Scientists have
known for decades that small changes in climate can have significant impacts on
the massive Antarctic Ice Sheet.
Now a new study
suggests the opposite also is true. An international team of researchers has
concluded that the Antarctic Ice Sheet actually plays a major role in regional
and global climate variability - a discovery that may also help explain why
sea ice in the Southern Hemisphere has been increasing despite the warming of
the rest of the Earth.
Results of the
study are being published this week in the journal Nature.
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| View of the Riiser-Larsen Ice Shelf in Antarctica. By Ben Holt (NASA), via Wikimedia Commons |
Global climate
models that look at the last several thousand years have failed to account for
the amount of climate variability captured in the paleoclimate record,
according to lead author Pepijn Bakker, a former post-doctoral researcher at
Oregon State University now with the MARUM Center for Marine Environmental
Studies at the University of Bremen in Germany.
The research
team's hypothesis was that climate modelers were overlooking one crucial
element in the overall climate system - an aspect of the ocean, atmosphere,
biosphere or ice sheets - that might affect all parts of the system.
"One thing
we determined right off the bat was that virtually all of the climate models
had the Antarctic Ice Sheet as a constant entity," Bakker said. "It
was a static blob of ice, just sitting there. What we discovered, however, is
that the ice sheet has undergone numerous pulses of variability that have had a
cascading effect on the entire climate system."
The Antarctic
Ice Sheet, in fact, has demonstrated dynamic behavior over the past 8,000
years, according to Andreas Schmittner, a climate scientist in Oregon State's
College of Earth, Ocean, and Atmospheric Sciences and co-author on the study.
"There is
a natural variability in the deeper part of the ocean adjacent to the Antarctic
Ice Sheet - similar to the Pacific Decadal Oscillation, or El Niño/La Niña but
on a time scale of centuries - that causes small but significant changes in
temperatures," Schmittner said. "When the ocean temperatures warm, it
causes more direct melting of the ice sheet below the surface, and it increases
the number of icebergs that calve off the ice sheet."
Those two
factors combine to provide an influx of fresh water into the Southern Ocean
during these warm regimes, according to Peter Clark, a paleoclimatologist in
OSU's College of Earth, Ocean, and Atmospheric Sciences and co-author on the
study.
"The
introduction of that cold, fresh water lessens the salinity and cools the
surface temperatures, at the same time, stratifying the layers of water,"
Clark said. "The cold, fresh water freezes more easily, creating
additional sea ice despite warmer temperatures that are down hundreds of meters
below the surface."
The discovery
may help explain why sea ice has expanded in the Southern Ocean despite global
warming, the researchers say. The same phenomenon doesn't occur in the Northern
Hemisphere with the Greenland Ice Sheet because it is more landlocked and not
subject to the same current shifts that affect the Antarctic Ice Sheet.
"One
message that comes out of this study is that the Antarctic Ice Sheet is very
sensitive to small changes in ocean temperatures, and humans are making the
Earth a lot warmer than it has been," Bakker said.
Sediment cores
from the sea floor around Antarctica contain sand grains delivered there by
icebergs calving off the ice sheet. The researchers analyzed sediments from the
last 8,000 years, which showed evidence that many more icebergs calved off the
ice sheet in some centuries than in others. Using sophisticated computer
modeling, the researchers traced the variability in iceberg calving to small
changes in ocean temperatures.
The Antarctic
Ice Sheet covers an area of more than 5 million square miles and is estimated
to hold some 60 percent of all the fresh water on Earth. The east part of the
ice sheet rests on a major land mass, but in West Antarctica, the ice sheet
rests on bedrock that extends into the ocean at depths of more than 2,500
meters, or more than 8,000 feet, making it vulnerable to disintegration.
Scientists
estimate that if the entire Antarctic Ice Sheet were to melt, global sea levels
would rise some 200 feet.
Other authors
on the study include Nicholas Golledge of Victoria University of Wellington in
New Zealand and Michael Weber of the University of Bonn in Germany.
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