Defining Heatwaves and Measuring Their Intensity: How the Historic 2003 Heatwave Could Become Commonplace in France
- tlaconde
- 29 minutes ago
- 5 min read
The increasing frequency and severity of heatwaves are among the most evident consequences of climate change. According to the latest IPCC report, it is "virtually certain that heatwaves have become more frequent and more intense" and "every additional 0.5°C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes".
As these events grow more frequent and severe, anticipating them over the short, medium, and long term is essential to prepare and mitigate their most damaging impacts. This raises the question: what exactly constitutes a heatwave? And what makes one heatwave worse than another?
In this article, we will introduce one of the many definitions of a heatwave and heatwave severity, and demonstrate how it can be used in conjunction with climate projections to anticipate future events.
Heatwaves: A Universal Phenomenon Without a Universal Definition
Heatwaves are not just about high temperatures; they involve a complex interplay of factors that can include humidity, duration, regional climate norms, and even daylight duration. Consequently, various countries have developed their own criteria to issue alerts and prepare responses, resulting in no universal definition of a heatwave.
Among other examples:
The World Meteorological Organization defines an heatwave as "a period during which the daily maximum temperature exceeds for more than five consecutive days the maximum normal temperature by 9°F (5°C), the ‘normal’ period being defined as 1961-1990",
At European level, Copernicus defines an heatwave as a period of at least three consecutive days when both the daily surface air temperature minima and maxima are higher than the highest 5% of values for the day in question during the 1991–2020 reference period,
In Ireland, an heatwave is defined as five or more days when the maximum shaded air temperature exceeds 25°C,
In Greece, heatwave alert theresholds are much higher: it requires 3 consecutive days with a maximum temperature higher than 39°C and a minimum temperature higher than 26°C,
In Italy, the definition of a heatwave is based not on the temperature but on the "apparent temperature", a thermal comfort index combining temperature, humidity, wind speed and solar radiation with thresholds varying from city to city and from month to month.
Many countries, including the USA, Australia, India, France, and the UK, define a heatwave based on duration and daily temperature thresholds that vary from region to region.
Quantifying Heatwave Severity
A common feature of these definitions is that they are multifaceted, typically including factors such as duration, maximum daily temperature, and minimum daily temperature. This complexity makes it challenging to compare one heatwave to another. For example, consider the 2003 heatwave in France, which lasted 15 days. How does it compare to the much shorter June 2019 heatwave, during which temperatures reached a record 46°C over the course of just five days?
The answer may depend on the type of impact considered: some systems or organisms will respond quickly to intense heat while other have more inertia and will be more impacted by long-term, milder heat. For example heat-related illnesses will increase quickly with temperature while river cooling capacity, impacting the availability of industry and thermal electricity production, will need approximately 5 days of hot weather to decrease.
To capture these complexe realities, we suggest using a sophisticated methodology developed by French researchers at the Centre National de Recherche Météorologique and the Institut Pierre-Simon Laplace under the direction of Gaelle Ouzeau .
This approach involves three temperature thresholds:
Speak (Heat Peak Threshold): The 99.5th percentile of local daily temperatures between 1975 and 2005.
Sstart (Start Threshold): The 97.5th percentile of daily temperatures.
Sint (Interruption Threshold): The 95th percentile of daily temperatures.
A heatwave can be detected when the Speak threshold is exceeded. The duration of the event is then determined between first exceedance of Sstart and the first drop below Sint. If the duration is longer than 5 days the event is qualified as an heatwave.
The intensity of a heatwave is computed over the duration of the event. It is defined as the sum of degree-days above the start threshold (Sstart), divided by the difference between the peak threshold (Speak) and the start threshold (Sstart).
In other words, the intensity of a day with a daily average temperature T is calculated as follows:
0 if T < Sstart
(T - Sstart) / (Speak - Start) otherwise
The overall heatwave intensity is obtained with the sum of these daily intensities.
This method allows for a rigorous quantification of heatwave severity based on local climatology, making the results applicable across various geographic regions. This approach was developed to quantify the severity of a heatwave without targeting a specific sector, activity or type of impact. The intensity calculated in this way is intended to be proportional to the impacts of the event on any given sector.
Case Study: Future Heatwave in France
We applied this methodology to assess how climate change could affect the frequency and severity of heatwaves in France, using the French government’s reference scenario of a 4 °C average warming in metropolitan France. This corresponds to about 3 °C of global warming, roughly the level expected by the end of the century if current climate policies remain unchanged.
To make the results easier to interpret, we compared future heatwaves to the catastrophic 2003 event, which killed more than 15,000 people in France. The full study is available in French here:
The 2003 heatwave was extraordinary: in the climate of the early 2000s, its probability was estimated at just 0.4 % per year, meaning that if the climate had remained stable, such an event would be expected only once every 250 years on average.
Under a +4 °C warming scenario, heatwaves of comparable or greater severity would become common. For example:
Paris could experience heatwaves as severe as or more severe than 2003 about four times per decade.
Marseille, France's second largest city in terms of population, could face such events almost every year.
Some extreme episodes could last well beyond two months, with severity increasing by a factor of two to five depending on the region.
Even in a more optimistic scenario aligned with the Paris Agreement (+2 °C globally, approximately +2.7 °C in France), the frequency of such severe heatwaves would still rise sharply, occurring every five to ten years.
These findings highlight that while reducing greenhouse gas emissions is essential to limit future warming, it cannot prevent a sharp increase in the frequency of the most severe heatwaves. Adaptation strategies will be needed in any case, as infrastructure, urban planning, and public health systems must be prepared to cope with the increasing frequency and intensity of extreme heat events.
At Callendar, we specialize in evaluating climate risks and developing tailored solutions to help businesses and communities adapt. Contact us to learn more about our climate risk assessment services and how we can assist you in building resilience against future heatwaves and other climate risks.