High wet-bulb temperatures in Saudi Arabia in summerWhile the heat index – which measures heat stress due to high temperatures combined with high humidity – is often used to quantify dangerous heat, a more precise measure of heat stress is the wet-bulb temperature (TW), which can be measured by putting a wet cloth around the bulb of a thermometer and then blowing air across the cloth. The wet-bulb temperature increases with increasing temperature and humidity and is a measure of “mugginess”. The U.S. National Weather Service defines the “Danger” threshold for TW at 24.6 degrees Celsius (76.3°F), and “Extreme Danger” at 29.1 degrees Celsius (84.4°F), assuming a 45% relative humidity.
Observations from the Mecca weather station indicate a significant rise in average TW during the past 30 years – nearly 2 degrees Celsius (3.6°F). This increase is well above the global average, and can be largely attributed to human-caused global warming. High heat stress events are common when the Hajj occurs during summer; over the 30‐year period 1984-2013, the danger threshold (TW of 24.6 degrees Celsius) was exceeded in 58% of years. However, the “Extreme Danger” threshold of 29.1 degrees Celsius was not reached.
Figure 1. Air-conditioned tents in Mina City, Saudi Arabia, 1.2 miles away from Mecca. (Photo credit: Seeley International)
Elderly Hajj participants at high risk of heat-related illnessWhile the floor of the Great Mosque, its covered areas, and the surrounding tents that pilgrims stay in are all air conditioned, the ritual of Hajj involves spending about 20-30 hours outdoors over a period of five days. The main outdoor activities, which occur in and surrounding Mecca, are:
1) Tawaf, or praying outside the Great Mosque of Mecca (Alharam) for a few hours on two different occasions;
2) Wakuf, or standing on the side of Mount Arafat for one day between sunrise and sunset, recognized as the most important activity of the Hajj; and
3) Ramy Al‐Jamrat, or walking in Mina (outskirts of Mecca) for several hours per day (called Stoning of the Devil), repeated in a sequence of three days.
Muslims who are in good health and can afford it are obligated to participate in the Hajj at least once in their lifetimes, and their desire to participate becomes more urgent as their age advances. As a result, a disproportionate fraction of Hajj participants are elderly and at higher risk of heat-related illness.
Figure 2. Satellite image indicating the location of the Great Mosque of Mecca (Al Haram), Mina, and Arafat in and near Mecca, Saudi Arabia. (Image credit: Google Earth and Kang et al., 2019, “Future Heat Stress During Muslim Pilgrimage (Hajj) Projected to Exceed Extreme Danger Levels”, Geophysical Research Letters 46,16 (2019): 10094–10100)
Future summer Hajj events at high risk of dangerous heatThe Hajj occurs every year on the same days of the Muslim calendar, which follows the lunar cycle. Since the lunar year is shorter than the solar year by about 11 days, the Hajj shifts about 11 days earlier every year, and cycles back to the same date in the solar calendar after about 33 years. The danger of extreme heat during Hajj will wane this decade as the dates transition from July to June and then May. But during the years 2045-2053, and again in 2079-2086, Hajj will fall during August-October. These are the months when wet bulb temperatures peak in Mecca, as a result of the combination of extreme heat and prevailing westerly winds that bring humid air from the Red Sea.
Figure 3. Daily maximum wet-bulb temperature (TWmax) during Hajj in 1976-2013 (blue line), and predicted under a moderate global warming scenario (RCP 4.5) through 2100 (green line). Shading represents the atmosphere‐ocean global climate model ranges during Hajj, and the vertical dashed black lines indicate bounds of Hajj periods occurring during August through October. (Image credit: Kang et al., 2019, “Future Heat Stress During Muslim Pilgrimage (Hajj) Projected to Exceed Extreme Danger Levels”, Geophysical Research Letters 46,16 (2019): 10094– 10100)
A 2019 paper by MIT scientist Suchul Kang and colleagues, “Future Heat Stress During Muslim Pilgrimage (Hajj) Projected to Exceed Extreme Danger Levels,” painted a very concerning picture for future Hajj events in a warming climate. The researchers showed that under a moderate global warming scenario, the maximum wet bulb temperature could be expected to exceed the “Extreme Danger” threshold of 29.1 degrees Celsius 15% of the time during Hajj in the years 2045-2053, and exceed the “Danger” threshold 91% of the time (Figure 3).
Along similar lines, a 2021 paper led by Fahad Saeed (Climate Analytics) and colleagues, “
From Paris to Makkah: heat stress risks for Muslim pilgrims at 1.5 °C and 2 °C,” warns that the odds of exceeding the “danger” threshold at Mecca increase substantially for global warming of 1.5°C and 2°C – levels that are likely to be exceeded this century in the moderate global warming scenario discussed above – and that the “Extreme Danger” threshold may be surpassed during summer months.
Deadly Hajj stampedes may be more likely during extreme heatThe two deadliest stampedes during Hajj both occurred during days with extreme heat and humidity, when the maximum wet bulb temperature exceeded the 24.6 degrees Celsius “Danger” threshold. On July 2, 1990, 1,426 pilgrims died in a stampede when the maximum temperature (Tmax) reached 41.7 degrees Celsius (107°F) and wet‐bulb temperature (TWmax) hit 25.1 degrees Celsius (77.2°F). Similarly, on September 24, 2015, more than 2,000 pilgrims died in a stampede when Tmax and TWmax reached 48.3 degrees Celsius (118.9°F) and 27.3 degrees Celsius (81.1°F), respectively. The exact cause of these stampedes is unknown, but
extreme heat is known to increase aggressive human behavior.
