According to the University of Colorado-Boulder, Earth’s ice ages are caused by the wobbling of the planet’s orbit. This wobble adds up to a difference of up to 23.5 degrees to either side of the axis. The amount of tilt in the Earth’s rotation impacts the amount of sunlight hitting the different parts of the planet, like the polar regions. Greater tilt leads to more noticeable differences between summer and winter temperatures.

According to researchers from the University of Washington, the Northern Hemisphere’s last ice age was over approximately 20,000 years ago, and nearly all indications are that the ice age in the Southern Hemisphere was over approximately 2,000 years later, suggesting that the south was responding to warming in the north. New research indicates that Antarctic warming began at least two, possibly four, millennia earlier than previously thought.

Past evidence for Antarctic climate change has come from ice cores drilled in East Antarctica, the largest of Antarctica’s ice sheets. According to the British Antarctic Survey, ice cores are cylinders of ice drilled out of an ice sheet or glacier. The ice confines small bubbles of air that hold a sample of the atmosphere. From these bubbles it is possible to determine directly the past concentration of gases in the atmosphere.

Now, researchers studying a new ice core from West Antarctica found that warming in the area “was well under way 20,000 years ago.”

“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,” said T.J. Fudge, a University of Washington doctoral student in Earth and space sciences, in a statement released by the University of Washington.

The findings are the result of a careful analysis of an ice core acquired from the West Antarctic Ice Sheet Divide, an area where there is little horizontal flow of the ice so that data are perceived to be from a region that stayed consistent over long periods of time.

The ice core acquired from the West Antarctic Ice Sheet Divide is more than two miles deep and accounts for 68,000 years. Up until now, however, data have been studied only from layers going back 30,000 years. Nearer to the surface, one meter of ice accounts for one year, but at greater depths the annual layers are crammed into centimeters.

Fudge identified the annual layers by positioning two electrodes along the ice core to quantify higher electrical conductivity associated with each summer season. Evidence of greater warming revealed itself in layers associated with 18,000 to 22,000 years ago, the beginning of the last deglaciation.

“This deglaciation is the last big climate change that that we’re able to go back and investigate,” Fudge said. “It teaches us about how our climate system works.”

Rapid warming in West Antarctica in recent years has been documented in previous work by Eric Steig, a University of Washington professor of Earth and space sciences. In fact, Steig’s laboratory provided the oxygen isotope data used in the research. The new data verify that West Antarctica’s climate is more vigorously affected by regional conditions in the Southern Ocean than East Antarctica is.

“It’s not surprising that West Antarctica is showing something different from East Antarctica on long time scales, but we didn’t have evidence for that before,” Fudge added.

According to Fudge, the warming in West Antarctica 20,000 years ago is not explained by a change in the Sun’s intensity. How the Sun’s energy was scattered over the region was a much bigger factor. The Sun’s energy not only warmed the ice sheet but also warmed the Southern Ocean that circumscribes Antarctica.

Changes in Earth’s orbit today are not a significant factor in the rapid warming that has been observed recently, Fudge added.

“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,” he concluded.