2023/07/02-31 Western USA Heatwave
High temperatures in Western USA July 2023 Heatwave mostly strengthened by human-driven climate change
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.
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.
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.
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.
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.
Davide Faranda, IPSL-CNRS, France 📨email@example.com 🗣️French, Italian, English
Tommaso Alberti, INGV, Italy 📨firstname.lastname@example.org 🗣️Italian, English
Additional Information : Complete Output of the Analysis
NB1: The following output is specifically intended for scientists and contain details that are fully understandable only by reading the methodology described in Faranda, D., Bourdin, S., Ginesta, M., Krouma, M., Noyelle, R., Pons, F., Yiou, P., and Messori, G.: A climate-change attribution retrospective of some impactful weather extremes of 2021, Weather Clim. Dynam., 3, 1311–1340, https://doi.org/10.5194/wcd-3-1311-2022, 2022.
NB2: Colorscales may vary from the ClimaMeter figure presented above.
The figure shows the average of surface pressure anomaly (msl) (a), average 2-meter temperatures anomalies (t2m) (e), cumulated total precipitation (tp) (i), and average wind-speed (wspd) in the period of the event. Average of the surface pressure analogs found in the counterfactual [1979-2000] (b) and factual periods [2001-2022] (c), along with corresponding 2-meter temperatures (f, g), cumulated precipitation (j, k), and wind speed (n, o). Changes between present and past analogues are presented for surface pressure ∆slp (d), 2 meter temperatures ∆t2m (h), total precipitation ∆tp (i), and windspeed ∆wspd (p): color-filled areas indicate significant anomalies with respect to the bootstrap procedure. Violin plots for past (blue) and present (orange) periods for Quality Q analogs (q), Predictability Index D (r), Persistence Index Θ (s), and distribution of analogs in each month (t). Violin plots for past (blue) and present (orange) periods for ENSO (u), AMO (v) and PDO (w). Number of the Analogues occurring in each subperiod (blue) and linear trend (black). Values for the peak day of the extreme event are marked by a blue dot. Horizontal bars in panels (q,r,s,u,v,w) correspond to the mean (black) and median (red) of the distributions.