Western US heatwave in  July 2024 mostly exacerbated by human-driven climate change

Contact Authors

• Gianmarco Mengaldo, NUS, Singapore 📨mpegim@nus.edu.sg  🗣️Italian, English

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

Citation 

• Mengaldo, G., & Faranda, D. (2024). Western US heatwave in July 2024 mostly exacerbated by human-driven climate change. ClimaMeter, Institut Pierre Simon Laplace, CNRS. https://doi.org/10.5281/zenodo.14101435

Press Summary (First Published 2024/07/10)

• Heatwaves similar to the July 2024 Western US heatwave are up to 4°C warmer than those previously observed in the country

• This was a largely unique event

• Human driven climate change and natural climate variability both played a role in increasing the heat during the Western US July 2024 heatwave.  

Event Description

From July 05 to July 07 2024, extreme heat has affected several states in the western US, including California, Oregon, Arizona, Nevada, and Utah. Several record-high temperatures were recorded in different cities, such as Las Vegas (Nev.), Palm Springs (Calif.), Redding (Calif.), and Fresno (Calif.). Several other notable high temperatures were reported in Phoenix (Ariz.), Sacramento (Calif.), Medford (Ore.), and Portland (Ore.), among other cities. Death Valley (Calif.) also experienced an extreme-high temperature of 129°F (~53.9°C) on July 07, just shy of the all-time record of 130°F (~54.4°C) (based on reliable modern records – the measurement of 134°F (56.6°C) registered in 1913 has been cast into doubt recently). NWS issued excessive heat warnings across these states and cities, with extreme risk of heat-related illnesses, with several deaths linked to the heatwave already reported. In the Portland (Ore.) area, 4 people have been reportedly killed by extreme heat-related illnesses, where temperatures reached 100°F or 37.8°C (20°F or 6.7°C above the average for the date), which was particularly problematic given that many people lack air-conditioning in the region. In Sacramento, a man died from organ-failure linked to heat stroke. In Death Valley (Calif.) a motorcyclist died from heat exposure on Juluy 06, and another was treated for severe heat illness. The heatwave put the entire population at risk, not only the vulnerable population, given the persistent extreme high temperatures reached, with little relief at night. It also exposed the dangers of heatwave hitting areas that are not used to such high temperatures, such as Oregon, that may lack enough air-conditioning coverage or heat shelters. This event also highlighted how air-conditioning systems cast significant pressure on the power grid, due to extreme demand of electricity, and how a possible failure of the grid may have catastrophic consequences. 

The Surface Pressure Anomalies reveal a significant positive anomaly (heat dome) over Northern California, Oregon and Nevada, that is largely responsible for the extreme high temperature. Temperature anomalies indicate warm anomalies reaching up to +10°C in some parts of northern California and Oregon. Precipitation data shows absence of precipitation in a large part of the region analyzed with some light precipitation (up to 2.5mm/day) in southern California and Arizona. Windspeed data show light to moderate winds.

Climate and Data Background for the Analysis

The IPCC AR6 report provides a clear relationship between heatwaves and climate change. Climate change is significantly contributing to the increase in heatwaves through various mechanisms: Warming resulting from climate change has led to an increased frequency, intensity, and duration of heat-related events, including heatwaves, in most land regions, with high confidence (IPCC SR OC C6 - Page 27). Climate change is projected to alter land conditions, affecting temperature and rainfall in regions, which can enhance winter warming due to decreased snow cover and albedo in boreal regions, while reducing warming during the growing season in tropical areas with increased rainfall. Global warming and urbanization can enhance warming in cities and their surroundings, especially during heatwaves, with a higher impact on night-time temperatures than daytime temperatures (IPCC AR6 WGII FR - Page 1058). Observed surface air temperature has been increasing since the 20th century in North America, intensifying the threat of heatwaves across the region. In the western US specifically, the frequency and duration of heatwaves have increased, associated with persistent high-pressure anomalies leading to so-called heat-dome conditions in the region, especially in the summer season. The combination of global warming and population growth in already-warm cities, as well as exposed dry and possibly desertic areas that attract a high number of visitors in regions like the western US are major drivers for increased heat exposure.

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 low confidence in the robustness of our approach given the available climate data, as the event is largely unique in the database

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the high temperature in the western US July 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 [-125°E -109°E 31°N 48°N]. The Surface Pressure Changes show that similar events  do not display significant changes in the present climate than what they would have been in the past. The Temperature Changes show that similar events produce temperatures in the present climate at least 4°C warmer than what they would have been in the past, over a large area of the region analyzed. The Precipitation Changes do not show any significant variations. Windspeed Changes indicate almost 5 km/h windier conditions in certain parts of Arizona, northern California and Oregon. We also note that Similar Past Events previously mainly occurred in July, while in the present climate they are mostly occurring in June. Changes in Urban Areas reveal that Sacramento, Phoenix and Portland are  1.5-3°C warmer in the present compared to the past.

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

Conclusion

Based on the above, we conclude that heatwaves similar to the western US July heatwave are up to 4°C warmer than the heatwaves previously observed in the country. We interpret the western US July heatwave as a largely unique event whose characteristics can mostly be ascribed to human driven climate change