New ice core data helps explain Antarctica's last deglaciation


Warming in Antarctica began approximately 22,000 years ago, a few thousand years earlier than previously suggested by similar records, according to a new analysis of an ice core taken by the National Science Foundation-funded West Antarctic Ice Sheet (WAIS) Divide project.  

81613-WAIS Ken-Taylor-Ice-Wall-thbA West Antarctica Ice Sheet Divide project researcher stands in a snow pit next to an ice core with data from 68,000 years ago. Enlarge image for more.Some of the new, annually resolved ice-core measurements from West Antarctica used in this study were developed by researchers at Nevada’s Desert Research Institute. 

“One of the big questions in climate science is what sequence of events caused the last deglaciation – that is, the transition of global climate from a glacial state to the present interglacial or warm period,” said Joe McConnell, a research professor at the Desert Research Institute who leads an international team of experts staffing DRI’s unique ultra-trace ice core analytical laboratory. “The new data presented here indicates that the last deglaciation was somewhat different in West Antarctica than East Antarctica and that sea ice changes around Antarctica may have played a central role in that sequence of events.”

For more than a century scientists have known that Earth's ice ages are caused by the wobbling of the planet's orbit, which changes its orientation to the sun and affects the amount of sunlight reaching higher latitudes, particularly the polar regions.

The Northern Hemisphere's last ice age ended about 20,000 years ago, and most evidence has indicated that the ice age in the Southern Hemisphere ended about 2,000 years later, suggesting that the south was responding to warming in the north.

But research published online Aug. 14 in the journal Nature shows that Antarctic warming began at least two, and perhaps four, millennia earlier than previously thought.

Most previous evidence for Antarctic climate change has 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 under way 20,000 years ago.

WAIS Divide is a large-scale and multi-year glaciology project supported by the U.S. Antarctic Program (USAP), which the National Science Foundation (NSF) manages. Through USAP, NSF coordinates all U.S. science on the southernmost continent and aboard vessels in the Southern Ocean as well as providing the necessary logistics to make the science possible,

The 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 ice covers one year, but at greater depths the annual layers are compressed to centimeters.

"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 Nature paper. Fudge s 41 co-authors are other members of the WAIS project.

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.

"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."

West Antarctica is separated from East Antarctica by a major mountain range. East Antarctica has a substantially higher elevation and tends to be much colder, though there is recent evidence that it too is warming.

Rapid warming in West Antarctica in recent decades has been documented in previous research by Eric Steig, a professor of Earth and space sciences at the University of Washington who serves on Fudge's doctoral committee and whose laboratory produced the oxygen isotope data used in the Nature paper. The new data confirms that West Antarctica's climate is more strongly influenced 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 said.

He 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. 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.

Changes in Earth's orbit today are not an important factor in the rapid warming that has been observed recently, he 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," Fudge said.

Julie Palais, the Antarctic Glaciology Program Director in NSF s Division of Polar Programs, said new findings will help scientists to better understand not only what happened at the end of the last ice age but it should also help inform our understanding of what might be happening as the climate warms and conditions begin to change in and around the Antarctic continent.

She added  recent work showing large calving events in the Amundsen Sea Embayment (ASE) off the coast of West Antarctica and the supposition that warm ocean currents are playing a role is entirely consistent with these results from WAIS Divide.

Additional information:

Read the National Science Foundation press release here - http://www.nsf.gov/news/news_summ.jsp?cntn_id=128808

Read the full study published in Nature here - http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12376.html?WT.ec_id=NATURE-20130815

Desert Research Institute

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