Help
Quote
The last time the concentration of CO2 was as high as 400 ppm was probably in the Pliocene Epoch, between 2.6 and 5.3 million years ago. Until the 20th century, it certainly hadn't exceeded 300 ppm, let alone 400 ppm, for at least 800,000 years. That's how far back scientists have been able to measure CO2 directly in bubbles of ancient air trapped in Antarctic ice cores.
But tens of millions of years ago, CO2 must have been much higher than it is now—there's no other way to explain how warm the Earth was then. In the Eocene, some 50 million years ago, there were alligators and tapirs on Ellesmere Island, which lies off northern Greenland in the Canadian Arctic. They were living in swampy forests like those in the southeastern United States today. CO2 may have been anywhere from two to ten times higher in the Eocene than it is today. (See related: "Hothouse Earth.")
Over the next 45 million years, most of it was converted to marine limestone, as CO2-laden rains dissolved the ingredients of limestone out of rocks on land and washed them down rivers to the sea. CO2-belching volcanoes failed to keep pace, so the atmospheric level of the gas slowly declined. Some time during the Pliocene, it probably crossed the 400 ppm mark, as it's doing now-but back then it was on its way down. As a result, at the end of the Pliocene, it became cold enough for continental ice sheets to start forming in the northern hemisphere. The Pliocene, says geologist Maureen Raymo of Columbia University's Lamont-Doherty Earth Observatory, "was the last gasp of warmth before the slow slide into the Ice Ages."
What was Earth like then? In Africa, grasslands were replacing forests and our ancestors were climbing down from the trees. (See related: "The Evolutionary Road.") On Ellesmere, there were no longer alligators and cypress trees, but there were beavers and larch trees and horses and giant camels—and not much ice. The planet was three to four degrees Celsius warmer than it was in the 19th century, before man-made global warming began.
If anything, those numbers understate how different the Pliocene climate was. The tropical sea surface was about as warm as it is now, says Alexey Fedorov of Yale University, but the temperature gradient between the tropics and the poles—which drives the jet streams in the mid-latitudes—was much smaller. The east-west gradient across the Pacific Ocean—which drives the El Niño-La Niña oscillation—was almost nonexistent. In effect, the ocean was locked in a permanent El Niño. Global weather patterns would have been completely different in the Pliocene.
And yet the two main drivers of climate—the level of CO2, and the parameters of Earth's orbit, which determine how much sunlight falls where and at what season—were essentially the same as today. Fedorov calls it the Pliocene Paradox.
Climate scientists are just beginning to crack it, he says. Maybe clouds outside the tropics were darker in the Pliocene, such that they bounced less sunlight back to space. Maybe the warm ocean was stirred by a lot more hurricanes.
Hanging over this academic research is a very nonacademic issue: Could our climate be capable of flipping to a completely different state? "That's the big question—whether CO2 can move us to the Pliocene," says Fedorov.
But tens of millions of years ago, CO2 must have been much higher than it is now—there's no other way to explain how warm the Earth was then. In the Eocene, some 50 million years ago, there were alligators and tapirs on Ellesmere Island, which lies off northern Greenland in the Canadian Arctic. They were living in swampy forests like those in the southeastern United States today. CO2 may have been anywhere from two to ten times higher in the Eocene than it is today. (See related: "Hothouse Earth.")
Over the next 45 million years, most of it was converted to marine limestone, as CO2-laden rains dissolved the ingredients of limestone out of rocks on land and washed them down rivers to the sea. CO2-belching volcanoes failed to keep pace, so the atmospheric level of the gas slowly declined. Some time during the Pliocene, it probably crossed the 400 ppm mark, as it's doing now-but back then it was on its way down. As a result, at the end of the Pliocene, it became cold enough for continental ice sheets to start forming in the northern hemisphere. The Pliocene, says geologist Maureen Raymo of Columbia University's Lamont-Doherty Earth Observatory, "was the last gasp of warmth before the slow slide into the Ice Ages."
What was Earth like then? In Africa, grasslands were replacing forests and our ancestors were climbing down from the trees. (See related: "The Evolutionary Road.") On Ellesmere, there were no longer alligators and cypress trees, but there were beavers and larch trees and horses and giant camels—and not much ice. The planet was three to four degrees Celsius warmer than it was in the 19th century, before man-made global warming began.
If anything, those numbers understate how different the Pliocene climate was. The tropical sea surface was about as warm as it is now, says Alexey Fedorov of Yale University, but the temperature gradient between the tropics and the poles—which drives the jet streams in the mid-latitudes—was much smaller. The east-west gradient across the Pacific Ocean—which drives the El Niño-La Niña oscillation—was almost nonexistent. In effect, the ocean was locked in a permanent El Niño. Global weather patterns would have been completely different in the Pliocene.
And yet the two main drivers of climate—the level of CO2, and the parameters of Earth's orbit, which determine how much sunlight falls where and at what season—were essentially the same as today. Fedorov calls it the Pliocene Paradox.
Climate scientists are just beginning to crack it, he says. Maybe clouds outside the tropics were darker in the Pliocene, such that they bounced less sunlight back to space. Maybe the warm ocean was stirred by a lot more hurricanes.
Hanging over this academic research is a very nonacademic issue: Could our climate be capable of flipping to a completely different state? "That's the big question—whether CO2 can move us to the Pliocene," says Fedorov.
Conservatives do not want to take that risk, even if most of the damage will be born by future generations.
