Climate change made U.S. summer hotter for almost everyone

Nearly everyone in the U.S. experienced hotter temperatures driven by human-caused climate change this summer, per a new Climate Central analysis.

By the numbers: 326 million people — that's 97% of the U.S. population — experienced at least one summer day with temperatures notably influenced by human-caused climate change in 2023, per Climate Central, a climate research and communications nonprofit.

Zoom in: In 45 of the 244 U.S. cities the group analyzed, at least half of all summer days had temperatures made at least twice as likely by climate change.

• The three U.S. cities with the most hot summer days driven by climate change: San Juan, Puerto Rico (90 days); Victoria, Texas (80 days), and Lafayette, Louisiana (74 days).

The big picture: This is a U.S.-focused version of a similar global analysis Climate Central published last week, which found that nearly half the world's population experienced temperatures made more likely by global warming this summer.

• Meanwhile, Earth just experienced what was likely its hottest summer on record, with a global average surface temperature about 0.65°C (1.17°F) above the 1991-2020 average for June through August.

How it works: Climate Central's analysis is based on the group's Climate Shift Index (CSI), which compares observed or forecast temperatures with simulations of the same weather conditions minus excess atmospheric greenhouse gases.

• The idea is to compare real-world conditions with what might have been the case absent human-caused climate change.

• A CSI of 3, for example, means human-caused climate change made a given daily average temperature three times more likely.

Of note: While the analysis itself wasn't peer-reviewed, the methodology has been.

• "It's similar to the strategy for weather forecasts — your weather forecast has not been peer reviewed, but the models underlying it have been," Andrew Pershing, Climate Central's vice president for science, said in a media call about last week's global data, Axios Generate's Andrew Freedman reports.

The bottom line: Rapid attribution analyses like this one drive home a key point: Climate change is having a significant present-day impact on millions across the country and the planet.