CLIMATE
MODELS CHALLENGED BY BARNARD PROFESSOR JULIAN SACHS
IN SCIENCE MAGAZINE
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Wide
swings in ocean temperature during the last
ice age extended well south of the polar
and subpolar Atlantic region and all the
way into the warm, subtropical ocean, a
new study demonstrates, suggesting that
the effect of future global warming may
extend farther south than some previous
predictions.
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The
study, published in the Oct. 22 issue of Science,
was undertaken by Julian Sachs, assistant professor
of environmental science at Barnard College in New
York City, and Scott J. Lehman, associate research
professor of geological sciences at the Institute
of Arctic and Alpine Research at the University
of Colorado, Boulder. The two examined sediments
recovered northeast of Bermuda from under more than
two miles of water.
Measurements
in Greenland ice cores by Pieter Grootes and coworkers
at the University of Washington over the last decade
documented large, rapid swings in air temperature
during the last glacial period. Similar swings in
polar and subpolar sea temperatures have been inferred
from the distribution of micro-fossil shells in
those sediments. But Sachs and Lehman are the first
to demonstrate that dramatic temperature changes
of up to 5 degrees Celsius (or 9 degrees Fahrenheit)
occurred not only in the north, but well into the
warm ocean, during the period 60,000 to 30,000 years
ago.
"It
has been known for the better part of a decade that
Greenland and the polar Atlantic region experienced
ocean-driven flip flops in temperature every few
thousand years during the last glacial period approximately
80,000 to 100,000 years ago," said Sachs and
Lehman. "What is new here is the clear evidence
that, like the polar Atlantic, the warm Atlantic
was also undergoing related, very large, and very
rapid (in terms of degree per decade) temperature
changes."
Change
much greater than predicted
Where
climate models predict subtropical sea temperature
change of up to 5 degrees Celsius between maximum
glacial and warm interglacial (i.e., modern) conditions,
a period of 10,000 years, the large, rapid temperature
swings the two scientists observed over a scant
250 years can probably only be caused by disruption
or even halting of the North Atlantic conveyor-like
circulation, a concept pioneered by Columbia University's
Wallace Broecker, Newberry Professor of Earth and
Environmental Sciences. The circulation of the North
Atlantic conveyor transports warm, tropical water
north to the polar areas, via the Gulfstream and
North Atlantic Drift currents. Once north, the salty
warm water cools and then sinks to the bottom of
the ocean, a process that draws more warm surface
water from the south.
"This north-south conveyor is what keeps northern
Europe far warmer than the Canadian provinces at
the same latitude - in short, what keeps London
from having a climate like Newfoundland," said
Sachs.
But
if the salty water does not become cold enough to
sink, due to global warming, or is diluted with
too much freshwater, the North Atlantic conveyor
halts. This appears to have happened repeatedly
throughout the time period studied by Sachs and
Lehman, since no other mechanism appears capable
of producing the large, sudden temperature swings
they document.
Most
numerical models used to predict the climate response
to increased greenhouse gas concentration do not
predict the large temperature changes of the warm
ocean documented by the two researchers, suggesting
that those models may have to be altered.
"Most
climate models developed over the past 10-15 years
suggest the effects of a shutdown of the North Atlantic's
conveyor-like circulation, such as that due to global
warming, will be localized in the far northeast
Atlantic - Iceland and Scandinavia," said Sachs.
"Our data suggests the footprint may be much
larger.'
"The
fact that we observed such large temperature fluctuations
in connection with changes in ocean circulation
documented by Lloyd Keigwin, senior scientist at
the Woods Hole Oceanographic Institute, and Ed Boyle,
professor of chemical oceanography, at MIT, suggests
future climate changes may not only be severe for
Northern Europe but could affect more southerly
latitudes" Sachs added.
Observed
Keigwin, "Because the climate system can respond
this quickly means it could respond this quickly
to man's influence and it may respond unpredictably."
While
no single study will send climate modelers back
to the drawing board, said Keigwin, "the kinds
of changes they see may be greater than what the
models predicted and that may lead to some recalculation."
One
caution, according to Sachs, is that the climate
system today may have a different sensitivity than
it did 30-60 thousand years ago, when ice sheets,
solar radiation receipts and greenhouse gas concentrations
were different.
Next
step: analyze change in Pacific Ocean
According
to Sachs, "The next step is to determine if
similar changes occurred in the much larger Pacific
Ocean, studies he and Lehman are pursuing. If so,
any human-induced changes to the ocean's plumbing
are likely to affect much of the mid-latitudes,
where many large population centers are found."
Columbia
University's Broecker said the findings of rapid,
wide temperature swings underscore other, recent
research. "At first we all thought this was
a phenomena of the northern Atlantic, that it wouldn't
even extend down to Bermuda.
And
now, in the last five years, it has been shown that
these changes have been shown in the Santa Barbara
basin, in the Arabian sea off of India, in the Cariaco
Trench off of Venezuela, and now, in this beautiful,
beautiful record that Julian has gotten on the sediment
near Bermuda. These things confirm that these changes
were not restricted to the north Atlantic -- they
were global," said Broecker. "It beautifully
replicates the Greenland record and just adds piece
of information to indicate that the global system
we live in is a strange one that is capable of doing
outrageous things namely jumping form one state
of operation to another."
The
mechanism by which temperature change in the warm
Atlantic can affect climate globally is via the
water vapor feedback. Warm ocean temperatures raise
the water vapor content of the atmosphere and thus
its heat-trapping or "greenhouse" capacity,
with a one-degree rise in water temperature equating
to a 6 percent increase in water vapor pressure.
Underscoring
the importance of ocean currents, to the surprise
of the researchers, the ocean-driven temperature
fluctuations they observed were as great as those
caused by changes in the tilt of the Earth, and
changes in the Earth's orbit: factors thought to
have produced the series of ice ages in geological
history.
"The
warming at the end of the ice age was supported
by the disappearance of enormous ice sheets, a 1/3
increase in atmospheric CO2 levels, and changes
in the seasonal distribution of the suns energy,
whereas the abrupt changes we document here seem
to be almost entirely ocean driven," said Sachs
and Lehman.
Discovery
built on new technique
Although
there had been earlier indications of ocean temperature
changes further south of the Greenland ice cores,
those had been based on records of mineral deposits
left by plankton - a record that can be unreliable
because plankton growth can be influenced by factors
besides temperature.
For
their study, Sachs and Lehman used a 12-meter section
of a 52.7-meter core of sediment drilled 4,462 meters
underwater into the sea floor of the Bermuda Rise
by French scientists as part of IMAGES (International
Marine Global Change Study), an international coring
project. The Bermuda Rise is a sediment drift formation
where sedimentation rates of 10 to 100 centimeters
per 1,000 years far exceed the average for the ocean,
giving an unusually detailed picture of temperature
over time - providing a method is available to deduce
temperature from sediments.
Sachs
and Lehman were able to do just that by streamlining
and automating a technique developed in the 1980s
by researchers at the University of Bristol, England.
The technique is based on the observation that the
ratio of two molecules produced by certain phytoplankton
varies in direct proportion to the water temperature
in which they live.
Although
the biochemical function of the molecules - called
alkenones - is still a mystery, they are thought
to play a role in maintaining cell membrane stiffness.
Much like butter is stiff while margarine is soft
when removed from the refrigerator, plankton may
produce more of the unsaturated or margarine-like
variety of alkenone when in cold water and more
of the saturated or butter-like variety in warm
water.
Taking
samples every 1 or 2 centimeters throughout the
12-meter section of sediment, each representing
33 to 67 years of deposition on average, the two
extracted lipids (fatty substances) from each sample
and then used gas chromatography to measure the
ratio of alkenones.
Based
on the technique, the researchers were able to reconstruct
the surface sea temperature, showing that it varied
between 15.5 centigrade to 21 centigrade - close
to the current water temperature of 22.5 Celsius.
"Our
study shows that previously documented disruption
of ocean currents during the last ice age produced
unexpectedly large and rapid temperature changes
in the warm Atlantic Ocean. That implies that Greenhouse
warming - which could similarly disrupt ocean currents
- could have consequences more global than some
current predictions," said Sachs.
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