BOULDER, Colorado - Current levels of Earth's atmospheric carbon dioxide may be high enough to bring about "irreversible" shifts in Arctic ecosystems, according to new research published today by scientists from the United States, Canada and The Netherlands.

Ellesmere Island natural ice sculptures like this may soon be history. The Arctic climate system is more sensitive to greenhouse warming than previously known said the researchers, who gathered evidence on what is now Ellesmere Island in Canada's High Arctic. (Photo by Cam17) The Arctic climate system is more sensitive to greenhouse warming than previously known said the researchers, who gathered evidence on what is now Ellesmere Island in Canada's High Arctic from a time period 2.6 to 5.3 million years ago. This period, known as the Pliocene Epoch, occurred shortly before Earth was plunged into an ice age.

"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 zero degrees Celsius (32 degrees F)," said lead author Ashley Ballantyne of the University of Colorado at Boulder.

"As temperatures approach zero 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," Dr. Ballantyne warned.

The research team points out that the overwhelming majority of climate scientists agree Earth is warming due to increased atmospheric concentrations of heat-trapping gases generated by human activities like fossil fuel burning and deforestation.

Arctic temperatures have risen by about 1.8 degrees F (1 degree C) in the past two decades in response to human-caused 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.

Environmental advocates are calling on governments negotiating the next climate treaty to limit greenhouse gas emissions to 350 parts per million, the level many scientists say will help to avert the worst consequences of climate change.

The research paper is being published in the July issue of the journal "Geology." The study was funded by the U.S. National Science Foundation, the Natural Science and Engineering Research Council in Canada, the Netherlands Organization for Scientific Research and the European Research Council.

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.

"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."

At the Ellesmere Island research site, called the Beaver Pond site, organic materials have been "mummified" in peat deposits, allowing the researchers to conduct detailed, high-quality analyses, said Eberle.

They found that in the Pliocene, Ellesmere Island had forests of larch, dwarf birch and northern white cedar trees, as well as mosses and herbs.

The island was inhabited by fish, frogs and mammals now extinct, including tiny deer, ancient relatives of the black bear, three-toed horses, small beavers, rabbits, badgers and shrews.

But the research value of the site is now threatened by a proposed coal mine. 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., a mineral exploration company headquartered in Vancouver, British Columbia.

The Beaver Pond site is close to Strathcona Fiord on Ellesmere Island in the Canadian Territory of Nunavut. In the 1980s, reconnaissance exploration conducted by Petro-Canada and others described coal seams up to 12 meters (39 feet) thick close to the surface along the steep north shore of the fiord.

"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."

For this study, the team used three independent methods of measuring the Pliocene temperatures on Ellesmere Island.

They measured oxygen isotopes found in the cellulose of fossil trees and mosses that reveal temperatures and precipitation levels tied to ancient water.

They analyzed the distribution of lipids in soil bacteria which correlate with temperature.

And they inventoried ancient Pliocene plant groups that overlap in range with contemporary vegetation.

"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."

The scientists found that while the mean annual temperature on Ellesmere Island during the Pliocene was about 34 degrees Fahrenheit (19 degrees Celsius) hotter than it is today, levels of the greenhouse gas carbon dioxide were only slightly higher than present.

Elevated Arctic temperatures during the Pliocene 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."

Presently, Arctic sea ice is declining at a rate of 11.2 percent per decade according to the National Snow and Ice Data Center.

Some climate change experts are forecasting that the Arctic summers will become ice-free within a decade or two.