August 15, 2013
West Antarctica Deglaciation Began Much Earlier Than Thought
Lawrence LeBlond for redOrbit.com - Your Universe Online
One of the deepest ice cores ever drilled in Antarctica is revealing some interesting evidence about the southern continent’s turbulent past and the role Earth’s orbit played in the history of the ice ages.
Scientists have known for more than a century that ice ages come and go due to the wobbling of the Earth as it orbits the sun. Increases in the intensity of summer sunlight in the Northern Hemisphere melted the ice sheets in Canada and Europe starting 20,000 years ago. It was previously believed that the Southern Hemisphere followed suit about 2,000 years later. But a new study has laid that theory to rest.
West Antarctica began emerging from the last ice age around 22,000 years ago – long before other regions of the continent and, for that matter, the rest of the world, according to the team of researchers who drilled the West Antarctic ice sample. The scientists said changes in the amount of solar energy triggered the warming of West Antarctica and the subsequent release of carbon dioxide from the Southern Ocean amplified the effect, resulting in worldwide warming that eventually ended the ice age.
Originally, experts believed that the south responded to warming in the north, resulting in deglacation there. But the new evidence, published in the journal Nature, shows that the Southern Hemisphere actually began warming as much as a four thousand years earlier than assumed, nearly 2,000 years before the north began going through similar changes.
Previous evidence for Antarctic climate change has come from East Antarctic ice cores, which is the coldest part of the continent. But the new ice cores from West Antarctica paint a much different picture, indicating a 4,000-year margin of error in previous results.
TWO-MILE ICE CORE
The West Antarctic Ice Sheet Divide project, funded by the National Science Foundation, has provided a significant link in the understanding about how the Earth emerges from an ice age. Edward Brook, a paleoclimatologist with Oregon State University, who is part of the team, said this new understanding has surfaced from just one two-mile-long sample of ice.
"This ice core is special because it came from a place in West Antarctica where the snowfall is very high and left an average of 20 inches of ice or more per year to study," said Brook, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences. "Not only did it allow us to provide more accurate dating because we can count the layers, it gave us a ton more data – and those data clearly show an earlier warming of the region than was previously thought."
"The site of the core is near the coast and it conceivably feels the coastal influence much more so than the inland sites where most of the high-elevation East Antarctic cores have been drilled," Brook said. "As the sunlight increased, it reduced the amount of sea ice in the Southern Ocean and warmed West Antarctica. The subsequent rise of CO2 then escalated the process on a global scale."
"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 and lead corresponding author of the study.
The ice core taken from West Antarctica plays a significant role in determining past ice melt. The area where the sample was taken shows little horizontal flow of the ice so data are known to be from a location that remained fairly consistent over a long period of time. The sample represents more than 68,000 years of history, yet the team has only analyzed layers going back about 30,000 years. Near the surface of the core, about 3.3 feet of ice equals one year; at greater depths the annual layers are compressed to mere inches.
Fudge identified the annual layers by running two electrodes along the ice core to measure higher electrical conductivity associated with each summer season. Periods of greater warming were discovered in the layers between 18,000 and 22,000 years ago, around the time of the last deglaciation.
"This deglaciation is the last big climate change that that we're able to go back and investigate," he said. "It teaches us about how our climate system works."
A TALE OF TWO ANTARCTICAS
Scientists have observed rapid warming in West Antarctica in recent decades. UW professor of Earth and space sciences, Eric Steig, who is part of Fudge’s team, produced the oxygen isotope data used in the study. The new data pulled from these isotope data reveal that West Antarctica’s climate is more strongly influenced by regional conditions in the Southern Ocean than East Antarctica is, which is higher in elevation and typically colder. However, experts have observed some warming even here.
"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 said.
He said that the sun’s intensity cannot be held accountable for West Antarctica’s warming 20,000 years ago. Instead, how the sun’s energy was distributed over the region was the driving factor in deglaciation, at least for this particular region. It not only warmed the ice sheet but also warmed the Southern Ocean surrounding Antarctica, especially in the summer when more sea ice melting took place.
"What is new here is our observation that West Antarctica did not wait for a cue from the Northern Hemisphere before it began warming," Brook added. "What hasn't changed is that the initial warming and melting of the ice sheets triggered the release of CO2 from the oceans, which accelerated the demise of the ice age."
Fudge noted that changes in Earth’s orbit are not an important factor in the rapid warming seen today. "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 added.