← all scenarios

Lethal heat and humidity (wet-bulb)

Humans cool themselves by sweating, which stops working when the air is both hot and humid. The combined measure is wet-bulb temperature. A long-cited theoretical ceiling is 35 °C, but laboratory tests on healthy young adults found harm well below that, and older or ill people are more vulnerable. Heat waves are already among the deadliest weather events.

The tier rates the quality of the evidence, not how likely the event is.

How it could unfold

1. 1Rising temperature and humidity push wet-bulb readings up● strong evidence
2. 2Heat stress and deaths rise, especially without cooling or at night● strong evidence
3. 3Outdoor work becomes unsafe for parts of the year; power demand spikes◐ moderate evidence
4. 4Heat plus blackout is the dangerous combination◐ moderate evidence

Historical anchors

• Conditions approaching 35 °C wet-bulb have been recorded briefly in the Persian Gulf and South Asia.

What experts and models say

Expert estimates are personal judgements, not measurements, and experts disagree. This site never adds its own probabilities.

What you can do

• Know where a cool place is; check on older neighbours; drink water; avoid midday exertion.

Open the preparedness checklist for this scenario →

Related scenarios

• Climate tipping cascade
• Grid or cyber collapse

References

1. Sherwood, S.C. & Huber, M. (2010). An adaptability limit to climate change due to heat stress. PNAS 107:9552-9555. link ↗
2. Raymond, C., Matthews, T. & Horton, R.M. (2020). The emergence of heat and humidity too severe for human tolerance. Science Advances 6:eaaw1838. link ↗
3. Vecellio, D.J. et al. (2022). Evaluating the 35 °C wet-bulb temperature adaptability threshold for young, healthy subjects. J. Appl. Physiol. 132:340-345. link ↗