Earth has been through this before.
Not the same planetary fever exactly; it was a different world the last time, around 56 million years ago. The Atlantic Ocean had not fully opened, and animals, including perhaps our primate ancestors, could walk from Asia through Europe and across Greenland to North America. They wouldn't have encountered a speck of ice; even before the events we're talking about, Earth was already much warmer than it is today. But as the Paleocene epoch gave way to the Eocene, it was about to get much warmer still—rapidly, radically warmer.
The cause was a massive and geologically sudden release of carbon. Just how much carbon was injected into the atmosphere during the Paleocene-Eocene Thermal Maximum, or PETM, as scientists now call the fever period, is uncertain. But they estimate it was roughly the amount that would be injected today if human beings burned through all the Earth's reserves of coal, oil, and natural gas. The PETM lasted more than 150,000 years, until the excess carbon was reabsorbed. It brought on drought, floods, insect plagues, and a few extinctions. Life on Earth survived—indeed, it prospered—but it was drastically different. Today the evolutionary consequences of that distant carbon spike are all around us; in fact they include us. Now we ourselves are repeating the experiment.
The PETM "is a model for what we're staring at—a model for what we're doing by playing with the atmosphere," says Philip Gingerich, a vertebrate paleontologist at the University of Michigan. "It's the idea of triggering something that runs away from you and takes a hundred thousand years to reequilibrate."
Gingerich and other paleontologists discovered the profound evolutionary change at the end of the Paleocene long before its cause was traced to carbon. For 40 years now Gingerich has been hunting fossils from the period in the Bighorn Basin, a hundred-mile-long arid plateau just east of Yellowstone National Park in northern Wyoming. Mostly he digs into the flanks of a long, narrow mesa called Polecat Bench, which juts into the northern edge of the basin. Polecat has become his second home: He owns a small farmhouse within sight of it.
One summer afternoon Gingerich and I drove in his sky blue '78 Suburban up a dirt track to the top of the bench and on out to its southern tip, which affords a fine view of the irrigated fields and scattered oil wells that surround it. During the recent ice ages, he explained, Polecat Bench was the bed of the Shoshone River, which paved it with cobbles. At some point the river shifted east and began cutting its way down through the softer and more ancient sediments that fill the Bighorn Basin. Meanwhile the Clark's Fork of the Yellowstone River was doing the same to the west. Polecat Bench now stands between the two rivers, rising 500 feet above their valleys. Over the millennia its flanks have been sculpted by winter wind and summer gully washers into rugged badlands, exposing a layer cake of sediments. Sediments from the PETM are exposed right at the very southern tip of the bench.
It is here that Gingerich has documented a great mammalian explosion. Halfway down the slope a band of red sediment, about a hundred feet thick, wraps around the folds and gullies, vivid as the stripe on a candy cane. In that band Gingerich discovered fossils of the oldest odd-toed hoofed mammals, even-toed hoofed mammals, and true primates: in other words, the first members of the orders that now include, respectively, horses, cows, and humans. Similar fossils have since been found in Asia and Europe. They appear everywhere, and as if out of nowhere. Nine million years after an asteroid slammed into the Yucatán Peninsula, setting off a cataclysm that most scientists now believe wiped out the dinosaurs, the Earth seems to have undergone another shock to the system.
During the first two decades that Gingerich labored to document the Paleocene-Eocene transition, most scientists saw it simply as a time when one set of fossils gave way to another. That perception started to change in 1991, when two oceanographers, James Kennett and Lowell Stott, analyzed carbon isotopes—different forms of the carbon atom—in a sediment core extracted from the Atlantic seafloor near Antarctica. Right at the Paleocene-Eocene boundary a dramatic shift in the ratio of isotopes in fossils of minuscule organisms called foraminifera (forams for short) indicated that an immense amount of "fresh" carbon had flooded into the ocean in as little as a few centuries. It would have spread into the atmosphere too, and there, as carbon dioxide, it would have trapped solar heat and warmed the planet. Oxygen isotopes in the forams indicated that the whole ocean had warmed, from the surface right down to the bottom mud, where most of the forams lived.
In the early 1990s the same signs of a planetary convulsion began turning up on Polecat Bench. Two young scientists, Paul Koch of the Carnegie Institution and James Zachos, then at the University of Michigan, collected half-inch clumps of carbonate-rich soil from each of the sediment layers. They also collected teeth of a primitive mammal called Phenacodus. When Koch and Zachos analyzed the carbon isotope ratios in the soil and the tooth enamel, they found the same carbon spike seen in the forams. It was becoming clear that the PETM had been a global warming episode that had affected not just obscure sea organisms but also big, charismatic land animals. And scientists saw that they could use the carbon spike—the telltale stamp of a global greenhouse gas release—to identify the PETM in rocks all over the world.
Where did all the carbon come from? We know the source of the excess carbon now pouring into the atmosphere: us. But there were no humans around 56 million years ago, much less cars and power plants. Many sources have been suggested for the PETM carbon spike, and given the amount of carbon, it likely came from more than one. At the end of the Paleocene, Europe and Greenland were pulling apart and opening the North Atlantic, resulting in massive volcanic eruptions that could have cooked carbon dioxide out of organic sediments on the seafloor, though probably not fast enough to explain the isotope spikes. Wildfires might have burned through Paleocene peat deposits, although so far soot from such fires has not turned up in sediment cores. A giant comet smashing into carbonate rocks also could have released a lot of carbon very quickly, but as yet there is no direct evidence of such an impact.
The oldest and still the most popular hypothesis is that much of the carbon came from large deposits of methane hydrate, a peculiar, icelike compound that consists of water molecules forming a cage around a single molecule of methane. Hydrates are stable only in a narrow band of cold temperatures and high pressures; large deposits of them are found today under the Arctic tundra and under the seafloor, on the slopes that link the continental shelves to the deep abyssal plains. At the PETM an initial warming from somewhere—perhaps the volcanoes, perhaps slight fluctuations in Earth's orbit that exposed parts of it to more sunlight—might have melted hydrates and allowed methane molecules to slip from their cages and bubble into the atmosphere.
The hypothesis is alarming. Methane in the atmosphere warms the Earth over 20 times more per molecule than carbon dioxide does, then after a decade or two, it oxidizes to CO2 and keeps on warming for a long time. Many scientists think just that kind of scenario might occur today: The warming caused by the burning of fossil fuels could trigger a runaway release of methane from the deep sea and the frozen north.
Koch and Zachos concluded from their data that the PETM had lifted the annual average temperature in the Bighorn Basin by around nine degrees Fahrenheit. That's more than the warming there since the last ice age. It's also a bit more than what climate models predict there for the 21st century—but not more than what they forecast for the centuries to come if humans keep burning fossil fuels. Models also predict severe disruptions in the world's rainfall patterns, even in this century, especially in subtropical regions like the American Southwest. But how to test the models? "You can't wait 100 or 200 years to see what happened," says Swedish geologist Birger Schmitz, who has spent a decade studying PETM rocks in the Spanish Pyrenees. "That's what makes the PETM story so interesting. You have the end result. You can see what did happen."