Heat waves are among the deadliest natural disasters. Historically, the most lethal have been dry heat events.
However, as global temperatures rise due to climate change, a potentially even more dangerous form of heat is becoming increasingly prevalent. Extreme humid heat.
Unlike dry heat, which can be mitigated by shade, breezes, and hydration, humid heat poses a far greater risk to human survival.
When high temperatures combine with oppressive humidity, the humidity hinders the body's ability to cool itself by evaporating sweat.
As climate change progresses, lethal wet-bulb events are expected to become frequent. Particularly in densely populated regions.
Dry Heat vs. Humid Heat
Dry heat waves are driven by large high-pressure systems which trap hot air near the Earth's surface.
When these high-pressure systems remain stationary, heat accumulates over days or weeks without relief from cloud or wind. The 2003 European heat wave, and the 2021 Pacific Northwest "heat dome," both of which resulted in numerous deaths are examples.
Humid heat waves happen when the atmosphere holds more moisture. Regions with accessible moisture such as coastal areas are at risk
A devastating example if this kind of heat event happened in 2015 in Karachi, Pakistan Temperatures reached 45°C (113°F) humidity was extreme. Power outages and water shortages amplified the human misery. Over 1,200 people died in just ten days, from heatstroke and dehydration.
The Danger of Wet-Bulb Temperatures (WBT)
Wet-bulb temperature (WBT) measures heat stress by combining temperature and humidity into a single value.
The concept derives from a simple experiment. A thermometer wrapped in a wet cloth cools through evaporation. The more humid the air, the slower the evaporation, as cooling efficiency decreases. This reduction is recorded on a thermometer which indicates when conditions become dangerous or deadly by showing the temperature measured at maximum evaporation.
Human Physiological Limits
The human body regulates core temperature through evaporating sweat. But, when WBT exceeds a critical threshold, sweat can't evaporate. Dangerous increases in core body temperature result.
Heat stress occurs when the human body can no longer effectively cool itself through sweating and evaporation. The condition can become life-threatening, even for healthy individuals.
The compensable level refers to the maximum wet-bulb temperature (WBT) at which the human body can still regulate its core temperature through sweat evaporation. Once this threshold is crossed, the body enters uncompensable heat stress, leading to dangerous and potentially fatal overheating.
Wet bulb temperature
Wet bulb temperature (WBT) is a critical measure which combines air temperature and humidity into a single reading that reflects the body's ability to cool itself. It's not the same as regular air temperature. Instead, it represents the lowest temperature to which skin can be cooled by evaporating sweat under current conditions.
A WBT reading of 19°C may sound mild, but temperature producing it can vary drastically depending on humidity.
At low humidity, a WBT of 19°C could correspond to an air temperature of 46°C (115°F).
At high humidity,a WBT of 19°C could correspond to an air temperature of 32°C (90°F).
At a WBT of 32°C, even fit, healthy people in the shade with water and rest can die within a few hours. A WBT of 32°C are lethal conditions.
Current climate projections demonstrate that the Earth may reach 2°C above pre-industrial temperature levels within 20years. At this temperature 35% of Earth's land area will experience dangerous WBTs.
Highly populated regions, most of India, eastern China, and also the eastern U.S., will face frequent and lethal heat waves.
At 4°C of Warming
Over 60% of Earth's landmass could experience deadly WBTs.
Events like Karachi's 2015 heat wave will become 90% longer in duration.
The Limits of Air Conditioning as a Solution
While air conditioning (AC) is the primary defense against extreme heat, it presents several challenges:
Heat waves strain power grids, increasing blackout risks. A hurricane or cyclone knocking out power during a heat wave could be catastrophic.
Urban Heat Islands
AC units expel heat outdoors, increasing city temperatures.
Carbon Emissions
AC accounts for 3% of global greenhouse gas emissions now and the contribution will rise.
Adaptation and Mitigation Strategies
A PBS video without hopium is impossible. Hopium follows.
To address these challenges, a multi-pronged approach is necessary:
Transition to Renewable Energy
Powering AC with clean energy reduces emissions. <-- what does this video know of Jevons Paradox, not so much.
Urban Design Improvements
White/green roofs, tree planting, and reflective pavements reduce heat absorption.
Cooling centers can provide refuge during outages.
Grid Resilience
Modernizing infrastructure prevents overloads.
Protecting Vulnerable Populations
Elderly, low-income, and outdoor workers require targeted interventions.
Conclusion
As climate change progresses, extreme humid heat will pose an unprecedented threat to human survival. Air conditioning offers temporary relief, reliance on it is unsustainable. A rapid transition to renewable energy, coupled with smarter urban planning and grid resilience, is essential to mitigate the worst impacts. The coming decades will test humanity's ability to adapt. But proactive measures today can still determine how severe the consequences will be. The time to act is now.