It’s not the heat, it’s the humidity. That simple phrase sums up a major danger pilgrims face (in addition to COVID-19) during the coming week during the Hajj, the annual Islamic pilgrimage to Mecca (Makkah), the holiest city for Muslims. This year, the Hajj falls during the period July 17-22, which is typically among the hottest weeks of the year; levels of heat stress are predicted to approach the danger level on several of the days.

Mecca is located approximately 45 miles inland from the Saudi city of Jeddah, which lies on the coast of the Red Sea. Humid air from the Red Sea often penetrates inland to Mecca when winds blow out of the west, raising the heat stress to dangerous levels for the two million-plus pilgrims who typically attend the five-day Hajj. (This year’s Hajj is limited to just 60,000 participants because of the COVID-19 pandemic.)

High wet-bulb temperatures in Saudi Arabia in summer

While 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. The latest forecasts from the GFS model predict that TW will mostly remain below the “Danger” threshold during this year’s Hajj, but TW could exceed the “Danger” threshold on Sunday and Wednesday afternoon if moist winds blow off of the ocean. The high temperatures each day during the Hajj are predicted to be 36-39 degrees Celsius (97-102°F).

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.

Elderly Hajj participants at high risk of heat-related illness

While 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.

Future summer Hajj events at high risk of dangerous heat

The 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.

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 heat

The 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.

Rare ‘unsurvivable’ heat events will increase in a warming climate

Since human skin temperature averages close to 35 degrees Celsius (95°F), wet-bulb temperatures above that value prevent all people from dispelling internal heat, leading to fatal consequences within six hours, even for healthy people in well-ventilated conditions. A wet-bulb temperature a few degrees lower is fatal for most people, but not all.

A 2020 paper in the open-access journal Science Advances by Raymond et al., “Potentially Fatal Combinations of Humidity and Heat Are Emerging across the Globe,” identified 14 examples of 35°C wet-bulb readings that have already occurred since 1987 at five stations in Pakistan, Saudi Arabia, and the United Arab Emirates (UAE). These conditions generally lasted less than six hours (see Bob Henson’s May 2020 post for details).

Those researchers found that the frequency of TW values reaching 27°C, 29°C, 31°C, and 33°C across the world all showed doubling trends between 1979 and 2017. They predicted that dangerous wet-bulb readings will continue to spread across vulnerable parts of the world, affecting millions more people, as human-caused climate change continues.

Higher wet-bulb temperatures will be particularly dangerous in the Indus River valley along the India/Pakistan border, where thousands of laborers work outdoors in pre-monsoon heat that can reach dangerous levels during May, June, and July. Jacobabad, Pakistan (population 191,000), has already recorded six days when the wet-bulb temperature exceeded the limits of human survivability: 35 degrees Celsius. A 2015 heat wave killed 3,477 people in India/Pakistan, ranking as the fourth deadliest heat wave in world history, according to the international disaster database, EM-DAT. Four of the 10 deadliest heat waves on record in the EM-DAT database have affected India and/or Pakistan.

A 2015 paper by Pal and Eltahir, “Future temperature in southwest Asia projected to exceed a threshold for human adaptability,” warned that human habitability will be severely impacted in the nations of Pakistan, India, Saudi Arabia, the United Arab Emirates, Iran, Iraq, Yemen, Oman, Qatar, Bahrain, and Kuwait in coming decades. They suggested those nations would benefit by supporting strong efforts to rein in climate change and forsake a “business as usual” approach. A “business as usual” approach would likely lead to summertime high temperatures by 2100 reaching 60 degrees Celsius (140°F) in Kuwait and the United Arab Emirates, and 55 degrees Celsius (131°F) in Mecca, Saudi Arabia. Wet-bulb temperatures would likely exceed 36 degrees Celsius (97°F) – beyond the limit of human survival – at some locations in Iran, the UAE, and Qatar. The authors wrote: “A plausible analogy of future climate for many locations in Southwest Asia is the current climate of the desert of Northern Afar on the African side of the Red Sea, a region with no permanent human settlements owing to its extreme climate.”

The data point to a logical conclusion: It would be strongly in the interest of the nations of Southwest Asia, and of other regions, to support aggressive efforts to reign in climate change to protect the Hajj … and the future of human habitability in their countries.