Halfway to boiling point
Climate change is bringing with it extreme heat, and conditions that are in some cases literally unsurvivable
Why is this a cause for concern?
Recent years have seen temperature records repeatedly broken, all over the world. July 2021 was the hottest month ever recorded on Earth. Last summer saw temperatures of over 50°C in Iraq, Iran, Kuwait, the United Arab Emirates and Saudi Arabia. This spring, heatwaves in India and Pakistan resulted in temperatures exceeding 50°C. Much attention is given to the secondary effects of global warming, such as rising sea levels, wildfires, storms and other types of extreme weather. Sometimes overlooked is the phenomenon of high temperatures in themselves, and the effect they will have on the human body. Scientists often assess these by recording what are known as “wet bulb temperatures” or WBTs.
What are wet bulb temperatures?
Normal “dry bulb” readings record air temperatures in the shade. These do not take humidity into account. If humidity is low, extreme temperatures are tolerable. “If you’re sitting in the shade with unlimited drinking water in California’s Death Valley” – where the highest temperature ever, of 56°C, was recorded – “conditions may not be pleasant, but they’re survivable,” says Colin Raymond, a Nasa scientist. By contrast, humid heat reduces humans’ ability to cool down by sweating. These are measured by a WBT, taken using a thermometer wrapped in a moist cloth. The wetter the surrounding air, the less moisture is able to escape and the higher the wet bulb reading will be. At a WBT of 32°C, it’s dangerous to do anything that raises the body temperature, such as walking. At 35°C, “it doesn’t matter what you’re doing”, says Raymond. “You can’t survive for extended periods of time.”
Why can’t we survive high wet bulb temperatures?
Humans need to maintain a constant body temperature of around 37°C. Once the air temperature is higher than that, only latent cooling – sweating – can cool us down. If humidity prevents evaporation, even healthy people sitting in the shade will die within about six hours. When your core temperature gets too hot, heat stroke ensures. The body’s proteins start to break down. Its enzymes stop regulating organs’ functions, which then begin to shut down. Essentially, your body cooks and multi-organ failure results.
How common are high WBTs?
Climate models had projected that the first WBTs exceeding 35°C would be recorded by the mid-21st century. But a 2020 study published in Science Advances – “The emergence of heat and humidity too severe for human tolerance” – found that WBTs of over 35°C have already been recorded in many parts of the world. “Extreme humid heat overall has more than doubled in frequency since 1979,” the study’s authors wrote. And incidences of WBT values in the 32°-35°C range have more than tripled during that 40-year period. Wet bulb readings above 35°C are still far from common; and they usually only occur for 1-2 hours’ duration. But they are becoming significantly more frequent.
Where are the worst-hit places?
Humid coastal areas in the subtropical zone north and south of the tropics are the most vulnerable (near the Equator, cloud cover provides some protection). The Science Advances study revealed that such hotspots are concentrated in the Middle East (around the Persian Gulf and Red Sea), South Asia and coastal southwest North America. The deadly 35°C threshold has already been crossed in, for instance, Ras al Khaimah in the United Arab Emirates, Mecca in Saudi Arabia, Jacobabad in Pakistan (see box), Haryana in northern India, and Tabasco in southern Mexico. In future, eastern China, Southeast Asia and Australia are also likely to be severely affected.
Are 35°C WBTs the only worry?
No. Considerably lower WBTs still pose extreme danger, particularly for the elderly, those with medical conditions, and for those who are not acclimatised to extreme heat. People’s heat tolerance can change over time, meaning that heat waves in normally cooler places, or early in the summer, are more likely to be deadly than the same conditions in habitually hotter areas, or later in the summer. Last year’s “heat dome” in America’s Pacific northwest killed 200 people, though wet bulb readings never exceeded 25°C. The 2003 heatwave in Europe caused more than 50,000 deaths, at maximum WBTs of around 26°C.
What impact will climate change have?
Average global temperatures are currently 1.1°-1.3°C higher than pre-industrial levels, and are only going up. Another study, published in Nature Climate Change in 2017, found that while today about 30% of the world’s population is exposed to potentially deadly levels of heat and humidity for at least 20 days a year, that percentage will increase to nearly half by 2100 – even with drastic reductions in greenhouse gas emissions. The effects of more regular extreme heat, says Dr Radley Horton of Columbia University, will be “transformative for all areas of human endeavour – economy, agriculture, military, recreation”.
Can the effects be mitigated?
Yes, most effectively by air conditioning, which not only lowers indoor temperatures but also removes humidity from the air. However, in poorer badly-affected areas such as Pakistan, few have air conditioning, and there are frequent power cuts. Preparedness is important. Indian cities, like Ahmedabad in Gujarat, now have “heat action plans”, which involve early public warnings, regulations preventing outdoor work, public “cooling centres” in schools and temples, and a dedicated healthcare response. Longer-term responses include planting more greenery in affected cities, replacing heat absorbent materials such as tarmac, and painting roofs white to reflect heat. But, as the Science Advances study states, the trend towards “increasingly extreme humid heat” represents “a major societal challenge for the coming decades”.