsius.

White Paint

A recent study found that white-painted roofs reflect 80% of sunlight and are cooler by 31 degrees Celsius in the summer afternoons. This is not a new idea; cities in North Africa and Southern Europe have been applying it for centuries. Currently, this solution is being implemented in the U.S. New York City recently painted over 930,000 square meters of rooftops white, helping to reduce indoor temperatures by 30%. California has also updated its building codes to encourage heat-reflective roofs.

Scientists around the world are developing various cool-coating paints for sidewalks, roofs, and walls, containing special additives that reflect heat from the sun. These can help pedestrians feel up to 1.5 degrees Celsius cooler, while also reducing surface heat absorption, meaning that at night, lighter surfaces do not release stored heat from the day. Los Angeles has tested cool paints but faced some limitations. The type of paint the city used costs $40,000 per mile (1.6 km) and lasts only 7 years. In 2020, a study in Phoenix showed that covering sidewalks with cool paint helped lower street surface temperatures. Subsequently, the city decided to implement a permanent program.

Data Collection

Data collection plays a crucial role in future planning to cope with heat. In 2022, 60 volunteers measured morning, afternoon, and evening temperatures in Clark County, including Las Vegas, in a heat mapping study funded by the National Oceanic and Atmospheric Administration. The map created from that data showed that the highest temperature increases occurred in northern and eastern Las Vegas, as well as downtown, which could be up to 6 degrees Celsius hotter than other areas in the city. Clark County is currently using this data to find ways to mitigate heat, including building public cooling centers and planting trees.

The city of Albuquerque, New Mexico, is partnering with local volunteers to map temperature and humidity by distributing specially designed temperature sensors. Residents drive or bike around pre-defined routes twice a day to record over 67,000 temperature points. The map reveals temperature differences of nearly 9.4 degrees Celsius across different areas of the city, with the hottest areas being downtown and neighborhoods adjacent to highways, where low-income communities are most affected.