The Heat Is on – on Earth

It is only for the past 300,000 years that humans have been populating our 4.6-billion-year-old Earth – and we have altered it like no other living being before us. Due to man-made climate change, global temperatures have risen by roughly 1.1 to 1.2 degrees Celsius in the past 200 years alone. And they will continue to rise. For Earth, this is not unprecedented – for periods throughout the history of our planet, worldwide temperatures measured multiple degrees above our current average. The difference: humans did not exist.

Adapting to heat
At the same time, Homo sapiens is a tropical creature – thermophysiologist Hein Daanen from Vrije Universiteit Amsterdam is convinced. While humans only came into existence during times of life-sustaining temperatures, throughout evolution we adapted mostly to a warm climate; for example, primates in central Africa already possessed sweat glands. However, even if humans adapted to heat with their help, it does not mean that we can withstand all tropical climate conditions.

The first person who developed a measure of how much heat a human can withstand was Scottish physiologist John Scott Haldane. He wanted to find out the conditions under which humans are still able to work, and to this end, in 1905, he descended into a tin mine in Cornwall, England. Haldane noted that at around 31.7 degrees Celsius, in calm, water-saturated air, the limits of human endurance were reached as body temperature rose, pulse accelerated, and shortness of breath and headache soon followed.

Experiments in the climate chamber
According to Haldane, these conditions, known as the wet-bulb temperature, constitute the limit. It describes the lowest temperature that can be achieved through evaporation cooling. In order to maintain a body temperature of approximately 36.8 degrees Celsius, the temperature of the skin cannot exceed 35 degrees. Beyond that, the body begins to sweat. The problem: the higher the humidity, the less effective the evaporation of sweat and the resulting cooling effect. This explains why humans can survive in the dry desert heat while their cooling system will fail under conditions of combined heat and humidity because our body temperature will rise. “Insufficient evaporation of sweat leads to an increase in body temperature and may trigger events such as heat stroke which can be fatal”, summarizes Hein Daanen.

The exact wet-bulb temperature at which the human body reaches its stress limit is also the subject of research by scientists at George Mason University in Virginia. They asked study subjects to cycle on a bicycle ergometer inside a climate chamber while steadily increasing either temperature or humidity. “As a result, conditions inside the chamber became quite hot and humid”, reports physiologist and team member Kathleen Grace Fisher who, throughout the experiment, monitored blood pressure and thermal sensation of the participants.

A menacing outlook
The result: the maximum tolerated wet-bulb temperature at 100 percent humidity was 30.6 degrees Celsius, which is lower than previously assumed. Climatologists then devised a model to show where on Earth such potentially dangerous heat would develop first. Their map shows that in the case of an increase in temperature of two degrees Celsius, wet-bulb temperatures threatening human life would develop in large parts of India, Pakistan and eastern China, as well as in sub-Saharan Africa. A rise in temperature of three degrees would bring about such climate conditions in the center and eastern parts of the United States, and in the case of an increase of four degrees, the inhabitants of the river valleys of the Missouri and Mississippi rivers in the US, as well as those living by the Gulf of Mexico and along the East Coast, would regularly groan under perilous heat waves with high humidity. Other areas such as the port city of Al-Hudaydah in Yemen would be uninhabitable for most of the year.

“I worry”, says Kathleen Grace Fisher, “that the temperatures which we classify as dangerous in case of extended exposure to the outdoors will increasingly occur over time, and mostly in densely populated areas, or in locations where people will not have sufficient access to air conditioning.” For this reason, she recommends that people without air conditioning use electric fans, immerse their hands and feet in cold water, wet their clothing, spray themselves with cool mist and, of course, stay sufficiently hydrated. A drop in the ocean, and certainly not a permanent solution for all those hot months – never mind the rising energy consumption. Fisher: “Our results confirm: we urgently need to take action against climate change.”