Millions of Americans are stranded on an urban 'heat island' this summer

With scorching temperatures well past 100 degrees Fahrenheit pummeling much of the U.S. under record-breaking heat, residents in major cities like Houston, Phoenix and Miami are experiencing far more intense weather than those in rural areas thanks to urban "heat islands."

The phenomenon is affecting more than 40 million Americans living under conditions where temperatures are at least 8 degrees Fahrenheit warmer than nearby rural environments, according to a new analysis from the nonprofit research group Climate Central.

What is an urban ‘heat island?’

Urban heat islands are created in cities where open land has been replaced with expansive buildings, concrete sidewalks, paved parking lots, and other materials that absorb and retain heat. At night, a city with more than 1 million residents can experience temperatures of more than 22 degrees higher than surrounding areas, according to the Environmental Protection Agency. Cars and air-conditioners throughout these cities also produce and emit heat, adding to the sweltering temperatures.

"We're basically talking about a phenomenon where there's a measurable increase in urban air temperatures that's caused by the way the city is built," Kaitlyn Trudeau, a climate scientist with Climate Central, told NBC News. "It's additional warming that is caused by dark building materials, dark pavement, building heights, population density and lack of green space."

While the effects of a heat island may be most noticeable during the summer months, the hotter temperatures persist year-round.

The Climate Central analysis examined how populations across 44 cities are affected by global warming, or the ongoing increase in the global temperature, according to the United Nations Intergovernmental Panel on Climate Change.

Scientists believe that rising global temperatures are a direct consequence of climate change, caused by a steady increase in greenhouse gas emissions and the weather pattern known as El Niño — a band of warm air from the tropical Pacific Ocean that recurs every two to seven years. A new study published this week found that the current heat wave baking the U.S. and southern Europe would have been "virtually impossible" without human-caused climate change.

Which cities suffer the most

According to the Climate Central Analysis, at least nine cities, including New York, San Antonio and Los Angeles are cities with at least one million residents that experience the highest urban heat island (UHI) index of at least 8 degrees Fahrenheit or higher, which estimates how much hotter these areas are due to the characteristics of the built environment. This means on days that it's 90°F in an open park, in these cities it would feel at least 98°F. That's especially tough on cities like Houston, Phoenix and Chicago that have experienced daily July averages at or near 100°F.

Poor and communities of color suffer more

Due to fewer heat-reducing amenities like parks, yards and street trees, low-income neighborhoods and communities of color are more likely to be in severe heat islands.

A joint investigation by NPR and the University of Maryland's Howard Center for Investigative Journalism in 2019 found that for residents of dozens of major cities, low-income neighborhoods are more likely to be hotter and pose more health risks than wealthier communities. For example, Baltimore's Franklin Square, a majority-Black neighborhood, is hotter than about two-thirds of the neighborhoods in Baltimore and roughly 6 degrees hotter than the Baltimore's coolest neighborhood.

Many city residents — especially in low-income areas — also live near sources of air pollution like major roadways, industrial facilities and ports can lead to a variety of health effects ranging from asthma to reduced lung function to cardiovascular disease with often inadequate access to health care. Since heat places additional burdens on the cardiovascular system and heat also increases the formation of smog, these pollutants combine with heat to pose an extra health burden.

"Ultimately, I describe extreme heat as such a risk-multiplier," Sonal Jessel, director of policy at the Harlem-based nonprofit WE ACT for Environmental Justice, told CNN. "It's not an issue that exists in a vacuum at all, and lower-income communities or communities of color bear the brunt of all these different hardships."