By Janet Cushman, Gregg Marland, and Bernhard Schlamadinger

There is widespread concern that observed increases in the concentration of carbon dioxide and other greenhouse gases in the earth's atmosphere will ultimately lead to changes in the earth's climate. Although it is clear that the atmospheric concentration of carbon dioxide is increasing and that the increase is being driven in large measure by the burning of fossil fuels (coal, oil, and natural gas), the climatic consequences of increasing atmospheric carbon dioxide are not so clear. Recognizing that fossil fuels play a very important role in the economies and lifestyles of people throughout the world, and acknowledging that great uncertainty exists regarding the climatic consequences of burning fossil fuels, it is reasonable to ask if the global economy can be powered in ways that might have less impact on the environment because they discharge less carbon dioxide.

Janet Cushman and Gregg Marland examine the cross section of a tree, which began sequestering carbon from the atmosphere in 1648. Despite being harvested in 1972, some portions of this tree continue to store carbon in wood products. Bernhard Schlamadinger

Oak Ridge National Laboratory has long been involved in research on alternative energy systems. Much of this research, including the Department of Energy's Biofuels Feedstock Development Program in ORNL's Environmental Sciences Division, began as efforts to develop domestic sources of clean, inexpensive energy. Biomass fuels used in efficient ways might provide a sustainable source of such energy. Researchers in the Biofuels Feedstock Development Program are now studying the environmental and economic issues that stem from biomass production and use as well as developing efficient new biomass energy crops and cropping systems.

Carbon Storage vs Energy Use

The potential role of biomass energy acquired a new dimension when it was suggested that planting large areas of new forest could slow the increase in atmospheric carbon dioxide by removing carbon dioxide from the atmosphere. Two questions then arose: How does using trees to remove carbon dioxide from the atmosphere compare with using biomass as a fuel, and how do these possibilities compare with harvesting forests for conventional wood products?

There are two common, but mutually exclusive, impressions about biomass fuels and carbon dioxide. One first impression is that biomass fuels and fossil fuels are not different because, when burned, both yield carbon dioxide. This is true if land from which biomass is harvested for fuel is not replanted and instead is converted to other uses. However, if the biomass is produced sustainably, the growing trees and other plants remove carbon dioxide from the atmosphere during photosynthesis and store the carbon in plant structures. When the biomass is burned, the carbon released back to the atmosphere will be recycled into the next generation of growing plants. When biomass is used for fuel in place of fossil fuels, the carbon in the displaced fossil fuel remains in the ground rather than being discharged to the atmosphere as carbon dioxide. The productivity, or rate of growth of the trees, becomes an important consideration. While slow-growing trees can take a very long time before the released carbon is recaptured in the next generation of trees, fast-growing trees can recycle carbon rapidly and will displace fossil-fuel use with every cycle.

Wood chips from fast-growing trees are stored in Hawaii.
These chips will later be used as fuel.

A second impression is that biomass energy systems, because they recycle carbon, produce no net emissions of carbon dioxide. This is not strictly true either. It takes some energy, much of it now provided by fossil fuels, to grow and harvest biomass fuel crops and to haul the fuel to a power plant. The use of biomass fuels does result in some discharge of carbon dioxide. The extent to which biomass fuels can displace net emissions of carbon dioxide will depend on the efficiency with which they can be produced and used.

Forests that are not harvested do not continue to accumulate carbon indefinitely. They eventually approach maturity and achieve, over time, a balance between the carbon taken up in photosynthesis and the carbon released back to the atmosphere from respiration, oxidation of dead organic matter, and fires and pests. If fossil fuels continue to be used to meet society's energy needs, reforestation or afforestation of ever larger areas would be needed to prevent increasing concentrations of atmospheric carbon dioxide. Does it make more sense to use trees for energy and to recycle carbon than to store carbon in forests while continuing to burn fossil fuels? Although the system is complex and critical variables are different in different places, it is important to understand the choices available.

Land Use and Carbon Dioxide

Scientists at ORNL have begun to examine a variety of land management alternatives, including whether substituting biomass fuels for fossil fuels could be an effective strategy for reducing net emissions of carbon dioxide to the atmosphere. How can limited resources of land be used most effectively to minimize net emissions of carbon dioxide to the atmosphere while meeting the energy requirements of our global society? Should we preserve existing forests, plant new forests, or develop biomass-based energy systems, or should we encourage the use of long-lived wood products? Is there some other or mixed strategy that is most attractive for minimizing the net emissions of carbon dioxide?

In this discussion we focus on minimizing the risk of global climate change through minimizing carbon dioxide emissions, but we recognize that other criteria go into land-use decisions. Many of these are being evaluated in other portions of ORNL's assessment of biomass energy resources and opportunities. For example, in some regions of the world, deforestation is a major source of carbon dioxide emissions. Currently, an estimated 15 to 20% of atmospheric carbon dioxide emitted by human activities results from deforestation or, more generally, from changes in land use. Clearly, many motivations, including the need for food production, are involved in decisions on land use and will affect the amount of land available for reforestation or for biomass energy crops. Although we are considering the possibility of planting new areas of forest, the rate of growth in atmospheric carbon dioxide could also be reduced substantially by decreasing the current rate at which forest is being converted to other land uses. Coincidentally, the amount of carbon dioxide emitted annually from deforestation around the world is of the same order of magnitude as the amount of additional carbon dioxide that would be discharged if the 14% of primary energy now supplied by biomass fuels globally were instead supplied by oil and coal.

The net impact of land management and the use of biomass-based products on the cycling of carbon will depend on the type of land used, the management practices used on that land, how the biomass products are used, and the time frame of the analysis. Especially important are how much carbon is stored on the land (including in trees and other plants and in the soil and plant litter on the ground) at the beginning and end of the analysis, how much fossil-fuel use is displaced, how much carbon is stored in durable wood products, and how much energy is required for forest and other land-management operations. Also important are how efficiently forest and other biomass products are used and the alternate products for which they substitute, including whether biomass fuels are substituted for coal, oil, or natural gas; whether they are used to produce liquid fuels, heat, electric power, or some combination of these; and the efficiency with which they are used. The net impact on carbon cycling will depend on the mix of forest and other biomass products used for short-lived products like paper, long-lived products like construction lumber, and fuels. It will depend on whether the lumber displaces aluminum, concrete, glass, or plastic. It will depend ultimately on whether the waste products are reused, buried in landfills, burned for energy, or incinerated.

Although we have focused on trees and forest products in our analyses to date, the most advantageous land use for confronting the carbon balance may not necessarily involve trees. If the primary intent is to store carbon on site, the obvious choice is a high-density forest. On the other hand, if production of biomass energy is the goal, a fast-growing herbaceous crop such as switch grass may be the best choice for some biomass energy technologies and some types of land. Under other circumstances, wood, biodiesel, or another fuel may be able to displace the most fossil fuel. And, if we broaden consideration to include other biomass products, we may find other alternatives. It is important to examine the full range of the affected system and to see how the carbon balance is affected.

Modeling Carbon Flows

To illustrate the impacts of some land-management alternatives on net carbon emissions, we use a simple mathematical model to compare two scenarios. In the first scenario (top half of the figure) 1 hectare of land is used to grow trees to store carbon for 50 years. During this period, a coal-fired power plant is used to generate electricity. In the second scenario (bottom half of the figure), the trees are harvested each time they reach an appropriate size and are used to displace some of the coal that would otherwise be burned. At the end of 50 years, there will be more carbon stored in living trees in the first scenario, but there will also have been more coal burned than in the second scenario. The net difference in carbon dioxide added to the atmosphere depends on how fast the trees grow and how efficiently they are harvested and converted into useful energy. The net carbon balance also depends on the amount of biomass on the land at the beginning of the analysis. If, for example, the land were already occupied by mature forest, carbon would continue to be stored but little or no additional carbon would be accumulated. On the other hand, unforested land could have a very large capacity to accumulate additional carbon in trees. Both the rate of accumulation of carbon in the forest (scenario 1) and the amount of coal displaced (scenario 2) depend on the growth rate of the trees.


Top: Growing forest accumulates carbon until it achieves, over time, a balance between the carbon taken up in photosynthesis and the carbon released back to the atmosphere from respiration, oxidation of dead organic matter, and fires and pests. In the meantime, fossil fuels are used to meet society's energy needs. Bottom: In productive forests, trees can be harvested for use in producing heat or power. Although harvesting may result in less carbon stored in standing biomass and forest soils, biomass fuels replace some of the fossil fuel that would otherwise be burned. The carbon in that fossil fuel remains stored in the ground rather than being released to the atmosphere. In both scenarios there are some energy needs for gathering the resource and converting it into useful energy, but, as the arrows on the transportation system suggest here, these are generally comparatively small. Arrows provide a qualitative indication of the magnitude and direction of carbon flows.

By comparing the results of these and other scenarios under a variety of initial conditions, biomass growth rates, and end uses, we begin to get some clues to the most carbon-efficient ways to manage forest or other lands and to the potential for biomass fuels to mitigate the increase in atmospheric carbon dioxide. The comparisons show that when the amount of forest biomass on the land in the beginning is very large and the productivity of the land is low, the most effective strategy is to allow the trees to grow, to stand, and to store carbon. In other words, slow-growing old-growth forests are best left in place. Similarly, the net carbon balance on degraded lands with low productivity is best when they are reforested, without harvesting, to store carbon.

Results are quite different on lands that can support high growth rates. There, the net reduction in carbon dioxide emissions is far greater if the trees are harvested and used as a fuel, with prompt replanting, than if the trees are left unharvested for carbon storage. On such lands, several generations of fast-growing trees (such as poplars) can be harvested in 50 years, displacing additional fossil fuel with each harvest. There are also intermediate productivities where the choices are not so clear-cut and the sign of the net carbon balance depends on other variables such as the efficiency with which biomass is substituted for fossil fuels.

Using current technologies, the most efficient way to convert biomass to useful energy, and thus to maximize the carbon dioxide savings, is to burn the biomass for heat or electricity generation, displacing coal. In all scenarios, carbon dioxide benefits increase as biomass growth rates increase and as utilization efficiency increases. The Biofuels Feedstock Development Program at ORNL aims to increase the productivity of tree and grass crops and improve the efficiency of biomass feedstock supply systems. Improvements in these areas offer a large payback both in the economics of biomass fuels and in the potential for net reductions in carbon dioxide emissions.

A more comprehensive model of carbon flows is now being developed at Joanneum Research in Graz, Austria, in collaboration with ORNL. This spreadsheet model allows us to calculate the carbon balance of land management and biomass utilization strategies. It can consider different types of biomass fuels as well as other biomass-based products from forestry or agriculture. Input parameters for the model describe the growth rate, rotation length, management intensity, previous land use, carbon dynamics of the soil and litter, fate and life expectancy of the harvested products, efficiency of fossil-fuel substitution, and energy required for land management. Model output is shown in diagrams with time on the horizontal axis and cumulative net reduction in carbon emissions on the vertical axis.

To illustrate the variety of factors that come into consideration, the following figure shows model results for a scenario in which a forest is harvested for a conventional mix of long- and short-lived products and energy and is then replanted for production of fuel wood. This scenario assumes high forest productivity and high efficiency in use of the fuel wood. Note that the carbon in wood products is gradually released to the atmosphere over time as the products decay. Some carbon is lost from soils (reflected in the drop in the bottom line of the figure) as the forest is converted to shorter rotations with more frequent harvests. Most strikingly, the net savings of carbon emissions continues to build over time as coal consumption is displaced.

When a productive forest is harvested and the site replanted with energy crops, the initial harvest will yield some long-lived products, some short-lived products, and some energy products. Over time, the various products will gradually decay and release their carbon to the atmosphere and some carbon will be lost from the soil because of more intensive management. The energy products harvested periodically over time will displace fossil fuels, allowing them to be left in the ground. There will also be some fossil fuels left unburned because wood products typically require less energy for their production than do other materials for which they substitute. The cumulative reduction in carbon emissions over time will depend on the rate of energy-crop growth, the efficiency of biomass substitution for fossil fuels, and many other parameters being modeled in studies at ORNL and Joanneum Research. The carbon balance would look much different (better) if this scenario were implemented on surplus agricultural land because we would expect a buildup of soil carbon and no loss of carbon to the atmosphere as wood products are oxidized over time.

Much remains to be learned about the potential for producing and using biomass fuels to reduce carbon emissions. However, initial studies of the carbon balance suggest that biomass fuels could play a significant role in minimizing net emissions of carbon dioxide to the atmosphere. And, very importantly, the initial studies suggest that the optimal strategy will be different from place to place, determined by the quality of the land, its current uses, competing uses, and the demands for energy and other products. Continuing studies at ORNL and at Joanneum Research will explore the potential of biomass fuels as a strategy for confronting global climate change.


BIOGRAPHICAL Sketches

JANET CUSHMAN is project manager for the U.S. Department of Energy's Biofuels Feedstock Development Program at Oak Ridge National Laboratory. A staff scientist in ORNL's Environmental Sciences Division, she has been associated with ORNL's biofuels research since 1980, serving as a task manager in the Short Rotation Woody Crops Program, project manager for the Herbaceous Energy Crops Program, and deputy project manager for the Terrestrial Energy Crops Program. She holds a B.A. degree in biology from Hiram College and an M.S. degree in ecology and evolution from Yale University.

GREGG MARLAND is a senior staff scientist in ORNL's Environmental Sciences Division. He has a Ph.D. degree in geology from the University of Minnesota. Before coming to ORNL in 1987, he worked on net-energy analysis and carbon dioxide research at the Institute for Energy Analysis, Oak Ridge Associated Universities.

BERNARD SCHLAMADINGER is a scientific staff member at Joanneum Research, Institute for Energy Research, in Graz, Austria. He has an advanced degree in mechanical engineering and economics from Graz University of Technology. He spent six months at ORNL on a sabbatical during 1994.


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