Fates
of polar ice sheets appear to be linked
Intense warm climate intervals--warmer
than scientists thought possible--have occurred in the Arctic over the past 2.8
million years.
That result comes from the first
analyses of the longest sediment cores ever retrieved on land. They were
obtained from beneath remote, ice-covered Lake El'gygytgyn (pronounced
El'gee-git-gin) ("Lake E") in the northeastern Russian Arctic.
The journal Science published the
findings this week.
They show that the extreme warm periods
in the Arctic correspond closely with times when parts of Antarctica were also
ice-free and warm, suggesting a strong connection between Northern and Southern
Hemisphere climate.
The polar regions are much more
vulnerable to climate change than researchers thought, say the National Science
Foundation-(NSF) funded Lake E project's co-chief scientists: Martin Melles of
the University of Cologne, Germany; Julie Brigham-Grette of the University of
Massachusetts Amherst; and Pavel Minyuk of Russia's North-East
Interdisciplinary Scientific Research Institute in Magadan.
The exceptional climate warming in the
Arctic, and the inter-hemispheric interdependencies, weren't known before the
Lake E studies, the scientists say.
Lake E was formed 3.6 million years ago
when a huge meteorite hit Earth, leaving an 11-mile-wide crater. It's been
collecting layers of sediment ever since.
The lake is of interest to scientists
because it has never been covered by glaciers. That has allowed the
uninterrupted build-up of sediment at the bottom of the lake, recording
hitherto undiscovered information on climate change.
Cores from Lake E go far back in time,
almost 30 times farther than Greenland ice cores covering the past 110,000
years.
The sediment cores from Lake El'gygytgyn
reflect the climate and environmental history of the Arctic with great
sensitivity, say Brigham-Grette and colleagues.
The physical, chemical and biological
properties of Lake E's sediments match the known global glacial/interglacial
pattern of the ice ages.
Some warm phases are exceptional,
however, marked by extraordinarily high biological activity in the lake, well
above that of "regular" climate cycles.
To quantify the climate differences, the
scientists studied four warm phases in detail: the two youngest, called
"normal" interglacials, from 12,000 years and 125,000 years ago; and
two older phases, called "super" interglacials, from 400,000 and 1.1
million years ago.
According to climate reconstructions
based on pollen found in sediment cores, summer temperatures and annual
precipitation during the super interglacials were about 4 to 5 degrees C
warmer, and about 12 inches wetter, than during normal interglacials.
The super interglacial climates suggest
that it's nearly impossible for Greenland's ice sheet to have existed in its
present form at those times.
Simulations using a state-of-the-art
climate model show that the high temperature and precipitation during the super
interglacials can't be explained by Earth's orbital parameters or variations in
atmospheric greenhouse gases alone, which geologists usually see as driving the
glacial/interglacial pattern during ice ages.
That suggests that additional climate
feedbacks are at work.
"Improving climate models means
that they will better match the data that has been collected," says Paul
Filmer, program director in NSF's Division of Earth Sciences, which funded the
"Lake E" project along with NSF's Office of Polar Programs.
"The results of this collaboration
among scientists in the U.S., Austria, Germany and Russia are providing a
challenge for researchers working on climate models: they now need to match
results from Antarctica, Greenland--and Lake El'gygytgyn."
Adds Simon Stephenson, director of the
Division of Arctic Sciences in NSF's Office of Polar Programs, "This is a
significant result from NSF's investment in frontier research during the recent
International Polar Year.
"'Lake E' has been a successful
partnership in very challenging conditions.
These results make a significant contribution to our understanding of
how Earth's climate system works, and improve our understanding of what future
climate might be like."
The scientists suspect the trigger for
intense interglacials might lie in Antarctica.
Earlier work by the international
ANDRILL program discovered recurring intervals when the West Antarctic Ice
Sheet melted. (ANDRILL, or the ANtarctic geological DRILLing project, is a
collaboration of scientists from five nations--Germany, Italy, New Zealand, the
United Kingdom and the United States--to recover geologic records from the
Antarctic margin.)
The current Lake E study shows that some
of these events match with the super interglacials in the Arctic.
The results are of global significance,
they believe, demonstrating strong indications of an ongoing collapse of ice
shelves around the Antarctic Peninsula and at the margins of the West
Antarctica Ice Sheet--and a potential acceleration in the near future.
The Science paper co-authors discuss two
scenarios for future testing that could explain the Northern
Hemisphere-Southern Hemisphere climate coupling.
First, they say, reduced glacial ice
cover and loss of ice shelves in Antarctica could have limited formation of
cold bottom water masses that flow into the North Pacific Ocean and upwell to
the surface, resulting in warmer surface waters, higher temperatures and
increased precipitation on land.
Alternatively, disintegration of the
West Antarctic Ice Sheet may have led to significant global sea level rise and
allowed more warm surface water to reach the Arctic Ocean through the Bering
Strait.
Lake E's past, say the researchers,
could be the key to our global climate future.
The El'gygytgyn Drilling Project also
was funded by the International Continental Scientific Drilling Program (ICDP),
the German Federal Ministry for Education and Research, Alfred Wegener
Institute, GeoForschungsZentrum-Potsdam, the Russian Academy of Sciences Far
East Branch, the Russian Foundation for Basic Research, and the Austrian
Ministry for Science and Research.
-NSF-
Posts
No comments:
Post a Comment