Global warming of 2.7 degrees by the end of the century. This would become a reality if existing efforts to reduce greenhouse gas emissions were continued. This 2.7 degrees would have fatal consequences for nature, for our economy, but also for humans as living beings.
According to a study recently published in the renowned journal “Nature Sustainability”, about a third of humanity could no longer live where they live today. The main reason for this is the increasing heat – but not only: humidity also plays a decisive role, as it determines how well our body can cool down by sweating when it is hot.
A measure of this heat stress in humans is the so-called wet bulb temperature. We explain exactly what this is and why we will increasingly see this measure when it comes to the devastating effects of climate change.
What is wet bulb temperature?
The wet bulb temperature is also referred to as the cooling limit temperature. It links the temperature of the dry air (as you usually read it on a thermometer) to the humidity. Wet bulb temperature is specifically defined to serve as a measure of human heat stress.
To put it somewhat complicatedly, it is the lowest temperature that can be reached by evaporation and cooling in a specific environment. If you look at the measurement of the wet bulb temperature, it all becomes a bit easier to understand.
How do you measure the wet bulb temperature?
The wet bulb temperature is – just like the normal air temperature – measured with a kind of thermometer. However, this is a special meteorological measuring instrument: the so-called psychrometer (psychrós is Greek and means something like icy, cool or cold).
The psychrometer consists of two thermometers, one of which is more familiar to us. The other is wrapped in some kind of damp cloth or damp cotton wool. Because the moisture on this material constantly evaporates and because evaporation has a cooling effect, the air to be measured is also constantly cooled.
Now it is crucial that the moisture can evaporate faster as the air is drier. Or the other way around: if the ambient air is more humid, it can absorb less moisture from evaporating materials – or from the sweat of human bodies – and therefore cool less.
This also shows that the lower the humidity, the greater the difference between the air temperature and the wet-bulb temperature – or between the “normal” thermometer and the one with the damp cotton wool.
The cooling limit temperature is therefore always below the air temperature – unless there is a relative humidity of 100 percent, then both masses are exactly equal.
Why is this dimension becoming increasingly important?
Like many animals, humans regulate their body temperature by sweating. The wet bulb temperature plays an important role in this: when it reaches a critical threshold, we can no longer transfer heat to the environment through sweating to keep our body temperature constant.
This basically means that the limit of human adaptability to extreme heat has been reached. If we can’t escape the conditions, our body’s temperature can rise above the survivable range and organs can fail.
What is the difference with the felt temperature?
As described, the wet bulb temperature is measured directly. The wind chill, which many are probably more familiar with, is shown in the so-called heat index.
The heat index is a measure of how the air temperature feels to our bodies when you take relative humidity into account. It is calculated for shady areas and indicates how uncomfortable we feel when it is hot and humid.
However, as the word implies, the heat index is a (constructed) index that must be calibrated each time. So it is a somewhat subjective measure that is even used in different versions in different countries. For this reason, climate scientists are increasingly turning to another measure of heat stress: the wet-bulb temperature.
Where is wet bulb temperature used?
The measure is mainly used in science, it should (still) mean little for the general public. Especially when it comes to being able to express quantitatively the effects of climate change and global warming on humans, the wet bulb temperature is a useful measure.
But it is also used as an ingredient in indices, such as the Wet Bulb Globe Temperature Index (WBGT). This measures heat stress in direct sunlight and has its origins in the US military: the WBGT was developed in 1956 by the United States Marine Corps in Parris Island, South Carolina. The aim of this was to reduce heat stress injuries in recruits. In addition to the wet bulb temperature, this index also includes the prevailing wind force, the angle of incidence of the sun and the cloud cover.
The WBGT is also used at sporting events. For example, FIFA uses this index as a guideline when it comes to possible heat stress in football players: if the WGBT exceeds 32 degrees Celsius, the regulations prescribe cooling breaks in both halves of the game.
What wet-bulb temperature can humans withstand?
For a long time it was assumed that the limit for humans was a wet-bulb temperature of around 35 degrees – with an exposure of at least six hours. According to the British Guardian, this roughly corresponds to a dry temperature of 40 degrees in combination with a high relative humidity of 75 percent.
However, a new study from 2022 now suggests that the threshold is likely to be much lower. The researchers studied young and healthy adults who had to imitate basic activities of daily living at high wet-bulb temperatures.
“Our data is real human data and shows that the critical wet-bulb temperature is closer to 31.5 degrees,” said study co-author and professor of physiology Larry Kenney.
What does this realization mean? Bill McGuire, director of Britain’s Hazard Research Center warned The Guardian: “If the new findings are correct, we will find ourselves in a completely new situation in extreme heat. The number of people globally exposed to a potentially deadly combination of heat and humidity would be much higher than previously thought.”
This impression is now confirmed in the recently published study ‘Nature Sustainability’, which paints a critical picture of habitability towards the end of the century.
Where could the critical wet bulb temperature be exceeded?
Events with extremely high wet bulb temperatures of up to 35 degrees have increased worldwide, NASA writes. Thus, the events in which they reached a height between 32 and 35 are quadrupled. Furthermore, between 1979 and 2017, there were about 1,000 events above 31 degrees and about a dozen above 35 degrees, in Pakistan, India, Saudi Arabia, Mexico and Australia. It is striking that nine of them have been registered since 2005.
According to a 2020 study, South Asia, the Persian Gulf and the Red Sea will be the most threatened areas by 2050, and East China, parts of Southeast Asia and Brazil by 2070.
“Previous studies suggested this would be the case in a few decades, but our study shows that it is happening now,” said the study’s lead author, Colin Raymond, a climate scientist at NASA’s Jet Propulsion Laboratory. “The duration of these events will increase and the affected areas will grow in direct correlation to global warming.”
Soource :Watson
I am Amelia James, a passionate journalist with a deep-rooted interest in current affairs. I have more than five years of experience in the media industry, working both as an author and editor for 24 Instant News. My main focus lies in international news, particularly regional conflicts and political issues around the world.
