High temperatures in French late Heatwave mostly strengthened by human-driven climate change  

Press Summary (First published 2023/08/25, last updated on 2023/11/22)

• Heatwaves similar to the late August 2023 French heatwave are between 1 ºC and 4 ºC hotter in the present than they would have been in the past.

• This was a somewhat uncommon event.

• We ascribe the high temperatures of the late August 2023 French heatwave to human-driven climate change, and natural climate variability likely played a minor role.

Event Description

Starting from August 20th, Western and Central Europe experienced unusually high temperatures, peaking on the 22nd and 23rd August. Extremely high temperatures were recorded at many locations: with a national temperature indicator of 27.5°C,  Wednesday August 23rd was the second-hottest day ever recorded in French history.  A large number of daily maximum temperature records were broken. In Toulouse, for example, the thermometer reached 42.4 °C (previous record was 40.7 °C). Others were greatly surpassed, like in Auch, reaching 42.3 °C compared to the previous 40.9 °C, and in Narbonne, where the mercury climbed to 42.1 °C from the previous record of 39.8 °C. Additionally, the heatwave was extreme also in mountain areas with Aiguille du Midi (~3800 m.a.s.l. in the Mont-Blanc massif) recording over 10 °C maximum temperature. Finally, the minimum daily temperature of 30.4 °C in Menton, set a new absolute national record for minimum daily temperature in mainland France. This heatwave ended on August 24th, when fresher air from the Atlantic reaching the country caused severe thunderstorms.

The heatwave was associated with a persistent area of high pressure (anticyclone) over Western and Central Europe, on the background of a warm Atlantic ocean and warm Mediterranean sea and of a positive phase of the El Niño–Southern Oscillation. The Surface Pressure Anomalies pattern associated with the event consists of a high pressure area over the Alps and Central Europe and a low pressure area over the Atlantic. Temperature Anomalies show that temperatures were 10°C or more warmer than usual over most of the domain considered. 

Climate and Data Background for the Analysis

Chapter 11 of the IPCC AR6 report emphasizes that in Western Europe, there is strong evidence indicating a very likely increase in maximum temperatures and the frequency of heatwaves. Specifically, in Western Europe, climate warming has already reached 1.7°C compared to the pre-industrial era over the last decade, with 1.5°C of this increase occurring since the 1960s, particularly during the summer months. The number of heatwave days in Western Europe has multiplied by five, transitioning from an annual average of 2 days between 1960 and 2020 to about 10 days. 

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 similar to other past events in the data record.

ClimaMeter Analysis

We analyse here (see methodology for more details) how events similar to the French late August 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 [-10°E 20°E 30°N 52°N]. Surface Pressure Changes show that the pressure over Britanny pressure has becoming higher while is lower over Italy. We underline that suche changes are rather modest. Temperature Changes show that similar events produce temperatures which in the present climate are between 1 ºC and 4 ºC hotter than what they would have been in the past especially on the Mediterranean area. This coincided with temperatures in Lyon, Toulouse and Marseille being over 1.5 ºC hotter than what they would have been in the past. We also note that Similar Past Events have become more common in the month of August, while they previously occurred largely in July. However, the differences in average maximum temperatures between these two months are limited in many French cities.

Finally, we find that sources of natural climate variability did not influence 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 late August 2023 French heatwave have become between 1 °C and 4 °C warmer in the present than in the past. We interpret this heatwave as a largely unique event whose characteristics can be mostly ascribed to human driven climate change.

Contact Author

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