# How Heat Waves Work

> On 23 October 2026 researchers at Imperial College London and the London School of Hygiene & Tropical Medicine estimated that summer heat caused more than 50,000 deaths in 854 European urban areas, about two-thirds of them because of climate change. This film explains what lies behind such a number: why there is no single definition of a heat wave, how a stalled high-pressure 'heat dome' warms sinking air and clears the sky, how dry soil and cities add heat, how the body sheds heat by blood flow and sweat, why humid heat is so dangerous and what wet-bulb temperature measures, how heat exhaustion differs from heatstroke, who is most at risk, why heat deaths are estimated as excess mortality, and how scientists show that climate change is making heat waves more frequent and more intense.

Interactive film: https://www.planetpast.com/stories/how-heat-waves-work/  
Publisher: PlanetPast (https://www.planetpast.com/) · d => T('readPublished', l, { d: dateIn(d, l) }) 2026-10-25 · Earth science, Biology, Society

## Prologue

### The weather that kills quietly
*Earth · Biology · Society*

How heat waves form, and what they do to us.

No storm, no flood, no wreckage. Just air that stays hot for days. So how does weather like that kill? On 23 October 2026, researchers in London estimated that this summer's heat caused more than 50,000 deaths in 854 European cities, the most they have found for those cities since 2000. It is a model estimate for cities only, not comparable with older Europe-wide counts. Let's follow a heat wave from the sky, to the ground, to the body.

- **>50,000**: estimated heat-related deaths, June–August (Imperial College London, LSHTM)
- **854**: urban areas in 30 countries

- **~40%**: of those countries’ people; rural areas are left out
- **~29,000**: the same cities in summer 2022, by the same method

- What counts as a heat wave
- Heat domes, dry soil and hot cities
- How the body sheds heat, and fails
- Counting deaths, and the role of warming

Prologue

#### A toll measured after the fact

LSHTM, 23 October 2026: a study in Nature Health led by Dr Clair Barnes (Imperial College London) estimates more than 50,000 heat-related deaths in summer 2026 (June–August) across 854 urban areas of the 27 EU countries, the UK, Norway and Switzerland, home to about 40% of their population. It is the highest toll since 2000, almost double summer 2022 (about 29,000) and 2003 (about 28,000) in the same urban areas. About two-thirds is attributed to climate change. Nearly a third of the deaths fell between 17 June and 1 July.

The figures are modelled, not counted: weather data and climate models are combined with past evidence on how deaths rise with temperature. The study compares today’s climate, about 1.4 °C warmer, with one without human-caused warming. Rural areas are not covered, so news reports note that the true toll is likely higher, and the figure cannot be compared directly with Europe-wide studies such as those of 2003 (>70,000) or 2022 (61,672), which covered whole countries. Country figures reported from the study include Italy 13,303, France 7,268, Spain 6,824 and Germany 6,340 (Trending Topics).

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026](https://www.lshtm.ac.uk/newsevents/news/2026/more-50000-heat-related-deaths-estimated-european-urban-areas-during-summer); [Trending Topics (23 Oct 2026): heat killed 50,000 people in Europe’s cities, two-thirds due to climate change](https://www.trendingtopics.eu/heat-deaths-europe-summer-2026/); [Democracy Now! headlines (23 Oct 2026): study, European heat waves killed 50,000 in 2026](https://www.democracynow.org/2026/10/23/headlines)

## Chapter I · What counts as hot

### What counts as hot
*Chapter I*

There is no single line for the whole world.

There is no single global definition of a heat wave. The World Meteorological Organization describes unusually hot weather lasting at least two or three days, with a noticeable impact on people. Each country draws its own line. Britain's Met Office needs three days in a row at or above a local threshold: 25 °C in Scotland and Wales, 28 °C around London. India's weather service needs at least 40 °C on the plains, and well above normal. Hot is measured against what a place is used to.

> “Unusually hot and dry or hot and humid weather that has a subtle onset and cessation” (World Meteorological Organization, via UNDRR)

- **3 days**: in a row at or above the local threshold
- **25–28 °C**: thresholds, coolest to warmest areas

- **≥40 °C**: and 4.5 °C or more above normal
- **≥45 °C**: counts as a heat wave whatever the normal

Chapter I · What counts as hot

#### Hot is relative

UNDRR’s hazard profile, drawing on the WMO, describes a heatwave as a marked, unusual period of hot weather persisting for at least two to three days and nights, with a discernible impact on human activities, and says universal numeric limits are not possible because impacts vary locally.

Met Office: at least three consecutive days with maximum temperatures at or above the local threshold, which varies by county; thresholds were raised in eight counties in the 1991–2020 update. British Red Cross summary: 25 °C in places such as Scotland and Wales, 28 °C in London and parts of the East Midlands. IMD: at least 40 °C for plains (37 °C coastal, 30 °C hills) with a departure of 4.5–6.4 °C from normal (severe above 6.4 °C), or an actual maximum of 45 °C (severe 47 °C), at two stations on two consecutive days.

Try it: the curve is a schematic spread of summer afternoon temperatures; the buttons move it and its threshold to a cooler or a hotter climate.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [UNDRR / WMO hazard information profile: heatwave](https://www.undrr.org/understanding-disaster-risk/terminology/hips/mh0047); [Met Office (2026): UK heatwaves, how the records tell a changing story](https://www.metoffice.gov.uk/blog/2026/uk-heatwaves-how-the-records-tell-a-changing-story); [British Red Cross: what is a heatwave? (Met Office thresholds)](https://www.redcross.org.uk/stories/disasters-and-emergencies/uk/what-is-a-heatwave); [India Meteorological Department: heat wave criteria (morning heat bulletin, annexure)](https://mausam.imd.gov.in/pdfs/heatcolduser/morning_heat_bulletin.pdf)

## Chapter II · The heat dome

### The heat dome
*Chapter II*

Air that sinks, and warms as it falls.

High pressure means air slowly sinking over a wide region. When such a high parks in one place for days, forecasters call it a heat dome. Sinking air is squeezed by the air around it, and squeezing warms it: dry air heats up by nearly ten degrees for every kilometre it falls. Watch a pocket of air come down. Sinking air also stops clouds forming, so the sun beats down all day and the heat stays put.

$$\Delta T \approx 9.8\,^{\circ}\mathrm{C} \times \Delta z\,(\mathrm{km})$$
- $\Delta z$: how far the air sinks, in kilometres
- $\Delta T$: how much it warms, with no heat added from outside

Air sinking from 3 km up arrives about 29 °C warmer.

- **5 °C → 34 °C**: from 3 km up to the ground at the dry rate, an illustration

- Sinking air: warmer and drier
- No clouds: full sunshine
- Weak winds aloft: heat lingers
- Rain is suppressed

Chapter II · The heat dome

#### Why high pressure means heat

The American Meteorological Society glossary describes a heat dome as an exceptionally warm air mass at middle latitudes during the warm season, under high pressure aloft that suppresses cloud and rain and can add warming as air sinks; because winds aloft are often weak, the heat can linger for days.

Dry adiabatic lapse rate: 9.8 °C per km. Air moving downward is compressed; work is done on it, so its temperature rises even though no heat is added from outside. Moist air that is condensing cools more slowly as it rises, which is why the dry rate applies to the clear, sinking air under a dome.

Try it: drop a parcel from 3 km and watch its temperature as it sinks.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [American Meteorological Society, Glossary of Meteorology: heat dome](https://glossary.ametsoc.org/wiki/Heat_dome); [Wikipedia: lapse rate (dry adiabatic lapse rate)](https://en.wikipedia.org/wiki/Lapse_rate)

### 

Highs usually drift east with the jet stream, the fast wind high above the mid-latitudes. When the flow buckles, a high can get stuck: this is called blocking. In 2010 a blocking high sat over western Russia from July into August. Crops failed on nine million hectares, about the size of Portugal, and fires smouldered in the peat bogs near Moscow. Moscow counted nearly 5,000 more deaths that July than the July before, a first glimpse of how heat deaths are measured. In 2021 a heat dome over the Pacific Northwest pushed Lytton, Canada, to a national record of 49.6 °C. British Columbia's coroners counted 619 heat deaths.

- **44.0 °C**: Yashkul, 11 July
- **9 million**: hectares of crops lost, close to the 92,000 km² of Portugal

![In July 2010 Moscow’s air pollution reached about five times normal. AI-generated illustration.](https://assets.planetpast.com/a/9a/9ad7a7dba806f813.jpg)

- **>14,000**: deaths in the month
- **~5,000**: more than in July 2009: the baseline

- **49.6 °C**: Lytton; the old Canadian record was 45 °C
- **619**: heat deaths in the province (coroners’ review)

![Days above 40 °C can brown even the needles of a temperate rainforest. AI-generated illustration.](https://assets.planetpast.com/a/2d/2db97dfc598365a8.jpg)

Chapter II · The heat dome

#### When the jet stream stalls

Met Office case study: the 2010 Russian heatwave was driven by a blocking high-pressure system that stalled the low-pressure systems that would normally break it up; the worst lasted from July into the first half of August. Moscow recorded over 14,000 deaths in July 2010, nearly 5,000 more than the previous July; pollution was about five times normal; more than 40 peat fires burned south and east of Moscow; about 9 million hectares of crops were lost in the worst drought in roughly 40 years. Highest reading in the table: Yashkul, 44.0 °C, 11 July. Nine million hectares is 90,000 km², close to Portugal’s 92,225 km² (Wikipedia). Comparing with the July before is a simple baseline: the extra deaths over a normal month, which chapter VI calls excess deaths. Peat is waterlogged soil made of old plant remains; when it dries it can smoulder for weeks.

2021: Lytton, British Columbia, set a Canadian record of 49.6 °C, far above the previous 45 °C, and was largely destroyed by a wildfire shortly afterwards (IPSL, on the World Weather Attribution study). British Columbia’s coroners’ review counted 619 heat deaths in late June and early July 2021 (CTV News).

Try it: switch between a flowing jet stream and a blocked one. The wave shape is schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [Met Office: the Russian heatwave of summer 2010](https://weather.metoffice.gov.uk/learn-about/weather/case-studies/russian-heatwave); [IPSL: North America heatwave almost impossible without climate change, World Weather Attribution study (2021)](https://www.ipsl.fr/?p=1542487); [CTV News: coroners’ review reveals true death toll of B.C.’s 2021 heat dome](https://bc.ctvnews.ca/coroners-review-reveals-true-death-toll-of-b-c-s-2021-heat-dome-1.5936108); [Wikipedia: Portugal (area 92,225 km²)](https://en.wikipedia.org/wiki/Portugal)

## Chapter III · Ground and city

### Ground and city
*Chapter III*

The surface underneath can make it worse.

The ground can feed a heat wave. On moist soil, much of the sun's energy evaporates water, which cools the surface. When the soil dries out, that energy heats the air instead. A 2014 study of Europe's 2003 and Russia's 2010 heat waves found both began over dry soils. The hot layer of air grew over four kilometres deep, holding so much heat that nights stayed warm and each day started hotter.

- Wet soil: into evaporation, which cools
- Dry soil: straight into heating the air
- Drier ground, hotter days, drier ground again

- **>4 km**: depth of the heated air layer, about double its usual depth

> “The heat doesn’t dissipate at night, so you start from where you left off the day before.” (Diego Miralles, Ghent University, 2014)

Chapter III · Ground and city

#### The soil moisture feedback

Scientific American, reproducing Nature (April 2014), on a study by Diego Miralles (Ghent University) in Nature Geoscience: each of the 2003 European and 2010 Russian mega-heatwaves started over areas with low soil moisture. The ground dried further in the first heat and added more heat to the air near the surface; the heated air layer roughly doubled to over 4 km. “The heat doesn’t dissipate at night, so you start from where you left off the day before.”

Try it: switch the field between wet and dry. The arrows show where the sunshine goes: blue for evaporation, orange for heating the air. Proportions are illustrative.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [Scientific American / Nature (2014): mega-heatwaves mechanism pinpointed (Miralles et al., Nature Geoscience)](https://www.scientificamerican.com/article/mega-heatwaves-mechanism-pinpointed)

### 

Cities add heat of their own. Dark roofs, roads and walls soak up sunlight and release it as heat, and fewer trees and less moist ground cool the air. The US Environmental Protection Agency puts cities about one to four degrees Celsius warmer than the countryside by day, and one to three degrees warmer at night. Cities are where most people live, and where the 2026 estimate looked.

![White and planted roofs reflect or shade sunlight that dark tar roofs absorb. AI-generated illustration.](https://assets.planetpast.com/a/40/40c6c3cec757d63a.jpg)

- By day: 0.6–3.9
- At night: 1.1–2.8

- **854**: European urban areas
- **~40%**: of those countries’ people live there

Chapter III · Ground and city

#### Why cities run hot

US EPA: heat islands form where structures are concentrated and greenery is limited; buildings, roads and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas are about 1–7 °F (0.6–3.9 °C) warmer than outlying areas by day and about 2–5 °F (1.1–2.8 °C) warmer at night.

The 2026 European estimate covers 854 urban areas only (LSHTM).

Try it: switch between day and night. The heat colours over the city and the fields are schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [US EPA: learn about heat island effects](https://epa.gov/heatislands/learn-about-heat-island-effects); [LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026](https://www.lshtm.ac.uk/newsevents/news/2026/more-50000-heat-related-deaths-estimated-european-urban-areas-during-summer)

## Chapter IV · The body

### The body
*Chapter IV*

A furnace that must keep cool.

Your body makes about 100 watts of heat at rest and keeps its core near 37 °C. It sheds the surplus in two main ways. First, vessels in the skin widen and bring warm blood to the surface, where heat passes to the air, but only while the air is cooler than the skin, about 35 °C. Second, sweat: it cools only as it evaporates, taking heat from the skin. In air hotter than the skin, that is the only way out.

- **100 W**: about this much heat from a resting body
- **37 °C**: core temperature
- **35 °C**: skin, normally at or below

- **Air 25 °C**: cooler than skin: heat leaves the body
- **Air 40 °C**: hotter than skin: heat flows in

- Blood to the skin: needs air cooler than about 35 °C
- Evaporating sweat: needs air dry enough to take the vapour
- Air hotter than skin: heat flows in, not out

![A drop that runs off takes almost no heat with it; only the water that evaporates cools. AI-generated illustration.](https://assets.planetpast.com/a/aa/aa1a93708db540dc.jpg)

Chapter IV · The body

#### How the body sheds heat

Sherwood and Huber (PNAS, 2010): a resting human body generates about 100 W of metabolic heat and keeps a core temperature near 37 °C; skin temperature is strongly regulated at 35 °C or below under normal conditions, so that heat can flow from core to skin. US National Weather Service: blood is circulated closer to the skin’s surface, and sweating does nothing to cool the body unless the water is removed by evaporation.

Try it: set the air to 25 °C or 40 °C and watch the arrows. In 40 °C air, heat flows into the skin and only evaporation carries it away. The figure is a schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [Sherwood & Huber (2010), PNAS: an adaptability limit to climate change due to heat stress](https://pmc.ncbi.nlm.nih.gov/articles/PMC2906879); [US National Weather Service: heat safety (how the body cools, heat index)](https://www.weather.gov/media/pah/WeatherEducation/heatsafety.pdf)

### 

When heat comes in faster than it can leave, trouble starts. Heat exhaustion brings heavy sweating, dizziness, headache, cramps and a fast pulse; with rest and cooling, most people start to feel better within about 30 minutes. Heatstroke is an emergency. The skin can be hot and may stop sweating, and the person may become confused, have a seizure or lose consciousness. Health services say: call for emergency help. Heat also strains the heart and kidneys; the World Health Organization warns it worsens heart disease, diabetes, asthma and kidney disease.

- Heavy sweating, pale clammy skin
- Dizziness, headache, nausea
- Cramps, fast breathing or pulse, thirst

- Still unwell after 30 minutes of cooling
- Hot skin that may not be sweating
- Confusion, seizure, loss of consciousness
- Call the emergency number

> “Untreated heat stress can lead to heat stroke, which can be a fatal medical emergency.” (World Health Organization)

Chapter IV · The body

#### Heat exhaustion and heatstroke

NHS 111 Wales: heat exhaustion usually does not need emergency help if the person cools down within 30 minutes; heatstroke is an emergency, and heat exhaustion can turn into it. Signs of heatstroke include still feeling unwell after 30 minutes of rest, cooling and fluids; a very high temperature; hot skin that is not sweating; fast breathing or heartbeat; confusion, poor coordination, a seizure or loss of consciousness. In those cases the service says to call the emergency number. In Thailand, the Department of Disease Control advises calling 1669 for severe cases.

This is general education, not medical advice. WHO (July 2024): heat stress worsens cardiovascular disease, diabetes, asthma and kidney disease, and in extreme heat untreated heat stress can lead to heat stroke.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [NHS 111 Wales: heat exhaustion and heatstroke](https://111.wales.nhs.uk/Heatexhaustionandheatstroke); [WHO (25 Jul 2024): UN Secretary-General calls for global action on extreme heat](https://www.who.int/news/item/25-07-2024-united-nations-secretary-general-calls-for-global-action-on-extreme-heat); [กรมควบคุมโรค สคร.12 สงขลา (7 มี.ค. 2567): เตือนระวังฮีทสโตรก โทรสายด่วน 1669](https://ddc.moph.go.th/uploads/files/4136520240307071304.pdf)

## Chapter V · Humid heat

### Humid heat
*Chapter V*

When sweat can no longer evaporate.

Now add humidity. When the air already holds a lot of water vapour, sweat evaporates slowly and drips off instead of cooling. Wet-bulb temperature measures this. It is what a thermometer wrapped in a wet cloth reads: evaporation cools it, unless the air is too humid. It is not the number on an ordinary thermometer. A wet-bulb of 35 °C needs air of about 40 °C at 70% humidity. In 2010 two scientists put the theoretical survival limit near that 35 °C, even for a person resting in shade and wind. Lab tests at Penn State in 2022 found young, healthy adults doing light activity reached their limit sooner, around 31 °C in humid heat.

- **≈24 °C**: wet-bulb at 34 °C air and 40% humidity
- **≈33 °C**: wet-bulb at 34 °C air and 90% humidity

- **40 °C + 70%**: air temperature and humidity that give a wet-bulb of about 35 °C

- **35 °C**: wet-bulb, for a person out of the sun, in gale-force wind, doused, unclothed, resting
- **31 °C**: highest wet-bulb in the data the authors studied

- Theory, at rest (2010): 35
- Lab, humid heat, light activity (2022): 30–31

Chapter V · Humid heat

#### Why humid heat is so dangerous

Sherwood and Huber (2010) estimated the survivability limit for peak wet-bulb temperature at probably close to 35 °C, “though this could be a degree or two off”, for a person out of the sun, in gale-force winds, doused with water, unclothed and not working, and said this likely overestimates what could practically be tolerated. In the data they studied, wet-bulb temperature never exceeded 31 °C. Penn State (March 2022, Journal of Applied Physiology): 24 people aged 18–34 doing light activity in an environmental chamber could no longer hold their core temperature steady at wet-bulb temperatures of 30–31 °C in warm, humid conditions and 25–28 °C in hot, dry ones; older and more vulnerable people likely have lower limits. The lower figure for dry heat is not a contradiction: to reach a wet-bulb of 25–28 °C in dry air, the air itself must be far hotter than skin, so the body gains heat directly from it. Wet-bulb is not the reading on an ordinary thermometer: by Stull’s formula, 34 °C air gives about 24 °C wet-bulb at 40% humidity and about 33 °C at 90%, and about 35 °C needs roughly 40 °C air at 70% humidity. The formula: Tw ≈ T·atan[0.151977(RH+8.313659)^½] + atan(T+RH) − atan(RH−1.676331) + 0.00391838·RH^1.5·atan(0.023101·RH) − 4.686035. The heat index, a different measure, is explained in the next step.

Try it: change the air temperature and humidity. The surface plots wet-bulb temperature from Stull’s 2011 formula, an approximation at standard sea-level pressure; the heat index in the readout uses the US National Weather Service equation. The red sheet marks 35 °C, the orange one 31 °C.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [Sherwood & Huber (2010), PNAS: an adaptability limit to climate change due to heat stress](https://pmc.ncbi.nlm.nih.gov/articles/PMC2906879); [Penn State (2022): humans can’t endure temperatures and humidities as high as previously thought (Vecellio et al., J. Appl. Physiol.)](https://psu.edu/news/research/story/humans-cant-endure-temperatures-and-humidities-high-previously-thought); [Stull (2011), J. Appl. Meteor. Climatol.: wet-bulb temperature from relative humidity and air temperature (calculator and validity range)](https://rechneronline.de/air/wet-bulb-temperature.php); [NOAA Weather Prediction Center: the heat index equation](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml); [US National Weather Service: heat safety (how the body cools, heat index)](https://www.weather.gov/media/pah/WeatherEducation/heatsafety.pdf)

### 

Thailand knows humid heat well. In April 2023 the country set its all-time record, 45.4 °C in Tak province. Weather services also give a heat index: not the thermometer reading, nor the wet-bulb, but how hot it feels with humidity counted. In Bangkok's Bang Na it reached 54 °C. That is the top of the 'danger' band in the Thai Meteorological Department's four levels; beyond 54 °C comes 'extreme danger'.

- **45.4 °C**: Thailand’s record air temperature, Tak, 15 April

- **42 °C**: air temperature on the thermometer
- **54 °C**: heat index: how hot it felt

- Watch: 27–32
- Caution: 32–41
- Danger: Bang Na’s 54: 41–54
- Extreme danger: >54

![Laos also set its record that month: 42.9 °C. AI-generated illustration.](https://assets.planetpast.com/a/c8/c84dfbcb526dbcf9.jpg)

Chapter V · Humid heat

#### Thailand’s hottest month on record

Tak recorded 45.4 °C on 15 April 2023, the highest temperature ever measured in Thailand (Wikipedia; Yale Environment 360). On 22 April 2023 the Thai Meteorological Department reported 42 °C in Bang Na district, Bangkok, and a record heat index of 54 °C; at least 28 provinces were forecast above 40 °C (AFP via The Jakarta Post). The Thai Meteorological Department grades the heat index in four levels: watch 27–32 °C, caution 32–41 °C, danger 41–54 °C, extreme danger above 54 °C (Manager Online, April 2024). Bang Na’s 54 °C therefore sits at the top of the danger band. Three different numbers: air temperature is what a thermometer in the shade reads; the heat index is how hot it feels once humidity is counted; the wet-bulb is what a wet thermometer reads.

World Weather Attribution (May 2023, reported by Yale Environment 360): the April 2023 humid heat in Laos and Thailand would have been virtually impossible without climate change; at 2 °C of global warming such heat would become about ten times more likely again.

The scene starts at 40 °C and 45% humidity as an example, not a measurement. The readout shows the wet-bulb temperature, the heat index and the Thai level for whatever you set.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [Wikipedia: Tak, Thailand (national record 45.4 °C, 15 April 2023)](https://en.wikipedia.org/wiki/Tak,_Thailand); [The Jakarta Post / AFP (22 Apr 2023): Thai authorities issue extreme heat warnings for dozens of provinces](https://www.thejakartapost.com/world/2023/04/22/thai-authorities-issue-extreme-heat-warnings-for-dozens-of-provinces); [ผู้จัดการออนไลน์ (13 เม.ย. 2567): ระดับดัชนีความร้อนของกรมอุตุนิยมวิทยา](https://mgronline.com/science/detail/9670000032515); [Yale Environment 360 (17 May 2023): April heat wave in South Asia made 30 times more likely by climate change](https://e360.yale.edu/digest/april-2023-heat-wave-india-thailand-climate-change); [NOAA Weather Prediction Center: the heat index equation](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml)

## Chapter VI · Counting the toll

### Counting the toll
*Chapter VI*

The deaths that should not have happened.

How many people does a heat wave kill? Death certificates rarely say. Heat is often a hidden cause behind a fatal heart attack or kidney failure. So epidemiologists count excess deaths: the deaths that happened, minus those normally expected at that time of year. That is how Europe's 2003 heat wave was measured: in 16 countries, more than 70,000 extra deaths, most in early August. The 2026 estimate covered only cities; there, by the same method, 2003 came to about 28,000 deaths.

$$\text{excess} = \text{observed} - \text{expected}$$
- $\text{expected}$: deaths normally seen on those dates, from earlier years

- **>70,000**: extra deaths in the 16 countries studied
- **1998–2002**: the reference years for “expected”

![The crisis ran from 3 to 16 August; 12 August was the deadliest day. AI-generated illustration.](https://assets.planetpast.com/a/b6/b6c4fc89a860b3f6.jpg)

- **>70,000**: all regions of 16 countries (Robine, 2008)
- **~28,000**: only the 854 urban areas of the 2026 study

Chapter VI · Counting the toll

#### Why heat deaths are estimated, not counted

AP (August 2023): coroners may not link contributing conditions to heat, and confirming heat’s role can take weeks or months. The excess-deaths method compares deaths in a period with the number expected; epidemiologist Kristie Ebi says it captures “people who would not have suffered a fatal heart attack or renal failure without the heat”.

Robine et al. (2008): daily regional death counts in 16 European countries for summer 2003 were compared with the same days in the reference period 1998–2002, giving more than 70,000 additional deaths; the August crisis ran from 3 to 16 August, with 12 August the deadliest day. The 2026 study, which covers only 854 urban areas and uses a modelled temperature–mortality method, puts 2003 at about 28,000 deaths in those cities (LSHTM): different coverage and method, not a contradiction.

Try it: show the expected line, then the observed one; the gap between them is the excess. The curves are schematic, not the 2003 data.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [AP via PBS NewsHour (13 Aug 2023): inconsistent methods for counting U.S. heat deaths](https://www.pbs.org/newshour/health/inconsistent-methods-for-counting-u-s-heat-deaths-stymie-public-health-efforts); [Robine et al. (2008), Comptes Rendus Biologies: death toll exceeded 70,000 in Europe during the summer of 2003](https://comptes-rendus.academie-sciences.fr/biologies/articles/en/10.1016/j.crvi.2007.12.001/); [LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026](https://www.lshtm.ac.uk/newsevents/news/2026/more-50000-heat-related-deaths-estimated-european-urban-areas-during-summer)

### 

The toll falls unevenly. A study of Europe's 2022 summer estimated 61,672 heat deaths. Most were people over 80, and more were women than men. After Canada's 2021 heat dome, British Columbia's review found two-thirds of those who died were 70 or older, more than half lived alone, and almost all died indoors. Babies, people with long-term illness, outdoor workers, pregnant women and anyone without a cool place to go are also at higher risk.

- Under 65: 4,822
- 65–79: 9,226
- 80 and over: 36,848

- **67%**: aged 70 or older
- **56%**: lived alone
- **98%**: died indoors

![Most who died in British Columbia lacked adequate cooling at home. AI-generated illustration.](https://assets.planetpast.com/a/a9/a96d66769bb28b48.jpg)

- Infants and young children
- People with chronic illness
- Outdoor workers and athletes
- Pregnant people; people without air conditioning

Chapter VI · Counting the toll

#### Who heat kills

ISGlobal (July 2023), on Ballester et al. in Nature Medicine: 61,672 heat-attributable deaths in 35 European countries between 30 May and 4 September 2022, from epidemiological models fitted to temperature and mortality data for 823 regions. Estimated deaths: 4,822 under 65, 9,226 aged 65–79 and 36,848 over 79; 35,406 women and 21,667 men.

British Columbia coroners’ review of the 2021 heat dome (CTV News): 619 deaths; 67% aged 70 or older; 56% lived alone; 98% died indoors; most lacked adequate cooling. CDC lists older adults, infants and young children, people with chronic conditions, people without air conditioning, athletes, outdoor workers and pregnant people as at increased risk. Different studies use different areas and methods, so their totals are not directly comparable: the 2026 study counted about 29,000 deaths in 2022 for its 854 urban areas alone.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [ISGlobal (10 Jul 2023): record-breaking heat in the summer of 2022 caused more than 61,000 deaths in Europe (Ballester et al., Nature Medicine)](https://isglobal.org/en/-/el-calor-record-del-verano-de-2022-causo-mas-de-61-000-muertes-en-europa); [CTV News: coroners’ review reveals true death toll of B.C.’s 2021 heat dome](https://bc.ctvnews.ca/coroners-review-reveals-true-death-toll-of-b-c-s-2021-heat-dome-1.5936108); [US CDC: people at increased risk for heat-related illness](https://www.cdc.gov/heat-health/risk-factors/index.html); [LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026](https://www.lshtm.ac.uk/newsevents/news/2026/more-50000-heat-related-deaths-estimated-european-urban-areas-during-summer)

## Chapter VII · A warming world

### A warming world
*Chapter VII*

A small shift in the average, a big change at the edge.

The IPCC, the United Nations' panel of climate scientists, found in 2021 that hot extremes have grown more frequent and intense over most land since the 1950s. Globally, it says, greenhouse gases from human activity are virtually certainly the main driver. Why they warm the planet is told in our film on the greenhouse effect. A small shift in the average does a lot at the edge: slide the curve warmer and the hot tail grows. A once-in-50-years heat extreme now comes about once a decade.

> “The frequency and intensity of hot extremes (including heatwaves) have increased.” (Sixth Assessment Report, Working Group I; virtually certain at global scale)

- **1.1 °C**: global surface warming by 2011–2020, compared with 1850–1900 (IPCC, 2021)

- **1 in 50**: years, in the climate of 1850–1900
- **~1 in 10**: years, today (IPCC, via Reuters)

- Today, ~1.1 °C: about once a decade: ~5
- At 1.5 °C: 8.6
- At 2 °C: 13.9
- At 4 °C: 39.2

Chapter VII · A warming world

#### What the IPCC found

IPCC AR6 Working Group I (2021), as reported by the WMO Bulletin and Carbon Brief: hot extremes, including heatwaves, have become more frequent and more intense across most land regions since the 1950s; it is virtually certain that human-induced greenhouse gas forcing is the main driver of observed changes in hot and cold extremes globally; some recent hot events would have been extremely unlikely without human influence. Hottest days in some mid-latitude and semi-arid regions are projected to warm at about 1.5 to 2 times the rate of global warming.

Reuters (9 August 2021), on the IPCC report: heatwaves that used to occur once every 50 years now happen about once a decade. IPCC AR6 Figure SPM.6, as reported by Actuaries Digital: a hot extreme that came once in 50 years in 1850–1900 becomes 8.6 times as frequent at 1.5 °C of warming, 13.9 times at 2 °C and 39.2 times at 4 °C. The “~5” for today is about once a decade instead of once in 50 years. Three warming figures appear in this film because they measure different periods: about 1.1 °C for 2011–2020 against 1850–1900 (IPCC, 2021); about 1.3 °C for the present climate used by World Weather Attribution; about 1.4 °C in the 2026 European study. How greenhouse gases warm the planet is explained in PlanetPast’s film on the greenhouse effect.

Try it: switch between the past and today. The bell curve is a schematic of daily summer temperatures; the share of days past the line is worked out from it and is not an IPCC figure.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [WMO Bulletin 71(1), 2022: regional trends in extreme events in the IPCC 2021 report](https://wmo.int/resources/wmo-bulletin/wmo-bulletin-vol-71-1-2022/regional-trends-extreme-events-ipcc-2021-report); [Carbon Brief (2021): what the new IPCC report says about extreme weather and climate change](https://www.carbonbrief.org/explainer-what-the-new-ipcc-report-says-about-extreme-weather-and-climate-change); [Reuters via TimesLIVE (9 Aug 2021): once-in-50-year heatwaves now happening every decade, UN climate report](https://www.timeslive.co.za/news/sci-tech/2021-08-09-once-in-50-year-heatwaves-now-happening-every-decade-un-climate-report/); [IPCC AR6 WGI, Figure SPM.6: projected changes in hot temperature extremes (baseline 1850–1900)](https://www.ipcc.ch/report/ar6/wg1/figures/summary-for-policymakers/figure-spm-6); [Actuaries Digital (Aug 2021): IPCC AR6, hotter, wetter, faster, stronger (50-year hot extremes at 1.5, 2 and 4 °C)](https://actuaries.digital/2021/08/25/ipcc-ar6-hotter-wetter-faster-stronger)

### 

Can one heat wave be blamed on climate change? A 2004 study found human influence had very likely at least doubled the risk of Europe's 2003 heat. World Weather Attribution and other teams now do this within weeks: define the event, check decades of observations, then run climate models for today's world and for one without human-caused warming. For the 2021 Pacific Northwest heat, climate change made it at least 150 times more likely; in the old climate it was practically impossible. The 2026 European estimate used the same two-world comparison for deaths: about two-thirds would not have happened without warming.

- **≥×2**: risk of 2003-like heat, very likely (>90% confidence)
- **1500**: 2003 was probably Europe’s hottest summer since at least this year

- Define the event: place, days, measure
- Observations: how often it now happens, and the trend
- Models: today’s world (≈1.3 °C warmer than 1850–1900) vs one without that warming
- Combine, with 95% confidence ranges

- **≥×150**: more likely: the lower bound of the estimate
- **5–10**: years between such heat waves at 2 °C of warming

- **≈2/3**: of the estimated deaths attributed to warming (about 1.4 °C, per the authors)

Chapter VII · A warming world

#### Blaming the weather on the climate, carefully

Stott, Stone and Allen (Nature, 2004): the summer of 2003 was probably the hottest in Europe since at latest AD 1500, and it is very likely (confidence level >90%) that human influence has at least doubled the risk of a heatwave exceeding that threshold magnitude. World Weather Attribution methods: select and define the event; use observations to estimate how often it occurs today and how that has changed; use many climate models to simulate today’s world (about 1.3 °C of warming) and a cooler world without it; combine the results with 95% confidence intervals. WWA says its methods are peer reviewed and have been applied in more than 100 rapid studies, 26 of which were also published in peer-reviewed journals.

Pacific Northwest 2021 (IPSL, on the WWA study): the heat would have been virtually impossible without human influence and was made at least 150 times more likely; even at 2 °C of warming, such a heatwave would occur about once every 5 to 10 years. The 2026 European estimate applies the same two-worlds idea to deaths: it compares modelled deaths in today’s climate with those in a climate without human-caused warming (LSHTM).

Try it: switch between a world without warming and today’s world. The curves are schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: [Stott, Stone & Allen (2004), Nature: human contribution to the European heatwave of 2003](https://ideas.repec.org/a/nat/nature/v432y2004i7017d10.1038_nature03089.html); [World Weather Attribution: methods](https://www.worldweatherattribution.org/methods/); [IPSL: North America heatwave almost impossible without climate change, World Weather Attribution study (2021)](https://www.ipsl.fr/?p=1542487); [LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026](https://www.lshtm.ac.uk/newsevents/news/2026/more-50000-heat-related-deaths-estimated-european-urban-areas-during-summer)

## Epilogue

### 

A stalled high, sinking air, dry ground, a hot city, humid air: a heat wave is a chain. Health agencies say much of its toll is preventable, with warnings, heat-health plans, shade and cool places to go. Often, the person most at risk is not out in the street, but an older person alone in a hot room, whom no one thought to check on.

- 1950s→: Hot extremes grow more frequent and intense (IPCC)
- 2003: Europe: >70,000 excess deaths
- 2004: First attribution study of a heat wave
- 2010: Russia’s blocking high; 35 °C wet-bulb limit proposed
- 2021: Pacific Northwest heat dome: 49.6 °C
- 2023: Thailand’s record: 45.4 °C
- 2026: Europe’s cities: >50,000 estimated heat deaths

- Stills are AI-generated
- 3D scenes and curves are schematic
- Wet-bulb and heat index are approximations
- Health content is education, not medical advice

- Early warning systems
- Heat-health action plans
- Urban planning: shade, greenery
- Access to cooling; checking on people alone

- **56%**: of those who died lived alone
- **98%**: died indoors

Epilogue · Sources

#### Sources

Every fact in this film comes from the sources below. The chapter stills and pictures are AI-generated illustrations. The globe uses NASA Blue Marble imagery. The 3D scenes, bell curves and mortality curves are schematic; wet-bulb temperatures use Stull’s 2011 approximation and heat index values the US National Weather Service equation. Health content is general education, not medical advice: if someone may have heatstroke, call the emergency number (1669 in Thailand, per the Department of Disease Control). Much of heat mortality is preventable: the Global Heat Health Information Network summarises WHO’s call for heat-health action plans, early warning systems, urban planning and access to cooling.

Sources: [LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026](https://www.lshtm.ac.uk/newsevents/news/2026/more-50000-heat-related-deaths-estimated-european-urban-areas-during-summer); [Trending Topics (23 Oct 2026): heat killed 50,000 people in Europe’s cities, two-thirds due to climate change](https://www.trendingtopics.eu/heat-deaths-europe-summer-2026/); [Democracy Now! headlines (23 Oct 2026): study, European heat waves killed 50,000 in 2026](https://www.democracynow.org/2026/10/23/headlines); [UNDRR / WMO hazard information profile: heatwave](https://www.undrr.org/understanding-disaster-risk/terminology/hips/mh0047); [Met Office (2026): UK heatwaves, how the records tell a changing story](https://www.metoffice.gov.uk/blog/2026/uk-heatwaves-how-the-records-tell-a-changing-story); [British Red Cross: what is a heatwave? (Met Office thresholds)](https://www.redcross.org.uk/stories/disasters-and-emergencies/uk/what-is-a-heatwave); [India Meteorological Department: heat wave criteria (morning heat bulletin, annexure)](https://mausam.imd.gov.in/pdfs/heatcolduser/morning_heat_bulletin.pdf); [American Meteorological Society, Glossary of Meteorology: heat dome](https://glossary.ametsoc.org/wiki/Heat_dome); [Wikipedia: lapse rate (dry adiabatic lapse rate)](https://en.wikipedia.org/wiki/Lapse_rate); [Met Office: the Russian heatwave of summer 2010](https://weather.metoffice.gov.uk/learn-about/weather/case-studies/russian-heatwave); [IPSL: North America heatwave almost impossible without climate change, World Weather Attribution study (2021)](https://www.ipsl.fr/?p=1542487); [CTV News: coroners’ review reveals true death toll of B.C.’s 2021 heat dome](https://bc.ctvnews.ca/coroners-review-reveals-true-death-toll-of-b-c-s-2021-heat-dome-1.5936108); [Scientific American / Nature (2014): mega-heatwaves mechanism pinpointed (Miralles et al., Nature Geoscience)](https://www.scientificamerican.com/article/mega-heatwaves-mechanism-pinpointed); [US EPA: learn about heat island effects](https://epa.gov/heatislands/learn-about-heat-island-effects); [Sherwood & Huber (2010), PNAS: an adaptability limit to climate change due to heat stress](https://pmc.ncbi.nlm.nih.gov/articles/PMC2906879); [US National Weather Service: heat safety (how the body cools, heat index)](https://www.weather.gov/media/pah/WeatherEducation/heatsafety.pdf); [NHS 111 Wales: heat exhaustion and heatstroke](https://111.wales.nhs.uk/Heatexhaustionandheatstroke); [WHO (25 Jul 2024): UN Secretary-General calls for global action on extreme heat](https://www.who.int/news/item/25-07-2024-united-nations-secretary-general-calls-for-global-action-on-extreme-heat); [Global Heat Health Information Network: WHO heat and health](https://heathealth.info/resources/who-heat-and-health/); [Penn State (2022): humans can’t endure temperatures and humidities as high as previously thought (Vecellio et al., J. Appl. Physiol.)](https://psu.edu/news/research/story/humans-cant-endure-temperatures-and-humidities-high-previously-thought); [Stull (2011), J. Appl. Meteor. Climatol.: wet-bulb temperature from relative humidity and air temperature (calculator and validity range)](https://rechneronline.de/air/wet-bulb-temperature.php); [NOAA Weather Prediction Center: the heat index equation](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml); [Wikipedia: Tak, Thailand (national record 45.4 °C, 15 April 2023)](https://en.wikipedia.org/wiki/Tak,_Thailand); [The Jakarta Post / AFP (22 Apr 2023): Thai authorities issue extreme heat warnings for dozens of provinces](https://www.thejakartapost.com/world/2023/04/22/thai-authorities-issue-extreme-heat-warnings-for-dozens-of-provinces); [ผู้จัดการออนไลน์ (13 เม.ย. 2567): ระดับดัชนีความร้อนของกรมอุตุนิยมวิทยา](https://mgronline.com/science/detail/9670000032515); [Yale Environment 360 (17 May 2023): April heat wave in South Asia made 30 times more likely by climate change](https://e360.yale.edu/digest/april-2023-heat-wave-india-thailand-climate-change); [AP via PBS NewsHour (13 Aug 2023): inconsistent methods for counting U.S. heat deaths](https://www.pbs.org/newshour/health/inconsistent-methods-for-counting-u-s-heat-deaths-stymie-public-health-efforts); [Robine et al. (2008), Comptes Rendus Biologies: death toll exceeded 70,000 in Europe during the summer of 2003](https://comptes-rendus.academie-sciences.fr/biologies/articles/en/10.1016/j.crvi.2007.12.001/); [ISGlobal (10 Jul 2023): record-breaking heat in the summer of 2022 caused more than 61,000 deaths in Europe (Ballester et al., Nature Medicine)](https://isglobal.org/en/-/el-calor-record-del-verano-de-2022-causo-mas-de-61-000-muertes-en-europa); [US CDC: people at increased risk for heat-related illness](https://www.cdc.gov/heat-health/risk-factors/index.html); [WMO Bulletin 71(1), 2022: regional trends in extreme events in the IPCC 2021 report](https://wmo.int/resources/wmo-bulletin/wmo-bulletin-vol-71-1-2022/regional-trends-extreme-events-ipcc-2021-report); [Carbon Brief (2021): what the new IPCC report says about extreme weather and climate change](https://www.carbonbrief.org/explainer-what-the-new-ipcc-report-says-about-extreme-weather-and-climate-change); [Reuters via TimesLIVE (9 Aug 2021): once-in-50-year heatwaves now happening every decade, UN climate report](https://www.timeslive.co.za/news/sci-tech/2021-08-09-once-in-50-year-heatwaves-now-happening-every-decade-un-climate-report/); [IPCC AR6 WGI, Figure SPM.6: projected changes in hot temperature extremes (baseline 1850–1900)](https://www.ipcc.ch/report/ar6/wg1/figures/summary-for-policymakers/figure-spm-6); [Actuaries Digital (Aug 2021): IPCC AR6, hotter, wetter, faster, stronger (50-year hot extremes at 1.5, 2 and 4 °C)](https://actuaries.digital/2021/08/25/ipcc-ar6-hotter-wetter-faster-stronger); [Stott, Stone & Allen (2004), Nature: human contribution to the European heatwave of 2003](https://ideas.repec.org/a/nat/nature/v432y2004i7017d10.1038_nature03089.html); [World Weather Attribution: methods](https://www.worldweatherattribution.org/methods/); [กรมควบคุมโรค สคร.12 สงขลา (7 มี.ค. 2567): เตือนระวังฮีทสโตรก โทรสายด่วน 1669](https://ddc.moph.go.th/uploads/files/4136520240307071304.pdf); [Wikipedia: Portugal (area 92,225 km²)](https://en.wikipedia.org/wiki/Portugal)