The new research shows that Antarctic warming began at least two, and perhaps four, millennia earlier than had been previously believed.
Most previous evidence for Antarctic climate change had come from ice cores drilled in East Antarctica, the highest and coldest part of the continent. However, a U.S.-led research team studying the West Antarctic core found that warming there was well underway 20,000 years ago.
National Science Foundation- (NSF) funded West Antarctic Ice Sheet (WAIS) Divide site is in an area where there is little horizontal flow of the ice, so the data are known to be from a location that remained consistent over long periods.
The WAIS Divide ice core is more than two miles deep and covers a period stretching back 68,000 years, though so far data have been analyzed only from layers going back 30,000 years. Near the surface, one meter of snow is equal to a year of accumulation, but at greater depths the annual layers are compressed to centimeters of ice.
T.J. Fudge, a University of Washington doctoral student in Earth and Space Sciences and lead corresponding author, said that sometimes we think of Antarctica as this passive continent waiting for other things to act on it. But here it is showing changes before it 'knows' what the North is doing.
Fudge identified the annual layers by running two electrodes along the ice core to measure higher electrical conductivity associated with each summer season.
Evidence of greater warming turned up in layers associated with 18,000 to 22,000 years ago, the beginning of the last deglaciation.
Fudge said that this deglaciation is the last big climate change that we're able to go back and investigate. "It teaches us about how our climate system works", he said.
He said that it is not surprising that West Antarctica is showing something different from East Antarctica on long time scales, but we didn't have direct evidence for that before.
Fudge noted that the warming in West Antarctica 20,000 years ago is not explained by a change in the sun's intensity. Instead, how the sun's energy was distributed over the region was a much bigger factor.
He said that it not only warmed the ice sheet but also warmed the Southern Ocean that surrounds Antarctica, particularly during summer months when more sea ice melting could take place.
Fudge explained that changes in Earth's orbit today are not an important factor in the rapid warming that has been observed recently.
He added that Earth's orbit changes on the scale of thousands of years, but carbon dioxide today is changing on the scale of decades so climate change is happening much faster today.
The research has been published online in the journal Nature.
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