2023/08/20-23 Late Summer French Heatwave
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.
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.
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.
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.
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.
Davide Faranda, IPSL-CNRS, France 📨email@example.com 🗣️French, 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.