Thursday, 5 April 2012
Carbon dioxide was the big driver that ended the last Ice Age, according to a new study of ice core data from around the world.
About 10,000-20,000 years ago, Earth started to emerge from a quarter million years of deep freeze as the terrestrial ice sheet rolled back and warmer temperatures prevailed.
What caused the end of this age, known as the Pleistocene, has long been debated.
Until now, the main evidence has come from ice cores drilled in Antarctica whose air bubbles are a tiny time capsule of our climate past.
Traces of CO2 in Antarctic ice show that carbon concentrations in the atmosphere rose after temperatures were on the rise.
This timing has been used by sceptics as proof that man-made carbon gases either do not cause global warming or at least do not make it as bad as mainstream scientists say.
But the new study, published in Nature , indicates that the Antarctic record doesn’t reflect global temperature rise.
The study is based on 80 ice cores and sedimentary samples taken from Greenland, lake bottoms and sea floors on every continent.
The data suggests that while changes in CO2 concentration did not trigger deglaciation, they were either synchronous with, or led global warming during the various steps of deglaciation.
“Putting all of these records together into a reconstruction of global temperatures shows a beautiful correlation with rising CO2 at the end of the Ice Age,” says lead author Dr Jeremy Shakun of Harvard University.
A rise in carbon dioxide “actually precedes global temperature range, which is what you would expect if CO2 is causing the warming.”
The study refutes theories by sceptics that natural changes in Earth’s orbit, bringing the planet closer to the Sun, caused the warming, not a rise in CO2.
“Our study shows that CO2 was a much more important factor and was really driving worldwide warming during the last deglaciation,” says Shakun.
“Orbital changes are the pacemaker. They’re the trigger, but they don’t get you too far.”
Shakun and colleagues theorise that orbital shift boosted sunlight that warmed the northern hemisphere between 21,500 and 19,000 years ago, causing some of its icesheet to melt and spill gigatonnes of chilly freshwater into the North Atlantic.
The big gush had a dampening effect on the Atlantic meridional overturning circulation, a well-known ‘conveyor belt’ of current by which warm water travels northwards on the surface of the Atlantic before cooling and returning southwards at depth.
When the current braked, warm water began to build up in the southern Atlantic, where it swiftly started to warm up Antarctica and the Southern Ocean.
Warming the south in turn shifted the wind and melted sea ice, releasing some of the vast amounts of CO2 that had been absorbed by the ocean and stored in its depths, according to their hypothesis.
“CO2 was a big part of bringing the world out of the last Ice Age and it took about 10,000 years to do it,” says Shakun.
“Now CO2 levels are rising again, but this time an equivalent increase in CO2 has occurred in only about 200 years, and there are clear signs that the planet is already beginning to respond.”
Shakun and colleagues work provides a “firm data-driven basis for a plausible chain of events for most of the last termination,” writes Eric W Wolff from the British Antarctic Survey in an accompanying commentary in the same issue of Nature.
But Wolff says the theory that warming was initially triggered by orbital shift should be “treated with caution”.
He says there are few temperature records in the northernmost latitude band. There are only records for Greenland, which shows warming also occurred between 62,000 and 60,000 years ago that didn’t lead to deglaciation.
“In short another ingredient is needed to explain the link between insolation and termination, and the triggers for the series of events.”
That ingredient could be the extent of the ice sheet, he says.
“It has been proposed that terminations occur only when northern ice-sheet extent is particularly large,” he writes.