Taking Credit for Forest
CARBON SINKS
Is the policy getting ahead of the science?
JANET PELLEY
T
he use of forests to store carbon has tremendous appeal as a quick tool to help slow global climate change while the world waits for the introduction of new energy technologies and stringent limits on greenhouse gas emissions. The concept is simple—plant new forests or enhance the growth of existing ones to rapidly soak up and store CO2 from the atmosphere. However, the approach requires estimating how much carbon forests absorb or emit, and there is significant uncertainty in the current forest carbon models that make those predictions. Whether forests are a carbon source or sink is an especially important issue for countries that have ratified or plan to ratify the Kyoto Protocol and that have been allowed to claim generous credits. The 2000 Bonn Accord of the Kyoto Protocol set a forest carbon credit ceiling for developed countries, with Canada, Russia, and Japan winning the largest concessions. Assuming the Kyoto Protocol is ratified, these countries must decide by 2006 whether or not they will claim a forest credit under the treaty’s first commitment period from 2008 to 2012. As decision makers turn to scientists for answers, the scientific uncertainties over forest carbon measurement have come to the forefront, exemplified by a controversy in forest-rich Canada. Two leading models of Canada’s forest carbon balance draw opposite conclusions, says John Stone, senior climate change adviser with Environment Canada. One finds that Canada’s 404 million hectares of forest are a modest sink for carbon, whereas the other concludes that they are a modest source. How this controversy is resolved could inform worldwide efforts to model forest carbon storage.
PHOTODISC
Looking for credits Boosting carbon storage in forests can earn “credits” under the proposed Kyoto Protocol for limiting greenhouse gas emissions, but forest managers must be able to measure carbon flux over intervals as short as four years. No standardized method has been developed to track forest carbon, and, as seen in the
© 2003 American Chemical Society
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Canadian case, competing methods and models yield different results. These scientific uncertainties could mean that companies and countries that are hoping to gain credit for their forest investments may be stymied by their inability to measure just how much carbon they have sequestered, some researchers say. In the meantime, the science of forest carbon sinks is improving, and companies and some researchers say it is better to invest in sinks now than wait until all the answers are in. The Kyoto Protocol also allows countries to claim and trade emissions credits for new forests planted on lands, such as farms, that weren’t forested before 1990. Commercial forests that are managed to promote tree spacing and that are carefully maintained to prevent fire and disease outbreaks are also eligible for sink credits (1). Such regulation has sparked hopes that a market for trading forest carbon credits may be created once the Kyoto Protocol is fully implemented, says Ken Ogilvie, executive director of Pollution Probe, a Toronto environmental group. Forest carbon sinks have a significant potential to mitigate the rise in mean global temperatures caused by the increase in atmospheric CO2 concentrations, according to the Intergovernmental Panel on Climate Change (IPCC). Of the roughly 8 gigatons (Gt) carbon released into the atmosphere each year by fossil fuel burning and deforestation, plant growth on land absorbs about 2.5 Gt of carbon, or ~30% of all of the annual anthropogenic emissions of greenhouse gases (2). Overall, the world’s forests store two-thirds of all terrestrial carbon, nearly 1 trillion tons (3). Preservation of existing forest stores, along with recovery of cut forests and planting of new ones, could mean that an additional 60–87 Gt of carbon could be sequestered worldwide by 2050, according to IPCC estimates. FIGURE 1
Global forest carbon pools Estimates of where forests sequester carbon find that more of the carbon is trapped in the soil (brown section) than in the tree’s biomass (green section).
Gigatons carbon
600 500 400 300 200 100 0 Boreal
Temperate
Tropical
Source: Adapted from Reference (4).
Forest ecosystems are able to transfer carbon from the air into plant tissue and, eventually, into the soil, explains Art Fredeen, forest ecologist at the University of Northern British Columbia. Over time, a greater proportion of the carbon is accumulated as decaying plant material in the soil than as tree biomass 60 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / FEBRUARY 1, 2003
(Figure 1). For instance, the boreal forest that encircles the northern latitudes of the globe contains roughly 7–8 times as much carbon in the soil as it does in the trees, says Fredeen. On the other hand, tropical forest carbon is split roughly even between trees and soils. As long as photosynthesis outpaces respiration, the ecosystem acts as a sink. But carbon released by fires, insect outbreaks, and elevated rates of respiration can sometimes turn the forest into a carbon source, Fredeen notes. These numbers have caught the eye of the Canadian government. Environment Minister David Anderson has stated that he expects that forest and agricultural carbon sinks will account for 10–15% of Canada’s effort to reach its Kyoto target (5). In June, Canada pumped more than C$12 million into a new carbon sink research program. Canada is anxious to make the sink policy work because the federal government sees it as an easy way to sell the Kyoto Protocol, which the country ratified on December 16. Under Canadian law, the federal government cannot implement the Kyoto Protocol without provincial cooperation, and, says Heather Smith, political scientist at the University of Northern British Columbia, two powerful provinces, Ontario and Alberta, currently oppose the treaty. Determining which forest carbon model is most accurate will have a significant impact on Canada’s strategy to cut its greenhouse gas emissions, Stone says. “Scientists have been charged with making predictions on the basis of the best available data and knowledge, but the ‘best available’ may not, in fact, be all that good,” warns Sean Thomas, tree ecophysiologist at the University of Toronto.
Models disagree According to the Integrated Terrestrial Carbon Cycle Model (InTEC), Canada’s forests absorb about 50 megatons (Mt) of carbon per year, says Jing Chen, biogeochemical modeler at the University of Toronto. InTEC integrates remote sensing images with climate, soil, and forest inventory data from 1901 to 1998 to show the distribution of carbon sources and sinks in Canada’s forests in 1-square kilometer (km2) grids (Figure 2) (6). The model found that stores of carbon in Canadian forests have declined over the past two decades because of increasing levels of disturbance from fire and insect damage, Chen explains. But at the same time, a warming climate and elevated levels of the plant nutrients nitrogen and CO2 from fossil-fuel burning have accelerated growth, and these factors outweigh the negative effects of disturbance. Over the past 100 years, the average spring temperature in Canada has risen by 1.2 °C, lengthening the growing season by six days and boosting net ecosystem productivity, Chen says. Although the longer growing season only adds 30 grams of carbon per square meter per year to net ecosystem productivity, when summed across the whole country, it makes forests a sink, instead of a source, for carbon, he says. Whereas Chen’s model is driven by remote sensing data, the Carbon Budget Model of the Canadian Forest Sector uses detailed on-the-ground inventories
of forest biomass, says Werner Kurz, ecosystem modeler at the Canadian Forest Service’s Pacific Forestry Center in Victoria, British Columbia. The model surveyed more than 12,000 records, each characterizing a specific forest ecosystem type and age, to simulate changes in ecosystem carbon pools and fluxes from 1920 to 1989 (7 ). Kurz’s model showed that in 1920, Canada’s forests sequestered roughly 250 Mt of carbon per year, but from 1985 to 1989, the forest released ~70 Mt of carbon to the atmosphere each year. The forest’s transition from a sink to a source of carbon came about during the 1980s when the area affected by fire and insect damage more than doubled, Kurz explains. The key difference between the two models is that Kurz has derived a relationship between forest age structure and biomass by averaging all available inventory data from the past 100 years, Chen says. Because growth factors such as temperature and CO2 concentration are higher now than they were 100 years ago, Kurz’s relationship doesn’t represent current conditions, he claims. Chen and his colleagues have modified the forest age structure and biomass relationship to account for a stronger growth response to present-day temperature and CO2 levels. However, Kurz says that the weak point in Chen’s model is that the cumulative effect of a longer growing season and CO2 and nitrogen fertilization amount to a 25% increase in net primary productivity. “Many scientists doubt that net primary productivity could be increased so much and haven’t observed it in natural forest ecosystems,” he says. It is likely that other limiting factors, such as precipitation and nutrient availability, are interacting to curtail the optimum tree growth resulting from rising temperature and CO2 levels, Kurz adds.
Lots of uncertainties Other researchers are more circumspect about their predictions. “Satellite data show that there is a clear trend of longer growing seasons and increased productivity in northern latitude forests,” says Steve Running, forest ecologist at the University of Montana. The growing season has lengthened by as much as two weeks in parts of Western Canada, he says. Inventory data from intact, unmanaged forests in Europe and the United States also reveal accelerated growth over the past 20–30 years, he adds. “This suggests there is a potential to store more carbon on the landscape,” Running says. But a warmer and longer growing season could also elevate respiration from plants and soil decomposers that release carbon to the atmosphere. It could also favor more carbon-generating fires and insect outbreaks. Whether a longer growing season means more carbon sequestration is still an open question, especially for Canada, he concludes. “I think that the jury is still out on whether CO2 fertilization has a big effect on carbon sequestration,” adds Chris Field, an ecologist at Stanford University. Recent experiments on CO2 fertilization of forests are inconclusive, with different sites responding with either higher or lower growth rates for unexplained reasons, he says.
Two major unanswered questions are how long does the response to higher CO2 concentrations last and what environmental conditions are compatible with stimulation of growth, Field says. New research on grasslands indicates that carbon fixation increases as CO2 concentration rises from preindustrial levels but then levels off at present-day levels (8), he says. Recent findings hint that ecosystems adapt to climate change by decreasing their response to higher CO2 and temperature levels, adds Tony King, ecosystem modeler with Oak Ridge National Laboratory in Tennesee. “Higher CO2 concentrations will stimulate carbon uptake, everything else being equal,” King says. But everything else, such as rainfall, temperature, and nuFIGURE 2
Possible carbon sources and sinks in Canada’s forests. This map was generated by the Integrated Terrestrial Carbon Cycle Model from Jing Chen’s group at the University of Toronto and describes sinks and sources over the period 1994–1998. The model takes into account carbon absorption by vegetation and releases arising from fires, insect damage, and decomposition of dead organic matter in soils. 175°W
165°
150°
120° 90°
60°
30°
15°
10°
65°N 60°N
60° 55° 55° 50° 50° 45° 45° 40° 40°
1.5 1.0 0.5
Sinks 120°W
0 –0.5 –1.0 –1.5
t C/ha/yr
Source
105°
35° 90°
75°
65°
Source: Jing Chen, University of Toronto.
trient availability, is rarely equal, and scientists don’t know how those variables interact, he says. For instance, CO2 fertilization of grasslands stimulates growth only when soils are dry, Field adds. “One of the most challenging issues is how to assess the amount of organic matter that is not in standing trees,” says Steven Wofsy, atmospheric scientist at Harvard University. Most forest inventories track only commercially valuable timber, leaving a lot of carbon unaccounted for. More than 75% of the carbon sequestered in the United States is found in organic matter that is not inventoried, such as woody debris, soil, wood in landfills, and woody plants that have invaded grasslands protected from fire, according to Steve Pacala, ecologist at Princeton University (9). “You can’t determine if a forest is a source or sink for carbon unless you can identify all the carbon pools, especially the underground pools,” Wofsy says. FEBRUARY 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 61 A
SEAN THOMAS, UNIVERSITY OF TORONTO
One of the best ways to do this is by using eddy covariance to directly measure carbon flux in the ecosystem, he adds. Eddy covariance, the comparison of updraft and downdraft CO2 concentrations in tiny puffs of wind moving down into and up out of the forest, can tell you precisely and continuously how much CO2 is taken up by photosynthesis and how much is released by respiration, says Thomas. But the calculations assume that an area has uniform topography, which is not
Researchers head into the treetops to measure carbon flux of an old-growth temperate rain forest located in Oregon near the Pacific Ocean.
helpful in mountainous forests, he says. If there is no breeze, measurements can be thrown off by a factor of 2. Although eddy covariance may not have the precision needed for international treaties or carbon trading schemes, it is giving scientists new information about the way carbon flux responds to changes in light, temperature, and rainfall, Thomas says.
Flux studies challenge old assumptions “Before policy makers argue about the details of carbon credits and trading, we need to get an unbi62 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / FEBRUARY 1, 2003
ased measurement of what the biosphere is doing,” Running says. This was the impetus behind Fluxnet, an international cooperative network of 200 sites across the globe combining flux towers with remote satellite measurements of photosynthesis (10). Canada reinvigorated its flux program this June with a $12 million investment in 20 towers nationwide, according to Hank Margolis, ecologist at Laval University in Quebec. The international network is already yielding surprising results, he says. By comparing eddy covariance with inventory and biomass sampling measures, Thomas and his colleagues have found that old-growth forests in North America’s Pacific Northwest, once thought to be neither sources nor sinks for carbon, are, in fact, modest sinks. While a young, vigorous forest may take up carbon at a greater rate, it doesn’t mean you should cut down old-growth forests for carbon management, Thomas says. It could take 200 years for that young forest to accumulate as much carbon as is stored in the old-growth forest, he notes. To determine the source/sink status of forests, it is essential to focus on how individual ecosystems respond to climate change, Wofsy says. He and his colleagues found that hydrology, not forest biomass, is the key factor determining the carbon balance of the forest based on nine years of eddy covariance measurements at a flux tower in Manitoba’s boreal forest (11). With a warmer winter and longer growing season, models predicted the ecosystem should be storing more carbon. Indeed, the trees were growing faster than 30 years ago, but flux measurements revealed the ecosystem was losing carbon to the atmosphere from elevated rates of soil decomposition. Warmer temperatures meant the top level of the permafrost was melting, leaving more of the two-meter blanket of peat soils to decompose in the summer. But over the past few years, higher rainfall has waterlogged the soils, which has slowed decomposition and brought the forest into carbon balance, Wofsy says. Research in Brazilian tropical forests also demonstrated that wetness or changes due to climatic variability are crucial for carbon storage, Wofsy says. He and hundreds of scientists are participating in the Large-Scale Biosphere–Atmosphere Experiment in Amazonia, a cooperative international project led by Brazil beginning in the late 1990s. Results from a flux tower site near Santarém suggest that the old-growth forest ecosystem is losing carbon, even though the trees are taking up large amounts of carbon, and there is now abundant moisture. El Niño droughts over the past 10 years have dumped a lot of decaying wood onto the forest floor. The present wet conditions that foster rapid tree growth also favor high rates of decomposition, releasing CO2 into the atmosphere. But it is hard to generalize from this one site, Wofsy says. “Over the whole forest, you expect to see patchiness in terms of areas that are losing or gaining carbon, which makes it hard to make general statements about long-term trends,” he cautions. Hydrology plays a different role in the aspen forests of Saskatchewan, says Andy Black, micrometeorologist at the University of British Columbia. In
SEAN THOMAS, UNIVERSITY OF TORONTO
warm El Niño years, the forest absorbs three tons of carbon per hectare per year, thanks to a longer growing season. In cool, La Niña-like years, carbon accumulation slips to 0.5–0.75 tons of carbon per hectare per year. But in 2001, which was warmer and drier than usual, carbon sequestration surpassed three tons per hectare per year. The findings suggest that although soil moisture was sufficient for tree growth in 2001, soil microorganisms didn’t have enough water, and respiration Close-up of treetop carbon flux measurement being conducted. dropped below normal levels, Black says. This accuracy,” says Sandra Brown, senior program officer new information on how different ecosystems vary with Winrock, International, a consulting firm. Her in their response to changes in temperature and moisture has not yet been incorporated into carbon firm has provided design and measurement services sequestration models, he adds. to sequestration projects in the tropics and North America. Inventory data coupled with models deterIs the risk acceptable? mined aboveground carbon storage, while analysis “Would you buy something no one knows how to meaof the percentage of carbon in soil combined with essure?” Thomas asks. With all the uncertainties undertimates of soil volume yielded figures for belowground lying the measurement of carbon sequestration, carbon storage, she says. policies that encourage trading and credit for forest “Forest carbon storage buys time to make carbon sinks are getting ahead of the science, he warns. changes in the energy system because it takes carNevertheless, many companies are forging ahead bon out of the global cycle quickly,” says Tony with investments in forest carbon sinks. American Janetos, vice president for science and research at Electric Power has invested more than $75 million in the World Resources Institute. Although there are so-called Kyoto forests, General Motors paid $10 milsignificant issues to be resolved concerning estilion to help regenerate Brazilian rain forests, and AES, mates of biomass accumulation over large areas, the a power utility, has purchased 14.1 million tons of profits to be made from trading will help direct atcarbon offset credits in Guatemala, according to the tention toward solutions to the technical problems, World Resources Institute. he predicts. Companies are well aware of the risks, says Paul Vickers, engineer with Trans Alta, an energy firm. “A Janet Pelley is a contributing editor to ES&T and is based in Toronto. number of companies have looked down the road 10 to 30 years and have seen that there is a very significant chance that we will have to decrease or seReferences quester our greenhouse gas emissions,” he says. (1) The Convention and Kyoto Protocol; http://unfccc.int/ Reforesting degraded land to sequester carbon, as resource/convkp.html. Trans Alta recently did in the Mississippi River Delta, (2) Intergovernmental Panel on Climate Change Third Assessment Report, Climate Change 2001; www.ipcc.ch. helps companies gain experience before rules are (3) Totten, M. Getting It Right: Emerging Markets for Storing put in place and helps manage the risk of looming Carbon in Forests; World Resources Institute: Washington, greenhouse gas laws, he says. These laws include DC, 1999. new requirements to offset CO2 emissions in the (4) Intergovernmental Panel on Climate Change Special states of Oregon, Washington, New Hampshire, Report, Land Use, Land-Use Change, and Forestry; https:// www.grida.no/climate/ipcc/land_use/index.htm. Massachusetts, and California, Vickers notes. “I be(5) Environment Canada. Forests and Agriculture Carbon lieve that investing in carbon sinks and trading sink Sinks & the Kyoto Protocol; www.climatechange.gc.ca/ credits [are] desirable because it takes CO2 out of english/whats_new/forests_e.html. the air and is a powerful mitigation tool at a good (6) Chen, J. M.; et al. Tellus Special Issue of the 6th International CO2 Symposium 2003, in press. cost,” says Richard Sandor, chief executive officer of (7) Kurz, W. A.; Apps, M. J. Ecol. Appl. 1999, 9, 526–547. Environmental Financial Products and chair of the (8) Gill, R. A.; Polley, H. W.; Johnson, H. B.; Anderson, L. J.; Chicago Climate Exchange, a greenhouse gas emisMaherali, H.; Jackson, R. B. Nature 2002, 417, 279–282. sion trading market. (9) Pacala, S. W.; et al. Science 2001, 292, 2316–2320. “There is a lot of interest in forest carbon sinks, and (10) Fluxnet; www-eosdis.ornl.gov/FLUXNET. (11) Goulden, M. L.; et al. Science 1998, 279, 214–217. total carbon can be measured with a high degree of
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