How Cities Can Cool Down Urban Heat Islands

More than 80% of Americans live in cities, which tend to be hotter than rural areas due to the “urban heat island” effect—where the high density of buildings, parking lots, and pavement absorb and retain heat. These higher temperatures can exacerbate health problems like asthma, create dangerous conditions for vulnerable populations, and escalate energy consumption and greenhouse gas emissions from people using more air conditioning.   

As climate change makes heat waves more frequent, cities are particularly susceptible to temperature spikes because of their urban heat islands. Mayors have asked residents to stay inside on hot days, high temperatures have forced school closures, and cities are setting up dedicated cooling centers for residents that lack AC. In short, heat is becoming a crisis in cities, and municipal leaders are searching for solutions.

Vivek Shandas, a professor at Portland State University who has written extensively about urban heat islands, said that cities can take a variety of approaches to cooling things down. The first step? Collect good data.

“A lot of cities recognize that where there’s a high volume of large buildings and a lot of asphalt, that’ll amplify temperatures more locally,” Shandas said. “But cities are usually using satellite-derived images of urban heat to identify hot spots, which might not give the best granular data.”

Shandas and a team of researchers recently worked on a new process for measuring temperature across a city, one that recruits volunteers through community groups to get on-the-ground data. Volunteers in 20 U.S. cities attached thermal monitors to their cars and drove slowly down streets in their cities this summer, collecting data on a hot day with no clouds during the morning, afternoon, and evening. They found some surprising results. In many cities, there was a 15 to 20-degree temperature difference between different neighborhoods. In West Palm Beach, Florida, Shandas said, the heat index hit 120 degrees in some places, while other areas didn’t even break 90. 

“There’s a lot of variation even street to street,” Shandas said. “A neighborhood could have industrial land with a lot of big box designs for buildings, lots of parking with black asphalt, and that could make it hotter. But we also found that a dry field can act like asphalt sometimes, and we pick up that heat as we go by.”

Once cities have granular data, they can start to understand where the heat islands are worst—because not all parts of a city are equally impacted. In forthcoming research, Shandas said he and a team mapped 108 different cities in the U.S. and found that 92% showed an association between heat and past redlining practices. Redlined neighborhoods that were home to people of color, and where people had trouble getting loans, were also often passed by when cities made investments in parks and green space, which mitigate urban heat, Shandas noted. And these communities often ended up as the places where industrial, boxy buildings with large parking lots were located. 

Shandas explains that there are six main factors that influence hyperlocal heat islands: the height of the tree canopy, the volume of the tree canopy, the amount of vegetation on the ground, the volume of buildings, the difference in height between those buildings, and the buildings’ coloring. Many low-income neighborhoods or communities home to people of color often lack older trees and ample lawns, as well as being the location for some of the most dense building patterns in a city. 

As a result, these communities are often the first to feel the impacts of rising temperatures. In many cities, residents of poorer neighborhoods are more likely to be hospitalized or die as a result of heat waves. Black and Hispanic residents are already more likely to have asthma, a condition worsened by increased heat. The number of shootings also rise during heat waves in most cities, and there are more incidences of road rage that causes traffic accidents.

“Vulnerable communities are the first to undergo changes,” Shandas said. “But we can use granular data to come up with comprehensive plans for a city.”

Armed with data, city governments have taken a variety of approaches to cooling down hot neighborhoods. They’re whitewashing asphalt and roofs to reflect the sun instead of absorbing heat and planting trees to provide more shade. One area that’s still ripe for reform, Shandas said, are municipal codes. 

“We should apply a level of scrutiny to building design through the permitting process,” Shandas said. “As mundane as that seems, cities have to make sure that a building that will be there in 100 years, when the climate looks very different, has done its due diligence to make sure the people living and working there will be safe. We’ve done that for earthquakes—why not local climate stressors?” 

Shandas is in favor of municipal code reforms that require developers to consider designs that reduce—or at least don’t amplify—hyperlocal temperatures. A grocery store, for example, could be required to do an underground parking lot, or shade the asphalt with trees. In Shandas’ home base of Portland, Oregon the city council has already taken up the issue with their new Better Housing By Design code changes.

The Oregon legislature this session voted to essentially ban single-family zoning to allow for greater density in response to the growing housing crisis in the state. But even if density increases in Portland and other cities across the state, there are a variety of code changes that can ensure heat islands also don’t grow. Shandas pointed to simple changes like reducing the amount of a lot that can be covered from 100% to 85%, which allows room for green space, or limiting the amount of impervious surfaces by only allowing a single driveway for two buildings. Cities can also incentivize developers to install green roofs and separate buildings to allow for greater air circulation between them.

The process for writing these new code stipulations in Portland was “incredibly robust,” Shandas said, because it involved members of the community, researchers, developers, and municipal staff. 

“We all recognize the danger of heat, it’s ultimately just a question of what we do about it,” he said. “It’s the government’s responsibility to address climate change and heat islands in the interest of public health and safety.”