As the Northern Hemisphere approaches summer’s peak, heat is testing the limits of human survival in Earth’s hottest spots — and demonstrating the extremes that are increasingly possible and probable against the backdrop of accelerating global warming.
In recent days, China set an all-time high of nearly 126 degrees Fahrenheit, while Death Valley hit 128 degrees, two shy of the highest reliably measured temperature on Earth.
Phoenix was expected to observe a record-breaking 19th consecutive day at or above 110 degrees Tuesday. And in the Middle East, the heat index reached 152 degrees, nearing — or surpassing — levels thought to be the most intense the human body can withstand.
Such conditions are more than enough to overwhelm the body’s ability to regulate its internal temperature, experts said, and offer a glimpse of dangers only expected to become more prevalent as global warming increases extremes in heat and humidity.
“We know these extreme temperatures are killing people right now,” said Cascade Tuholske, an assistant professor at Montana State University.
Without the help of air conditioning, fans or shade, the body only has its own cooling system to withstand heat. Some body heat can escape through convection and radiation, though that is only effective if the air temperature is lower than body temperature.
Otherwise, sweating is the only way to cool down, transferring heat from the body to the air as it turns from a liquid to a vapor. But that, too, has its limits.
“Sweating is only effective in cooling our bodies if it evaporates,” said Larry Kenney, a professor at Pennsylvania State University who studies physiological responses to heat. Sweat that pools on the skin or drips off “represents dehydration, without any cooling effect,” he said.
Research has shown the human body loses its ability to cool itself via sweating at 95 degrees (35 degrees Celsius) on a scale known as the wet bulb global temperature, which factors in a combination of temperature, humidity, wind speed, sun angle and cloud cover. Unlike the heat index, which rises above the air temperature based on humidity, the wet bulb globe temperature is not designed to be interpreted as a measure of how hot it feels outside.
