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American cities are running hotter than the countryside around them, and the gap is growing. Using 36 years of Landsat satellite data for 50 major cities, the U.S. Geological Survey found the ground surface in cities averaging 2.88 °C (5.19 °F) warmer than the land around them — and that difference rising by about 0.32 °C (0.58 °F) every decade since 1985. In 47 of the 50 cities, the city was warmer.
What a heat island is
Buildings, roads and other infrastructure absorb the sun's heat and give it back more than forests or water do. Replace one with the other and a city's surface ends up hotter than the rural land nearby: a surface urban heat island (SUHI).
It matters because more than 80 percent of Americans live in urban areas, and the urban population has grown by more than 6.3 percent in 40 years. Heat islands make residents uncomfortable, raise energy demand, and add to the risk of illness and death from heat — a risk climate change is already raising, with more frequent hot days and nights, and longer, more intense heat waves. As cities grow denser and wider, the effect is expected to strengthen.
How it was measured
| Step | What the USGS did |
|---|---|
| Surface temperature | processed every available Landsat thermal image into annual mean surface temperature maps at 30-metre resolution, 1985–2020 |
| Land cover | used the National Land Cover Database and the LCMAP annual land-change products to map each city's land cover year by year |
| Urban and rural | drew each urban core from Census Bureau metropolitan extents, and a 5-kilometre ring of surrounding rural land |
| Intensity | for every urban pixel, the difference between its temperature and the mean of the rural ring |
| Hotspots | places hotter than surroundings of the same urban land-cover type, in more than half of the years |
Hotspots matter because heat is local: a person's experience can differ from one block to the next, or between a bench in the shade and one in full sun.
Mean surface urban heat island intensity, 50 cities, 1985–2020.
What it found
Heat island intensity in 1985 and 2020: Washington, D.C. (top) and Los Angeles (bottom).
| City | Heat island trend, per decade |
|---|---|
| Raleigh–Durham | +0.56 °C (+1.01 °F) |
| Los Angeles | +0.51 °C (+0.92 °F) |
| Seattle | +0.38 °C (+0.68 °F) |
| Washington, D.C. | +0.29 °C (+0.52 °F) |
| All 50 cities, average | +0.32 °C (+0.58 °F) |
Only three of the 50 cities showed a falling trend. In the Washington area, comparing the 1985 and 2020 maps with aerial photographs shows how the change in intensity, and the spread of hotspots, follow different kinds of urban structure.
Trends in heat island intensity, 1985–2020.
What it is for
The result is an operational dataset, published as a USGS data release (Xian and others, 2022), with GIS layers and trend analyses for all 50 cities, and intended for federal climate resources such as the U.S. Global Change Research Program's indicators and the EPA's climate change indicators. Tracking where and when urban heat builds can show where heat hazards are likely, protect vulnerable and underserved communities, and guide strategies to reduce extreme heat at national, regional and local scales.
Sources
Written from Characterizing Urban Heat Islands Across 50 Major Cities in the United States, U.S. Geological Survey Fact Sheet 2023–3048, a work of the United States government in the public domain. It cites the U.S. Census (2020), the EPA (2023), the U.S. Global Change Research Program (2023), Manoli and others (2019), and Xian and others (2022a, 2022b).
Licens: CC0 1.0 (offentligt eje) · Bearbejdet efter pubs.usgs.gov
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