A new study shows the Arctic climate system may be more
sensitive to greenhouse warming than previously thought, and that
current levels of Earth's atmospheric carbon dioxide may be high
enough to bring about significant, irreversible shifts in Arctic
ecosystems.
Led by the University of Colorado at Boulder, the international
study indicated that while the mean annual temperature on Ellesmere
Island in the High Arctic during the Pliocene Epoch 2.6 to 5.3
million years ago was about 34 degrees Fahrenheit, or 19 degrees
Celsius, warmer than today, CO2 levels were only
slightly higher than present. The vast majority of climate
scientists agree Earth is warming due to increased concentrations
of heat-trapping atmospheric gases generated primarily by human
activities like fossil fuel burning and deforestation.
The team used three independent methods of measuring the
Pliocene temperatures on Ellesmere Island in Canada's High Arctic.
They included measurements of oxygen isotopes found in the
cellulose of fossil trees and mosses that reveal temperatures and
precipitation levels tied to ancient water, an analysis of the
distribution of lipids in soil bacteria which correlate with
temperature, and an inventory of ancient Pliocene plant groups that
overlap in range with contemporary vegetation.
"Our findings indicate that CO2 levels of
approximately 400 parts per million are sufficient to produce mean
annual temperatures in the High Arctic of approximately 0 degrees
Celsius (32 degrees F)," Ballantyne said. "As temperatures approach
0 degrees Celsius, it becomes exceedingly difficult to maintain
permanent sea and glacial ice in the Arctic. Thus current levels of
CO2 in the atmosphere of approximately 390 parts per
million may be approaching a tipping point for irreversible
ice-free conditions in the Arctic."
A paper on the subject is being published in the July issue of
the journal Geology. Co-authors included David Greenwood of
Brandon University in Manitoba, Canada, Jaap Sinninghe Damste of
the Royal Netherlands Institute for Sea Research, Adam Csank of the
University of Arizona, Natalia Rybczynski of the Canadian Museum of
Nature in Ottawa and Jaelyn Eberle, curator of fossil vertebrates
at the University of Colorado Museum of Natural History and an
associate professor in the geological sciences department.
Arctic temperatures have risen by about 1.8 degrees F, or 1
degree C, in the past two decades in response to anthropogenic
greenhouse warming, a trend expected to continue in the coming
decades and centuries, said Ballantyne. Greenhouse gases in the
atmosphere have risen from about 280 parts per million during the
pre-industrial era on Earth to about 390 parts per million
today.
During the Pliocene, Ellesmere Island hosted forests of larch,
dwarf birch and northern white cedar trees, as well as mosses and
herbs, including cinquefoils. The island also was home to fish,
frogs and now extinct mammals that included tiny deer, ancient
relatives of the black bear, three-toed horses, small beavers,
rabbits, badgers and shrews. Because of the high latitude, the
Ellesmere Island site on the Strathcona Fiord was shrouded by
darkness six months out of the year, said Rybczynski.
Fossils are often preserved in a process known as
permineralization, in which mineral deposits form internal casts of
organisms. But at the Ellesmere Island site known as the "Beaver
Pond site," organic materials -- including trees, plants and mosses
-- have been "mummified" in peat deposits, allowing the researchers
to conduct detailed, high-quality analyses, said Eberle.
Ballantyne said the high level of preservation of trees and
mosses at Ellesmere Island allowed the team to measure the ratio of
oxygen isotopes in plant cellulose, providing information on water
absorbed from precipitation during the Pliocene and which yielded
estimates of past surface temperatures. The team also compared data
on the width of tree rings in larch trees at the Beaver Pond site
to trees at lower latitudes today to help them estimate past
temperatures and precipitation levels.
The researchers also analyzed the distribution of ancient
membrane lipids from soil bacteria known as tetraethers, which
correlate to temperature. The chemical structure of the fossilized
tetraethers makes them highly sensitive to both temperature and
acidity, or pH, said Ballantyne.
The last line of evidence put forward by the CU-Boulder-led team
was a comparison of Pliocene ancient vegetation at the site with
vegetation present today, providing a clear "climate window"
showing the overlap of the two time periods. "The results of the
three independent temperature proxies are remarkably consistent,"
said Eberle. "We essentially were able to 'read' the vegetation in
order to estimate air temperatures in the Pliocene."
Today, Ellesmere Island is a polar desert that features tundra,
permafrost, ice sheets, sparse vegetation and a few small mammals.
Temperatures range from roughly minus 37 degrees F, or minus 38
degrees C, in winter to 48 degrees F, or 9 degrees C, in summer.
The region is one of the coldest, driest environments on Earth.
"Our findings are somewhat disconcerting regarding the
temperatures and greenhouse gas levels during the Pliocene," said
Eberle. "We already are seeing evidence of both mammals and birds
moving northward as the climate warms, and I can't help but wonder
if the Arctic is headed toward conditions similar to those that
existed during the Pliocene."
Elevated Arctic temperatures during the Pliocene -- which
occurred shortly before Earth plunged into an ice age about 2.5
million years ago -- are thought to have been driven by the
transfer of heat to the polar regions and perhaps by decreased
reflectivity of sunlight hitting the Arctic due to a lack of ice,
said Ballantyne. One big question is why the Arctic was so
sensitive to warming during this period, he said.
Multiple feedback mechanisms have been proposed to explain the
amplification of Arctic temperatures, including the reflectivity
strength of the sun on Arctic ice and changes in vegetation
seasonal cloud cover, said Ballantyne. "I suspect that it is the
interactions between these different feedback mechanisms that
ultimately produce the warming temperatures in the Arctic."
In 2009, CU-Boulder's National Snow and Ice Data Center showed
the September Arctic sea ice extent was 649,000 square miles, or
1,680,902 square kilometers, below the 1979-2000 average, and is
declining at a rate of 11.2 percent per decade. Some climate change
experts are forecasting that the Arctic summers will become
ice-free summers within a decade or two.
In addition to its exceptional preservation of fossil wood,
plants, insects and mollusks, the Beaver Pond site on Ellesmere
Island is the only reported Pliocene fossil site in the High Arctic
to yield vertebrate remains, said Rybczynski.
Eberle said there is high concern by scientists over a proposal
to mine coal on Ellesmere Island near the Beaver Pond site by
WestStar Resources Inc. headquartered in Vancouver, British
Columbia. "Paleontological sites like the Beaver Pond site are
unique and extremely valuable resources that are of international
importance," said Eberle. "Our concern is that coal mining
activities could damage such sites and they will be lost
forever."
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