Extreme temperatures in Eastern United States  likely influenced by both human-driven climate change and natural variability

Press Summary (First Published 2024/06/25)

• Heatwaves similar to the Eastern US heatwave are up to 2 °C warmer than the warmest heatwaves previously observed in this area

• This was a somewhat uncommon event

• We mostly ascribe the high temperature in the Eastern US heatwave  to human driven climate change and natural climate variability likely played a modest role

Event Description

From June 18th to June 23th the Eastern United states experienced a severe heatwave. Over 75 million people across the eastern U.S. were under heat warnings or advisories as cities on the East Coast braced for record-breaking temperatures. Baltimore and Philadelphia  reached near 40°C, while calendar-day records were broken across the east.  Philadelphia region faced dangerous conditions due to high humidity.  New York City health officials reported a 500% increase in heat-related emergency room visits. Flooding has also been experienced in the mid-west, with some regions experiencing both extreme heat and flooding. In Iowa, a breached levy prompted evacuations in Rock Valley, assisted by Sioux City Fire Rescue.

The Surface Pressure Anomalies reveal a strong positive anomaly over the Atlantic Coast of the United States. These conditions favor Temperature Anomalies up to +7°C. Precipitation Data show an absence of precipitation in the southern U.S. and large amounts of precipitation, reaching up to 20mm/day, in the northern U.S. and along the Canadian-U.S. border. Windspeed data show low to moderate winds, especially in coastal areas and the Great Lakes region.

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 medium-high confidence in the robustness of our approach given the available climate data, as the event is somewhat unusual in the data record

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the high temperature in the Eastern US heatwave changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [90°E 68°E 32°N 50°N]. The Surface Pressure Changes show that similar events  display significant changes (up to +1 hPa) in the present climate than what they would have been in the past over the region of interest. The Temperature Changes show that similar events produce temperatures in the present climate up to 2 °C warmer than what they would have been in the past, over a large area of the region analyzed. The Precipitation Changes show modest variations (up to 3 mm/day). Windspeed Changes indicate up to 4 km/h windier conditions over Northeast regions. We also note that Similar Past Events previously mainly occurred in June, while in the present climate they are mostly occurring in May. Changes in Urban Areas reveal that New York, Buffalo, and Washington are at least 1 °C warmer in the present compared to the past. 

Finally, we find that sources of natural climate variability, mainly the Pacific Decadal Oscillation (PDO), influenced the event. This means that the changes we see in the event compared to the past may be primarily due to human driven climate change.

Conclusion

Based on the above, we conclude that heatwaves similar to the Eastern US heatwave are up to 2 °C warmer than the heatwaves previously observed in the country. We interpret the Eastern US heatwave as a somewhat uncommon event whose characteristics can mostly be ascribed to human driven climate change.

Contact Authors

-Tommaso Alberti, INGV, Italy +393461692775 📨tommaso.alberti@ingv.it 🗣️ Italian, English

-Erika Coppola, ICTP, Italy 📨coppolae@ictp.it  🗣️ Italian, English

-Rachel White, University of British Columbia, Canada  📨rwhite@eoas.ubc.ca  🗣️English

-Davide Faranda, IPSL-CNRS, France +33681038467 📨davide.faranda@lsce.ipsl.fr 🗣️French, Italian, English