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What happens on a hot day when the sun bakes buildings and asphalt parking lots in areas that contain little, if any, grass or trees? The heat absorbed into these impervious surfaces goes back into the surrounding air, keeping the area warmer even throughout the night.

These hotspots, called urban heat islands, affect more than our comfort—they can put the health and even lives of vulnerable residents at risk. Seniors, children and those with medical conditions exacerbated by heat may live in these areas with no air conditioning and no relief, especially during heat waves. Extreme heat is the deadliest weather-related event in the United States, according to the U.S. Environmental Protection Agency.

Some cities have sought ways to reduce the environmental danger by, for example, adding green spaces to existing neighborhoods and planning them into new ones. Other cities may not know where to start: Which neighborhoods are truly the hottest? Where could limited resources be used to the greatest benefit?

To help answer those questions, USGS Earth Resources Observation and Science (EROS) Center Research Physical Scientist George Xian has been working with several colleagues on a project that started with a handful of cities and now has grown to 50 across the United States. A new data release version and new fact sheet describe the trends in changing average surface temperatures and hotspot locations from 1985 to 2020­­ using Landsat satellite data.

The dataset with temperature measurements across the United States offers a unique look at overall temperature trends. “People did trends analysis for individual cities, not the entire U.S. this way,” Xian said.

US map with multiple colored points, and grid of 4 smaller maps with colored bands
A screenshot of one page of the "Characterizing Urban Heat Islands Across 50 Cities in the United States" fact sheet produced by the Earth Resources Observation and Science (EROS) Center.

He expects the data to help communities work on resilience plans and methods to mitigate heat and to help regions that historically haven’t experienced much extreme heat.

“For example, for the northwestern part of the U.S., traditionally they don't have any air conditioning. But with this heat continuing in the future, how are they going to deal with this challenge? So we expect our data can provide valuable information for society to deal with climate challenges,” Xian said.

The Data and the Meaning

Landsat satellites, which provide complete coverage of the Earth’s land surface every 8 days, contain a thermal sensor that provides an estimate of land surface temperature. This is useful for comparing temperatures in nearby areas that differ in the type of substance covering the land. The long, unparalleled Landsat record is also valuable for comparing temperatures in a single spot back in time.  

To determine the stories of temperature change and hotspot locations in the 50 varied cities, which range from Seattle to Miami and San Diego to Boston, the annual average temperature of each was determined for more than three decades. Two trusted EROS land cover datasets also derived from Landsat data, the National Land Cover Database (NLCD) and Land Change Monitoring, Assessment and Projection (LCMAP), were used to outline annual land cover and change for the cities. To find the hotspot locations with persistently higher temperatures than surrounding areas with the same urban land cover type, Xian and his colleagues determined which sites had higher annual averages more than 50 percent of the time between 1985 and 2020.

In that time span, they found 47 of 50 cities experienced warming urban heat island intensities, with an average intensity of 5.19 degrees Fahrenheit. The highest intensities generally occurred in cities in the eastern third of the United States and near the West Coast.

EROS urban heat information has already found its way into the newly released Fifth National Climate Assessment, and Xian talked about it at the 2023 American Geophysical Union conference the week of December 11 in San Francisco. Other key EROS colleagues involved in the urban heat project include contractors Hua Shi, Chase Mueller and Reza Hussain, who provide additional science and data engineering expertise.

Xian is eager to keep tabs on who downloads and uses the data to benefit their cities. And the coming year holds a new challenge: extending the urban heat island analysis to all cities in the United States with more than 50,000 residents—hundreds of them—for years 1985-2023.

By starting with urban heat islands in 50 cities across the country, “now people can see this is not just a regional or local issue,” Xian said. “This is a national challenge."

A December 15, 2023, Newsweek article highlighted the results of the study as well.

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