3 Ways Planting Trees Helps Reduce The Urban Heat Island Effect

Heat Waves And Rising Air Temperatures Are Underscoring the Need for Urban Trees and the Shade They Provide

At a time when countries around the world are experiencing record-high temperatures and heat waves, the need to address urban heat and climate change is front and center. Compounded by the heat island effect, cities worldwide are experiencing days of extreme heat exposure at 3x the rate they did in 1980 — and with that comes a sharp rise in heat-related illnesses.

In fact, NOAA data shows that heat already kills more people in the US than hurricanes, tornadoes and other extreme weather events. A study by Duke University estimates that this adds up to an estimated 12,000 premature deaths per year.

An important part of the solution? Trees. Scientists have found that when done thoughtfully, planting urban trees and enhancing urban forests and urban green canopy can lower air temperatures in city neighborhoods by up to 10 degrees. And in a rapidly warming world, that could mean the difference between life and death for some 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 areas. When homes, businesses and industrial buildings are built close together, they generate, trap and store heat, which significantly increases 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 that people burn off 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 pretty significant temperature increase.

In heavily populated areas with tall, close-together buildings and busy roadways, 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 rise of heat waves and record-setting temperatures, we probably don't need to tell you that that's a big deal. And as you may have guessed, Urban Heat Islands are especially pronounced in larger cities — but smaller cities and towns also experience them.

Understanding Heat Island Types

In general, temperatures are different at the earth's surface and at the atmospheric level, which is the air high above the city. To help us understand, climate scientists talk about two types of heat islands: surface heat islands and atmospheric heat islands.

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 as compared to cooler air in less heavily settled areas. Typically weaker during the late morning and throughout the daytime hours, they form as a result of the slow release of 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 be high.

Case Study: Urban Heat in Miami

In 2021, members of 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 rise above 100 degrees F, Phoenix, Arizona is considered America's hottest city. In fact, they're already experiencing temperatures that are 2.5 degrees hotter than they were in the middle of the last century, and this is only projected to increase. Researchers at the School of Geographical Sciences & Urban Planning at Arizona State University ran decade-long climate simulations and found that, when compared with conditions in 2000, Phoenix and other fast growing Sunbelt cities will see the “largest relative changes in population heat exposure."

To help address this, our awesome 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 many asphalt-heavy industries. As part of the effort, students learned about the importance of planting trees, and each class spent 30 minutes helping to get them in the ground. They even got to name the trees they planted, which will help foster a deeper connection.

Our partner plans to have a lasting relationship with the elementary school and will continue to engage with staff and students long after the planting. Teachers will be given free access to environmental education resources to continue student engagement with their school’s canopy, including a database with over 200 resources, lesson plans, and tree inventory activity boxes.

phoenix arizona urban tree planting elementary school

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 to 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 down 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 4 pounds of air pollutants like ozone, carbon monoxide, and other particulates
Reduce atmospheric CO2 emissions by 2,360 pounds.

In the context of a rapidly changing climate, planting trees is an essential strategy to protect urban populations and reduce climate change. Looking for a way to make an impact today? Consider supporting our Fund for Urban Forestry!