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.

• Natural climate variability likely played an important role in driving the pressure pattern and the increase in temperatures linked to the September 2023 heatwave.

Event Description

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.

Climate and Data Background for the Analysis

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.

ClimaMeter Analysis

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. 

Finally, we find that sources of natural climate variability, notably the El-Niño–Southern Oscillation, the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation, may have partly influenced the event. This means that the changes we see in the event compared to the past may be primarily due to natural climate variability.

Conclusion

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.

Contact Authors

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

Tommaso Alberti, INGV, Italy  📨tommaso.alberti@ingv.it  🗣️Italian, English

Gabriele Messori, Uppsala University, Sweden  📨gabriele.messori@geo.uu.se 🗣️Swedish, French, Italian, English