Wet-Bulb Temperature: The Climate Threshold Humans Cannot Ignore

Why heat and humidity together may become one of the greatest climate risks of the 21st century

When people hear weather forecasts, they usually focus on air temperature.

A day with temperatures above 40°C is generally considered extremely hot.

However, temperature alone does not tell the whole story.

In reality, one of the most important indicators of heat-related risk combines two factors: temperature and humidity.

This indicator is known as wet-bulb temperature.

Although largely unknown outside scientific and meteorological circles, wet-bulb temperature may become one of the most important climate indicators of the 21st century.

As global temperatures continue to rise, understanding wet-bulb temperature is becoming increasingly important for public health, climate adaptation and human survival.

What Is Wet-Bulb Temperature?

Wet-bulb temperature is a measure that combines air temperature and humidity.

It represents the lowest temperature that can be reached through the evaporation of water.

The concept is directly linked to how the human body cools itself.

Under normal conditions, the body regulates temperature by producing sweat.

When sweat evaporates, it removes heat from the skin and helps maintain a safe internal body temperature.

However, when humidity levels are very high, evaporation becomes less effective.

The body loses its ability to cool itself efficiently.

As a result, the risk of heat stress increases dramatically.

Why Humidity Matters

Many people assume that dry heat is always more dangerous because temperatures can become extremely high.

Yet humid heat often poses a greater threat to human health.

For example:

• 45°C in a very dry environment may still allow effective sweat evaporation.

• 35°C combined with extremely high humidity may severely limit the body's cooling capacity.

This is why wet-bulb temperature provides a more realistic measure of physiological heat stress than air temperature alone.

It reflects what the human body actually experiences.

The Critical 35°C Threshold

Scientists have identified an important physiological limit.

A wet-bulb temperature of approximately 35°C is often considered the upper boundary of human survivability for extended exposure.

At this level, even healthy individuals resting in the shade with unlimited drinking water may struggle to regulate their body temperature.

Prolonged exposure can lead to:

• heat exhaustion

• heat stroke

• organ failure

• death

Fortunately, such conditions remain relatively rare.

However, climate projections suggest they may become increasingly frequent in certain regions during the coming decades.

Climate Change and Rising Risks

Climate change is increasing both global temperatures and atmospheric moisture.

A warmer atmosphere can hold more water vapor.

This means that many future heatwaves may combine:

• higher temperatures

• higher humidity

• longer duration

Together, these factors increase wet-bulb temperatures.

As a result, heatwaves are becoming more dangerous, even in regions that have historically adapted to hot weather.

The challenge is not only heat.

It is heat combined with humidity.

Regions Most at Risk

Why heat and humidity together may become one of the greatest climate risks of the 21st century

Mediterranean countries such as Spain, Italy and Greece are already experiencing more frequent and intense heatwaves.

As temperatures continue to rise, episodes of dangerous heat stress may become increasingly common.

Spain and the Mediterranean Challenge

Spain faces a particularly complex climate situation.

The country is already experiencing:

• rising average temperatures

• longer heatwaves

• more frequent droughts

• increasing water stress

At the same time, humid air masses associated with Mediterranean weather patterns can occasionally create conditions favorable to elevated wet-bulb temperatures.

Although Spain is unlikely to experience the most extreme wet-bulb conditions seen in parts of South Asia, the combination of heat and humidity may still pose significant health risks for vulnerable populations.

DANA, Humidity and Human Health

Weather phenomena such as DANA (Isolated High-Level Depression) can increase atmospheric humidity and create unstable weather conditions.

While DANA events are primarily associated with heavy rainfall and flooding, they also highlight the important role that moisture plays in the atmosphere.

Humidity is not simply a weather variable.

It is a key factor influencing thermal comfort, human health and climate resilience.

Understanding the interaction between heat and humidity is becoming increasingly important as extreme weather events become more frequent.

Adapting to a Warmer World

As wet-bulb temperatures rise, societies will need to adapt.

Possible measures include:

• improved heat warning systems

• urban cooling strategies

• climate-resilient infrastructure

• public health preparedness

• better water management

Protecting vulnerable populations will become a growing priority in many parts of the world.

The challenge is not only environmental.

It is also social, economic and public health related.

Conclusion

Wet-bulb temperature reminds us that climate change is not simply about rising temperatures.

It is about the interaction between heat, humidity and the limits of human physiology.

As the atmosphere warms and holds more moisture, understanding wet-bulb temperature may become increasingly important for assessing future climate risks.

The most dangerous heat is not always the hottest heat.

It is often the heat that the human body can no longer escape.

In the emerging Water Era, understanding the role of water in the atmosphere may become just as important as understanding water on the ground.

Could Marine Aquifers Help Reduce Future Heat Stress?

Some researchers and independent analysts are beginning to explore whether large-scale water management strategies could play a role in mitigating extreme heat conditions.

One possible area of interest involves marine aquifers, which contain significant volumes of freshwater beneath many coastal regions around the world.

If these resources can be better understood and managed sustainably, they may eventually contribute to new approaches for landscape hydration, ecosystem restoration and local climate adaptation.

The underlying idea is relatively simple.

Dry soils absorb and release heat differently than moist soils.

Maintaining healthier soil moisture levels may help reduce surface temperatures, support vegetation growth and improve local microclimates.

As climate change increases the frequency of extreme heat events, future research may explore whether strategic freshwater management could become part of broader climate adaptation strategies.

Although many scientific, technical and environmental questions remain unanswered, the relationship between water availability, soil moisture and thermal regulation deserves increasing attention in a warming world.

In the Water Era, managing freshwater resources may not only be about supplying water to people.

It may also become part of the effort to reduce the impacts of extreme heat on ecosystems and human societies.

Pere Castells Teulats

Researcher · Science Communicator

Water vs Oil