Heat Stress

Heat stress is a growing global risk, exacerbated by climate change, leading to health crises, reduced labor productivity, and increased mortality worldwide.

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The map above shows areas with 90 or more days per year above 32° C under a scenario of 1° C warming (compared to 1850-1900 average), which we passed in 2017. Explore more about the changes in global temperature distribution under other scenarios presented in Probable Futures’ predictions.

Hot, humid conditions contribute to heat stress, heat-related deaths, and productivity loss. The rapid decrease of the albedo effect associated with the Arctic will result in rising global temperatures and create a hotter climate. A seasonally ice free Arctic may contribute to a rise in frequency of severe El Nino episodes – worsening heatwaves, especially in tropical areas (Jiping Liu et al., 2022).

The World Meteorological Organization reported that 2025 was one of the three warmest years on record, with global temperatures averaging 1.44°C above the 1850–1900 baseline and the 2023–2025 average, reaching 1.48°C above pre-industrial levels.

With global temperatures now approaching the Paris Agreement's 1.5°C threshold, understanding how extreme heat may expand geographically is becoming increasingly important for climate adaptation and public health planning

Dangerously Hot Climates

Millions living in dangerously hot climates

According to data from the European Drought Observatory, in summer 2022 63% of European Union and United Kingdom land – approximately the size of India – were given drought warnings or alerts (Said-Moorhouse and Dewan, 2022).

A review of research led by researchers from Arctic Basecamp highlights how extreme heat is responsible for a 167% surge in heat-related deaths in 2023 compared with the 1990s (Romanello et al. 2024), A 1% rise in early term births (Thompson et al.,2023). Extreme heat could also cause damage to the gut lining – leading to sepsis, inflammation, organ failure, and death (Fung et al., 2021).

Read the full study here

Research finds (Lenton et al., 2023) that climate change has already put around 9% of people outside the ‘human climate niche’ — the temperature conditions where population density shows humans have historically thrived. That’s equivalent to more than 600 million people.

High temperatures are linked to increased mortality, decreased productivity, problems with agriculture, increased conflict, and the spread of infectious diseases.

Recent events underscore the growing human cost of extreme heat. During the 2025 European heatwave, researchers estimated around 2,300 heat-related deaths across 12 major cities in just 10 days, with approximately 1,500 deaths attributed to human-caused climate change. In 2026, Western Europe again experienced temperatures exceeding 40°C, triggering health alerts, transport disruptions, and heat-related fatalities.

The 2026 Lancet Countdown Europe report further confirms that heat-related mortality in Europe continues to rise compared with the 1990s, increasing by about 23% (roughly one additional death in every five compared to the 1990s baseline period), alongside increasing heat-related health risks and system pressures.

The 2025 Lancet Countdown also reported that heat-related mortality has increased by 23% since the 1990s, reaching an average of 546,000 deaths annually worldwide, while climate-driven heat exposure contributed to record losses in labour productivity and growing health risks for vulnerable populations.

As temperatures rise, the research estimates that by 2030 more than 2 billion people could be outside the niche. By 2090, that’s estimated to increase to around 40% of people, or around 3.7 billion.

It found that India, Nigeria and Indonesia are set to have the largest populations exposed – all nations that, today, are above the median poverty rate and below the median per-capita emissions.

The research shows that reducing global warming from 2.7 to 1.5 °C – the Paris Agreement target – would reduce the population left outside the niche from 3.7 to 2.7 billion people.

Extreme Heat, Real Consequences

Recent extreme heat events show how heat stress is intensifying and costing lives. In Europe’s summer 2025 heatwave, around 2,300 heat‑related deaths were estimated across 12 major cities during a 10‑day period, highlighting the severe human impact of soaring temperatures above 40 °C.

In Spain alone, health authorities reported 1,180 heatwave deaths between mid‑May and mid‑July 2025, a dramatic rise compared with the previous year.

In South Asia, studies show India recorded more than 40,000 suspected heatstroke cases and at least 110 confirmed deaths between March and mid‑June 2025, with heatwave days more than double the usual count in several regions.

And in Australia in early 2026, forecast models warn of its most significant heatwave in years, with widespread temperatures expected in the mid‑40s °C, posing acute heat stress risks to communities, elderly populations, and outdoor workers.

These events underscore how rising heat extremes are already translating into serious health threats and mortality across different continents.

Credit: Data: ERA5. Map: Copernicus Climate Change Service (C3S)/ECMWF. Image produced under the Programme of the European Union.

This map shows surface air temperature anomalies for July 2025 compared to the 1991–2020 reference period. Areas in red shades indicate temperatures above the long-term average, highlighting regions experiencing extreme heat, while blue shades indicate cooler-than-average areas.

Intensification of Global Heat Stress

A recent global analysis using the Universal Thermal Climate Index (UTCI) shows that human-experienced heat stress has intensified worldwide since the 1970s. The hottest days are warming at 0.27°C per decade, while the hottest nights are warming faster at 0.32°C per decade, reducing nighttime recovery.

Compared with the 1970s, many regions now experience up to ~50 additional heat stress days per year, and extreme heat stress occurs 2–2.5 times more frequently across large parts of Europe, North and South America.

At the population scale, exposure has expanded sharply: the share of people experiencing at least 90 days of strong heat stress has increased from 55% to 70%, and exposure to at least one day of extreme heat stress has risen from 16% to 22% (~1 billion additional people), driven by both climate change and population growth (Emerton et.al, 2026)

Cities too hot to live and work in

The poles are the planet’s heatsink – and loss of ice and snow amplifies the risk of global heatwaves, meaning more intense heat stress in already-hot parts of the world. Arctic amplification of global warming can alter jet stream patterns, which in 2022 led to extended periods of heat stress throughout much of Europe and Asia.

One study found that in South Asia, where one quarter of the world’s population lives, 2°C of global warming will more than double the population’s exposure to unsafe labour temperatures, and exposure to lethal temperatures will rise 2.7 times.

Recent heatwaves show that several major cities are already experiencing conditions that severely constrain workability and habitability. Baghdad (the capital of Iraq in central Mesopotamia) and Basra (a major port city in southern Iraq near the Persian Gulf) are now regularly among the world’s most heat-vulnerable cities, with temperatures exceeding 49–51°C in recent summers (2023 -2025) and escalating risks for outdoor workers and urban residents.

Phoenix (USA) has also experienced record-breaking heat, including 122 days at or above 100°F (38°C) in 2025 following a record 113-day streak in 2024, making sustained outdoor activity increasingly hazardous even in high-income urban settings.

Karachi (Pakistan) has repeatedly exceeded 44°C in recent heatwaves (2025-2026), with high humidity amplifying heat stress and contributing to hospital overloads and disruptions to daily labour during peak heat events.

In Delhi (India), recent heatwaves (2025–2026) have pushed daytime temperatures into the 44–45°C range, with isolated peaks up to ~45°C and heat index values exceeding 50°C, while night-time temperatures have remained around 30–34°C, intensifying urban heat island–driven heat stress and limiting recovery for outdoor workers.

These combined extremes indicate that some cities are intermittently approaching the upper bounds of human heat tolerance, where high temperature and humidity can overwhelm the body’s ability to regulate core temperature, and recent evidence shows that life-threatening heat stress can occur even below the traditionally cited 35°C wet-bulb threshold under real-world urban exposure conditions.

Economic Disruption

As well as the humanitarian consequences, South and Southeast Asia’s role as a manufacturing hub means major disruptions both the local economy and global supply chains if its cities periodically become too hot to work in.

A separate analysis, looking at the likely rise in labour days lost because workers will be unable to perform physical activity, found that by 2050, Southeast Asia could lose 16% of its labour capacity due to rising heat stress (Verisk Maplecroft, 2017). The report estimates Singapore would see a 25% productivity drop, with losses of 24% for Malaysia, 21% for Indonesia, and 16% for Cambodia and the Philippines. It also identified risks for Manaus in Brazil, Cartagena in Colombia, and Panama City, among other cities.

More recent evidence (2025–2026) indicates that these losses are already materializing, with new economic modelling showing that heat stress is currently reducing effective work capacity by up to 30–40% in exposed outdoor sectors across South and Southeast Asia, especially in construction and manufacturing supply chains, and is beginning to disrupt production continuity in export-oriented industrial hubs.