The Last Time Our Planet Was This Hot, Woolly Mammoths Roamed the Earth

If you could go back to the Eemian period—from 116,000 to 129,000 years ago—you’d feel right at home. OK, the woolly mammoths lumbering about might take you aback, as might the hippopotami roaming freely across what would one day be the streets of Europe. But when it comes to climate, things would not be all that different. Mean global temperatures today are about 1ºC warmer than they were in the pre-industrial era, leading to the extreme weather and other events we’ve been experiencing: heat waves, wildfires, droughts, floods, super storms, savage hurricanes, and more.

In the Eemian, things were warmer still, close to 2ºC hotter than in the pre-industrial era, surely leading to even more severe conditions. Individual weather events like hurricanes are too brief to be preserved in the so-called climate archive that Earth scientists use to study climate history, particularly deep cores drilled from ice sheets, the ocean floor, lake silt, and the land. But computer models coupled with the data from the cores do suggest a turbulent Eemian.

“We are not exclusively tied to the climate archive,“ says Syee Weldeab, professor of Earth Science at the University of California, Santa Barbara. “We can run [computer] models that change [the weather] as we increase the energy in the atmosphere and the ocean.”

One study in Research Gate found that Eemian hurricanes were stronger and more northerly than those observed today—even increasing the incidence of winter storms, which lasted well beyond the contemporary hurricane season. Another, in Scientific Reports, found Eemian droughts and brush fires in Australia that lasted multiple centuries at a time. Yet more research in the Proceedings of the National Academy of Sciences reports that the Eemian was characterized by “‘superstorms’ more intense than any observed historically.” If the Eemian is Earth’s past, it is also Earth’s portent—a potential warning of the kind of climatological upheaval we face if we allow our global temperature to creep past the 1ºC threshold to the 2ºC that defined the Eemian.

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No matter how violent the Eemian was, planetary scientists today are alarmed that our current era marks the warmest the planet has been since a period that occurred so long ago. After all, it took the Eemian more than 16 millennia to unfold and fade. Human-caused climate change required less than 300 years—since the dawn of the fossil-fueled industrial age in 1760—to cause such disruption in the one world we’ve got.

“It’s amazing,” says Gifford Miller, distinguished professor emeritus of geological sciences at the University of Colorado, Boulder. “I think a lot of people struggle to imagine that us little, tiny human beings can actually alter the energy balance so much that it will fundamentally change the climate.”

Why the Earth Ran a Fever

Unlike contemporary climate change, the global warming and knock-on weather effects of the Eemian had little to do with greenhouse gasses in the atmosphere. Analysis of the archive cores indicates that the concentration of carbon dioxide and other greenhouse gasses in the atmosphere back then was about 280 parts per million (ppm), according to Miller. Today, the National Oceanic and Atmospheric Administration puts the figure at an alarming 417.06 ppm. Even if humans turned off the CO2 spigot today, the emissions already in the system would continue to warm the world for decades.

So if greenhouse gasses were little more than a bit player in the extreme warming of the Eemian, what was responsible? The answer is the angle of the Earth and the relative positions of the planet and the sun. Earth does not spin evenly around its axis, but rather, can wobble like a top—a process called precession. At the beginning of the Eemian, that wobble pointed the North Pole toward the sun, slightly increasing the 23.7 degree angle the Earth usually maintains, and exposing the northern hemisphere to more sunlight than it would usually get.

“The north leaned closer to the sun,” says Miller, ”and there was about 9% more solar energy being absorbed by the planet.”

Then too, there was the proximity of the Earth and the sun. The average distance between the two bodies is 150 million km (93 million mi.). But that figure changes over the course of the year. Once every 12 months, the Earth reaches what is known as its aphelion—or furthest approach—drifting out to about 150 million km (94.5 million mi.). Six months later, it reaches its perihelion, drawing closer to 147 million km (91.4 million mi.).

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But eccentricities in the Earth’s orbit can sometimes disturb this cycle. Periodically the planet will linger close to the perihelion distance—generally for a few thousand years or so. A handful of millennia are nothing on a cosmic scale, but, as with hemispheric tilt, the phenomenon can dramatically affect energy absorption. “Those two combinations,” says Miller, “a higher tilt and being closer to the sun resulted in an overall increase of 12% of the sun’s energy received.”

The Parallels Between Then and Now

That 12% made a big difference in a lot of ways similar to the ones we’re seeing with contemporary global warming. ​​For starters, there’s the oceans, which absorb enormous amounts of heat and evaporate more water vapor in the process—a sort of feedback loop since water vapor is itself a potent greenhouse gas.

Northward migration of plant life exacerbated climate change in the Eemian too, something that ancient, preserved DNA from the Canadian Arctic revealed—and something that’s happening today as well. At temperatures warm, Arctic areas that once weren’t hospitable to trees begin to support them. Leaf canopies cover up bright, white snow, which would ordinarily reflect sunlight back into space. Instead the leaves absorb the heat, warming up the Arctic forests and causing them, like the oceans, to release temperature-raising water vapor into the atmosphere.

Then too there is the lesser-known matter of methane hydrates—again both an Eemian and likely contemporary problem. A combination of methane and water, methane hydrates usually remain in a frozen state in the deep ocean. As the ocean warms, however, the deposits thaw and separate, releasing the methane alone—another powerful greenhouse gas, allowing it to rise up in the water and escape into the atmosphere.

“There’s a long-term climatic feedback process in this,” says Weldeab, “something that amplifies the warming.”

The Eemian ultimately came to an end after the Earth straightened its tilt a little and returned to its usual aphelion-perihelion cycle; by the time that happened—13,000 years after the Eemian began—the ice from the prior glaciation had been lost. Compare that slow melt to the mere decades it’s taken human-induced climate change to do such damage to vast expanses of ice in Greenland and Antarctica and create the likelihood of an ice-free Arctic summer as early as 2030. Overall, humans have needed just 263 years, since the dawn of the Industrial Age, to create their own overheated Eemian. Unlike the last one, there is no natural process like the realignment of the planet that will step in and set things to rights. We made the mess—and it’s ours to clean up.