U. COLORADO (US)—Current levels of the Earth’s atmospheric carbon dioxide may be high enough to bring about significant irreversible shifts in Arctic ecosystems.
The region appears to be more sensitive to greenhouse warming than previously thought, according to a new international study.
The research indicates 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.
“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),” says Ashley Ballantyne of the University of Colorado at Boulder.
“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.”
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.
Details of the study are published in the July issue of the journal Geology.
Scientists agree global warming is largely due to increased concentrations of heat-trapping atmospheric gases generated primarily by human activities like fossil fuel burning and deforestation.
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 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, says coauthor Natalia Rybczynski, of the Canadian Museum of Nature in Ottawa.
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, explains Jaelyn Eberle, associate professor of geological sciences at CU-Boulder.
Ballantyne says 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, explains Ballantyne.
The last line of evidence put forward 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,” says Eberle. “We essentially were able to ‘read’ the vegetation in order to estimate air temperatures in the Pliocene.”
Today, the region is one of the coldest, driest environments on Earth—a polar desert featuring 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.
Warming up, again?
“Our findings are somewhat disconcerting regarding the temperatures and greenhouse gas levels during the Pliocene,” says 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, says 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, says 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, Rybczynski says.
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.
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