In my previous post, I briefly mentioned that there are parts of the world becoming too hot and humid for long-term human habitation and that further climate change will exacerbate this trend. Extreme heat waves have become more frequent over the past few decades as the global climate continues to warm. This extreme heat can cause injury and death, particularly in medically vulnerable people such as the very young and very old. But there is a point where no human being, regardless of physical fitness, can survive such extreme heat.
Wet bulb temperature is defined as the lowest temperature that an area can reach using only evaporative cooling. Evaporation of a liquid into a gas naturally cools the surrounding environment. When energy is added to a liquid, its temperature increases. But once it reaches its boiling temperature, it doesn’t immediately turn into a gas without absorbing some extra energy. For water, the amount of energy it takes to turn 100°C water into a gas is over five times as much as it takes to turn 0°C water into 100°C water. So the act of evaporation absorbs a lot of heat from the surrounding environment. Additionally, this gaseous water vapor moves away from where it was and takes the heat it absorbed with it. This is why humans sweat; we secrete water on our skin when it's hot, this water absorbs heat from our skin, causing it to evaporate and take this heat with it. Wet bulb temperatures are measured with wet bulb thermometers; thermometers wrapped in a wet cloth that cools the thermometer via evaporative cooling. Wet bulb temperatures are useful for measuring apparent temperature, i.e. what it ‘feels like’ for humans, since it replicates how the human body experiences heat. (Wet bulb temperature is similar to heat index, though heat index is a calculated measurement based on a given humidity and temperature.)
Wet bulb temperature is influenced by temperature and humidity. Air can only hold a finite amount of water vapor, so the closer it gets to 100% humidity (the maximum capacity of air to hold water vapor), the harder it is for water to evaporate. Conversely, at low humidities, water evaporates quickly since it’s easy to find room for evaporating water molecules. As such, wet bulb temperature is highest when both temperature and humidity is highest. The higher the humidity, the less effective evaporative cooling is, so the more heat is trapped where it is. For humans, this means that sweating becomes ineffective at cooling the body. This is why humid heat feels worse than dry heat; the mechanism the human body uses to cool itself is less effective in humid environments.
Chart showing wet bulb globe temperatures of given temperatures and humidities in Celcius and Fereinheit. Note, these charts show wet bulb globe temperature instead of wet bulb temperature, which also takes into account sunlight and wind speed.
When the body gets too hot, it cools down by sweating and by diverting blood toward the skin. Blood carries a lot of body heat, so dilating blood vessels in the skin to allow more blood to flow through them puts more heat in a position to escape the body. This is why one’s skin may become redder when one is very hot. But diverting blood to the skin pulls it away from major organs. Blood pressure drops, so the heart must work harder to keep blood circulating. Internal organs such as the intestines get less oxygen, which might mean toxins they normally keep contained start to slip out into the body. Also, sweating removes water and salt from the body. Losing water causes the blood to thicken, compounding the heart’s problems to keep blood circulating. The nervous and muscular systems rely on electrolytes to function, so losing salt causes them to falter. This is why heat stroke has multiple neurological symptoms, such as headache, dizziness, delirium, aggression, and vomiting. All these symptoms worsen as temperatures get hotter or one is exposed to them for longer. As the body becomes stressed, inflammation and immune system shutdown make these problems worse. Kidney failure becomes a risk as they receive less water and are made to filter even more toxins while cardiac arrest becomes a risk as the heart works even harder to pump thicker blood. In extreme cases, muscle cells start to die and their insides spill out into the bloodstream, putting even more stress on the kidneys and other organs. Without relief, one would eventually die of a heart attack or another form of organ failure.
The wet bulb temperature that would cause such lethal hyperthermia varies from person to person depending on their age, size, heat acclimation, and pre-existing medical conditions. A wet bulb temperature of 35°C (heat index of 71°C or 160°F) has long been considered to be the threshold of what even the fittest, most heat-adapted humans can survive for longer than six hours, even in the shade, with access to water, and with fans to keep cool. At this temperature/humidity, there’s no effective way for the body to get rid of excess heat and instead the body can only get hotter until it fails. But recent research has suggested the real limit might actually be lower at only 31°C. To put this into perspective, this would be 31°C (87°F) at 100% humidity or 38°C (100°F) at 60% humidity.
Wet bulb temperatures exceeding 35°C have occasionally been observed in the past, particularly during extreme heat waves in the tropics. They are rare, but a recent study compiling readings from weather stations around the world for the past four decades found that these extreme temperature events are becoming more frequent faster than originally anticipated. It found the number of instances where WBT exceeded 30°C has doubled since the 1970s to roughly 1,000 instances a year. These instances were only for a few hours at a time in very specific locations in the tropics, but this is still more instances of dangerously high temperatures than climate models originally predicted for our warming climate. As the planet continues to warm, the length, severity, and affected area of these instances will expand. The locations these extreme temperatures were measured were along the coast of the Gulf of Mexico, the Middle East, West Africa, India, and Southeast Asia. As climate change worsens, these tropical regions could see temperatures regularly reach these deadly highs while subtropical regions might see occasional heat waves that reach these lethal temperatures.
Data points show the highest recorded wet bulb temperatures in a given area. Reds denote temperatures near or surpassing 31°C.
Do keep in mind that a region does not have to become lethally hot at all times to become depopulated; if a place becomes uninhabitable during the summer, few people are going to move back for just the winter. Also keep in mind that 31-35°C is when even the most physically fit will die; children, the elderly, and the infirm will be at risk at WBTs as low as 28°C. Also chronically high temperatures can be a problem too; those who experience frequent higher body temperatures, such as those who do manual labor outside in warm climates, are at greater risk of developing heart or kidney disease due to the frequent stress their organs are put under. Outside of individual human health, manual labor becomes less efficient at higher temperatures which can impact regional economies in such regions. And the increased need for air conditioning in these conditions puts more strain on electrical infrastructure. Since air conditioning and electricity would become more expensive in these regions, impoverished people will be more affected by these heat waves. And of course there is the consideration of climate refugees; the logistical challenges of rehoming the millions of people living in regions that are at risk of becoming uninhabitable.
I’m sure by now, you don’t need me to tell you that climate change is bad and that we should work to avoid its worst effects. The unfortunate truth is that I cannot give you much in the way of actionable advice on how to mitigate climate change, the only thing most individual people can realistically do is to vote for policies and people who want to fix the problem. I come back to describing the potential ramifications of a warming world because the best I can do is provide perspective. In the past year, OSHA has begun establishing new safety regulations for working in high heat environments and enforcing existing rules more strictly, such as mandating regular breaks and access to water and PPE. NASA is using satellites to gather WBT data in regions with fewer weather stations, useful information for determining what regions are most at risk. The practical effects of lethal heat are already manifesting and having to be considered by governments in day-to-day decision making. We might be more motivated to support climate policy when we have a realistic understanding of how climate change will impact us on an individual level. And what I hope these past few entries have done is give you all a greater appreciation for our climate by showing how intricate it is and how it impacts our society.
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