Every year, hundreds of thousands of Muslims make Hajj – the sacred pilgrimage to Mecca. In 2024, the pilgrimage took place in mid-June, the start of summer in Saudi Arabia, amid several humid heat waves.
But this year, more than 1,300 pilgrims never returned home. Deadly heat combined with humidity proved dangerous.
Our new research shows that the upper limit of human heat tolerance was breached for a total of 43 hours during the six days of the Hajj. During these times, the temperature and humidity exceed the level at which our bodies can cool down.
Scientists are increasingly worried about the death toll caused by humid heat waves and how this number will escalate in the near future. This year is currently the hottest year on record, surpassing the previous hottest year of 2023.
So why did so many people die during the pilgrimage? And what does it mean for us as the climate changes?
THE PLANET RECEIVES MORE MOISTURE
As the planet gets hotter, it also gets wetter in many places, including arid Saudi Arabia. Since 1979, extreme heat waves have more than doubled in frequency globally, increasing the risk of deadly events like this one.
To perform Hajj, you must walk from 6 to 21 km per day. Many pilgrims are older and in poor health, making them more vulnerable to heat stress.
This year’s pilgrimage began on June 14. Over the next six days, temperatures reached 51°C, while the “wet bulb temperature” (a combination of temperature and humidity) rose as high as 29 ,5°C.
June is typically the driest month in Saudi Arabia with an average relative humidity of around 25% and an average wet bulb temperature of 22°C. But during this year’s Hajj, average humidity reached 33% and rose as high as 75% during the most extreme heat periods.
Our study found that heat tolerance limits for older adults were breached on all six days of the Hajj, including four periods lasting more than six hours. On one intense day, June 18, the humid heat reached levels considered dangerous even for young and healthy pilgrims. The point at which wet bulb temperatures enter the lethal zone depends on the exact combination of temperature and humidity, because our bodies respond differently to dry or humid temperatures.
Saudi authorities have installed shelters with air conditioning and other cooling methods. But these are only available to pilgrims with official permits. Most of those killed were unlicensed, meaning they were unable to access cool relief.
The weather will be even more dangerous in the future. In 25 years, Hajj will return to peak summer in August and September. At 2°C warming, the risk of heatstroke during Hajj will be 10 times higher.
LIMITS ON HUMAN HEAT CAPABILITY
In 2010, researchers first proposed a theoretical “survival limit,” which is a wet bulb temperature of 35°C.
But now we know the real limit is much lower. Experiments testing the limits of human physiology inside controlled heat chambers, supported by complex models, have revealed new limits of heat tolerance.
These limits vary depending on your age and humidity. For example, the tolerance limit for young people is about 45°C at 25% humidity but only 34°C at 80% humidity. For older people, the limit is still lower – 32.5°C at 80% humidity is dangerous.
These limits are the point at which the temperature is so hot and humid that your body cannot cool itself, even while resting. Prolonged exposure will cause increased body temperature, heat stroke and death within a few hours.
Many of us are familiar with air temperatures of 34°C or higher. But we tolerate dry heat much better than moist heat. Wet conditions make it much more difficult for us to use our main way of radiating heat – sweat.
We rely on air to evaporate sweat from our skin and carry heat with it. But humidity changes this. When there is a lot of water in the air, it is more difficult for sweat to evaporate.
Heat is a silent killer. This is not a visible threat, unlike fires, floods, droughts and other climate-driven extreme weather events. Heat-related deaths are difficult to track and may be underestimated. But what we know shows that heat is the deadliest climate hazard in many parts of the world. To date, many studies have focused on one variable – air temperature. Only recently have scientists begun to tackle the little-known threat of deadly humidity.
WHAT INCREASES MOISTURE?
Moisture comes from evaporation of oceans and large bodies of water. As climate change warms the oceans, they create more moisture. That means coastal regions – home to many of the world’s largest cities – are extremely vulnerable. That’s why arid Saudi Arabia and other countries on the Arabian Peninsula are especially at risk – they are surrounded by shallow, warming seas.
But moisture can also travel far inland, through a phenomenon known as “atmospheric rivers,” rivers of moisture in the air. Here’s how deadly humidity waves can hit landlocked areas like northern India.
The threat of hot and humid weather will become more serious. We have seen deadly heat and humidity in the Arabian Gulf, across Bangladesh, northern India and parts of Pakistan as well as in Southeast Asia.
People are dying from these events, but the extent has not been fully documented. This year’s heat waves closed schools in the Philippines, India and Bangladesh and killed dozens of people during India’s elections.
WHAT IS THE WAY FORWARD?
If we don’t quickly phase out fossil fuels, we could see deadly, humid heat strike multiple times a year in every major economy, including the United States, India, China, South America, Asia, and Europe. Europe and much of Africa.
We like to think we can adapt to change. But there is a hard limit to our ability to adapt to deadly humidity and heat.
Technological adaptations such as air conditioning help. But they are not available to everyone. They are also not safe.
During a heat wave, many of us turn on our air conditioners at the same time, using a lot of electricity and increasing the risk of power outages. Power outages during heat waves can have deadly consequences.
In the famous first chapter of Kim Stanley Robinson’s novel The Ministry For The Future, an American aid worker struggles to survive an intense humid heat wave in India that kills millions. The book is set just a few years in the future.
Hajj deaths warn us that the deadly heat and humidity are not a fiction. That’s another reason to rapidly reduce greenhouse gas emissions by ending our dependence on fossil fuels.
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