PARIS • Up till now, scientists have disagreed over the cause of the Ice Age glacier decline, with some attributing it mainly to solar radiation and regional influences such as ice sheets and ocean currents.
But the authors of a new study say that their results are unequivocal. An increase in greenhouse gases was the driving force behind global glacier retreat at the end of the last Ice Age, echoing current climate change, the authors say.
Using new techniques, the researchers showed that it was a 55 per cent increase of carbon dioxide in the atmosphere - from 180 to 280 parts per million (ppm) - over some 7,000 years that melted the glaciers to a level that remained stable till the start of the industrial era.
Higher levels of greenhouse gasses, such as carbon dioxide, trap more of the Sun's heat on Earth, causing global temperatures to rise.
Modern carbon dioxide levels have risen much more rapidly, from 280 ppm at the start of the Industrial Revolution to 400 ppm today. Since 1880, average global temperatures have risen by 0.8 deg C and the rate of warming is increasing.
The team of scientists, led by Dr Jeremy Shakun of Boston College, re-examined the ages of more than 1,100 previously studied glacial boulders by measuring a particular isotope - beryllium-10 - produced by exposure to cosmic rays.
They compared their findings, the most accurate so far, to the timing of the rise of carbon dioxide in the atmosphere, data that can be gleaned from ice bubbles trapped in ice cores.
"The only factor that explains glaciers melting all around the world in unison during the end of the Ice Age is the rise in greenhouse gases," said Dr Shakun.
More than 11,000 years later, the researchers say, global warming is on track to wipe out 80 per cent to 90 per cent of the remaining glaciers within a few hundred years unless carbon dioxide emissions are held in check.
Rapid melting would raise sea levels and rob hundreds of millions of people in Asia and South America of a critical source of water.
Scientists are still not sure what triggered the gradual release of carbon dioxide into the atmosphere starting 19,000 years ago.
"It's fair to say that the reason carbon dioxide went up and down over the Ice Ages is one of the biggest palaeo-climate mysteries out there," Dr Shakun said by e-mail.
The most likely scenario, according to co-author Peter Clark of Oregon State University, is that huge quantities of carbon dioxide bubbled up from the sea.
"The carbon was likely released because of changes in the ocean and its circulation that were triggered by changes in the Earth's orbit around the Sun," he said in an e-mail exchange.
The impact of greenhouse gas emissions on glaciers today, both scientists say, is similar to the Ice Age endgame, except that - after 10,000 years of relative stability - the process is happening 50 times faster.