The eight years between 2015 and 2022 were the hottest on record. In 2022, 850 million people lived in regions that experienced all-time high temperatures. Globally, killer heat waves are becoming longer, hotter, and more frequent. One study found that a heat wave like the one that cooked the Pacific Northwest in 2021 is 150 times more likely today than it was before we began the atmosphere with CO2 at the beginning of the industrial age.
Just look at the events of this year: wildfire smoke from Canada turned the skies on the east coast an apocalyptic orange; sea ice in Antarctica hit a record low; all-time temperature records were shattered in Puerto Rico, Siberia, Southeast Asia, Mexico, and Texas (I live in Austin, where, as I write this in late June, it’s 106 degrees F). In the North Atlantic ocean, sea surface temperatures in late June are the highest ever recorded.
The truth is, extreme heat is remaking our planet into one in which large swaths may become inhospitable to human life. One recent study projected that over the next fifty years, one to three billion people will be left outside the climate conditions that gave rise to civilization over the last six thousand years. Even if we transition fairly quickly to clean energy, half of the world’s human population will be exposed to life-threatening combinations of heat and humidity by 2100. Temperatures in parts of the world could rise so high that just stepping outside for a few hours, another study warned, “will result in death even for the fittest of humans.”
Life on Earth is like a finely calibrated machine, one that has been built by evolution to work very well within its design parameters. Heat breaks that machine in a fundamental way, disrupting how cells function, how proteins unfold, how molecules move. Yes, some organisms can thrive in higher temperatures than others. Roadrunners do better than blue jays. Silver Saharan ants can run across superhot desert sands that would kill other insects instantly. Microbes live in 170-degree hot springs in Yellowstone National Park. A thirty-year-old triathlete can handle a 110-degree day better than a seventy-year-old man with heart disease. And yes, we humans are remarkable creatures with a tremendous capacity to adapt and adjust to a rapidly changing world.
But to understand the dangers of extreme heat today, it helps to understand how we have lived with heat in the past. Among other things, we evolved clever ways to manage the heating and cooling of our bodies that gave our ancestors an evolutionary edge over competitors. To tell you about it, though, I have to go way back, because you can’t separate heat from the beginning of things.
Fourteen billion years ago, the universe was compressed into a stupendously hot, incredibly dense nugget, which then rapidly expanded. This nugget cooled as it swelled; its particles gradually slowed their frenzied motion and aggregated into clumps, which over time formed stars, planets — and us. How exactly life emerged out of the hot mess of the universe is only dimly understood. The most widely accepted theory is that life began around the volcanoes that rose above the ocean shortly after the earth formed, probably within the first hundred million years. The volcanoes were surrounded by hot geyser‑fed ponds and bubbling hot springs, which were loaded with organic compounds from the asteroids and meteors that bombarded the planet. Volcanoes acted as chemical reactors, creating a hot volcanic soup. Somehow, RNA molecules grew, eventually growing longer and more complex and folding into true proteins and double‑stranded DNA. They formed microbes that floated in thick mats on the volcanic ponds. When the ponds dried out, winds picked up their spores and spread them for miles. Rains eventually washed microbes into the ocean.
Evolution’s next trick was developing a way for animals to deal with temperature fluctuations. In the long arc of evolution, two strategies have emerged: one is to let your body’s temperature change with the temperature around it, which is what creatures did for the first three and a half billion years or so. If necessary, these animals warm themselves by basking in sunlight or sitting on warm rocks. This heat management strategy survives today in fish, frogs, lizards, alligators, and all the reptiles and amphibians. Scientists call them ectotherms; you and I call them cold‑blooded.
But around 260 million years ago, a new heat management strategy emerged. Some animals found a way to control their own internal temperature that was not dependent on the temperature of their environment. In effect, it turns their bodies into little heat engines, allowing them to operate independently of the world outside — as long as they can maintain a steady temperature inside. This heat management strategy remains alive and well in animals that scientists call endotherms but that everyone else calls warm‑blooded: dogs, cats, whales, tigers, and virtually every other mammal on the planet, including us. Birds, which are flying dinosaurs, are also warm‑blooded.
The birth of warm‑bloodedness was an evolutionary leap, and one that scientists still don’t fully understand. For one thing, the traits of warm‑bloodedness do not transfer well to fossils, so you can’t just look at the bones of a long‑ago creature and determine whether it was warm‑ or cold‑blooded. For another, the transition from cold‑bloodedness to warm‑ bloodedness didn’t happen with a single quick jump. Many species — especially dinosaurs — had attributes of both.
A Guest Editorial