More than half of Seattle lives on "heat islands"

More than half of Seattle residents live on "urban heat islands" with temperatures that can get at least 8 degrees higher than surrounding areas, per a new analysis by the nonprofit climate research group Climate Central.

Why it matters: Heat islands — where warmth is trapped by hard surfaces such as parking lots and pavement — can make neighborhoods less livable and increase the risk of heat-related health complications, Axios' Alex Fitzpatrick writes.

Driving the news: Climate Central analyzed surface heat absorption, tree cover and other factors in 44 U.S. cities and found that 41 million Americans live in urban areas that can be at least 8 degrees warmer than neighboring tracts of land.

• In Seattle, more than 80% of the population live on heat islands: 54% in tracts that are at least 8 degrees warmer than surrounding areas, 19% in areas that are at least 9 degrees warmer and 7% on islands that are at least 10 degrees warmer, per the data.
• That makes Seattle one of five cities with the highest average Urban Heat Island Index per capita, along with New York, San Francisco, Chicago and Miami, per the data.

The intrigue: While the number of Seattle homes with air conditioning nearly doubled between 2013 and 2021, Seattle remains one of the nation's least air-conditioned metros, making it even harder for residents to stay cool.

Catch up quick: Low-income neighborhoods have historically tended to be more vulnerable to heat islands than wealthier ones.

• Seattle's Lawton Park and Madison Park are affluent neighborhoods that be can be as much as 20 degrees cooler at night than Renton, Kent and Auburn, located along commercial-industrial corridors, according to a 2022 abstract that looked at extreme heat in Washington.

Yes, but: Increasingly, as summers get hotter and heat waves last longer, heat islands are engulfing entire metro regions, as a 2020 heat mapping project by Seattle and King County shows.

• The project mapped not only the high temperatures in the county but also how quickly an area cooled down at night, which is a critical indicator of how dangerous a heat wave could be to humans.

• With the exception of a few neighborhoods that are close to both water and large green spaces, such as The Highlands and Carkeek, most of Seattle and western King County are in a regional heat island that's bordered by Puget Sound to the west, the Cascade foothills to the east and is most intense along Interstate-5.

What they're saying: "It's fairly evident that where there are fewer mature trees and more blacktop, the more people will feel the heat," Douglas Williams, a spokesperson for the county Department of Natural Resources and Parks, told Axios.

What's next: King County Metro Transit is using the Seattle/King County heat mapping data to make sure bus shelters offer shade, either through design or location, and both the county and city have committed to planting trees: 3 million trees for King County by 2025; 48,000 in the next five years for Seattle.