The heat wave risk measures how many days per year the site spends in periods that are abnormally hot for the local climate, and how rare such a year would have been twenty years ago. If the future is worse than any year seen in 1995-2014, the level is the highest.
Indicator. Warm Spell Duration Index (WSDI), ETCCDI index: days per year that are part of runs of at least 6 consecutive days with a daily maximum temperature above the 90th percentile of the same time of year in 1961-1990. Unit: days.
Data. CMIP6 climate extreme indices from the Copernicus Climate Data Store (base period 1961-1990, version 2.0), one run per model (21 to 27 models depending on the scenario), each model bilinearly interpolated onto a 1° grid. The site value is that of the nearest land point. Scenarios: SSP1-2.6 / SSP2-4.5 / SSP3-7.0 / SSP5-8.5.
Calculation.
(1) For each scenario and model the mean of the 5 years around each year is taken, then the median of the models (half of the models lie above it, half below); the 10th and 90th percentiles give the uncertainty range.
(2) The reference climate is that of twenty years ago: all the years 1995-2014 of all the models together (about 520 model-years per point), the recent reference period of IPCC AR6.
(3) The reference years with at least as many heat wave days as the future year are counted. The return period is the number of years divided by that count (for example 520 years with 26 cases give 20 years), capped at 250. If it never happened in those years, the return period is 250 and the level is F.
Thresholds → level A–F.
• return period < 5 years → A
• 5–20 → B
• 20–50 → C
• 50–100 → D
• 100–250 → E
• ≥ 250 years, i.e. at most twice in about 520 model-years, or never → F.
From physical to impact. The physical level (A–F) is combined with the production process's sensitivity through the risk matrix (6 physical levels by 5 sensitivity bands) → impact level. Version 2025 also turned the impact level into a value at risk with a damage factor per class, a xNatura estimate without a published source: it is no longer used. Economic damage, Version 2026 xNatura estimate: 0 €: the production loss of hot days is in the Heat stress (working hours lost, ILO 2019) and Extreme heat (OECD 2024) risks, where the site total counts the larger of the two, and the extra energy cost in the Climate change risk (van Ruijven et al. 2019).
Economic damage. The production damage of heat (working hours lost) is counted only once, in the heat stress risk, so that the same hours are not counted three times.
Sources.
• [Climate extreme indices derived from CMIP6 (Copernicus CDS)](Climate extreme indices derived from CMIP6 (Copernicus CDS))
• [Indices for monitoring changes in extremes based on daily temperature and precipitation data](Indices for monitoring changes in extremes based on daily temperature and precipitation data)
• [ETCCDI climate extremes indices](ETCCDI climate extremes indices)
• [Observed coherent changes in climatic extremes during the second half of the twentieth century](Observed coherent changes in climatic extremes during the second half of the twentieth century)
• [IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate](IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate)
• [Guidelines on Analysis of Extremes in a Changing Climate in Support of Adaptation to Climate Change](Guidelines on Analysis of Extremes in a Changing Climate in Support of Adaptation to Climate Change).
Indicator. Warm Spell Duration Index (WSDI), ETCCDI index: days per year that are part of runs of at least 6 consecutive days with a daily maximum temperature above the 90th percentile of the same time of year in 1961-1990. Unit: days.
Data. CMIP6 climate extreme indices from the Copernicus Climate Data Store (base period 1961-1990, version 2.0), one run per model (21 to 27 models depending on the scenario), each model bilinearly interpolated onto a 1° grid. The site value is that of the nearest land point. Scenarios: SSP1-2.6 / SSP2-4.5 / SSP3-7.0 / SSP5-8.5.
Calculation.
(1) For each scenario and model the mean of the 5 years around each year is taken, then the median of the models (half of the models lie above it, half below); the 10th and 90th percentiles give the uncertainty range.
(2) The reference climate is that of twenty years ago: all the years 1995-2014 of all the models together (about 520 model-years per point), the recent reference period of IPCC AR6.
(3) The reference years with at least as many heat wave days as the future year are counted. The return period is the number of years divided by that count (for example 520 years with 26 cases give 20 years), capped at 250. If it never happened in those years, the return period is 250 and the level is F.
Thresholds → level A–F.
• return period < 5 years → A
• 5–20 → B
• 20–50 → C
• 50–100 → D
• 100–250 → E
• ≥ 250 years, i.e. at most twice in about 520 model-years, or never → F.
From physical to impact. The physical level (A–F) is combined with the production process's sensitivity through the risk matrix (6 physical levels by 5 sensitivity bands) → impact level. Version 2025 also turned the impact level into a value at risk with a damage factor per class, a xNatura estimate without a published source: it is no longer used. Economic damage, Version 2026 xNatura estimate: 0 €: the production loss of hot days is in the Heat stress (working hours lost, ILO 2019) and Extreme heat (OECD 2024) risks, where the site total counts the larger of the two, and the extra energy cost in the Climate change risk (van Ruijven et al. 2019).
Economic damage. The production damage of heat (working hours lost) is counted only once, in the heat stress risk, so that the same hours are not counted three times.
Sources.
• [Climate extreme indices derived from CMIP6 (Copernicus CDS)](Climate extreme indices derived from CMIP6 (Copernicus CDS))
• [Indices for monitoring changes in extremes based on daily temperature and precipitation data](Indices for monitoring changes in extremes based on daily temperature and precipitation data)
• [ETCCDI climate extremes indices](ETCCDI climate extremes indices)
• [Observed coherent changes in climatic extremes during the second half of the twentieth century](Observed coherent changes in climatic extremes during the second half of the twentieth century)
• [IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate](IPCC AR6 WGI, Chapter 11: Weather and Climate Extreme Events in a Changing Climate)
• [Guidelines on Analysis of Extremes in a Changing Climate in Support of Adaptation to Climate Change](Guidelines on Analysis of Extremes in a Changing Climate in Support of Adaptation to Climate Change).