1.1.1Active indicatorExposure of Vulnerable Populations to Heatwaves
1.1Heat and health
1.2Health and extreme weather-related events
1.3Climate suitability for infectious disease transmission
Indicator 1.1.1 · Hazards
Exposure of Vulnerable Populations to Heatwaves
Exposure to extreme heat has a range of health consequences including heat stress and heat stroke, worsening heart disease, and acute kidney injury and adverse mental health outcomes. Infants and older adults are particularly vulnerable to adverse health effects. This indicator tracks the number of heatwave days and the exposure of these vulnerable populations, those under 1 and over 65, to heatwaves.
Headline finding
Globally, 83% (11.3 of 13.5) of the heatwave days to which people were exposed on average in 2025 would not have been expected to occur without climate change
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Figure 1
Heatwave Days Attributable to Human-Induced Climate Change
Annual number of heatwave days by country under observed (factual) climate conditions and under a counterfactual scenario assuming no influence of human-induced climate change on global temperatures, from 2006 to 2025. The difference between the two scenarios represents the number of heatwave days attributable to human-induced climate change. Average number of heatwave days attributable to human-induced climate change, shown globally and by country, Human Development Index group, WHO region and Lancet Countdown region, from 2006 to 2025. The sum of expected exposure days and days attributable to climate change gives the total average number of heatwave days to which people were exposed in a given year.
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Figure 2
Heatwave Exposure Among Vulnerable Populations
Total heatwave person-days, total heatwave counts, heatwave days per person, and heatwave counts per person among vulnerable groups, shown globally and by country, Human Development Index group, WHO region and Lancet Countdown region. Heatwave person-days represent the total number of heatwave days experienced across the population. Heatwave counts total represent the total number of heatwave events experienced across the population, calculated as population multiplied by the number of heatwaves per year. Heatwave days per person show the average number of heatwave days experienced per person, and heatwave counts per person show the average number of heatwave events experienced per person.
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Figure 3
Exposure of Vulnerable Populations to Heatwaves
Change in average annual heatwave exposure among infants under 1 year of age and adults aged 65 years and older, comparing 2006–2015 with the 1986–2005 baseline. Measures include the total change in heatwave person-days, calculated as the change in annual heatwave days relative to the baseline multiplied by the population of each vulnerable group, and the change in heatwave days per person, calculated by normalising the population-weighted change.
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About this indicator
Description
This indicator has two parts. Part (1) attributes heatwave days to anthropogenic climate change by estimating temperatures in a counterfactual climate without human-caused forcing. The difference in heatwave days between observed and counterfactual datasets gives the number attributable to climate change. Part (2) estimates vulnerable populations' exposure to heatwaves by combining annual heatwave days per grid cell with gridded demographic data for older adults (≥65 years) and infants (<1 year) to calculate exposure in person-days. A heatwave is defined as at least three consecutive days where minimum and maximum temperatures exceed the 95th percentile for the area based on a 1986–2005 baseline.
Authors
Part 1
Dr. Andrew J Pershing, Joseph Giguere
Part 2
Dr Federico Tartarini, Dr Mitchell Black, Prof Ollie Jay
Data downloads
Caveats
Counterfactual temperatures (conditions without climate change) cannot be observed and must be estimated, though multiple pathways are used following best practice. The binary heatwave comparison also obscures nuance: counterfactual days below the threshold were likely still very warm, and days qualifying in both climates are more dangerous today due to warming. ERA5-Land reanalysis data may contain biases relative to observations, and its resolution cannot capture urban microclimates such as heat islands. The heatwave definition may miss relevant events and ignores humidity. Demographic data has limited validation, and WorldPop's unconstrained mapping may misestimate population densities.
Data sources
Part 1
- ECMWF ERA5 reanalysis dataset and corresponding daily counterfactual temperatures computed a multi-method approach
- WorldPop2025 (averaged over 2016-2025) were used for population weighting
Part 2
- ECMWF ERA5-Land reanalysis dataset.
- Demographic Data (1980–2000): Hybrid gridded demographic dataset from the Lancet Countdown 2023 (0.25° resolution).
- Demographic Data (2000–2015): WorldPop Age and Sex Structure Unconstrained Global Mosaic.
- Demographic Data (2015–2025): WorldPop Age and Sex Structure Unconstrained Global Mosaic.