The medium, not the maximum
Public attention fixes on the lethal extreme — the wet-bulb temperature at which the body can no longer cool itself. But that threshold is rare and episodic. The damage that actually accumulates is to the human medium: the productive coefficient of labor hours, cognition, learning and health. Months of merely functional heat — too hot to work safely outdoors, too hot to think clearly indoors without cooling — erode output and human capital long before anyone reaches a physiological limit.
The binding constraint is not the day the body fails. It is the months the mind and the workday degrade.
Three metrics, three questions
These are not interchangeable, and conflating them is the most common error in heat discourse:
- Air temperature (dry-bulb) — how hot the day is. Drives device thermals and headline numbers.
- Feels-like (heat index) — air plus humidity, for human discomfort. What a person experiences.
- WBGT (wet-bulb globe temperature) — the work-safety metric. Above ~30.5°C in sun, sustained outdoor labor becomes unsafe.
- MAT (mean annual temperature) — the chronic-niche metric. Crossing ~29°C MAT pushes a city outside the historical human niche.
Late-century, weak-path peaks reach feels-like values of 55–72°C in the Gulf and South Asia, with WBGT in the 34–42°C band — well past the work-safety line for much of the year. See the heat dashboard for the full regional bands.
Cognition, learning and pollution
Heat and PM₂.₅ act on the same target: the brain. Elevated temperature degrades attention, working memory and decision quality; chronic particulate exposure compounds the effect and suppresses learning. With ~94% of the world breathing air above the WHO PM₂.₅ guideline, the cognitive tax is near-universal and largely invisible — it shows up as slower development, not as a disaster.
Heat and the devices we work on
The infrastructure of modern work has its own thermal envelope. Lithium-ion batteries throttle or pause charging above ~35°C ambient to protect chemistry; silicon down-clocks to avoid damage, so sustained workloads fall below spec; datacenters lose efficiency exactly when cooling demand and grid stress peak together. In cities where afternoon ambient sits at 38–45°C, phones and laptops operate in the throttle-or-pause band for hours daily.
Device thermals close the loop: heat raises cooling demand → grid stress rises → blackouts remove cooling and compute together → the work that adaptation depends on slows precisely when it is needed most. This is why grid · power · compute is the central coupling hub of the index.
The wet-bulb tail
Noncompensable wet-bulb heat (≈35°C Tw) remains a tail — low exposure share, but maximal medium impact where it strikes. It is episodic today in the Persian Gulf, parts of South Asia and coastal Central America, and could become seasonal in patches by late century on weak paths. In the index it ranks lower on Exposure but maxes Medium: a tail inside Tier II, not the global center of gravity. The center of gravity is the chronic envelope, not the lethal spike.