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Arctic nearing greenhouse gas tipping point

Doug O'Harra

The Arctic just about sizzled 3.5 million to 4 million years ago during the "warm" geologic period that preceded the Pleistocene ice ages -- temperate enough to grow trees on Alaska's North Slope, says a University of Alaska Anchorage scientist.

Basked in temperatures 18 to 28 degrees Fahrenheit above the present, the boreal ice cap would have completely melted away every summer, while regional glaciers and ice caps shriveled, according to a new analysis of prehistoric mollusk shells recovered from the ancient remnants of a beaver pond on Ellesmere Island.

But these surprising beach-front temperatures and meltdown of polar ice may not be the most dramatic findings from this ingenious new study that pioneered a high-tech method to analyze fossils for more accurate clues to the ancient climate of the Pliocene Epoch.

Modern levels of the greenhouse gas carbon dioxide are now flirting with the same CO2 concentrations that triggered this hot period, the researchers say. We're collectively toeing the same threshold -- and could face the same consequence.

In other words, without dramatic reductions in greenhouse gas concentrations, and soon, our fate might be already cooked.

"Our data from the early Pliocene, when carbon dioxide levels remained close to modern levels for thousands of years, may indicate how warm the planet will eventually become if carbon dioxide levels are stabilized at the current value of 400 parts per million," said Aradhna Tripati, one of the study's seven authors and an atmospheric scientist at the University of California Los Angeles, in this story.

For Alaskans, the findings offer one startling glimpse into a possible future for the state's Arctic regions, says lead author Adam Csank, a new UAA researcher who led the project for his Ph.D. studies at the University of Arizona.

Climate clues from an ancient beaver pond

"This study highlights that temperatures (18 to 28 degrees F) warmer on Ellesmere Island results in switch from high-Arctic tundra to an ecosystem much like we have here in Anchorage, with boreal forest, beavers and bears," Csank told Dispatch in an email message. "Ellesmere Island is much further north than Barrow, so for Alaskans, what this could mean is a forested North Slope."

The study, to be published in April 15 in the print issue of Earth and Planetary Science Letters, describes the latest results from the "Beaver Pond," a deposit of frozen peat on Strathcona Fiord in far northeastern Canada. Its nickname came from the gobs of the prehistoric beaver-chewed branches trapped inside.

Since 1988, paleontologists have been mining this deposit, uncovering a extraordinary trove of Pliocene fossils that includes an extinct species of beaver, three-toed horse, badgers, black bears, shrew, rabbit, fish, frogs, insects, mollusks, beaver-chewed trees, plants, cones, seeds and pollen. These fossils date from a time that's particularly intriguing to climate scientists because continental geography, intensity of sunlight hitting earth and CO2 levels closely resemble modern conditions.

The natural presence of gases like CO2 keeps the home planet warm enough to sustain life. The sun heats the surface, and these gases slow the loss of that heat back into space. They essentially act like glass in a greenhouse. As you might expect, the overall concentration of these gases in the air exerts great influence over the particulars of the Earth's climate, pushing it toward steaming, fetid swamps when levels peak, or chilling it down toward bleak continental glaciers and steppes when levels plunge. 

After thousands of years of hovering at about 300 parts per million, CO2 levels began a sudden sustained rise in the 19th century, a change that most scientists now blame on the increased burning of fossil fuel during the Industrial Revolution and the onset of modern, population-dense life. The Earth's CO2 level is now almost 392 parts per million -- levels last seen 4 million years ago during the Pliocene.

"The Intergovernmental Panel on Climate Change identifies the early Pliocene as the best geological analog for climate change in the 21st century and beyond," explained Tripati, in this story. "The climate-modeling community hopes to use the early Pliocene as a benchmark for testing models used for forecasting future climate change."

"Although there are differences between the Pliocene and today, the Pliocene is the most recent geologic period when global temperatures were the order of those predicted by the IPCC for the future," Csank added. 

But how do you get a fix on air temperature that occurred something like 1.2 billion days ago? Analyzing air bubbles trapped in Greenland and Antarctic ice sheets can offer climate data going back about 800,000 years. But to look much deeper into the past, scientists must find other clues.

To conjure glimpses of the Arctic climate for the entire Pliocene, 2.6 million to 5.3 million years ago, many of the same researchers previously deployed three different sources from the same Ellesmere beaver pond -- oxygen molecules fixed inside fossil trees and moss, fat content of prehistoric soil bacteria and an inventory of what plants were present, according to a study published last summer.

They found that the average annual temperature of Ellesmere Island was much warmer than it is today, according to a story published by the University of Colorado at Boulder. That difference was produced by CO2 levels at about 400 parts per million -- a concentration that meant the High Arctic region hovered right at the freezing point, said Ashley Ballantyne of the University of Colorado Boulder's geological sciences department.

That bit of data was not particularly good news for the present fate of polar ice. The average annual surface air temperature of the Arctic has been rising over the past century, from a few degrees below freezing to a few degrees above, according to the latest estimates.

If overall temperatures remain above freezing long enough, permanent and glacial ice will become "exceedingly difficult to maintain," Ballantyne explained last year.

"Our findings are somewhat disconcerting regarding the temperatures and greenhouse gas levels during the Pliocene," added co-author Jaelyn 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."

The Arctic is more sensitive than expected

"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," the authors of that earlier study concluded. The latest research goes a step further by taking a new angle in quest of even more "robust" measurements. It draws on the clues hidden inside the shells of mollusks that last tasted air some 3 million to 4 million years ago.

Csank, a graduate student at Arizona in the 2000s, and Tripati, one of the original researchers, came up with a plan to use two independent techniques to measure the proportion of certain molecules inside the fossil shells as a way to pin down the Pliocene climate with more accuracy. Csank gathered specimens in 2004, and others dug up more material in later years. Then the old clam fragments went to the laboratories at four different universities in two countries.

They were looking for the tiniest of hints: molecular isotopes. Different kinds of oxygen and carbon molecules -- these isotopes -- get fixed inside the shells of freshwater mollusks at different levels and ratios, depending on the temperature when they were formed.

So -- pin down the isotope ratios and levels, and you can figure out how warm or cold it was when the mollusks were alive. 

In this case, Csank and Tripati and their co-authors "pioneered a new method for measuring past temperature using only the calcium carbonate found in fossilized shells" that didn't require any plant specimens.

Working on the project with Csank, Tripati, and Ballantyne were William Patterson of the University of Saskatchewan, Natalia Rybczynski, a paleobiologist at the Canadian Museum of Nature, Robert Eagle and John Eiler, at the California Institute of Technology.

So why go to all this trouble?

Sorting out what might happen to the Arctic and Antarctic -- and the fate of polar ice -- is one of the most important issues in the science of climate change. The cryosphere functions sort of like a planetary air conditioner, reflecting solar energy back into space and cooling off the atmosphere. But the health of the ice is sensitive.

The Beaver Pond results suggests that "the carbon dioxide threshold for maintaining year-round Arctic ice may be well below modern levels," Tripati said. "In some sense, the Arctic serves as the proverbial canary in the coal mine, the first warning sign of fast-approaching danger."

Lately, the canary has been gasping, so to speak. Last September, Arctic ice covered the third smallest extent seen the age of satellite monitoring began in 1979, continuing a string of near-record melt-backs. As of early March, the sea ice rebounded to the one of the smallest winter maximums ever recorded. It has since begun to shrink.

Alaska offers more clues to Pliocene climate

Csank, who will become postdoctoral researcher next month at the Environment and Natural Resources Institute at UAA in Anchorage, is now working on a study of modern carbon cycling in northwest Greenland. But he plans to keep digging for clues into Alaska’s prehistoric climate.

On May 6, Csank will give a presentation on what these ancient fossils can tell us about the future climate, during the Classrooms for Climate symposium at UAA.

“There are abundant Pliocene fossil forest deposits in Alaska that I have been interested in working on for some time. In fact I have some samples sitting here in the lab from Circle,” he said in an email to Dispatch. 

"Stay tuned for my future work, which will look at what Alaska specifically looked like during the Pliocene (and) could provide more clues to what the Alaska of the future may look like."

Contact Doug O'Harra at doug(at)alaskadispatch.com