What are urban heat islands?

Mirroring their increasing populations, the United States’ current 10 largest cities have gradually become warmer over the past century, according to data from the U.S. Census Bureau and the National Oceanic and Atmospheric Administration (NOAA).

Such rising temperatures can be attributed to growing intensity of urban heat islands, or urban regions that are noticeably hotter than surrounding areas. And with expected increases in urban populations and temperature from climate change, cities are looking for new ways to address the causes and impacts of heat islands.

Of the 10 largest U.S. cities, Phoenix has warmed the most: the average yearly temperature from 2010 to 2017 was 6.7 degrees Fahrenheit higher than it was just before the turn of the century.

Additionally, it is one of the fastest growing cities in the country. Population growth is one important factor contributing to urban heat islands in Phoenix and similar cities.

Urban heat islands are intensified by conventional buildings and roads that trap heat, and growing populations typically mean more infrastructure, said Kamil Kaloush, director of the National Center of Excellence for SMART Innovations at Arizona State University (ASU).

“The expansion of the built environment to accommodate population growth will continue to challenge our selection of materials and infrastructure design to mitigate the (urban heat island),” Kaloush said in an email, noting that climate change’s warmer weather could further increase the intensity of heat islands.

Dallas is in a similar situation. Its average temperature from 2010 to 2016 was 3.2 degrees higher than its baseline year of 1900, and the city is expecting to grow significantly in the coming decades. According to the North Central Texas Council of Governments (NCTCOG), the Dallas-Fort Worth population is projected to increase from 7 to 10 million by 2040.

One of the areas the NCTCOG is focusing on is reducing ground-level ozone levels, a harmful air pollutant. Because heat is an important agent in ozone's formation, anything that increases the temperature, like heat islands, are a concern, said Derica Peters, an environment and development planner at the NCTCOG.

Heat islands can have a variety of other impacts on a community. According to a 2015 article in the Annual Review of Environment and Resources, heat islands can increase the energy intensity of buildings, negatively impact water quality and increase water consumption. This is particularly problematic because heat islands tend to occur in semi-arid and arid regions.

Cities and researchers have explored various strategies to address different aspects of urban heat islands.

In northern Texas, for example, the NCTCOG developed principles that local communities can follow, including recommendations for efficient growth, diverse land-use patterns and walkability in cities, Peters said.

“Our various programs and efforts, they were developed from consensus having come about in the region,” Peters said. “That way cities can know that they’re being supported throughout the region and that as a whole we devised what we think are opportunities that are best suited for our region.”

Other common heat island mitigation strategies include green or cool roofs and urban forestry programs. Adding vegetation to a roof or creating roofs that are more reflective and can reduce building energy intensity, while increasing forestry can provide shade and help cool urban areas.

Additionally, pavements, which represent the "largest percentage of a community's land cover," according to an Environmental Protection Agency report, are another important factor in reducing heat island intensity.

ASU's Kamil Kaloush focuses his research on permeable and modified pavements, which he said will help maintain durable highways and help decrease urban temperatures.

“They are able to dissipate the heat more quickly than dense pavements,” Kaloush said. “They can also allow for water flow, which has a positive effect on storm water management, and cool the surface temperature.”

Peters said she thinks cities should be proactive in creating an environment that incentivizes building in ways that reduce heat island intensity and impacts.

“As we are well aware that the urban heat island is here, and as we’re simultaneously trying to prevent it and mitigate it, we have to be able to respond to it as it does impact the public and our infrastructure,” Peters said.