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(4) SCIENTISTS SPECULATE ABOUT SUDDEN END OF ICE AGE
From Andrew Yee
Scripps Institution of Oceanography
University of California-San Diego
Media Contacts:
Janet Howard or Cindy Clark, (619) 534-3624
E-mail:jehoward@ucsd.edu;
cclark@ucsd.edu
SCIENTISTS DETERMINE TRIGGER OF END OF ICE AGE
Using a new technique to analyze air trapped in ancient ice
cores,
scientists at Scripps Institution of Oceanography at the
University
of California, San Diego and Washington State University have
determined that the end of the last ice age was triggered by an
abrupt period of warming that began in the North Atlantic rather
than
in the tropics. Scientists have debated for years which region of
the
globe is responsible for triggering periods of rapid warming.
The researchers report in the Oct. 29 issue of Science that
Greenland
temperatures rose by 16 degrees Fahrenheit in less than several
decades, bringing the last ice age to a close about 15,000 years
ago.
They were surprised to learn, however, that the dramatic period
of
warming appears to have been triggered by changes in circulation
in
the North Atlantic rather than in the tropics, commonly believed
to
be the region that drives global climate change because it is
central
to the Earth's heat and water vapor budgets.
"There has been a lot of interest lately in the role the
tropics
might play in climate change -- everything from causing El Ninos
to
speculation that the abrupt warmings during the last ice age were
caused by changes in the tropics," said Jeffrey
Severinghaus, an
assistant professor in the Scripps Geosciences Research Division
who
is the paper's lead author. "These findings sort of go
against that
grain because they suggest that the North Atlantic climate
changed
first."
Using a new technique based on analysis of nitrogen and argon
isotopes in air trapped in Greenland ice, Severinghaus and Edward
Brook of Washington State University determined that the
Greenland
Summit warmed some 16 degrees Fahrenheit over a period of several
decades beginning 14,672 years ago. Analysis of methane
concentrations trapped in the ice -- an indicator of tropical
climate
-- showed, however, that temperatures in the tropics began to
climb
20 to 80 years after the onset of the abrupt Greenland warming.
"Our data clearly shows that tropical climate lags the
Greenland
climate by at least 20 years, so that means the tropics could not
have triggered the event," Severinghaus said. "Our work
doesn't,
however, rule out that the tropics could play a role in
amplifying
warming once it begins."
Determining what may trigger rapid periods of warming is of key
interest to scientists who are trying to predict what effect
adding
carbon dioxide to the atmosphere from the burning of fossil fuels
may
have on future climate.
"We know that over the next 100 years the Earth will
probably warm
because of the greenhouse effect and there is a remote
possibility
that we might trigger one of these abrupt climate events,"
Severinghaus said. "So, it is very important that we
understand the
underlying mechanisms behind them."
In order to trace how Greenland temperatures changed over time,
Severinghaus analyzed tiny air bubbles trapped within the layers
of
snow-turned-to-ice deposited in Greenland over the past hundreds
of
thousands of years. Similar to the way in which sediments are
laid
down at the bottom of the ocean, the ice cores are made up of
compacted snow layers that become older with depth. Severinghaus
focused his efforts on analyzing samples of ice taken from the
Greenland Ice Sheet Project Two (GISP 2) core covering a period
from
15,000 to 10,000 years ago.
Current methods of analyzing ice cores are plagued by the problem
that air trapped in bubbles within the ice is younger than that
of
the surrounding matrix, making the air sample difficult to
accurately
date. The problem stems from the fact that air is only trapped
within
the consolidated snow at a depth of several hundred feet below
the
surface at which point the ice may be thousands of years old.
Because
of this, it has been difficult for scientists to relate the
precise
timing of rapid warming events with concurrent changes in
atmospheric
gases.
To overcome this dilemma, Severinghaus developed a new technique
based on thermal diffusion in which he used changes in the
isotopic
ratios of argon and nitrogen in the GISP 2 core to develop a
gas-based temperature record for Greenland near the end of the
last
ice age. Brook then analyzed methane concentrations within the
same
ice samples in order to track changes in tropical climate over
the
same period.
Because the period of rapid warming was seen first at high
northern
latitudes, the two scientists propose that the end of the last
ice
age was triggered by changes in circulation of the North Atlantic
that caused heat to be transported from the equator towards
Europe
and the poles.