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."
The study was funded by the National Science Foundation, the Natural
Science and Engineering Research Council in Canada, the Netherlands
Organization for Scientific Research and the European Research Council.
Contact: Ashley Ballantyne
[email protected]
University
of Colorado at Boulder