|August 16, 2013|
New Antarctic ice core reveals secrets of climate change
|A new Antarctic ice core that's more than 10,000 feet long suggests that West Antarctica may have begun melting more than 2,000 years earlier than believed. The secret? Sea ice.|
Most ice we see melts quickly, from ice cubes melting into a soda to icicles disappearing on a sunny winter day. But in Antarctica, ice can stick around for hundreds of thousands of years
A newly revealed cylinder of ancient ice could change the way we think about climate change. A study published August 14 in the scientific journal Nature looked at 30,000 years of ice in more detail than has ever before been possible.
"We're trying to home in on how our climate changes, on the scale of years and decades rather than on the scale of thousands of years, which we've never been able to do in Antarctica before," says T.J. Fudge, the lead author on the paper. "I think that's going to tell us so much about how our climate system works, at the short time scales that are relevant to modern climate change."
Most of Antarctica's oldest ice is up in the eastern mountains, where cold air and steep terrain insulate the ice fields from the surrounding ocean and atmosphere. Climate researchers have focused on the 800,000-year climate record preserved there, discounting two previous ice cores taken in the west. One of the western cores was taken on a glacier, so its ice had moved away from the spot that the snow first fell; the other was taken at the edge of the Ross Ice Sheet, so its ice layers are vulnerable to ocean changes that can mask climate changes.
"So many good records came out of East Antarctica that the West Antarctic records have been forgotten a little bit," says Mr. Fudge, who is a graduate student in glaciology at the University of Washington.
But because West Antarctica is more influenced by storms and changes in sea ice, it's also much more sensitive to atmospheric changes -- the very changes that make for a detailed and useful climate record.
In 2005, a team of scientists discovered the best of both worlds: a stable location in the west, which they found at the divide between two glaciers on the West Antarctic Ice Sheet (WAIS). Not only was the location right, but the WAIS Divide gets 3 feet of snow a year that then compacts down into about 9 inches of ice, which then compacts further and further over thousands of years, as it gets buried under tons of more snow and ice.
"This is basically the best site in Antarctica -- in the Southern Hemisphere -- to be able to look at the abrupt changes of the last glacial period," says Fudge.
With the ability to "zoom in" to the climate record -- to look at changes from year to year instead of epoch to epoch -- Fudge's team discovered that the timeline of warming and cooling, established from the eastern cores, wasn't telling the whole story.
How Antarctica began to melt
"The thought prior to our work is that Antarctica began to warm about 18,000 years ago, after the Northern Hemisphere had started to warm" about 24,000 years ago, says Fudge.
Climatologists debated exactly how the north triggered the south, though most agreed that the global ocean conveyor belt played a role, but they agreed that "Antarctica took its signal from the north to get it going." Fudge's team agrees that warming kicked into high gear 18,000 years ago, but they found evidence of warming in West Antarctica beginning between 2,000 and 4,000 years before the northern "trigger."
It now appears that both the northern and southern hemispheres were affected by orbital changes that made for longer summer days. In Antarctica, the increased sunlight melted the sea ice.
Sea ice has huge implications for climate change, for one simple reason: air above ice can get much, much colder than air above flowing water. "There's this great ability to amplify changes with sea ice. This is what we see happening in the northern hemisphere today, with the sea ice loss near the North Pole: You change the temperature a little, that changes the amount of sea ice a little, which then changes the amount of sea ice a lot. It creates a feedback loop," says Fudge.
Don't confuse sea ice with icebergs, warns Fudge. Icebergs are chunks of continental glaciers that calve into the ocean and float away. Sea ice is more like lake ice, or river ice: even when the whole surface doesn't freeze over, a rime of ice can grow on the edge of a lake or river. Similarly, a continent-scale "rime" of sea ice grows around the edge of Antarctica every winter, reaching 3 to 6 feet thick and expanding over 5 million square miles.
Two miles, straight down
The team took the first, short ice cores from the site in 2005, and then began this drilling project in 2006. Because of Antarctica's brutally short summers, they only had 30 to 35 drill days each year, so it took until December 2011 to extract the more than 2 miles of ice that made up the 68,000-year record, which they're still analyzing.
"You can only drill about 3 meters of ice each time you send the drill down," says Fudge, "so by the time you're drilling the deepest ice, you're 2 miles down, so you have to send the drill down 2 miles, grab 10 feet, come back up 2 miles, and then send it back down an extra 10 feet to grab the next one... It takes about 3 hours for one drill run." Then the ice begins its 6-month journey, first by plane to the coast, then by ship to Los Angeles, then overland to Denver, when the analysis finally begins -- all while keeping it deep-frozen.
"It is not an inexpensive process, so we feel a great responsibility," Fudge acknowledges. "I think the paper we just published is the first of many really tremendous records that are changing our perception about how our climate system works."