High temperatures in the June 2025 Eastern USA heatwave exacerbated by human-driven climate change

Contact Authors

• Davide Faranda, IPSL-CNRS, France,📨davide.faranda@lsce.ipsl.fr, 🗣️English,French, Italian

• Tommaso Alberti, INGV, Italy,📨tommaso.alberti@ingv.it, 🗣️English, Italian

Citation

• Faranda, D., & Alberti, T. (2025). High temperatures in the June 2025 Eastern USA heatwave exacerbated by human-driven climate change. ClimaMeter, Institut Pierre Simon Laplace, CNRS. https://doi.org/10.5281/zenodo.15746087

Press Summary

• Meteorological conditions similar to those causing the June 2025 Eastern USA heatwave are up to 1.5 °C warmer and 4 mm/day locally wetter in the present than they were in the past. These changes mean that “wet heat”—the dangerous combination of high temperatures and elevated humidity—is becoming more intense.

• This event was associated with rare meteorological conditions. 

• We ascribe the high temperatures causing the June 2025 Eastern USA Heatwave to human driven climate change and natural climate variability likely played a minor role.

Event Description

From June 23 to 25, an intense heatwave swept across the eastern United States as a powerful heat dome stalled over the region, pushing temperatures to record-breaking levels. Cities like Newark (39.4 °C / 103 °F), Boston (38.9 °C / 102 °F), Baltimore (40.6 °C / 105 °F), and New York City (37.8 °C / 100 °F) endured extreme heat and high humidity, with heat index values exceeding 43.3 °C (110 °F) in many locations. The event was fueled by a stagnant high-pressure system that trapped warm air and blocked cloud formation, creating ideal conditions for prolonged heat. The combination of high humidity and record daytime highs created dangerous conditions, prompting the National Weather Service to issue heat advisories and excessive heat warnings for over 125 million people. 

In Connecticut, where the heat index reached 45.5 °C (114 °F), CT Insider reported surges in heat-related emergency calls and nearly 90 patients treated by Hartford HealthCare for symptoms like dehydration and heat stroke. In New York City, over 100 people were treated for heat-related illnesses, according to the New York Times. Meanwhile in New Jersey, the heat led to the hospitalisation of more than 150 people at a school graduation ceremony, despite measures to reduce exposure, the BBC reported.

Infrastructure buckled under the strain: highways cracked and warped in New Jersey and Illinois, Amtrak imposed speed restrictions to prevent track damage, and in Chesapeake, Virginia, a drawbridge malfunctioned due to heat-induced expansion, as detailed by AP News. Blackouts were reported in several states. In Washington, D.C., The New York Post described overwhelmed cooling centers and the cancellation of outdoor events, while People Magazine reported that Senate Majority Leader Chuck Schumer was briefly hospitalized for dehydration after becoming lightheaded. The organization Climate Central noted that the heatwave was made 3 to 5 times more likely by climate change, underscoring the growing influence of global warming on extreme weather. 

The surface pressure anomalies reveal an anticyclonic anomaly over central-to-eastern USA, which is closely associated with the persistence of extreme high temperatures over Michigan, Ohio, Pennsylvania, and New York. This high-pressure system inhibited atmospheric mixing and cloud development, favoring heat accumulating near the surface. The temperature anomalies indicate widespread warm anomalies reaching up to +7 °C compared to the 1950–present climatology. The most intense anomalies are observed over Ohio, Pennsylvania, and New York, consistent with the observed heatwave conditions. The precipitation data show a marked absence of rainfall over most of eastern USA during the event, with the exception of localized convective precipitation over the central-to-western regions. This dry pattern reflects the stabilizing influence of the high-pressure system and the lack of moisture advection. The wind speed data show light winds across the affected regions, with relatively calm conditions over the full domain.

Climate and Data Background for the Analysis

Observations show that hot extremes, including heatwaves, have intensified in cities, aggravating air pollution events and limiting the functioning of key infrastructure ( IPCC AR6 WGII SPM). We have high confidence that, in urban areas, the temperature increases associated with urbanization (the heat island effect) combined with more frequent hot extremes due to climate change will increase the severity of heatwaves (IPCC AR6 WGI SPM- Page 25). The IPCC reports highlight several key points about heatwaves in the US. Past observed trends are inconclusive for the Eastern North-America region (IPCC, AR6, WGI, Chapter 11), but future projections by all model ensembles show a high confidence of an increase of extreme heat events in these regions, with a high probability that such a signal will emerge from the natural variability in the next decades (IPCC. WGI, Chapter 12)

Heatwaves are significantly impacting the Eastern United States, particularly in urban areas. The 2018 US National Climate Assessment has identified that southeastern USA is already experiencing more frequent and longer summer heatwaves and, by 2050, rising global temperatures are expected to mean that more cities in southeastern USA may experience extreme heat. This includes disadvantaged African American communities, who are more exposed and hence disproportionately experience the impacts of climate change.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event we have high confidence in the robustness of our approach given the available climate data, as the event is somewhat unusual in the data record

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event we have high confidence in the robustness of our approach given the available climate data, as the event is similar to other past events in the data record.

ClimaMeter Analysis 

We analyze here (see Methodology for more details) how events similar to the meteorological conditions leading to the Eastern USA heatwave of 24–25 June 2025 have changed in the present (1987–2023) compared to what they would have looked like if they had occurred in the past (1950–1986) in the region [95°W 65°W 35°N 50°N]. Surface pressure changes show no significant changes over the region of interest. This suggests that similar heatwave events today are associated with similar anticyclonic conditions to the past. Temperature changes reveal a substantial warming over Michigan, Ohio, Pennsylvania, and New York, with present-day conditions showing temperatures up to +1.5 °C higher than their historical counterparts. Precipitation changes show a reduction of up to 4 mm/day locally wetter in the present than they were in the past over central regions, indicating that similar meteorological conditions now result in drier surface conditions compared to the past. Increased precipitation (+3-4 mm/day) is observed over the eastern coast. Wind changes show a slight decrease in speed in central-to-eastern regions, up to -1.5 km/h.

Similar past events indicate a clear seasonal shift, with a higher fraction of cases now occurring in June, compared to a more even distribution between May and July for the 1950–1986 period. This shift supports the interpretation that extreme heat events are occurring earlier in the season under present-day conditions.

Changes in urban areas show that cities like Chicago, New York, and Philadelphia experienced significantly warmer conditions (up to +1,5  °C) during this event compared to similar past events. Precipitation levels were lower in Chicago, with increases over New York and Philadelphia, while wind speed changes were minor but slightly negative over the three cities.

These results suggest that weather situations similar to those of the June 2025 Eastern USA heatwave are now associated with significantly warmer and wetter conditions than in the past, consistent with the expected influence of anthropogenic climate change. The large-scale atmospheric configuration resembles previous events but is now intensified by background warming, while natural variability appears to have played only a secondary role in shaping the event.

Conclusion

Based on the above, we conclude that meteorological conditions similar to those causing the June 2025 Eastern USA heatwave are up to 1.5 °C warmer and 4 mm/day locally wetter in the present than they were in the past. These changes mean that “wet heat”—the dangerous combination of high temperatures and elevated humidity—is becoming more intense especially on the East coast. We interpret this event as an event driven by rare meteorological conditions, whose characteristics can be ascribed to human driven climate change.