Ancient ice cores indicate air temperatures in Antarctica rose up to 18 degrees Fahrenheit in just a few decades as the last ice age began to wane some 19,000 years ago, the largest and most abrupt warming spike ever recorded in the Southern Hemisphere.
University of Colorado at Boulder Associate Professor James White said the strong, rapid event belies previous evidence that Antarctic warming events were significantly more gradual than those documented from Greenland ice cores during the last deglaciation. "This Antarctic event is the first to rival a number of strong warming events in the Northern Hemisphere in both temperature and speed during the interglacial transition," said White, a fellow at CU’s Institute of Arctic and Alpine Research.
The research team analyzed ice samples from the Siple Dome core on coastal West Antarctica, drilled recently by U.S. researchers with funding from the National Science Foundation. The timing of the warming correlates with an abrupt sea-level rise documented by researchers at Australian National University and with less dramatic warming increases seen in the Byrd and Vostok ice cores from Antarctica.
"The signal we see in the Siple Dome core is so strong, we can speculate it may have been the trigger area for the end of the glacial period," said White. Because of its coastal location, Siple Dome would have been climatically sensitive to events like partial collapses of the West Antarctic Ice Sheet, which would have caused seas to rise globally.
White presented a paper on the subject at the Fall 2000 meeting of the American Geophysical Union held Dec. 15 to Dec. 20 in San Francisco. Paper co-authors included Jeff Severinghaus of the University of California at San Diego, Kendrick Taylor of the University of Nevada, Reno, Albert Brooks of Washington State University and Richard Alley of Pennsylvania State University.
A second abrupt warming event in Antarctica about 15,000 years ago also was documented by the team using the Siple Dome core. Nearly as strong and swift as the first, the second event appears to have preceded a rapid warming in the North Atlantic known as BOLLING/ALLEROD, said White.
"The BOLLING /ALLEROD event is widely considered the signal for when the North Atlantic ‘woke up’ and began to circulate like we see it today, pumping vast amounts of heat into Scandinavia and Europe,’" said White. "Over the course of a few decades we see a warming of air temperatures in that region ranging from about 18 degrees to 27 degrees F."
Both Antarctic warming events were charted by measuring changes in stable isotopes of nitrogen in air and hydrogen and oxygen isotopes in the ice itself. They were tentatively dated using a combination of annual ice-layer counting and atmospheric methane concentrations trapped in the ice. Annual ice layers in ice cores often can be distinguished from one another as distinct rings laid down, much like the annual rings of trees.
Severinghaus measured the ratio of two nitrogen isotopes in core sections tied to the two Antarctic warming events in order to calculate changes in the air temperature. White sampled the ice core for two stable isotopes of water, deuterium and oxygen-18.
"These isotope changes can be used to determine surface conditions such as air temperature, both in the Pacific Ocean where moisture originates as well as on the ice sheet," he said.
Excesses in deuterium in the cores indicated that sea-surface temperatures rose dramatically or the humidity in the region dropped significantly at the time, White said. Perhaps the most likely scenario is a decrease in humidity over the southern Pacific as water vapor rose into the atmosphere, triggered by a major change of air circulation.
"Water vapor is the prime greenhouse gas," said White. "The deuterium isotope changes may be an indication there was a swift and fundamental change in the atmosphere affecting the tropical Pacific Ocean. Perhaps adding greenhouse gases to the atmosphere was some kind of threshold event as reflected by the sudden temperature rise."
The researchers also were able to tease out "glimpses" of past El Nino/Southern Oscillation events, thought to occur every two to seven years when warming water wells up in the tropical Pacific and moves from east to west, causing worldwide weather changes. Evidence of ENSO events is visible in the Siple Dome core going back 10,000 years.
White said there is no doubt human activity has played a large part in global warming over the past several centuries. "Greenhouse gas concentrations and climate change are based on simple theories," he said. "By pumping vast amounts of greenhouse gases into the atmosphere, humans have set down a huge footprint that covers the entire planet."
Editors: White’s presentation is at 2:45 p.m. PST on Tuesday, Dec. 19. The AGU Press Room telephone number is (405) 905-1008.
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