Extreme heat

To calculate Extreme heat Risk, we use the variable [Copernicus Climate Data Store (CDS)]:

• daily near surface air temperature (tas): daily mean temperature (°C) at ground level (2m). The process is two-stage and based on analysis of daily mean temperature (tas). The main indicator is the annual count of Extreme Heat Days (Total Extreme Heat Days - TEHD). Phase 1: Definition of an 'Extreme Heat Day' A single day is classified as 'extreme' if it simultaneously meets two criteria: Relative Criterion: Its mean temperature is above the 95th historical percentile for that specific calendar day (e.g., hotter than 95% of all past August 19ths). Absolute Criterion: Its mean temperature is also above 32 °C. This dual condition ensures that only days that are both abnormally hot for local climate and absolutely hot at a level that has significant impacts on health, infrastructure and ecosystems are counted. Phase 2: Definition of Annual Risk A future year's risk level is not based on a fixed number of days, but compares its annual TEHD count with the historical distribution of annual TEHD counts. In practice, a future year is at higher risk the rarer and more severe its number of extreme heat days is compared to what occurred in the past.

The methodology described for Extreme Heat Risk assessment is a two-stage approach. It is considered best practice because it combines sensitivity to local climate with absolute impact thresholds.

Our definition of 'Extreme Heat Day' that combines a relative threshold (percentile) and an absolute one (fixed temperature) is an advanced practice recommended by major organizations for creating effective impact indicators.

• Heatwaves and health: guidance on warning-system development This document emphasizes that there is no universal definition of heat wave and that the most effective warning systems must be location-specific (captured by our 95th percentile criterion) and linked to health impacts (captured by our 32 °C criterion). The document explains that health impacts begin to accelerate when temperatures exceed thresholds to which population and infrastructure are acclimatized. Our hybrid approach implements exactly this recommendation.
• On the Measurement of Heat Waves This is a highly cited scientific article that examines different ways of defining a heat wave. It discusses pros and cons of methods based on absolute and relative thresholds (percentiles), concluding that the most robust approaches often combine them to capture both local climate anomalies and dangerous heat levels for humans. Our methodology embodies these conclusions.

The second stage of our methodology, where we assess a year's severity by comparing its total number of extreme days (TEHD) with historical distribution of the same, is a standard climatological method for characterizing season severity.

• Climate Change 2021: The Physical Science Basis Chapter 11 ('Weather and Climate Extreme Events...') is the reference. When IPCC discusses increasing heat waves, it doesn't just say 'temperatures will rise'. It analyzes precisely how frequency (number of days/events), intensity and duration of extreme events are changing. Our TEHD indicator is a direct measure of frequency. Classifying a year based on its TEHD percentile is a statistically robust way to answer the question: 'How anomalous was this year in terms of extreme heat day frequency?'

In conclusion, your methodology involves creating a hybrid and robust impact indicator (as recommended by WMO/WHO) and using this indicator to assess an entire year's severity with a standard statistical method in climatology, fully aligned with IPCC's approach for extreme analysis.