Have you ever noticed that some areas of Pittsburgh feel much hotter than a nearby park or tree-lined neighborhood on a summer day? That’s because of what’s known as an urban heat island. Urban heat islands are built-up areas that stay warmer than nearby rural or natural areas, especially during the summer months.
How do they happen?
Trees do a lot more than make our neighborhoods beautiful: they naturally cool the places where we live. But in many cities, trees have been removed in favor of roads, sidewalks, parking lots, and buildings. Without the shade and cooling effect of trees, temperatures in urban areas can increase by 3°F compared to nearby places with more vegetation.
The materials that make up our cities also play a role. Asphalt, concrete, and dark-colored buildings absorb heat from the sun throughout the day and slowly release it back into the air, making neighborhoods feel hotter long after the sun goes down.
The way our neighborhoods were built also matters. Decades of redlining and disinvestment left many communities—especially Black and other historically marginalized neighborhoods—with fewer parks, fewer street trees, and less investment in green space. Those patterns are still visible today. In Pittsburgh and cities across the country, neighborhoods that were historically redlined often have significantly lower tree canopy than wealthier, predominantly white neighborhoods. That lack of tree cover contributes to hotter streets, hotter homes, and greater exposure to extreme heat.
What Can We Do?: Increase Trees & Vegetation
If urban heat islands happen because we’ve lost tree canopy, then planting and caring for trees is one of the most effective ways to cool our communities.
Trees cool neighborhoods in two important ways. First, they provide shade that keeps streets, sidewalks, buildings, and other surfaces from absorbing as much heat during the day. Second, they cool the air through evapotranspiration, a natural process in which trees absorb water through their roots and release it through their leaves. Together, these processes help lower both surface and air temperatures.
The benefits don’t stop there. Strategically planted trees can shade homes and businesses, reducing the need for air conditioning during the hottest months. Green infrastructure has been shown to reduce energy demand by 10 percent, helping lower both energy costs and greenhouse gas emissions.
Trees also improve air quality. They store carbon dioxide, helping reduce the amount of greenhouse gases in the atmosphere, while their leaves capture pollutants like nitrogen oxides, sulfur dioxide, carbon monoxide, and ground-level ozone. Research shows that tall, dense vegetation planted near roadways and other major sources of pollution can reduce air pollutants by approximately 30 percent.
Their impact extends below ground, too. Trees intercept rainfall before it reaches the ground, reducing the amount of stormwater that becomes runoff. Urban trees can absorb 15–27 percent of annual rainfall, helping reduce flooding while filtering pollutants like fertilizers, pesticides, and animal waste before they reach local waterways. The shade that trees provide also help to keep stormwater cooler, protecting streams and rivers from harmful temperature spikes that can affect aquatic life.
Perhaps most importantly, planting trees is an investment that lasts for generations. A healthy urban forest doesn’t just make neighborhoods cooler today; it helps communities become healthier, more resilient, and better prepared for a warming climate in the future.
Impacts of Urban Heat Islands
Human Health
Hotter neighborhoods affect more than just comfort. They can have serious impacts on our health.
As temperatures continue to rise, heat-related illnesses and deaths are becoming more common. Older adults, young children, low-income communities, outdoor workers, and people living with chronic health conditions, disabilities, mobility limitations, or taking certain medications are especially vulnerable. Heat exhaustion and heat stroke become more common during extreme heat, and long-term exposure can even contribute to cardiovascular disease.
Research also shows that spending time around nature can lower heart rate and blood pressure while improving immune system function. When neighborhoods lack trees and green spaces, people miss out on those important health benefits.
Energy Consumption
When temperatures climb, so does our use of electricity.
Air conditioners work overtime during the hottest parts of the day, and homes and businesses use more energy to stay comfortable. As temperatures continue to rise, so will the demand for energy.
Air Pollution
More energy use also means more pollution.
To meet higher electricity demand, more fossil fuels are burned, releasing additional greenhouse gases into the atmosphere. That contributes to even warmer temperatures, creating a cycle that’s difficult to break. Urban heat islands don’t just result from climate change, they can also make its impacts even worse.
More Frequent and Harsher Heat Waves
Heat waves have become more common across the United States over the past several decades. As global temperatures continue to rise due to climate change, communities that already experience extreme heat are expected to see heat waves become even more frequent and more intense.
Urban heat islands aren’t just about rising temperatures. They’re about the neighborhoods where people live, work, and spend time every day. By planting and caring for trees, protecting existing canopy, and investing in greener communities, we can help make Pittsburgh cooler, healthier, and more resilient for generations to come.