Tundra test stuns scientists
The Toronto Star,Sept. 23, 2004
OTTAWA -- Dramatic results made public today from a unique 20-year American experiment are raising the spectre of runaway warming above the Arctic tundra that would accelerate global climate change.
The findings, if confirmed with additional studies, could also doom Canada's Kyoto plan targets for reducing emissions of carbon dioxide, the leading greenhouse gas.
This double whammy arises because U.S. researchers discovered climate warming might trigger conditions where tundra decomposition will dump carbon dioxide into the atmosphere faster than it's soaked up by accelerated plant growth.
This extra carbon dioxide could trigger a "positive feedback," speeding up the rate of global warming even more, warns a study published today in Nature, the influential British research journal.
The results also add extra urgency to a planned November meeting of ministers from Canada and seven other circumpolar countries, called to approve a program to tackle the impact of climate change in the Arctic. More than 40 per cent of the world's Arctic vegetation region lies in Canada.
Until now, most studies projected that the tundra would be a carbon dioxide "sink" rather than a source under climate change. Meeting Canada's Kyoto targets depends on getting credit for net reduction of carbon dioxide in the tundra.
But if all the carbon currently stored as peat, moss and other ancient vegetation in the top metre of tundra decomposed, that would boost global atmospheric levels of carbon dioxide by roughly 25 per cent, says Paul Grogan, a Queen's University expert in northern ecosystems.
"This study raises some big questions," said Grogan, holder of a Canada Research Chair who co-authored a commentary about the findings in Nature.
Grogan and the study's lead author, Michelle Mack, both emphasized that the experiments carried out at a long-term ecological research site in Alaska looked at just one key aspect -- the impact of more nutrients in the soil -- in the complex cycling of carbon between the atmosphere and the earth.
The findings might not hold true in different northern regions, like the immense boreal bogs or the so-called polar desert.
And other environmental influences need to be included in any calculation, such as permafrost thawing and warmer soil, the researchers say.
"There is a lot of diversity in tundra," cautioned Mack, a professor in the University of Florida's botany department who carried out the field work as a researcher at the University of Alaska in Fairbanks.
Despite such cautions, the experimental findings are so dramatic and unexpected that the study has already aroused great interest among Arctic ecologists and climate-change scientists.
"These results challenge some of our assumptions that more vegetation and trees mean that you're automatically storing net carbon, even if only temporarily," says Tim Moore, a McGill University geography professor investigating the carbon cycle in a bog near Ottawa.
Mack found that the artificially fertilized tundra plots near Toolik Lake in Alaska suffered a net loss of two kilograms of carbon per square metre in the 20 years between 2000 and 1981, when the experiment began. Most of the loss took place in layers deeper than five centimetres, and had been missed previously because measurements went no deeper than the root level.
"This was probably the most surprising thing that I and my colleagues had seen for a long time," Mack recalled.
The addition of nitrogen and phosphorus fertilizer mimicked nutrient conditions expected under the pronounced warming projected in the Arctic by climate-change models. Microbes become more active as soil warms and digest organic matter, transforming carbon to carbon dioxide and freeing up nutrients like nitrogen and phosphorous that spur plant growth.
The U.S. experiments found that plant growth doubled atop the tundra, with a knee-high species of woody shrub replacing low sedges. But the extra carbon locked up in this new vegetation was outweighed by carbon released through accelerated decomposition and leaching in the tundra.
"These are the first people to look at what happens by digging deeper in the soil," said Grogan.
The Queen's professor has just launched a similar research project at Daring Lake, some 480 kilometres north of Yellowknife in the Northwest Territories. Unlike the U.S. experiment, the Canadians are varying the concentrations of fertilizer applied, and using greenhouses to raise temperatures and snow fences to manipulate the snow cover.
Grogan estimated it would take at least five years to produce the first results.
Canada lags far behind the U.S. and other circumpolar countries in almost every area of Arctic science because low federal funding over the last two decades has stymied older northern researchers and discouraged new scientists from entering the field
National Science Foundation
Fairbanks, Alaska -- Institute of Arctic Biology (IAB) ecologists Donie Bret-Harte and Terry Chapin and colleagues working in northern Alaska discovered that tundra plants and soils respond in surprisingly opposite ways to conditions that simulate long-term climate warming.
Their findings are published in the September 23, 2004 edition of the leading science journal Nature and are featured in the journal's News and Views section.
Bret-Harte, Chapin, lead author Michelle Mack of the University of Florida, Gainesville, and colleagues set out to investigate whether the commonly held assumption that a warming climate will lead to bigger plants that can store more carbon and thereby reduce atmospheric carbon dioxide was indeed a silver lining in the global warming cloud that some people had hoped for.
"The broadest implication of this research is that climate warming could lead to a much greater release of carbon dioxide to the atmosphere and a greater positive feedback to further warming than we originally thought," Bret-Harte said.
In the experiment, conducted at IAB's Toolik Field Station, researchers measured the amount of carbon and nitrogen in plants and soils from plots of tundra that have been continually fertilized since 1980 -- a condition thought to simulate the increased nutrient availability expected as a result of a warmer climate. The plots are part of a 20-plus-year project by Terry Chapin of IAB, and Gus Shaver of The Ecosystems Center at the Marine Biological Laboratory in Massachusetts.
"One of the greatest values of IAB's Toolik Field Station is that it provides opportunities for long-term uninterrupted research in a pristine environment. We could never have gotten the results we did without such a long-term experiment," said Bret-Harte.
"The connection between fertilization and warming is that warmer temperatures should stimulate decomposition of dead plant material, releasing carbon to the atmosphere and nitrogen to plants. Nitrogen limits plant growth in most terrestrial ecosystems, said Bret-Harte"
"What's really surprising about this result is that we didn't expect that this big loss of carbon from the soils would be stimulated by nitrogen alone. Everyone had assumed increased decomposition would be caused by increased temperatures, and the main effect of increased nitrogen would be to stimulate plant growth and store more carbon. We expected that fertilization by itself would lead to increased carbon storage."
"Instead, nitrogen seems to stimulate decomposition and promote carbon dioxide release to the atmosphere from the soils," Bret-Harte said.
The researchers found that although the aboveground portion of tundra plants doubled their productivity under fertilization and, as expected, stored more carbon, the losses of carbon and nitrogen from the deep-soil layers was substantial and more than offset the increased carbon stored in the aboveground plants and plant litter.
Because more than one-third of the world's soil carbon is stored in northern ecosystems -- boreal forest and Arctic tundra -- and is equivalent to two-thirds of the carbon found in the atmosphere, the loss of deep-soil carbon could mean an even greater increase in atmospheric carbon dioxide concentrations than is caused by fossil fuel burning.
"The paradigm is that decomposers (microbes) are always limited by carbon availability and almost never limited by nitrogen availability, but this project suggests that we don't understand decomposition as well as we thought we did. Better understanding of decomposition is necessary to be able to predict what will happen with climate warming in northern ecosystems."
The project was supported by the National Science Foundation (NSF), the National Aeronautics and Space Administration, the Arctic Long-Term Ecological Research (LTER) programme and an NSF postdoctoral fellowship to Michelle Mack.