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Q. What Is Wet-Bulb Temperature?

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In simple terms, wet-bulb temperature reflects the heat our bodies actually have to withstand in humid air. A wet-bulb reading of 35°C has traditionally been considered the point at which the human body's ability to regulate its own temperature breaks down — a level at which even a healthy person, resting in the shade with plenty of water, can struggle to cool off.

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Q. Why Does Wet-Bulb Temperature Matter?

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▲ High heat and humidity together interfere with the body's ability to cool itself.Source: Sunhak Peace Prize

◎ Impact on Human Health

The human body has a remarkably effective cooling system: sweat. As temperatures rise, sweat evaporating off the skin carries heat away, keeping the body from overheating.

But when the air already holds too much moisture, that system starts to fail. Sweat sits on the skin instead of evaporating. Drinking enough water and resting in the shade may no longer be enough to release heat, and the results can range from heat exhaustion to heatstroke and organ damage.

As wet-bulb temperatures approach 35°C, the body's ability to cool itself can collapse within a matter of hours. And for people already vulnerable to heat — older adults, children, people with chronic illness, outdoor workers — real danger can set in at even lower readings.

◎ Impact on Ecosystems and the Environment

Rising wet-bulb temperatures aren't just a human problem. Extreme heat and humidity reshape habitats for plants and animals, and put pressure on crops and livestock. Surging demand for cooling can strain power grids and add further pressure on the environment.

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Q. How Is Wet-Bulb Temperature Measured?

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◎ Measurement Method

The principle behind it is simpler than it sounds. A traditional wet-bulb thermometer uses two thermometers side by side: a dry-bulb thermometer, which reads the actual air temperature, and a wet-bulb thermometer, whose bulb is wrapped in a wet cloth.

As water evaporates from the wet cloth, it draws heat away and lowers the reading. The drier the air, the faster that evaporation happens and the lower the wet-bulb reading drops. The more humid the air, the harder it is for water to evaporate, so the wet-bulb reading climbs closer to the actual air temperature. That resulting number is the wet-bulb temperature.

▲ A diagram showing how evaporation from the wet cloth draws heat away from the thermometer.Source: ClimateCheck

◎ Reading the Numbers

Knowing the wet-bulb temperature gives a much clearer picture of just how dangerous a hot day really is for the human body. In places like Delhi, India, where extreme heat and high humidity often occur together, it can matter as much as the air temperature itself.

The traditionally cited limit of human survivability is a wet-bulb temperature of 35°C. Those readings show just how dangerous Delhi's heat had become. More than 160 people are reported to have died in the extreme heat that hit Delhi that summer.

▲ In the summer of 2022, wet-bulb temperatures in Delhi, India, approached or exceeded the 35°C threshold considered the limit of human survivability.Source: Getty Images Plus

Unfortunately, similar crises keep recurring. In 2024, Pakistan and other parts of South Asia endured severe heat waves, and in 2025, millions of people across India and Pakistan faced prolonged, extreme heat stress.

The World Bank has warned that extreme heat could make parts of India extremely difficult places to live. Combined with water scarcity and soil degradation, it could also deal a heavy blow to agriculture and livestock farming.

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Q. How Are Climate Change and Wet-Bulb Temperature Connected?

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◎ A Warmer World Makes for Hotter, Wetter Air

As the planet's average temperature rises, the air can hold more moisture. As days with both high heat and high humidity become more common, wet-bulb temperatures naturally rise along with them.

In 2020, a research team at Columbia University analyzed global weather data from 1979 to 2017. They found that dangerously high wet-bulb temperatures were already being recorded in places including India's eastern coast and northwest, Pakistan, the Red Sea coast, the Gulf of California, and the southern Gulf of Mexico.

▲ A map showing regions worldwide that have recorded a daily wet-bulb temperature of at least 31°C. Deeper yellow and red indicate higher recorded values.Source: Science Advances

◎ Future Climate Scenarios

In 2021, the UK's national weather and climate service, the Met Office, analyzed how heat stress risk would change as global average temperatures rise.

The analysis found that at 2°C of warming above pre-industrial levels, the number of people exposed to extreme heat stress could grow from about 68 million today to roughly 1 billion. At 4°C of warming, close to half the world's population could be living in regions facing severe heat stress risk.

And this isn't a distant future problem. A 2025 joint report from the World Health Organization (WHO) and the World Meteorological Organization (WMO) found that 71% of the global workforce — about 2.4 billion people — are already exposed to excessive heat. Extreme heat threatens the lives and health of countless workers every year and is linked to tens of millions of workplace injuries.

▲ 71% of the global workforce — roughly 2.4 billion people — are exposed to excessive heat.Source: WHO/WMO, Climate Change and Workplace Heat Stress, 2025

◎ Europe Is No Longer Safe, Either

Extreme heat isn't just a South Asian problem anymore. Europe, once considered to have a relatively mild climate, has been hit by repeated record-breaking heat waves. Temperatures above 45°C have been recorded in southern Europe, and multiple countries have set new all-time highs.

There are structural reasons Europe is especially vulnerable. Many homes were built to retain heat through cold winters, and air conditioning remains uncommon in parts of the continent. That leaves older adults, low-income residents, and people living alone at greater risk during prolonged heat waves.

The WHO considers Europe the fastest-warming region in the world. Recent studies on heat-related mortality confirm that climate change is making dangerous heat both more frequent and more intense.

The Met Office warns that even at wet-bulb temperatures of 32°C and above, vulnerable groups and outdoor workers can face serious danger. As warming continues, these conditions are likely to recur across more continents and more cities.

▲ A map showing regions that could experience more than 10 days a year with wet-bulb temperatures above 32°C if global average temperatures rise 4°C above pre-industrial levels.Source: Met Office

Climate scientist Matthew Huber has warned that by the end of this century, extreme conditions approaching a wet-bulb temperature of 35°C could become far more common than they are today. A number once treated as a theoretical ceiling is edging closer to everyday reality.

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We Need to Act

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▲ Protecting those most vulnerable to extreme heat today, and slowing global warming for the long term, both need to happen together.Source: Sunhak Peace Prize

Rising wet-bulb temperatures show just how directly climate change affects the human body. If heat and humidity keep climbing at their current pace, more regions could become difficult places to work, live, and grow food.

Governments and cities need heat-warning systems, cooling centers, safe labor standards, medical response plans, and climate-resilient buildings. At the same time, we need to cut greenhouse gas emissions to prevent an even more dangerous future. In other words, protecting lives today and lowering tomorrow's temperatures have to happen at the same time.

By this point, you might be wondering: can recycling or cutting back on electricity really make a difference? Honestly — individual action alone can't stop the climate crisis. But it can help protect someone from extreme heat, strengthen a community's preparedness, and add momentum to bigger policy change.

One small action today can protect someone's life in a heat wave. And when enough of us raise our voices together, it can drive the bigger changes that lower the temperature the next generation will inherit.

Protecting one another today, while slowing how fast our planet is heating, is both a climate responsibility and a promise of peace for future generations.

Written by Sharon Choi
Director of Planning, Sunhak Peace Prize Secretariat