July 7, 2023 – If you clicked on any major news site this week, there was one consistent headline that was hard to miss, let alone ignore: “Hottest Day Ever on Earth.”
It was on July 3 that the common global temperature reached 16.1 °C. But this record was short-lived, as on July 4 temperatures rose again to 16.2 °C. On closer inspection, this seemingly unspectacular average temperature led to highs of fifty °C in Africa. In Antarctica, where it's currently winter, temperatures reached 8.5 °C. And within the USA, The Washington Post warned that as much as 54 million Americans were susceptible to dangerous (or “extreme”) heat that day alone. Scientists warn that we're in uncharted territory. And humanity is about to achieve the height of its adaptive capability.
“When we talk about evolution, we are talking about millions of years for [humans] to develop this ability to regulate temperature,” said Dr. Camilo Mora, a professor of data analysis at the University of Hawaii-Manoa. “So if the outside temperature rises, it will take millions of years for us to logically adapt to it,” he said.
Mora and his colleagues have spent decades modeling the risk of extreme heat associated with global climate change, showing that the Earth has warmed by about 1°C (1.8°F) over the past decade, leading to an over 2,300% increase in deaths from heat waves alone.
Heat can be deadly in many ways
When most people think of the effects of extreme heat, they naturally think of things like fatigue, headaches, a slight feeling of weakness, or nausea. But these symptoms are just the tip of a melting iceberg: heat exposure is associated with many things that can damage vital organs, sometimes permanently.
It all starts with thermoregulation, a concept that describes how the body maintains a constant internal (or core) temperature of 37°C. Thermoregulation is controlled by a gland in the brain called the hypothalamus, which responds to higher air temperatures by signaling blood vessels to expand and divert blood, salt and fluids to the skin to cool down through the process of evaporation (sweating).
But is there such a thing as “too hot”?
“About 10 or 12 years ago, a bunch of meteorologists developed an idea: the wet bulb temperature, which represents the upper limit of human adaptability or sustainability,” said W. Larry Kenney, PhD, professor of physiology and kinesiology at Pennsylvania State University in University Park.
He explained that the term “wet-bulb” comes from an experiment in which scientists wrapped a wet cloth around the bulb of a thermometer and used it as a substitute for human skin. When the moisture evaporated from the cloth, the thermometer reading dropped. However, when the air was too humid, less or no evaporation occurred.
“The wet bulb temperature is a certain temperature of the air when it's 100% saturated. So in case your skin is 95°F (35°C) and the air temperature is 95°F, but it surely is totally saturated with moisture, the steam and sweat can now not evaporate. And so we lose our primary way of cooling the body,” Kenney said.
The result is not pretty.
In a 2017 study, Mora and his colleagues identified 27 ways in which heat exposure can lead to organ failure and death. Simply put, when blood is diverted to the surface of the skin, blood flow (and oxygen) to other organs (the brain, heart, gut, liver, and pancreas) is reduced. When the body and cells exceed the limits of how much heat they can tolerate, cells die and their protective membranes break down.
Several organs then stop working properly. In the heart, loss of cardiac function can lead to a heart attack, and dehydration thickens the blood, increasing the risk of blood clots and strokes. Kidney failure can occur. Injury to the lung lining means that the lungs and bloodstream eventually lose oxygen, leading to shortness of breath.
When cell membranes break down, pathogens and toxins can enter organs, causing things like increased inflammation in the pancreas, neurological damage in the brain, and the leak of bacteria and toxins from the gut into the bloodstream, which in turn can cause sepsis and a systemic inflammatory response that disrupts cellular balance. Combined with injury to the lining of veins and other parts of the vascular system, inflammation can trigger blood clots that cut off blood supply to vital organs and lead to fatal bleeding. Damaged muscle tissue can leak electrolytes and proteins into the blood, causing kidney failure.
The consequences are devastating. When the body's core gets too hot during extreme heat events – like those currently occurring in Texas and other states – a vicious cycle of multi-organ failure and breakdown occurs, leading to permanent disability and death.
Heat sensations and risks
On June 23 of this year, a 31-year-old man and his 14-year-old stepson died in Big Bend National Park in Texas, where air temperatures had reached 49 °C.
What were they thinking as they began their hike that morning?
Nicholas Ravanelli PhD, assistant professor of kinesiology at Lakehead University in Thunder Bay, Ontario, and clinical exercise physiologist, studies how people perceive, cope with, and behave during heat waves—an area that has only recently received attention in research circles.
“Perception is an important factor in our physiology [in terms of] how well we adapt to heat,” he said. “There is a missing link for which we do not yet have enough evidence; namely, how people perceive the environment and make proactive or reactive decisions to cool or protect themselves.”
“If you become involved [place] “End-organ damage typically occurs when the temperature is above 104 to 106 degrees Fahrenheit and the body can no longer cool itself sufficiently. This affects the brain,” says Dr. Sara Andrabi, assistant professor of emergency medicine at Baylor College of Medicine and associate medical director at Ben Taub Hospital Emergency Center in Houston.
“You see things like dysfunction in people's mindsets. They may not answer questions, they may (have trouble walking). I always tell parents, if you see your kids really irritable and not acting like themselves, that can be a sign [of heat-related illnesses]because they are unable to put into words what is going on,” she said.
When this happens, people may lose the flexibility to make rational decisions that would save their lives, said Kenney of Pennsylvania State University.
We may never know exactly what was happening that morning in Big Bend, nor the the explanation why the person and his stepson set out on the hike despite heat warnings from park authorities and the National Weather Service.
However, not everyone reacts to heat in the identical way.
Kenney and his colleagues recently found that wet bulb temperatures are significantly lower in older people and other vulnerable populations.
Older individuals are also more vulnerable to extreme heat because they've difficulty moving, which limits their ability to maneuver and dissipate heat from their bodies. Some prescribed drugs also affect temperature, including certain medications for depression and hypertension.
In younger children, the body surface area to body size ratio is smaller, allowing heat to penetrate to the core of the body way more quickly, resulting in dehydration, electrolyte imbalance and warmth stress.
Aside from these obvious groups, Kenney says some people can simply tolerate certain heat conditions higher than others.
“There are four main factors,” he said. “The first is genetics. The second is heat acclimation or acclimation, which means that the body slowly goes through a series of adaptations over time, making it more tolerant of high heat and humidity conditions. The third would be aerobic fitness. And the fourth is fluid balance, so whether people are well hydrated or not.”
But even with a jump start, everyone seems to be ultimately exposed to the results of worldwide warming.
“This isn't something that happens to people in other parts of the world,” Mora said. “It's happening to us, and it's happening everywhere. We're screwed.”