Climate Change and Heat-Related Mortality

How much of today's heat mortality is due to climate change?

A landmark 2021 study in Nature Climate Change gave the first large-scale, empirical answer. Researchers in the Multi-Country Multi-City (MCC) Collaborative Research Network used mortality and temperature data from 732 locations in 43 countries between 1991 and 2018, then compared observed "factual" temperatures with a "counterfactual" world in which only natural climate forcings had operated. The difference, translated to deaths, is a direct estimate of the mortality burden of human-induced warming.

Across all countries, the study attributed 37.0% of warm-season heat-related deaths to human-induced climate change, or about 9,702 such deaths per year on average across the study locations. The percentage varied widely by region: highest in southern and western Asia (Iran, Kuwait), southeast Asia (Philippines, Thailand), and Central and South America (up to about 76% in countries such as Ecuador and Colombia); lowest in the northernmost subregions of Europe, Asia, and the Americas.

The regional gradient is striking. Even under recent, modest warming, tropical and subtropical regions already carry a much larger climate-attributable share than northern regions do.

Record heat-mortality burden in older adults

Beyond attribution studies, an annual health-and-climate assessment tracks the trend line year by year. In its 2024 report, the Lancet Countdown on health and climate change reported that heat-related deaths among people over 65 in 2023 had reached a new record: 167% higher than the 1990s average, and 102 percentage points higher than the roughly 65% increase that would have been expected from population aging alone had temperatures not changed.

Exposure has grown in step. In 2023, infants and adults over 65 experienced an average of 13.8 heatwave days per person, another record. Adults over 65 were exposed to about 9.3 more heatwave days in 2023 than in the 1986-2005 baseline. Older adults dominate heatwave mortality worldwide in part because thermoregulation weakens with age, and because chronic conditions such as cardiovascular disease, respiratory disease, and kidney disease that are common in older adults make heat exposure more dangerous.

Zhao et al. (2021) estimated the global mortality burden of both cold and hot temperatures using data from 750 locations across five continents between 2000 and 2019. Their totals give the widest available scale for temperature-related deaths, and show that both extremes matter, though the trend is toward more heat and less cold.

Why heat kills

Heat kills through both direct and indirect mechanisms. Directly, if the body cannot shed enough metabolic heat, core temperature rises. Sweating and moving blood to the skin are the main cooling mechanisms; when air temperature is high and humidity is high enough that sweat cannot evaporate, both mechanisms fail. That produces heat exhaustion or, at the extreme, heat stroke, a medical emergency.

Indirectly (and more commonly) heat pushes people with existing cardiovascular, respiratory, or kidney disease over a threshold. Death certificates in these cases typically list the underlying disease rather than heat, which is one reason coded heat-death counts substantially undercount the true toll. Modeled excess-death studies (of the kind used in Vicedo-Cabrera 2021 and by the Lancet Countdown) look at how mortality rises during hot periods regardless of the certified cause, and consistently find several times more heat-related deaths than the certified count.

The trajectory: what published projections say

Institutional bodies do not project a single global heat-death figure, but the trend is unambiguous. The Intergovernmental Panel on Climate Change's Sixth Assessment Report (WGII, 2022) concludes with high confidence that climate change has increased heat-related mortality, with the largest impacts on vulnerable groups including the elderly, outdoor workers, and low-income communities. The World Health Organization has published a widely quoted institutional estimate of an additional 250,000 deaths per year between 2030 and 2050 attributable to climate change (from heat, undernutrition, malaria, and diarrhoea combined), while noting this figure is conservative and does not capture the full range of exposures.

The Lancet Countdown's most recent report shows heat exposure indicators (health-threatening days, heatwave person-days, and heat-related mortality of over-65s) all setting new records year after year. Whether the future toll follows a linear extrapolation depends heavily on emissions, adaptation (cooling access, early-warning systems, urban design), and population aging.

How to read these numbers

Several cautions keep the figures honest. First, all three headline numbers are modeled estimates, not tallies of death certificates. They rest on statistical comparisons of observed mortality with counterfactual scenarios, and carry the usual model uncertainty (Vicedo-Cabrera 2021 reports a range of 20.5-76.3% by country; Zhao 2021's global 5.08 million total has a wide confidence interval).

Second, the studies measure different things: Vicedo-Cabrera 2021 asks what share of heat-related deaths is due to climate change; Zhao 2021 asks how many total deaths are associated with any non-optimal temperature (mostly cold); Lancet Countdown tracks trends year over year among adults 65+. They are not additive.

Third, the studies describe populations, not individuals. Individual heat risk depends on age, chronic conditions, occupation, housing, cooling access, and local climate. What the evidence supports is directional and well documented: climate change has already increased heat mortality, older adults and hot-region populations bear most of the additional burden, and both exposure and mortality are trending upward. Extreme heat is a medical emergency; anyone with confusion, fainting, a very high temperature, or hot dry skin needs urgent care.

Methodology and limitations

This page relies on three peer-reviewed sources and one institutional report. Country- and region-level attribution comes from Vicedo-Cabrera et al. (Nature Climate Change 2021), covering 732 locations in 43 countries with a factual-versus-counterfactual climate design. Year-over-year trends and the record for adults 65+ come from the 2024 Lancet Countdown on health and climate change (Romanello et al., The Lancet 2024). Global cold and heat mortality shares come from Zhao et al. (Lancet Planetary Health 2021), covering 750 locations, 2000-2019. Institutional framing draws on the IPCC Sixth Assessment Report (WGII 2022) and the World Health Organization's fact sheet on climate change and health.

Limitations: attribution and excess-death studies are modeled, not counted; regional shares depend on the mix of study locations; heat-related deaths are undercounted on death certificates because heat often kills by worsening cardiovascular, respiratory, or kidney disease; and studies measure different quantities (climate-attributable share, total temperature-related deaths, over-65 trend) that cannot be summed. This page is general educational data journalism, not medical or policy advice.

Frequently asked questions

How much of heat-related mortality is due to climate change?

A large multi-country study estimated that 37.0% of warm-season heat-related deaths across 43 countries between 1991 and 2018 were attributable to human-induced climate change, with country-level values ranging from about 20.5% to 76.3%. Regions with the highest attributable shares included parts of Central and South America, southeast Asia, and southern and western Asia.

Are heat deaths of older adults really at record highs?

According to the 2024 Lancet Countdown on health and climate change, heat-related deaths of adults over 65 in 2023 were 167% higher than the 1990s average, and 102 percentage points higher than the roughly 65% increase that would have been expected from population aging alone if temperatures had not changed.

How many people die from heat globally each year?

A 2021 Lancet Planetary Health study estimated that about 489,000 deaths per year between 2000 and 2019 were attributable to heat (roughly 0.91% of all deaths), while cold accounted for about 4.6 million (8.52%). Confidence intervals are wide, and the heat share has been rising while the cold share has been falling.

Why do heat-death counts differ so much between studies?

Because studies measure different things and use different methods. Certified death counts capture only cases where heat is written on the death certificate. Excess-mortality models estimate how much death rates rise during hot periods regardless of certified cause. Attribution studies then estimate how much of that rise is due to human-induced climate change. These approaches produce different numbers, all of which can be correct for their own question.

Who is most at risk from climate-driven heat?

Older adults dominate the heat-mortality toll in most high-income settings, and heat exposure is rising fastest for them. Outdoor workers, infants, pregnant people, people with cardiovascular or kidney disease, and communities with limited cooling access are also more vulnerable. Populations in hot, humid regions face the largest climate-attributable share of heat-related deaths.

Is extreme heat a medical emergency?

Yes. Heat stroke (a very high core temperature with confusion, fainting, or hot dry skin) is a medical emergency. Heat exhaustion (heavy sweating, weakness, nausea, dizziness) can progress to heat stroke without cooling and rest. This page is educational; anyone with these symptoms needs urgent medical care.

Sources

Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).

1. Vicedo-Cabrera AM, Scovronick N, Sera F, et al. The burden of heat-related mortality attributable to recent human-induced climate change. Nat Clim Chang. 2021;11:492-500. 732 locations in 43 countries, 1991-2018; 37.0% of warm-season heat-related deaths attributable to human-induced climate change (range 20.5-76.3%). Nature Climate Change
2. Romanello M, Walawender M, Hsu SC, et al. The 2024 report of the Lancet Countdown on health and climate change: facing record-breaking threats from delayed action. Lancet. 2024;404(10465):1847-1896. Heat-related deaths of adults 65+ up 167% vs the 1990s, 102 pp above the +65% expected without temperature rise. PubMed
3. Zhao Q, Guo Y, Ye T, et al. Global, regional, and national burden of mortality associated with non-optimal ambient temperatures from 2000 to 2019: a three-stage modelling study. Lancet Planet Health. 2021;5(7):e415-e425. 5,083,173 deaths/yr from non-optimal temperatures (9.43% of all deaths); heat 0.91%; cold 8.52%. Full text
4. IPCC (Intergovernmental Panel on Climate Change). Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report. Cambridge University Press, 2022. Concludes with high confidence that climate change has increased heat-related mortality. (secondary) IPCC AR6 WGII
5. World Health Organization. Climate change and health (fact sheet). WHO estimates an additional 250,000 deaths per year between 2030 and 2050 from climate change (heat, undernutrition, malaria, diarrhoea). (secondary) WHO

How to cite this page

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