3 Ways Planting Trees Helps to Reduce The Urban Heat Island Effect

Heat Waves And Rising Air Temperatures Are Presenting Challenges — Especially When It Comes to Urban Heat Islands

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, research shows that heat already kills more people in the US than hurricanes, tornadoes and other extreme weather events, contributing to an estimated 12,000 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 around 10 degrees. And in a rapidly warming world, that could mean the difference between life and death for many 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 add up to 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 have them. 

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 different 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 studies have shown 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 tend to be most intense during the day when the sun is shining.

• 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.