High temperatures in the October European Heatwave likely influenced by both human-driven climate change and natural variability  

Press Summary (First published 2023/10/16)

• Heatwaves similar to the October 2023 European Heatwave are between 1 ºC and 4 ºC hotter in the present than they would have been in the past.

• This heatwave was a very uncommon event.

• Natural climate variability likely played a role in driving the pressure pattern and the associated increase in temperatures linked to the October 2023 heatwave.

Event Description

In early October 2023, a large part of Europe, from Iberia to Eastern Europe, experienced unseasonally warm temperatures. Madrid Airport recorded a high of over 33 °C on the 8th October, while Seville Airport exceeded 35 °C on the same day and recorded highs above 30 °C for 5 days in a row. On the 13th October, several French cities exceeded 30 °C. Further East, Slovenia broke its national temperature record for October with 31 °C recorded in Crnomelj. It was the last of a series of 5 European countries to break their October records in 2023. Indeed, while we focus here on the period 8th-13th October, the early days of the month have also been exceptionally warm. These record-breaking temperatures were in sharp contrast to the weather in Northern Europe, with Stockholm recording a below-climatology low of -1  °C on the 10th October.

The heatwave had serious consequences especially on agriculture: orchards in Southern France are currently in bloom, while the first buds would normally expected for next spring, in five months' time. The autumnal warmth has thus disrupted the seasonality of fruit trees. In Italy, Coldiretti has emphasized a particularly dire situation for bees, which are unable to leave their hives to gather nectar and pollen, and for cows which, due to the heat, eat less and drink twice the average amount of water consumed normally, 140 liters per day instead of 70. The same is happening in poultry farms, with a reduced egg production.

The Surface Pressure Anomalies show a large positive (anticyclonic) anomaly over the Western Mediterranean and a negative pressure anomaly (cyclonic) over the British Isles and Northern Europe. This configuration is prone to advect warm air from Africa to central Europe contributing to the large Temperature Anomalies seen across the continent, reaching up to 10°C in Spain, the Alpine region and Eastern Europe. It also led to a sharp break between cooler temperatures in the northern part of the continent and anomalous warmth further south.

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 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 October 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 [-30°E 20°E 33°N 60°N]. Surface Pressure Changes are limited to increases over the Alps, which may favour a more intense warming due vertical air motions. A similar feature was observed for the September 2023 European heatwave. Temperature Changes show that similar events produce temperatures which in the present climate are between 1.5 ºC and 4 ºC hotter than what they would have been in the past, with higher differences in Central and Eastern Europe. This coincided with temperatures in Zurich, Paris and Milan being over 1.2 ºC hotter than what they would have been in the past. We also note that Similar Past Events have become more common in the months of October and November, while they previously largely occurred in September. This suggests that the weather configuration associated with the October heatwave is increasingly becoming a late Autumn to early Winter-type phenomenon.

Finally, we find that sources of natural climate variability, notably the the Atlantic Multidecadal Oscillation and 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 due to human driven climate change, with a contribution from natural variability 

Conclusion

Based on the above, we conclude that heatwaves similar to the October 2023 European heatwave are showing locally increased pressure over the Alps and between 1 ºC and 4 ºC warmer temperatures in the present than in the past. We interpret this heatwave as an event for which for which natural climate variability played a role.

Contact Authors

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

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