2023/09/03-10 September European Heatwave
High temperatures in the September European Heatwave enhanced by both human-driven climate change and natural variability
Press Summary (First published 2023/09/12, Updated 2023/11/22)
Heatwaves similar to the September 2023 European Heatwave are between 2 ºC and 4 ºC hotter in the present than they would have been in the past.
This was a largely unique event.
In September 2023, the UK, as well as parts of France, the northern regions of Italy, and the Benelux countries, experienced an unexpected heatwave following a summer with varied weather patterns. The temperatures reached 36-39 °C in France and 30-33 °C in the United Kingdom. The heatwave in September was a significant contrast to the cooler and more unsettled weather experienced earlier in the meteorological summer across these areas. Indeed both London and Paris recorded the hottest day of 2023 (so far) with 32.7 °C in Heathrow and 36.5 °C in central Paris.
The effects of the heatwave ranged from direct impacts, notably on agriculture as part of prolongued Autumn warmth, to indirect impacts which included flash flooding and transport disruptions due to the thunderstorms associated with the unseasonally high temperatures.
The Surface Pressure Anomalies show moderate negative (cyclonic) anomalies over the Eastern Atlantic and a weak positive pressure anomaly (anticyclonic) over Central and Eastern Europe. The high-pressure system resulted in settled conditions and above-average temperatures for the time of year. This weather pattern was influenced by the cyclonic activity in the North Atlantic, allowing warm air to be drawn northward and affecting multiple countries in the region. This contributed to the large Temperature Anomalies seen across the continent, exceeding 10 °C in parts of Iberia, Italy and Eastern Europe.
According to the IPCC AR6 report, the “surface temperature in the Mediterranean region is now 1.5 °C above the pre-industrial level, with a corresponding increase in high-temperature extreme events (high confidence)”. The IPCC report further states that: “A growing number of observed impacts across the entire basin are now being attributed to climate change, along with major roles of other forcing of environmental change (high confidence). These impacts include multiple consequences of longer and/or more intensive heat waves.”
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 September heatwave, we have low confidence in the robustness of our approach given the available climate data, as the event is largely unique in the data record.
We analyse here (see Methodology for more details) how events similar to the September 2023 European 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 [-18°E 20°E 33°N 55°N]. Surface Pressure Changes show that the pressure over the mountains, as the Alps and the Pyrenees, has increased, favouring a more intense warming due vertical motions of the air. Similar Surface Pressure Changes have been observed for the late August 2023 French heatwave. Temperature Changes show that similar events produce temperatures which in the present climate are between 2 ºC and 4 ºC hotter than what they would have been in the past. This coincided with temperatures in Paris, Lyon and Milan 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, with an increased frequency in September, while they previously largely occurred in July and August.
Based on the above, we conclude that heatwaves similar to the September 2023 European heatwave are showing increasing pressure and between 2 ºC and 4 ºC warmer temperatures in the present than in the past. We interpret this heatwave as a largely unique event for which natural climate variability likely played an important role.
Davide Faranda, IPSL-CNRS, France 📨email@example.com 🗣️French, Italian, English
Tommaso Alberti, INGV, Italy 📨firstname.lastname@example.org 🗣️Italian, English
Gabriele Messori, Uppsala University, Sweden 📨email@example.com 🗣️Swedish, 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.