High temperatures in Western USA July 2023 Heatwave mostly strengthened by human-driven climate change

Press Summary (First published 2023/12/18)

• Heatwaves similar to Western USA July 2023 Heatwave are between 1 °C to 3 ºC warmer, 0 to 4 mm/day drier, and 3 to 6 km/h windier in the present than they would have been in the past across much of Western USA.

• Western USA July 2023 Heatwave was a largely unique event.

• We mostly ascribe the increase of temperatures in the Western USA July 2023 Heatwave to human-driven climate change and natural climate variability likely played a modest role.

Event Description

During July 2023 in the United States, an unprecedented surge of intense heat swept across numerous states, impacting regions such as Texas, New Mexico, Arizona, Nevada, and California. The scorching temperatures soared to staggering heights, with Death Valley experiencing an astonishing 53 °C, while Phoenix faced a blistering 48 °C on multiple days. The sweltering conditions shattered previous records, particularly in Phoenix, where the mercury soared above 43 °C for a record-breaking 31 consecutive days throughout July, eclipsing the previous 18-day streak set back in 1974. Heat warnings reverberated across the southern states, extending as far east as Florida, where unprecedentedly high ocean temperatures were recorded. On July 17, 2023, the U.S. National Weather Service (NWS) issued its inaugural excessive heat advisory for Miami, Florida, underscoring the severity of the heatwave.

July 19 marked a historic moment as Phoenix set a new record for its all-time high minimum temperature, reaching 36.1 °C, surpassing the previous milestone set on July 15, 2003. The scorching conditions also triggered ecological challenges, exemplified by the Eagle Bluff Fire on July 31, which necessitated the evacuation of Osoyoos, British Columbia. Both U.S. and Canadian officials estimated that approximately 890 hectares on the Canadian side of the border were engulfed in flames due to prolonged dryness and heightened temperatures from June into July. 

During Western USA July 2023 Heatwave we observe a rich and complex structure of Surface Pressure Anomalies with respect to the climatology, with negative anomalies on the southern Rocky Mountains and positive anomalies elsewhere. Temperature Anomalies are up to 5 ºC higher than the climatology. The heatwave developed on the background of the warmest month of July ever in the instrumental record, and of a positive phase of the El Niño–Southern Oscillation. Precipitation Data shows that the entire month has been completely dry over the region examined except for the Mexican Sierra Madre Occidental. Windspeed data show low winds over the entire region.

Climate and Data Background for the Analysis

The IPCC report states that the frequency and intensity of hot extremes, including heatwaves, have increased in most land regions, including the USA, since the 1950s. This increase is attributed to human influence on the climate system, particularly through the emission of greenhouse gases. The report also notes that the number of warm nights has increased in most land regions, including the USA, and that the frequency and intensity of heavy precipitation events have also increased in many regions, including parts of the USA. These changes in temperature and precipitation patterns have led to a range of impacts, including increased risk of heat-related illnesses and deaths, reduced crop yields, and increased risk of flooding and landslides. In summary, the IPCC report indicates that hot extremes, including heatwaves, have increased in frequency and intensity in the USA since the 1950s, and that this increase is due to human influence on the climate system. The report also notes that changes in temperature and precipitation patterns have led to a range of impacts, including increased risk of heat-related illnesses and deaths, reduced crop yields, and increased risk of flooding and landslides.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For the Western USA July 2023 Heatwave, we have medium-low confidence in the robustness of our approach given the available climate data, as the event is unusual in the data record.

ClimaMeter Analysis

We analyse here (see Methodology for more details) how events similar to the Western USA July 2023 Heatwave have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000)  in the region [-120°E -105°E 25°N 40°N].  Surface Pressure Changes show small negative changes lower than 1hPa. Temperature Changes show that similar events are 1 °C to 3 °C warmer in the present than in the past. Precipitation Changes show that present events are 0 to 4 mm/day drier than in the past, while Windspeed changes show  3 to 6 km/h windier conditions in the present than in the past. All combined these changes suggest that these events contribute to warmer, drier and windier conditions in the present than in the past with the higher hydrological stress and more favorable conditions to the development of wildfires. We also note that Similar Past Events have become more common in June and July, while they previously mostly occurred exclusively in August and September. This has likely contributed to the hotter temperatures discussed above. The analysis for Changes in Urban Areas shows that Las Vegas, Phoenix and Tucson being hotter, drier and windier than what they would have been in the past in similar events.

Finally, we find that sources of natural climate variability, notably Atlantic Multidecadal Oscillation, may have heavily influenced the event. This means that the changes we see in the event compared to the past may be influenced by the natural climate variability.

Conclusion

Based on the above, we conclude that heatwaves similar to the Western USA July 2023 Heatwave are between 1 °C to 3 ºC warmer, 0 to 4 mm/day drier, and 3 to 6 km/h windier in the present than in the past. We interpret this heatwave as a largely unique event whose characteristics can mostly be ascribed to human driven climate change.

Contact Authors

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

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