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New Science: Should Carbon Credits Grow on Trees?

Alice Kenny

Scientists in Europe recently discovered trees may release large amounts of methane into the atmosphere. The Ecosystem Marketplace finds out how the study is changing our understanding of ecosystem services in forests.

Scientists in Europe recently discovered trees may release large amounts of methane into the atmosphere. The Ecosystem Marketplace finds out how the study is changing our understanding of ecosystem services in forests. Ever since a young German scientist and his colleagues released a study announcing that forests, prized for absorbing the greenhouse gas carbon dioxide, may simultaneously spew out methane, a fellow-greenhouse-gas, the carbon-trading world has convulsed. From the Brazilian rainforest to Manhattan skyscrapers, scientists, economists and politicians are scrambling to weigh in on this possible breakthrough. Some say the study's results pull the rug out from under the carbon-credits-for reforestation-trading system approved under the Clean Development Mechanism (CDM) of the Kyoto Protocol, highlighting the limits of paying others to clean up one's own mess. Meanwhile, whiplashed by media frenzy, the scientists who published the study in Nature in January caution that this interpretation may be exaggerated. Still others question the study's conclusions, adding that they detract little from the science surrounding carbon trading. Now, just a month after the 132 signatory nations celebrated the first anniversary of the legally binding enforcement of the Kyoto Protocol to reduce human-generated greenhouse gas emissions, this new controversy adds to the debate over one of the Protocol's mechanisms which allows countries and companies to buy stock in new or restored forests to offset emissions from burning fossil fuels like coal and oil. It also underscores complexities surrounding the larger issue of how to measure and monitor the results of environmental markets. With best-available science constantly changing — especially in a new field like ecosystem services — markets need to find a way to meet disparate but essential goals: regulatory consistency, flexibility to accommodate scientific advances and assurance that they answer ecosystem objectives. Referring to the forest-methane-emissions discovery in particular and ecosystem marketplaces in general, Todd Dawson, professor of integrative biology at the University of California Berkeley sums up the controversy. "This is a new idea that will raise a lot of eyebrows. The finding challenges the whole idea of using forests as a place where carbon is only sequestered. Then again, that's what science is all about; we discover new things."

Never Simple

CO2, methane and other gases stored in the Earth's crust have warmed the planet since its formation. Exhaled into the atmosphere through volcanoes, rotting vegetation, and flatulent animals, the gases create a greenhouse effect, warming the Earth's surface by 93 degrees Fahrenheit. School children learn that through the process of photosynthesis, plants use sunlight to absorb CO2, the primary greenhouse gas, while creating sugar and oxygen. They do this so efficiently that forests suck up about a third of human-induced carbon emissions, according to recent studies. And so, once scientists discovered that burning fossil fuel dangerously accelerates the natural greenhouse gas effect, planting more forests to absorb increased carbon emissions seemed like a logical solution. The carbon trading market born out of the Kyoto Protocol capitalizes on this solution. It prompts industries and nations exceeding their carbon-emissions allowances to purchase credits from developing countries where new forests have been planted. At first glance, Keppler's report appears to cast doubt on underlying assumptions supporting this market. According to his measurements, the same forests that absorb CO2 may simultaneously emit methane, the gas that comes in second after CO2 in contributing to the greenhouse effect. This adds a significant twist to scientific understanding. Scientists had believed that plant-related methane formed only in oxygen-scarce environments such as swamps and leaking landfills. Yet Keppler measured methane in isolated chambers containing live plants. Perhaps even more astonishing was Keppler's extrapolation that live plants may contribute from 10 to 30 percent of global methane emissions. That means, says National Institute atmospheric chemist David Lowe that "we now have the specter that new forests might increase greenhouse warming through methane emissions rather than decrease it by being sinks for carbon dioxide."

Skeptical Scientists

Keppler's revolutionary theory has met with skepticism within the scientific community. From laboratories in California to field studies in rain forests, scientists who spent their careers studying methane are scratching their heads. They are questioning whether Keppler's discovery it accurate; speculating about the impact it will have on the forest-for-carbon trading market; and wondering how they could have overlooked such a significant methane source. Biogeo-chemist at the University of California Berkeley Robert Rhow comments, "the process is incredibly novel and scientifically fascinating but the extrapolation (that plants produce 10-30 percent of methane in the atmosphere) is a little hard to swallow. It cries out for further study." Since scientists believed that methane emissions came from oxygen-free sources and not from live plants, few before Keppler measured plant emissions for this gas. But when Keppler and his colleague's detected methane while experimenting with dead leaves, they decided to investigate further. They put fresh leaves in sealed glass vials that had been purged of methane and live plants in Plexiglas chambers that had been similarly purged. Then they transferred the gas every 25 minutes into an analytical system. Each time they found methane. Coincidentally, Michael Keller, a scientist with the U.S. Forest Service International Institute of Tropical Forestry, recently completed a not-yet-published study that comes up with methane results consistent with Keppler's. Yet Keller is circumspect about its implications. "That our results were consistent does not prove that methane emissions come from plants," he says when reached by phone in San Paolo, Brazil. "The methane could have come from live plants or it could have been from non-oxygenic sources such as termites, decomposing plants or soil sediment that we already know produce methane. It will be quite a challenge to tease that out." But what if Keppler's discovery is correct? Will it undermine the value of using forests as sinks to absorb carbon dioxide? The answer is "no," says Keppler, his coauthors, and fellow scientists studying their records. Paging through data spread across his sunlit desk in New York City, Chief Scientist of Environmental Defense Bill Chameides assigns numbers to this equation. The warming caused by methane emissions from a temperate forest in the United States amounts to only 1 – 10 percent of the cooling gained from carbon dioxide sequestration, he says. Methane emissions by plants would only slightly diminish the benefits gained by their ability to absorb carbon, contends Thomas Rockman, a coauthor of the original study. "Climatic benefits gained through carbon sequestration by reforestation far exceed the relatively small negative effect (of methane production)," he says from his office at Utrecht University's Institute for Marine and Atmospheric Research in the Netherlands..

Buying Time

In environmental markets where scientific discoveries can transform economic and environmental bottom lines, Keller's discovery spotlights uncertainties surrounding the new field of ecosystem services. Some say Keller's work offers enough ammunition to throw out the whole forest-for-carbon trading scheme. But others say that while additional steps can be taken, the carbon trading market has already been designed with enough flexibility to accommodate evolving knowledge. University of Berkeley scientist Dawson stands among the critics. "We think of trees in a static way, as if they were a pump," he says. "The problem is the carbon sink has a knob on it. Sometimes it is turned in a positive direction that absorbs carbon; sometimes it's turned in a negative direction when it emits carbon." Forests can burn down, frozen peat moss can thaw, and now, it appears that trees may emit the greenhouse gas methane. These types of risks, others counter, are no reason to abandon a market. "Farming futures, for example, are always uncertain," says Chameides, "but the market thrives despite uncertainty." What makes markets successful, he adds, are mechanisms developed to account for uncertainties. From his perspective, the carbon market already has allowances to accommodate leakage and additions such as the newly discovered information concerning methane emissions. Additional mechanisms can be incorporated to keep the market nimble enough to take advantage of new discoveries, said Steve Pacala of Princeton University's Climate Mitigation Initiative. First, to increase accurate projections, nations should undertake comprehensive forest inventories that determine greenhouse gas uptakes and forest-sequestering longevity. Then, to foster investor confidence, these inventories could be certified nationally. To provide room for new discoveries, certifications would be renegotiated every ten years. And to offer insurance to investors, certifications would be grandfathered during the ten-year contract period. "We absolutely have enough information to get started," Pacala says. "The rest is just wrinkles." Despite the brouhaha Keppler's report sparked, scientists from across the globe generally seem to agree with Pacala's assessment, stressing that the key is to start now and refine later. "We have to take the greenhouse effect very seriously," says Keller. "The climate will warm, sea level will rise and this will have enormous consequences on people's livelihoods. This is unprecedented in the history of our species." To avoid disaster, emissions will have to be decreased by 70 percent within the next century, Chameides says, an enormous goal that new and rebuilt forests cannot meet. This means that new technologies that sequester carbon or provide alternative, non-fossil fuel energy sources will have to be established by mid century. This would likely trigger a new market and income source that could replace carbon-for-forest trading. "We need a bridge to get us there," Chameides says. "Forests represent that bridge. Yes, they're a band aid. They don't solve the problem but they buy us time to solve the problem." Alice Kenny is a prize-winning science writer and a regular contributor to the Ecosystem Marketplace. She may be reached at

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