Key takeaways: How Trees Cool Cities, Lower Energy Use, and Support Well-Being
- Urban extreme-heat exposure nearly tripled worldwide between 1983 and 2016, driven by rising temperatures, the urban heat-island effect, and population growth in cities.
- Urban heat islands form when pavement, rooftops, buildings, traffic, industry, and other human activity absorb, store, and release heat — making cities substantially warmer than nearby rural areas.
- Trees cool cities through shade and evapotranspiration: shaded surfaces can be 20–45°F cooler at peak heat, while vegetation can help reduce peak summer air temperatures by an estimated 2–9°F.
- Strategically placed urban trees can lower cooling-energy demand, reduce strain on electric grids during heat waves, and limit additional waste heat from air-conditioning systems.
- Urban forestry can advance climate resilience and tree equity. A tree planting at a Title I school in heat-burdened West Phoenix was designed to cool school grounds, reduce energy costs, improve air quality, manage stormwater, and connect students with environmental education.
Heat Waves And Rising Air Temperatures Are Underscoring the Need for Urban Trees and the Shade They Provide
In 2021, researchers published a landmark study in the journal Proceedings of the National Academy of Sciences that put hard numbers to a reality billions of people are already feeling: cities are becoming dramatically hotter. Using detailed temperature records combined with population data, the team mapped how often, and how many people, were exposed to extreme heat in 13,115 cities worldwide over more than three decades, from 1983 to 2016. Their focus was on “extreme heat exposure” — a measurement that includes both the frequency of intense heat and where people actually live.
By combining these fine‑resolution temperature and population datasets, the scientists found that urban extreme heat exposure increased by nearly 200% over that period — meaning it's about three times higher than it was in the early 1980s — driven by both climate change and the urban heat island effect, and now affecting roughly 1.7 billion people living in cities around the world. This surge isn’t simply because more people moved into cities; total urban warming pushed exposure rates 52% higher than population growth alone would have caused, showing how rising temperatures and expanding concrete landscapes compound risk together.
In the United States, a 2020 study led by researchers at Duke University estimated that heat exposure caused about 12,000 premature deaths per year during the 2010s, and projected that this toll would rise sharply under both high and moderate future warming scenarios.
An important part of the solution? Trees. When done thoughtfully, planting urban trees, growing urban forests and enhancing urban green canopy can make a big difference. In one U.S. city, researchers found that on hot summer days, neighborhoods with plenty of trees can be several degrees cooler than nearby streets with little shade — and that the biggest cooling benefits show up once tree cover exceeds 40%. In a rapidly warming world, that could mean the difference between life and death for vulnerable city dwellers.
What Are Urban Heat Islands?
Simply put, urban heat islands are areas within urban and suburban environments that experience elevated temperatures — especially when compared to rural zones. When homes, businesses and industrial buildings are built close together, they generate, trap and store heat, significantly increasing surrounding air temperatures.
Another source of heat that may come as surprise? People. Whether it's driving a car or powering a factory, any time people burn energy, they are probably generating heat. When there are a lot of people engaged in a lot of activities close together, this "waste heat" can combine with other heat sources to create a significant temperature increase.
In heavily populated areas with busy roadways and tall, close-together buildings, there's little opportunity for hot air to escape, thus creating Urban Heat Islands.
According to the US EPA, "the annual mean air temperature of a city with one million or more people can be 1.8 to 5.4°F (1 to 3°C) warmer than its surroundings — and on a clear, calm night, this temperature difference can be as much as 22°F (12°C)." Given the rising prevalence of heat waves and record-setting temperatures, that's a big deal.
Understanding Heat Island Types
- Surface Heat Islands: Man-made surfaces like roadways and rooftops absorb and emit significantly more heat than natural surfaces like grass and soil. And data shows that on a warm day with a temperature of 91°F, conventional roofing materials may be as much as 60°F warmer than surrounding air temperatures. Surface heat islands are typically strongest during the daytime.
- Atmospheric Heat Islands: These heat islands are defined as the warmer air found in urban areas, compared with cooler air in less heavily settled areas. Typically weaker during the late morning and throughout the daytime hours, they form as a result of slowly releasing solar heat that has been absorbed by buildings, concrete surfaces and other built materials throughout the day. This is why, even after the sun has set, temperatures can remain high.
Case Study: Urban Heat in Miami
In 2021, our Urban Forestry team conducted research at an urban forest installation in Miami, FL. Using heat traps, they recorded temperatures both outside of the installation and inside to determine how much the trees reduced temperatures. Read the PDF Case Study to learn about their findings!
Case Study: Planting Trees at a Title I Elementary School in Phoenix, AZ
With temperatures that regularly soar above 100° F, Phoenix is considered America's hottest city. In fact, Phoenix is already experiencing temperatures 2.5° hotter than they were in the middle of the last century — and this is only expected to increase. Researchers at Arizona State University’s School of Geographical Sciences and Urban Planning used decade-long climate simulations to compare population heat exposure in 2000–2009 with projections for 2090–2099. They found that rapidly growing Sunbelt cities — including Orlando, Austin, Miami, and Atlanta — are projected to experience the largest relative increases in population exposure to locally extreme heat.
To help address this, our partner Trees Matter planted 33 15-gallon trees and engaged approximately 1,000 students and staff at a Title I school that's located near a highway and surrounded by many asphalt-heavy industries. Through the program, students learned about the importance of planting trees, and each class got 30 minutes of hands on experience. They even got to name the trees they planted, fostering a deeper connection with nature.
Our partner plans to have a lasting relationship with the elementary school and will continue to engage staff and students long after the planting. Teachers will also be given free access to environmental education resources, including a database with over 200 resources, lesson plans, and tree inventory activity boxes.
Thanks to a history of under-investment — and a high concentration of asphalt concrete and other heat-absorbing materials — West Phoenix is greatly affected by tree inequity and the Urban Heat Island effect. As a result, heat illness is critically high and air conditioning is a major expense. Fortunately, the planted trees will help reduce costs, as well as the pressure put on these systems at the school. In addition to cooling the school buildings, trees will help cool the playground and fields, allowing kids to play more safely outside. In this area, our partner has seen that trees can reduce surface temperatures by up 10-15 degrees!
In addition to the above mentioned benefits — and with the help of the iTree Design tool — our partners determined that in the first year after planting, the new trees would:
- Intercept 1,117 gallons of rainfall
- Conserve energy from AC needs (1,698 kWh) and heating needs (3.5 therms)
- Remove four pounds of air pollutants like ozone, carbon monoxide, and other particulates
- Reduce atmospheric CO2 emissions by 2,360 pounds.
Planting trees is an essential strategy to protect urban populations from the dangers of urban heat. Looking for a way to make an impact today? Consider supporting our Fund for Urban Forestry!
