User Tools (skip):
The case of the missing sink
| There's a mystery in the science of global warming. Of the seven gigatons of carbon produced annually by us earthlings, about one half of it is absorbed by "sinks," such as the oceans and the land. Which ought to mean that the rest is in the atmosphere in the form of carbon dioxide. But that's not the case. Close to two gigatons of this most important greenhouse gas are missing and Nigel Roulet is following a lead on its whereabouts.
PHOTO: BRONWYN CHESTER
Director of the Centre for Climate and Global Change Research, Roulet — and many other scientists trying to solve the "global carbon budget"— got his first clue when atmospheric models located the missing carbon in the 30-to-90 degree range of latitude. he notes that of all the land sinks, such as forests, agricultural systems and peatbogs, the latter were particularly concentrated in that northern half of the world.
"Peatbogs occupy only two to three percent of the global landscape but at 30 to 60 degrees north they comprise five to 10 percent of the land," explains the geographer from his office.
"Essentially, bogs are found in the boreal forest, in Canada, Alaska, Fino-Scandinavia, the British Isles and Russia." They were formed 8,000 to 10,000 years ago during the last glaciation. While many of the peatbogs in Canada's south have been drained for urban or agricultural development, a huge number remain in the vast boreal forest, which occupies much of Canada. Mer bleue, for instance, the site under analysis by Roulet, geography chair Tim Moore, 10 graduate and post-doc students, as well as colleagues from Trent University, UQAM and the Université de Montréal, is on the outskirts of Gloucester, a suburb of Ottawa.
Walking along the narrow boardwalks, built on stilts overtop of the Sphagnum moss and the bushy leatherleaf plants typical of the bog's flora, Roulet notes that 25 percent of the carbon in land plants and soils is stored in peat deposits. Which is perhaps not surprising considering the age and depth — three to 15 metres — of the peat and the size of some bogs. Finland, for instance, whose Finnish name, "Suo," means peat, is largely bogland.
What gives this soggy ecosystem such carbon-retaining capacity? Precisely its sogginess. For when the Sphagnum moss, which grows on the surface of the bog, or the leaves of plants or trees die in this wet environment, the detritus eventually sinks the 50 cm to two metres below the water level to the dense peat where it decomposes in the absence of oxygen. Because anaerobic (without oxygen) decomposition is far slower than aerobic, the loss of carbon dioxide to the atmosphere, a usual result of decomposition, is greatly slowed.
The result: peatbogs, the world over, act as storage for approximately 10 percent of all fixed carbon, meaning the carbon taken up by mosses and flora. In the forest, for instance, explains Moore, the carbon photosynthesized, or taken in, by the flora is equal to the carbon respired (at night). In the peatbog, however, one of every 10 molecules of carbon photosynthesized remains in the bog. That's a gift to a biosphere — our own — that is well beyond its capacity to absorb all the carbon dioxide produced by its citizens' propensity for burning fossil fuels.
While this trait of peat has been known about for some time in a general way, no one had gone to the bog to measure directly and continuously the exchange of carbon dioxide with the atmosphere. As Moore, an expert in the regeneration of harvested peatbog, puts it: "How the bog stores carbon, in what quantity and how that storage capacity will change with changes in the climate is completely unknown."
"The study of peatland ecology is really behind forest ecology, for instance," he notes. "The areas are inaccessible and of relatively little commercial interest."
Furthermore, it's expensive working in a bog. Given the fragile and wet nature of the environment, there is considerable infrastructure needed long before beginning measurements. At Mer bleue, the boardwalks (to keep rubber boots off the moss and plants) cost $40,000; it cost $22,000 to run in a power line and the highly sophisticated measuring devices cost a further $100,000 —all of which took two years to set up and a $0.5 million, four-year strategic grant from NSERC, with additional funds from FCAR.
Funding and having the right idea at the right time aside, Roulet, principal investigator of the project, points out that he couldn't conduct this research without the highly sensitive technology. "We're measuring the average carbon dioxide exchange over half an hour produced by soil respiration in peat and, during the day, by photosynthesis," says Roulet. "We're doing this now because the technology has only existed for the past five to six years."
He points to the numerous sensors on "the tower" standing behind the project's work cabin. From a distance, there is nothing unusual about the arrangement but up close, the odd conglomeration of "eyes," sensors and the "eddy covariance" (to measure the effects of air currents in carbon dioxide exchange), all wired into the computer inside the cabin, is in stark contrast to this ancient and simple environment.
The two years' worth of measurements, not only of carbon exchange with the bog but how carbon exchange is affected by such variables as moisture, wind, temperature of the air and temperature of the bog's surface, have shown that the bog does retain more carbon dioxide than it gives off. However, what has been a surprise to Roulet is that the peatland turns out to be three times more carbon-rich than predicted. One day, while scratching their heads about this seeming anomaly, Roulet and Moore simultaneously and while teaching their respective courses had a moment of revelation.
"We both looked at our maps of nitrogen deposition for our course [one on global change, the other an introduction to environmental systems] and realized that nitrogen [a component of car exhaust] was falling on the same areas where peatbogs are found. "Peatlands get all their nutrients from the atmosphere," says Roulet. "So we speculate that the reason for larger carbon accumulation is a supply of nitrogen fertilizer." Canada's peatbogs fall in the same territory as the lakes that suffer from acid rain, which is caused, in part by nitrous oxide.
"Nitrogen is a potential explanation for the increase in carbon absorption," says Roulet.
"If we take the numbers from Mer bleue and apply them to all areas receiving nitrogen deposition — which is 50 per cent of peatlands in the north — then peatlands could explain 10 to 20 percent of the missing carbon sink.
Moore is now in the process of experimenting with nitrogen fertilizer on the bog's plants. He's comparing the absorption of carbon dioxide when there is no nitrogen, to an amount of nitrogen equivalent to rainwater to a concentration two times that amount. In the soils lab in Burnside Hall, undergraduate geography students working for Roulet are measuring peat respiration with different levels of nitrogen.
"Our hypothesis is that as nitrogen is increased, soil respiration [the giving off of carbon dioxide] goes down," says Roulet.
One of the next steps in the global understanding of the exchange of carbon dioxide in peatbogs will be to see if Mer bleue is a unique. "We have four to five papers now being reviewed," says Roulet. The papers should stimulate others in the area —and there are only a handful of people looking at peat in North America and Europe — to analyze other bogs.
What will the implications be if Roulet and Moore can show that fertilizing peatbogs with nitrogen greatly enhances the ecosystem's ability to hold carbon? Well, it won't be to recommend a wide-scale fertilizing of these largely ignored landscapes. "No," says Roulet, who religiously takes the train in to work from his home in Beaconsfield. "We can't look to ecosystems to take care of our excessive dependence on fossil fuels."
Rather, the results of this work will be used to help "policy-makers understand the global carbon budget."
Anyone interested in visiting the public section, which is separate from the research section, of Mer bleue can take highway 17 until Anderson Road, then go north for three kilometres. Every fall, Roulet gives a guided tour of this little known landscape.