1 of 1 Thermometer shows 40°C in Madrid, Spain Photo: GETTY IMAGES Thermometer shows 40°C in Madrid, Spain Photo: GETTY IMAGES
Temperature records were broken this year even before the start of the hot season in the northern hemisphere.
In Spain, for example, temperatures of 38.8 °C prevailed in April, which would be exceptional even in the height of summer.
South and Southeast Asia were particularly hard hit by a very prolonged heatwave, with countries such as Vietnam (44°C) and Thailand (45°C) setting alltime temperature records.
Singapore had a more modest record, which was also broken with temperatures reaching 37C. And Shanghai, China, recorded its highest temperature in more than a century in May: 36.7°C.
We know that climate change is making these temperatures more likely, but similarmagnitude heatwaves can have wildly different effects, depending on factors like humidity or how well a region prepares for extreme heat.
How does a humid country like Vietnam deal with a 44°C heatwave, and how does that compare to dry heat or a notsobig heatwave in Singapore, which is even wetter?
climate and physiology
The recent heat wave in Southeast Asia is perhaps remembered for the high levels of heat stress on the body.
Heat stress is mainly caused by temperature, but other climatic factors such as humidity, radiation and wind also play a role.
Our bodies gain heat from the air around us, the sun, or our own internal processes like digestion and movement. In response, our body must lose some of that heat.
We give off part of the heat directly to the air around us and through breathing. However, most of the heat is lost through sweating.
As sweat evaporates from the skin’s surface, it absorbs energy in the form of latent heat from the skin and air around us.
But meteorological factors influence this entire process. For example, the lack of shade exposes our body to direct sunlight, while higher humidity slows evaporation from our skin.
It’s this humidity that made the recent heat wave in Southeast Asia, an already extremely humid region of the world, so dangerous.
The limits of heat stress
Medical conditions and other personal circumstances can make some people more susceptible to heat stress.
But heat stress can reach a threshold above which all people, even those who are not clearly susceptible to heat hazards — that is, healthy, fit, and wellacclimatized people — simply cannot survive even with moderate exertion.
One way of determining heat stress is the socalled Global Wet Bulb Temperature (WBGT).
In full sunlight, this temperature equates to approximately 39°C combined with 50% relative humidity. During the most recent heat wave that swept Southeast Asia, this limit may have been exceeded in some places.
In less humid places outside of the tropics, the humidity and therefore the wetbulb temperature and risk are much lower.
The heat wave in Spain last April with maximum temperatures of 38.8 °C led to global wetbulb temperatures of “only” around 30 °C.
The 2022 UK heatwave, with temperatures above 40°C, had humidity below 20% and WBGT values around 32°C.
Two of the authors of this work (Eunice Lo and Dann Mitchell) were part of a team that recently used climate data to map heat stress around the world.
The research highlighted the regions most at risk of exceeding these limits, including India and Pakistan, Southeast Asia, the Arabian Peninsula, Equatorial Africa, the Equatorial Region of South America and Australia.
Due to increasing global warming, the thermal load limits are being exceeded more and more frequently in these regions.
In fact, most people are already at risk well below the survival limit. This is why large numbers of deaths occur during significantly weaker heat waves.
Furthermore, these global analyzes often fail to capture some very localized extremes caused by microclimatic processes.
For example, a neighborhood in a city may trap heat more efficiently than the neighboring neighborhood, it may be ventilated by a cool ocean breeze, or it may be in the “rain shadow” of a local hill, which lowers humidity.
variability and acclimatization
In the tropics there are usually fewer temperature fluctuations.
Singapore, for example, is almost on the equator and the maximum daily temperature is around 32°C all year round. In London, the typical high in midsummer is just 24°C. Nevertheless, the temperature record in London (40°C) is higher than that of Singapore (37°C).
As severe heat stress is already common in some regions such as Southeast Asia, this may indicate that people are well adapted to coping with heat.
Initial reports suggest that the severe heat stress from the recent heat wave caused surprisingly few direct deaths, although accurate reports of indirect deaths have yet to be released.
On the other hand, the relative stability of the heat yearround could mean people are less prepared for the large temperature swings associated with the recent heat wave.
Even in the absence of climate change, it is not unlikely that normal fluctuations could result in significant heat waves, beating local records by several degrees Celsius. Therefore, even approaching the physiological limit can represent a very high risk.
*Alan Thomas KennedyAsser is a climate researcher at the University of Bristol, UK.
Dann Mitchell is Professor of Climate Science at the University of Bristol, UK.
Eunice Lo is a climate science and public health researcher at the University of Bristol, UK.