Meltdown: Why ice ages don't last forever

BACK in 1993, a boy playing football near Nanjing, China, suddenly fell through the ground. He had inadvertently found a new cave, later named Hulu, which has turned out to be a scientific treasure chest. Besides two Homo erectus skeletons, it contains stalagmites that have helped solve one of the greatest mysteries in climate science: why the ice ages came and went when they did.

For more than 2 million years, Earth's climate has been oscillating wildly. Immense ice sheets slowly advance across northern lands, then suddenly melt away to leave the planet basking in a relatively brief period of warmth before the ice creeps back again. Climate scientists have long suspected that these glacial cycles are triggered by changes in our planet's orbit. Yet while this theory has had many successes, it fails to explain one critical fact: why the ice ages end every 100,000 years or so. "It's a big problem," says Larry Edwards of the University of Minnesota in Minneapolis.

Edwards is part of a group of researchers who may finally have the answer, thanks to Hulu and other nearby caves. If their conclusions are right, then the greatest ice sheets of the past were remarkably vulnerable, melting away when there was just a glimmer of extra sunlight.

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This was far from the end of the story, though. We now know that the polar ice caps started to form around 30 million years ago, as carbon dioxide levels fell. Around 2.5 million years ago, as it got colder still, a cycle began in which more extensive ice sheets repeatedly spread across the northern hemisphere and then retreated. At first, these ice ages were relatively minor and occurred roughly every 41,000 years - just as you would expect based on the changing tilt of Earth's axis.

But then, a little less than a million years ago, the pattern changed. A series of much more severe ice ages began that lasted 100,000 years. That is a big mystery, because although the shape of the Earth's orbit alters slightly over periods of 95,000 and 125,000 years, this has a far weaker effect on the seasons than the other orbital cycles. Why would the deepest ice ages be driven by the smallest changes in summer sunshine?

Faced with this conundrum, some researchers began to explore alternatives to the mainstream orbital theory. One idea is that Earth sometimes passes through interplanetary dust clouds that cut off some of the sun's heat. Or perhaps our star could be periodically getting brighter and dimmer.

Studies of ice cores from Antarctica, however, were starting to point in a different direction. The cores showed there was a close correlation between temperature and the levels of greenhouse gases in the atmosphere. This suggested a partial answer to the 100,000-year problem: small changes in sunshine might be greatly amplified by rises in CO₂ levels. But there was too much uncertainty about the timing of events to say what caused what.

To find out what really happened, researchers need accurate dates, especially for the ends of the ice ages. "I've been after the timing of these terminations for 25 years," says Edwards. While marine sediments and ice cores record the sequence of events, it is difficult to date those events precisely. ...

The stalagmites hold indirect clues to the climate in the form of oxygen isotopes, which record the strength of the summer monsoon. Water containing heavy oxygen condenses more easily, so the moisture-laden air of the monsoon loses most of its oxygen-18 as it moves inland. By the time it reaches central China, the rains are low in oxygen-18, and the stalagmites there record this depletion. But as the last ice age was ending, 11,000 to 17,000 years ago, the oxygen-18 content of the stalagmites increased - a sign that summer monsoon rains were much weaker than usual.

Wang then went looking for a cave with older stalagmites. He struck lucky at nearby Linzhu cave, despite a rather unusual hazard. "Bats stole our guide rope," says Wang. "The cave has many branches, and we lost our way out." When his team did eventually escape, around midnight, they brought out samples that held a much longer climate record. And stalagmites from another nearby cave called Sanbao provided even more precise dates.

... the new evidence points to a coherent story. Ice sheets build up until they near the brink of stability, at which point a modest rise in summer sunshine is enough to tip them over the edge. As the ice sheets melt, fresh water is released into the Atlantic, shutting down ocean circulation and pumping CO₂ into the atmosphere. As long as the combined effect of extra summer sunshine and rising CO₂ outweighs the regional cooling produced by the shutdown of ocean circulation, the ice keeps melting, pouring more fresh water into the Atlantic. And the melting of a really large ice sheet keeps ocean circulation shut down for a long time, eventually pumping so much CO₂ into the atmosphere that the ice sheets melt away in just a few thousand years. ...

via Meltdown: Why ice ages don't last forever - environment - 24 May 2010 - New Scientist.