As heat waves blanket the U.S. this summer, temperature records have been spiking. Earlier this week, the global average temperature reached just over 62°F. It was the hottest average temperature ever recorded by the National Centers for Environmental Prediction since the organization began keeping records in 1979.
But Earth has seen warmer days in its past and it will experience them again in the future. During so-called hothouse periods, when the atmosphere was supercharged with greenhouse gases, the planet was much warmer than it is today and the worst heat waves were correspondingly nightmarish. And while human carbon emissions haven’t pushed Earth into a new hothouse state yet, climate change is making heat waves more frequent and severe. Earth won’t be as scorching and uninhabitable as Venus anytime soon—temperatures there are hot enough to melt lead—but heat that challenges the limits of human tolerance will occur more often as the century wears on, scientists say.
And in the very, very distant future, Earth might actually become like Venus.
The scorching past
Though it may not feel like it, Earth is currently in what geologists consider an icehouse climate: a period cold enough to support an ice-age cycle, in which large continental ice sheets wax and wane near the poles. (Right now the one in the northern hemisphere has retreated to Greenland.) To get a glimpse of what a much warmer world would look like, we need to go back at least 50 million years to the early Eocene.
“That was sort of the last really warm climate the Earth experienced,” says Jessica Tierney, a paleoclimatologist at the University of Arizona.
Today, Earth’s average temperature hovers around 60 degrees Fahrenheit. During the early Eocene, it was closer to 70 degrees and the world was a different place. The poles were free of ice; the tropical oceans simmered at spa-like temperatures of 95 degrees Fahrenheit. Palm trees and crocodiles hung out in the Arctic. Several million years before that, at the Paleocene-Eocene Thermal Maximum (PETM), things were even warmer.
More extreme hothouse periods lurk in the deeper recesses of geologic time. During the Cretaceous Hot Greenhouse 92 million years ago, global surface temperatures rose to around 85 degrees Fahrenheit and remained hot for millions of years, allowing temperate rainforests to flourish near the South Pole. Some 250 million years ago, the boundary between the Permian and the Triassic period is marked by an extreme global heating event where Earth's average temperature flirted with 90 degrees Fahrenheit for millions of years, according to a preliminary reconstruction from the Smithsonian Institution.
In that hellish interval, Earth experienced the worst die-off of life in its history. The tropical oceans were like a hot tub. We don’t have daily weather data from the Permian (or any other ancient chapter in Earth’s history), but it’s likely that in the vast, dry interior of the supercontinent Pangea this week’s Death Valley heat wave would have been just another day.
“The warmer these average conditions are, the more often you’ll see really extreme heat events,” Tierney says. On the hottest days during the hottest times, “places like a desert would just be unbelievably hot.”
The warming future
All of Earth’s recent hothouse periods seem to have one thing in common: They were preceded by a massive pulse of greenhouse gases into the atmosphere, whether that was volcanic eruptions spewing carbon dioxide or methane bubbling up from beneath the seabed. Humans are conducting a similar planetary experiment today by burning through enormous reserves of fossil carbon, raising atmospheric carbon dioxide levels at a rate unseen since the extinction of the dinosaurs, 65 million years ago, and perhaps far earlier.
“Usually when we see a rapid change in climate [in the past], it’s driven by similar mechanisms to what we’re doing today,” says MIT earth scientist Kristin Bergmann. “There’s a fairly quick change in the greenhouse gases that warm our planet.”
As in the past, global average temperatures are once again rising quickly. And extremely hot days are also on the uptick, with study after study concluding that recent record-breaking temperatures would have been nearly impossible without our influence.
It’s difficult to forecast exactly how hot Earth might get if we keep jamming carbon into the atmosphere, experts say. As Michael Wehner, an extreme weather researcher at Lawrence Berkeley National Laboratory, put it in an email: “The increase in temperatures of future heat waves depends a lot on how far into the future and how much more carbon dioxide we emit.”
But recent research by Wehner and his colleagues offers a peek into what the heat waves of tomorrow could look like if we don’t curb our carbon emissions at all: By the end of the century, heat waves in California could top out at temperatures about 10 to 14 degrees Fahrenheit higher than they do today.
A Venus-like fate?
If you’re a nihilist, you might point out that all of this is peanuts compared with what Earth will likely experience in the far future. Planetary scientists have long predicted that as the sun ages and grows brighter, Earth’s surface will eventually heat up to the point where the oceans start to simmer like water on a stove. Water vapor, a potent greenhouse gas, will pour into the atmosphere, triggering a runaway greenhouse effect that, in a billion years, could transform our world into something not unlike our neighbor, Venus. There, beneath a thick, toxic, and sulfurous atmosphere, surface temperatures are close to 900F.
“The assumption has been as the sun continues to brighten, the same thing will happen on Earth,” says North Carolina State university planetary scientist Paul Byrne, adding that billions of years ago, our planetary neighbor might have had an agreeable climate and oceans.
Venus might not have ruined by the sun at all. Recent modeling work suggests that the culprit might have been a series of volcanic paroxysms that caused “biblical releases of CO2 into the atmosphere,” Byrne says. But either scenario—planetary heat death by the sun or by volcanoes—points to a way that events far beyond our control might send Earth’s future climate into a harrowingly hot tailspin.
“Whether it’s going to be exactly 475 degrees Celsius or not I don’t know,” Byrne says, referring to the temperature at Venus’s surface. But if Earth goes through a Venus-like transition, “it will be really, really hot.”
Even if our Blue Marble manages to escape Venus’ fate, there’s no avoiding getting blow-torched in about five billion years. At that time, the sun will expand into a red giant star, subsuming the Earth in a fiery blaze.
“The prevailing view is that the sun will swallow earth,” Byrne says. “We’re getting [expletive deleted].”