■ Domain 03 · Heat Stress

Heat is the exposure the century has not yet decided to stop.

Air pollution bends with policy. Water bends with infrastructure. Poverty bends with growth. Heat bends only with the global energy system — and so far the global energy system is still pushing it upward, year after year.

DATA WINDOW 2000–2100 · BASELINE 2025 · IPCC AR6 · LANCET COUNTDOWN · ILO

Global temperature Wet-bulb threshold Heat mortality Labor productivity Cooling gap

1.2°C

of global warming above pre-industrial levels has already occurred.

IPCC AR6 / WMO, 2025

44%

of humanity now faces high heat-stress exposure in a typical year.

Author estimate, SSP2 baseline

0.5M

heat-related deaths were recorded in 2019, with deaths among over-65s up 167% since the 1990s.

Lancet Countdown, 2023

2.2%

of global working hours could be lost to heat stress by 2030.

ILO, 2019

The dose

The dose is set by physics — and physics does not negotiate

Every other exposure in this atlas has a policy lever that works within a decade. Heat does not. The warming already locked into the system continues for decades after emissions fall, because carbon dioxide stays in the atmosphere for centuries.

The spread between scenarios is the entire decision space. The low-emission pathway holds warming near 1.4°C by 2100. The high-emission pathway reaches 4.4°C. Current policies sit between them, near 2.7°C. Those differences are not abstract: they decide how many days a year outdoor work is possible, how many nights a body can cool, and how many regions cross the wet-bulb threshold where survival itself depends on artificial cooling.

Figure 1

Global mean temperature rise above pre-industrial, by scenario, 2000–2100

IPCC AR6 · °C

Read: the four SSP pathways bracket the century. The gap between SSP1-2.6 and SSP3-7.0 by 2100 — roughly 2.2°C — is the difference between adaptation pressure and adaptation failure in the hottest regions. Source: IPCC AR6 WG1 assessed warming ranges; midpoints shown for readability.

Takeaway

Heat exposure in 2100 was decided by the emissions trajectory chosen in the 2020s and 2030s. The delay between cause and effect is the reason heat feels unstoppable now — and the reason acting now still changes the back half of the century.

The threshold

Wet-bulb temperature is the metric that separates discomfort from danger

Heat stress depends on humidity as much as temperature. A dry 40°C is survivable with shade and water. A humid 35°C wet-bulb is not, because sweat stops evaporating and the body's cooling system shuts down.

The 35°C wet-bulb threshold was long treated as a theoretical limit. Recent measurements have brushed against it in South Asia and the Gulf, and modeling shows crossings becoming routine in parts of South Asia, the Middle East, and coastal West Africa under warming above 2°C. Well before the threshold is crossed, labor productivity falls: the body slows to protect its core temperature, and outdoor work stops being safe through the hottest hours.

That is why the economic damage of heat arrives years before the mortality statistics do. Read wet-bulb crossings as a labor-productivity variable first; the death toll is the trailing indicator.

Figure 2

High heat-stress days per year, by region, 2000–2050

SSP2-4.5 · days above high-stress threshold

Read: South Asia's count of high heat-stress days rises from a handful in 2000 toward two full months a year by 2050 under the middle pathway. Sub-Saharan Africa follows the same curve a decade behind. Source: author estimates from ISIMIP heat-humidity grids under SSP2-4.5; rounded.

Physiology

Heat attacks the body's three weak points at once

Point 1

Core temperature

The body keeps a narrow core range near 37°C. Sustained heat pushes it past 39°C, then 40°C — the zone where organs begin to fail. The young and the old fail first.

Point 2

Cardiovascular load

Cooling the body means pumping blood to the skin. That load lands on the heart. Most heat deaths are cardiovascular events wearing a heat label.

Point 3

Hydration

Sweating consumes water. Where water is already unsafe or scarce, heat converts dehydration from a nuisance into an emergency — the direct link between this page and the water page.

Lancet Countdown data shows heat-related deaths among people over 65 rose 167 percent compared with the 1990s — faster than the population aged.

The trend will steepen. Aging populations, urban heat islands, and rising nights all point the same direction. The only countervailing force is access to cooling, which is exactly what the exposed half of humanity lacks.

The adaptation frontier

The cooling gap is where heat exposure becomes heat harm

More than a billion people live at high heat risk without access to cooling — fans, refrigeration, or a cooled public space. Sustainable Energy for All puts the number at 1.1 to 1.2 billion and rising.

Cooling is not a luxury in a 2.7°C world. It is the difference between a heat wave and a mass-casualty event, between food that lasts a week and food that spoils in a day, between a vaccine that works and one that degrades. The gap maps onto the same regions as every other exposure in this atlas — which is why closing it is a polycrisis intervention, not a comfort purchase.

Figure 3

People at high heat risk without access to cooling, by region, 2025

millions · SEforALL baseline

Read: South Asia and sub-Saharan Africa hold the largest cooling-access deficits. Efficient cooling — not air conditioning alone — is the adaptation technology with the highest lives-saved per dollar in the hottest districts. Source: SEforALL Chilling Prospects; regional split is author estimate.

Takeaway

Heat exposure is a physical quantity; heat harm is a social one. The cooling gap, not the thermometer, is where the two separate — and where policy can act fastest.

Next: the full century

The final page assembles all four exposures into a single 2000–2100+ trajectory — and states the rule for reading it.

2000–2100+ Poverty Overview