Humans May Be Forced to Leave the Tropics

A new study claims that if global warming continues unabated, there could come a time when it will be too warm for humans to survive in the tropics!

What is wet-bulb temperature and why is it important?

The study focused on a measure called wet-bulb temperature. Wet-bulb temperature accounts for heat and humidity. It is similar to what weather watchers often refer to as the “heat index” or the “feels like” temperature that accounts for both heat and humidity.

Based on this measure of how the human body responds to humidity, the researchers postulated that if countries are able to cap warming at 1.5 degrees Celsius above pre-industrial levels, the tropics will be spared temperatures that surpass the “survival limit.”

However, they say that life in the world’s hottest latitudes could become intolerable if those controls aren’t met.

Scientists believe that a “wet-bulb” temperature of 95 degrees F is the upper limit of human tolerance. Of course, people vary in how much heat they feel they can stand. But there is a general consensus among climatologists and medical professionals that at a wet-bulb temperature of 95 degrees C, anyone lingering outdoors would be in trouble.

That limit of 95 degrees F may not sound like a lot, especially if you live in South Florida. However, scientists say that once the wet-bulb temperature crosses over that 95°F threshold, the air is so hot and humid that not even sweating can lower your body temperature to a safe level. With continued exposure above this limit, death by overheating can follow.

The body normally maintains a fairly stable internal temperature of 98.6 degrees F. Skin temperature has to be a little lower to allow core heat to flow to the skin. If it’s not, a person’s internal temperature could quickly rise, explained Yi Zhang, the lead researcher on the new study.

“High core temperatures are dangerous or even lethal,” said Zhang. Zhang is a graduate student in atmospheric and oceanic sciences at Princeton University in New Jersey.

The study

For their study, Zhang and her colleagues made projections as to how global warming could affect wet-bulb temperatures in the tropics. The tropics is the area defined by 20 degrees north and south of the equator. That includes the Amazon rain forest, a large share of Africa, the Indian peninsula, and parts of Southeast Asia.

The researchers started with the theory that fairly simple atmospheric dynamics control local wet-bulb temperatures. Then they used decades worth of weather-station data to confirm that was the case.

Once that was established, they were able to project that if global warming is limited to 1.5 degrees Celsius (as called for by the Paris Accords). This will prevent “most of the tropics” from reaching intolerable wet-bulb temperatures.

Averting intolerable wet-bulb temperatures does not, however, mean the planet is out of the woods. Human health can certainly suffer under less-extreme heat, Zhang noted.

Heatwaves routinely cause sometimes fatal heat illness. Warming also contributes to air pollution, which can exacerbate chronic health conditions like heart and lung disease, she added.

Zhang and her associates believe that the only way to stay below the 95o F (35o C) threshold is to reduce greenhouse gas emissions to net-zero.

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