Why is 40°C heat so dangerous in the tropics? – 06/10/2023 – Science

Even before the start of the hot season in the northern hemisphere, temperature records have already been broken this year.

Spain, for example, experienced temperatures in April —38.8°C— that would be out of the ordinary even in the height of summer.

South and Southeast Asia were particularly hard hit by a very persistent heat wave, with all-time temperature records recorded in countries such as Vietnam (44°C) and Thailand (45°C).

Singapore had a more modest record that was also broken, with temperatures reaching 37°C. And Shanghai, China, recorded the highest temperature in more than a century in May: 36.7°C.

We know that climate change makes these temperatures more likely, but heat waves of similar magnitude can have very different impacts, depending on factors such as humidity or how well a region is prepared for extreme heat.

So how does a humid country like Vietnam deal with a 44°C heat wave, and how does that compare to dry heat or a not-so-great heat wave in Singapore, which is even more humid?

climate and physiology

The recent heat wave in Southeast Asia may be remembered for its level of heat stress on the body.

Heat stress is mainly caused by temperature, but other climatic factors such as humidity, radiation and wind are also important.

Our bodies gain heat from the air around us, the sun, or our own internal processes such as digestion and exercise. In response, our bodies need to lose some of that heat.

We lose some heat directly to the air around us and through breathing. But most of the heat is lost through perspiration.

When sweat on the surface of the skin evaporates, it absorbs energy from the skin and the air around us in the form of latent heat.

But meteorological factors affect this whole process. The absence of shade, for example, exposes our body to direct sunlight, while higher humidity slows down the rate at which our skin evaporates.

It is this humidity that made the recent heat wave in Southeast Asia, which is already an extremely wet region of the world, so dangerous.

The limits of heat stress

Preexisting health conditions and other personal circumstances can make some people more vulnerable to heat stress.

But heat stress can reach a threshold above which all humans, even those who are not clearly vulnerable to heat hazards — that is, healthy, fit and well-acclimatized people — simply cannot survive, even at a moderate level. of effort.

One way to determine heat stress is the so-called Global Wet Bulb Temperature (WBGT).

In full sun conditions, this temperature equates to around 39°C, combined with 50% relative humidity. This limit will likely have been exceeded in some places during the recent heat wave that swept across Southeast Asia.

In less humid places, away from the tropics, the humidity and therefore wet bulb temperature and risk will be much lower.

The heat wave in Spain last April, with maximum temperatures of 38.8°C, had global wet bulb temperatures of “only” around 30°C.

The 2022 heat wave in the UK, with temperatures over 40°C, had humidity of less than 20% and WBGT values ​​of around 32°C.

Two of the authors of this paper (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 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.

In these regions, thermal stress limits are exceeded more and more frequently due to increasing global warming.

In fact, most people are already vulnerable at levels well below the limits of survivability. This is why we see large amounts of deaths in significantly weaker heat waves.

Furthermore, these global analyzes often do not capture some very localized extremes caused by microclimatic processes.

A neighborhood in a city, for example, may capture heat more efficiently than the neighboring neighborhood, or it may be ventilated by a cool sea breeze, or be in the “rain shadow” of a local hill, which decreases its humidity. .

Variability and acclimatization

The tropics typically have less temperature variation.

Singapore, for example, is located almost on the Equator and its maximum daily temperature is around 32°C throughout the year. In London, the typical high temperature in mid-summer is just 24°C. Even so, the temperature record in London (40°C) is higher than that of Singapore (37°C).

As some regions like Southeast Asia already experience high heat stress frequently, perhaps this indicates that people are well acclimatized to deal with heat.

Initial reports suggest that the intense heat stress of the recent heat wave caused surprisingly few direct deaths — but an accurate account of deaths from indirect causes has yet to be published.

On the other hand, the relative stability of heat throughout the year may make people less prepared for the large temperature swings associated with the recent heat wave.

It is not unlikely, even in the absence of climate change, that normal variations could produce significant heat waves that break local records by several degrees Celsius. Therefore, even being close to the physiological limit can represent a very high risk.

*Alan Thomas Kennedy-Asser is a climate science 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 health researcher at the University of Bristol, UK.

This article originally appeared on the academic news site The Conversation and republished under a Creative Commons license. Read the original English version.