Planet-warming carbon dioxide emissions - similar to those caused by burning fossil fuels and other human activities now - helped heat the planet and end the last ice age some 11,700 years ago, scientists reported on Wednesday.
In a finding that offers a response to those skeptical about human-caused global warming, researchers from Harvard University, Oregon State University and other institutions reported in the journal Nature that rising temperatures followed increases in carbon dioxide.
Climate scientists had long suspected this was the case, but the geologic record was murky. Earlier studies looked at air bubbles trapped in ancient ice in Antarctica that revealed carbon dioxide levels in the late Pleistocene Epoch, some 20,000 to 10,000 years ago, the period when the ice age tapered off.
In these previous studies, it appeared that carbon dioxide levels rose after temperatures did, leading climate change skeptics to question whether carbon dioxide was a driver of global warming, then or now.
This latest study managed to look at ice cores and samples of undersea sediments - the deeper the dig, the older the sediment, with biochemical information that indicates temperature variation through time - at 80 sites around the globe.
In Antarctica alone, the earlier studies were confirmed: temperatures there increased before carbon dioxide rose. But globally, a rising amount of carbon dioxide in the air preceded temperature change, according to this new report.
The rise in carbon dioxide over the end of the ice age was significant, from about 180 carbon dioxide molecules for every million in the atmosphere to 260, a measurement called parts per million or ppm, according to study author Jeremy Shakun.
The Big Melt
This increase took place over about 7,000 years, Shakun told a telephone news briefing. By contrast, the current level of atmospheric carbon dioxide is 392 ppm, a rise of about 100 ppm in the last century or two, he said.
"In this century, we're probably going to be going up about 100 (ppm) more," Shakun said, but added that Earth probably won't feel the total impact from this carbon dioxide rise for centuries.
"The system has a lot of inertia to it," he said. "To warm up the oceans takes quite a while, and we've also got ice sheets and you can't melt an ice sheet in 100 years."
But while carbon dioxide pushed temperatures up to accelerate the ice age's end, that was not the initial cause. Instead, the big melt was first prompted by a periodic wobble in the Earth's axis, the scientists said.
At some points in the wobble, the Northern Hemisphere leans slightly closer to the sun and this occurred at the beginning of the end of the Pleistocene, when ice sheets covered much of what is now North America and Europe.
That slight sun-ward tilt melted those northern ice sheets within a few hundred years, pushing global sea levels up by about 33 feet (10 metres), or by more than the total melting of the ice covering Greenland now would do, said co-author Peter Clark of Oregon State University.
Greenland's ice sheet covers most of the island, over 656,000 square miles (1.7 million square kilometres), three times the size of Texas. Summer melt of this ice sheet increased by 30 percent from 1979 through 2006, and reached a new record in 2007, according to the U.S. National Snow and Ice Data Center in Colorado.
Because ice sheets are made of compacted snow, they produce fresh water when they melt, and the gush of fresh water into the salty North Atlantic altered ocean chemistry enough to shut down the Atlantic Merdional Overturning Circulation, sometimes called the conveyor belt, which typically sends heat from the tropics northward, moderating northern Europe's climate.
When the conveyor belt slowed or stopped, cool temperatures stayed in the north and warmth stayed in the south, letting the Antarctic get warmer. That warming trend may also have shifted the winds and melted sea ice, drawing carbon dioxide out of the deep ocean, where quantities of it are stored, Shakun said.