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Testing Climate Projections Against Reality: Lessons from the June 2026 Western European Heatwave.

  • 1 day ago
  • 5 min read

The late June 2026 heatwave was one of the most severe ever recorded across Western Europe. New temperature records were set in several regions and infrastructure was disrupted. In the aftermath of the heatwave, a surprising shift emerged in public debate. For years, climate scientists had been criticized for being overly alarmist. Following this unprecedented event, however, some commentators argued the opposite: that climate projections had underestimated the severity of extreme heat.


According to these claims, climate models were too optimistic. They had failed to anticipate an event of such intensity, with some suggesting that the weather experienced in June 2026 matched what had only been projected for the middle of the century.


"No one had predicted a heatwave of this magnitude. Even scientists themselves are surprised by its severity and duration." Emmanuel Grégoire, Mayor of Paris on July 5.

In this article, we compare ClimateVision's projections with the temperatures actually recorded during the June 2026 heatwave to assess whether this event truly fell outside the range of what climate science had anticipated.


Example of a social media post implying that the June 2026 heatwave disprove climate projections
One of many viral posts suggesting that climate projections were too optimistic compared to the June heatwave in France

Data sources


Source of the projected extreme temperatures


The projections presented in this article were generated using ClimateVision V1.3. ClimateVision is Callendar's climate projection plateform. It combines the latest CMIP6 climate projections with automated bias correction and statistical extreme value analysis to estimate the probability of future temperature extremes (and other climate variables) for any location worldwide.


Although they were produced after the June 2026 heatwave, the projections do not incorporate any information from the event itself. The same projections would have been obtained before the heatwave occurred.

It is also worth noting that all the results presented in this article are generated directly by ClimateVision. No additional processing, calibration or post analysis has been applied.


Unless otherwise stated, all projected values correspond to the 2030 climate under the SSP2 4.5 intermediate greenhouse gas emissions scenario. At such a near term horizon, the choice of emissions scenario has only a limited influence on projected temperature extremes.


Source of the observed temperatures


The observed temperatures correspond to official measurements recorded at weather stations in the ten largest cities areas in mainland France during the June 2026 heatwave.


Comparing observed and projected peak temperatures


A projected maximum temperature is only meaningful when associated with a probability of exceedance. ClimateVision estimates extreme temperatures across a range of annual exceedance probabilities, from 20% (a 5 year event) to 1% (a 100 year event).


The table below compares the highest temperatures recorded during the June 2026 heatwave with two projected design values:

  • 10% annual exceedance probability (10 year return period): commonly used for applications where the consequences of failure are relatively limited, such as the design of cooling systems.

  • 2% annual exceedance probability (50 year return period): a more conservative design level, widely used for buildings or industrial facilities.


City / weather station

Peak temperature / date

Extreme temperature projection

10 % exceedance probability

2 % exceedance probability

Paris

Vélizy-Villacoublay

39,2 °C

24/06

39,5 °C

41.3°C

Marseille

Marignane

37,2 °C

29/06

35,1 °C

36,3 °C

Lyon

Lyon-Bron

39,0 °C

26/06

39,2 °C

39,7 °C

Toulouse

Toulouse-Blagnac

40,3 °C

23/06

40,3 °C

41,5 °C

Nice

Nice

33,5 °C

29/06

31,2 °C

33,3 °C

Nantes

Nantes-Bouguenais

42,2 °C

24/06

37,9 °C

40,4 °C

Montpellier

Montpellier-Aéroport

37,5 °C

29/06

38,7 °C

41,4 °C

Strasbourg

Strasbourg-Entzheim

40,4 °C

27/06

38,0 °C

38,9 °C

Bordeaux

Bordeaux-Mérignac

42,5 °C

23/06

40,5 °C

41,9 °C

Lille

Lille-Lesquin

37,0 °C

24/06

36,1 °C

38,3 °C


The comparison shows that, in cities such as Paris, Lyon and Toulouse, the highest temperatures recorded during the June 2026 heatwave closely match the projected 10 year extreme for the 2030 climate. In Montpellier, the observed maximum is below this threshold.

In Lille, the observed peak temperature corresponds to an event with an estimated return period of approximately 20 years according to ClimateVision.

In Marseille and Bordeaux, the observed temperatures slightly exceeded the projected 50 year extreme, but remained close to the 100 year values (36.7°C and 42.3°C respectively).


In Strasbourg and Nantes, observed temperatures exceeded the projected values by a greater margin. Even there, however, the measurements remain within the range of values projected by ClimateVision across warmer climate scenarios. In Nantes, for example, the projected 100 year maximum temperature in the 2030 climate reaches 45.6°C under the high emissions SSP5 8.5 scenario, well above the 42.2°C recorded during the heatwave.


Overall, the peak temperatures observed across France during the June 2026 heatwave were exceptional compared with past recoards, but they were not unexpected in today's climate. In most locations, they correspond to events with annual probabilities of occurrence between 10% and 2%, equivalent to return periods of roughly 10 to 50 years.

These are precisely the probability levels that should inform the design of infrastructure, buildings and public policies. A 2% annual exceedance probability may appear small, but it corresponds to nearly a one in five chance of experiencing at least one such event over a ten year period. These are far from negligible risks.


What about daily mean temperatures?


The peak temperatures observed during the heatwave are therefore consistent with ClimateVision projections. But does the same conclusion hold when considering the average temperature over an entire day?

This comparison is also possible, as ClimateVision provides estimates of extreme temperatures at both hourly and daily time scales.


City / weather station

Temperature of the hottest day

Extreme daily temperature projection

10 % exceedance probability

2 % exceedance probability

Paris

Vélizy-Villacoublay

32,9 °C

31,5 °C

33,6 °C

Marseille Marignane

30,7 °C

30,1 °C

31,0 °C

Lyon Lyon-Bron

31,9 °C

32,2 °C

33,0 °C

Toulouse Toulouse-Blagnac

32,3 °C

32,9 °C

34,3 °C

Nice Nice

29,8 °C

28,5 °C

29,4 °C

Nantes Nantes-Bouguenais

33,7 °C

30,6 °C

33,2 °C

Montpellier Montpellier-Aéroport

31,1 °C

31,8 °C

33,0 °C

Strasbourg Strasbourg-Entzheim

32,2 °C

31,1 °C

32,3 °C

Bordeaux Bordeaux-Mérignac

34,2 °C

31,8 °C

33,4 °C

Lille Lille-Lesquin

30,3 °C

28,8 °C

30,6 °C


The analysis of daily mean temperatures leads to the same conclusion. Across most of France's largest cities, the hottest day of the June 2026 heatwave corresponds to an event with an annual probability of occurrence between 2% and 10% in the projected 2030 climate.


Only Nice and Nantes exceeded the projected 50 year extreme, and by less than 0.5°C. The observed temperatures remain within the range of projected values for rarer events. In Nantes, for example, the projected 100 year daily mean temperature for the 2030 climate reaches 34.4°C.


Conclusion: putting climate projections to the test against the June 2026 heatwave


The June 2026 heatwave represents a real world test for climate projections. The temperatures observed were exceptional compared with the historical climate, but they were within the range of conditions identified as possible under 2030 climate. Across most of France's largest cities, the extreme temperatures recorded during the event corresponded to events with an estimated annual probability of occurrence between 2% and 10% in the current climate according to ClimateVision.


This analysis also highlights the value of extreme value analysis. By quantifying the probability of rare events, including events that may not have occurred in the available historical record, ClimateVision provides decision makers with a more robust basis for designing resilient projects while avoiding unnecessary overengineering against highly unlikely scenarios.


The purpose of climate projections is to quantify the range of conditions that a location, asset or project may reasonably face over a given time horizon. This comparison suggests that ClimateVision effectively fulfills this objective.

In practical terms, infrastructure, industrial assets or buildings designed using these results would have been better prepared for the June 2026 heatwave than those designed using historical weather data alone.


The June 2026 heatwave did not reveal the limitations of climate projections. Instead, it highlighted the limitations of decisions that continue to rely solely on historical climate conditions that no longer represent the climate we operate in.


Is your project exposed to current and future heat risks? ClimateVision can provide a reliable answer within hours.

Designed for industries and infrastructure operators, ClimateVision provides actionable climate projections and extreme weather risk assessments for any location worldwide. Contact us to learn more.

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