A simple interpretation of Antarctic ice-core records, the one preferred by global warming skeptics, portrays carbon dioxide as a weak, even inconsequential player in the warming that brought the world out of the last ice age. But the first continuous, near-global record of temperature as the last ice age ended now shows that carbon dioxide did indeed help warm the world.
The issue with Antarctic ice cores was that they recorded a rise in temperature ahead of the rise in carbon dioxide. How could the greenhouse gas be causing the warming, skeptics asked, if it wasn't in the atmosphere when the warming started?
But climate scientists know that no one region is representative of global climate trends. So Jeremy Shakun of Harvard University and his colleagues created a global temperature record. They combined 80 records of temperature over the past 22,000 years retrieved from around the world, ranging in latitude from Antarctica to Greenland. The seven types of records included ice cores whose oxygen isotopes record varying temperature. There were also pollen from lake muds and microfossils from ocean sediments, whose species and abundance reflect temperature.
Once a globally representative record came together, the data clearly showed carbon dioxide rising ahead of rising temperature, as it should if the greenhouse gas were helping drive the world out of the ice age. The warming of Antarctica ahead of carbon dioxide's rise was a red herring, Shakun and his colleagues conclude online today in Nature.
To see why, the researchers drew on a climate model as well as a variety of other climate records. They saw changes in the far north that triggered southward-marching changes in ocean and atmospheric circulation that eventually reached Antarctica. The immediate effect? There was an early warming as South Atlantic currents that normally carry heat away to the north stalled. But that warming came before the same changes triggered the release of much carbon dioxide from the deep ocean. As a result, Antarctic warming got a jump on the rest of the world, but carbon dioxide went on to warm the globe as a whole.
The new global temperature record "is quite an achievement," says Eric Wolff, a paleoclimatologist at the British Antarctic Survey in Cambridge, the United Kingdom. The early Antarctic warming "has been a thorn in the side of climate scientists," he says, but "one doesn't have to deal with that issue anymore."