Carbon offsets (CO₂-emission offsets)

Many human activities result in large emissions of carbon dioxide (CO₂) and other so-called greenhouse gases into the atmosphere . Especially important in this regard are the use of fossil fuels such as coal , oil, and natural gas to generate electricity, to heat spaces, and as a fuel for vehicles. In addition, the disturbance of forests results in large emissions of CO₂ into the atmosphere. This can be caused by the conversion of forests into agricultural or urbanized land uses, and also by the harvesting of timber.

During the past several centuries, human activities have resulted in large emissions of CO₂ into the atmosphere, causing substantial increases in the concentrations of that gas. Prior to 1850 the atmospheric concentration of CO₂ was about 280 ppm, while in 1997 it was about 360 ppm. Other greenhouse gases (these are also known as radiatively active gases) have also increased in concentration during that same period: methane (CH₄) from about 0.7 ppm to 1.7 ppm, nitrous oxide (N₂O) from 0.285 ppm to 0.304 ppm, and chlorofluorocarbons (CFCs) from zero to 0.7 ppb.

Many climatologists and environmental scientists believe that the increased concentrations of radiatively active gases are causing an increase in the intensity of Earth's greenhouse effect . The resulting climatic warming could result in important stresses for both natural ecosystems and those that humans depend on for food and other purposes (agriculture, forestry, and fisheries). Overall, CO₂ is estimated to account for about 60% of the potential enhancement of the greenhouse effect, while CH₄ accounts for 15%, CFCs for 12%, ozone (O₃) for 8%, and N₂O for 5%.

Because an intensification of Earth's greenhouse effect is considered to represent a potentially important environmental problem, planning and other actions are being undertaken to reduce the emissions of radiatively active gases. The most important strategy for reducing CO₂ emissions is to lessen the use of fossil fuels. This will mostly be accomplished by reducing energy needs through a variety of conservation measures, and by switching to non-fossil fuel sources. Another important means of decreasing CO₂ emissions is to prevent or slow the rates of deforestation , particularly in tropical countries. This strategy would help to maintain organic carbon within ecosystems, by avoiding the emissions of CO₂ through burning and decomposition that occur when forests are disturbed or converted into other land uses.

Unfortunately, fossil-fuel use and deforestation are economically important activities in the modern world. This circumstance makes it extremely difficult for society to rapidly achieve large reductions in the emissions of CO₂. An additional tactic that can contribute to net reductions of CO₂ emissions involves so-called CO₂ offsets. This involves the management of ecosystems to increase both the rate at which they are fixing CO₂ into plant biomass and the total quantity stored as organic carbon. This biological fixation can offset some of the emissions of CO₂ and other greenhouse gases through other activities.

Offsetting CO₂ emissions by planting trees

As plants grow, their rate of uptake of atmospheric CO₂ through photosynthesis exceeds their release of that gas by respiration . The net effect of these two physiological processes is a reduction of CO₂ in the atmosphere. The biological fixation of atmospheric CO₂ by growing plants can be considered to offset emissions of CO₂ occurring elsewhere—for example, as a result of deforestation or the combustion of fossil fuels.

The best way to offset CO₂ emissions in this way is to manage ecosystems to increase the biomass of trees in places where their density and productivity are suboptimal. The carbon-storage benefits would be especially great if a forest is established onto poorer-quality farmlands that are no longer profitable to manage for agriculture (this process is known as afforestation, or conversion into a forest). However, substantial increases in carbon storage can also be obtained whenever the abundance and productivity of trees is increased in "low-carbon ecosystems," including urban and residential areas.

Over the longer term, it is much better to increase the amounts of organic carbon that are stored in terrestrial ecosystems, especially in forests, than to just enhance the rate of CO₂ fixation by plants. The distinction between the amount stored and the rate of fixation is important. Fertile ecosystems, such as marshes and most agroecosystems, achieve high rates of net productivity, but they usually store little biomass and therefore over the longer term cannot sequester much atmospheric CO₂. A less extreme example involves second-growth forests and plantations, which have higher rates of net productivity than do older-growth forests. Averaged over the entire cycle of harvest and regeneration, however, these more-productive forests store smaller quantities of organic carbon than do older-growth forests, particularly in trees and large-dimension woody debris.

Both the greenhouse effect and emissions of CO₂ (and other radiatively active gases) are global in their scale and effects. For this reason, projects to gain CO₂ offsets can potentially be undertaken anywhere on the planet, but tallied as carbon credits for specific utilities or industrial sectors elsewhere. For example, a fossil-fueled electrical utility in the United States might choose to develop an afforestation offset in a less-developed, tropical country. This might allow the utility to realize significant economic advantages, mostly because the costs of labor and land would be less and the trees would grow quickly due to a relatively benign climate and long growing season. This strategy is known as a joint-implementation project, and such projects are already underway. These involve United States or European electrical utilities supporting afforestation in tropical countries as a means of gaining carbon credits, along with other environmental benefits associated with planting trees. Although this is a valid approach to obtaining carbon offsets, it can be controversial because some people would prefer to see industries develop forest-carbon offsets within the same country where the CO₂ is being emitted.

Afforestation in rural areas

An estimated 5–8 billion acres (2–3 billion ha) of deforested and degraded agricultural lands may be available world-wide to be afforested. This change in land use would allow enormous quantities of organic carbon to be stored, while also achieving other environmental and economic benefits. In North America, millions of acres of former agricultural land have reverted to forest since about the 1930s, particularly in the eastern states and provinces. There are still extensive areas of economically marginal agricultural lands that could be afforested in parts of North America where the climate and soils are suitable for supporting forests.

Agricultural lands typically maintain about one-tenth or less of the plant biomass of forests, while agricultural soils typically contain 60–80% as much organic carbon as forest soils. Because agricultural sites contain a relatively small amount of organic carbon, reforestation of those lands has a great potential for providing CO₂ offsets.

It is also possible to increase the amounts of carbon stored in existing forests. This can be done by allowing forests to develop into an old-growth condition, in which carbon storage is relatively great because the trees are typically big, and there are large amounts of dead biomass present in the surface litter, dead standing trees, and dead logs lying on the forest floor. Once the old-growth condition is reached, however, the ecosystem has little capability for accumulating "additional" carbon. Nevertheless, old-growth forests provide an important ecological service by tying up so much carbon in their living and dead biomass. In this sense, maintaining old-growth forests represents a strategy of CO₂-emissions deferral, because if those "high-carbon" ecosystems were disturbed by timber harvesting or conversion into another kind of land use, a result would be an enormous emission of CO₂ into the atmosphere.

Dixon et al. (1993) examined carbon-offset projects in various parts of the world, most of which involved afforestation of rural lands. The typical costs of the afforestation projects were $1–10 per ton of carbon fixed. These are only the costs associated with planting and initial tending of the growing trees; there might also be additional expenses for land acquisition and stand management and protection.

Of course, even while rural afforestation provides large CO₂-emission offsets, other important benefits are also provided. In some cases, the forests might be used to provide economic benefits through the harvesting of timber (although the resulting disturbance would lessen the carbon-storage capability). Even if trees are not harvested from the CO₂-offset forests, it would be possible to hunt animals such as deer, and to engage in other sorts of economically valuable outdoor recreation . Increasing the area of forests also provides many non-economic benefits, such as providing additional habitat for native species , and enhancing ecological services related to clean water and air, erosion control, and climate moderation.

Urban forests

Urban forests consist of trees growing in the vicinity of homes and other buildings, in areas where the dominant character of land use is urban or suburban. Urban forests may originate as trees that are spared when a forested area is developed for residential land use, or they may develop from saplings that are planted after homes are constructed. Urban forests in older residential neighborhoods generally have a relatively high density and extensive canopy cover of trees. These characters are less well developed in younger neighbourhoods and where land use involves larger buildings used by institutions, business, or industry.

There are about 70 million acres (28-million ha) of urban land in the United States. Its urban forest supports an average density of 20 trees/acre (52 trees/ha), and has a canopy cover of 28%. Nowak et al. (1994) estimated that urban areas of the United States contain about 225 million tree-planting opportunities, in which suboptimal tree densities could be subjected to fill-planting.

Urban forests achieve carbon offsets in two major ways. First, as urban trees grow they sequester atmospheric CO₂ into their increasing biomass. The average carbon storage in urban trees in the United States is about 13 tons per acre (33 tonnes per ha). On a national basis that amounts to 0.8 billion tonnes of organic carbon, and an annual rate of uptake of six million tonnes.

In addition, urban trees can offset some of seasonal use of energy for cooling and heating the interior spaces of buildings. Large, well-positioned trees provide a substantial cooling influence through shading. Trees also cool the ambient air by evaporating water from their foliage (a process known as transpiration ). Trees also decrease wind speeds near buildings. This results in decreased heating needs during winter, because less indoor warmth is lost by the infiltration of outdoor air into buildings. Over most of North America larger energy offsets associated with urban trees are due to decreased costs of cooling than with decreased heating costs. In both cases, however, much of the energy conserved represents decreased CO₂ emissions through the combustion of fossil fuels.

It is considerably more expensive to obtain CO₂-offset credits using urban trees than with rural trees. This difference is mostly due to urban trees being much larger than rural trees when planted, while also having larger maintenance expenses. In the survey of Dixon et al. (1993), the typical costs of rural CO₂-offset projects were $1–10 per ton of carbon fixed, compared with $15–30 per ton for urban trees.

Another study estimated the carbon savings associated with planting 100-million trees in urban areas in the United States (Nowak et al., 1994). In this case, the total CO₂-emission offsets were estimated to be 58.2 kg C per tree per year (for trees at least ten years old). About 90% of the total CO₂ offsets was associated with indirect savings of energy for cooling and heating buildings, and 10% with carbon sequestration into the growing biomass of the trees. The estimated costs of the carbon offsets were $6.6–27.5 per ton of carbon, but these costs would decrease considerably as the trees grew larger. This study estimated that planting trees in urban areas of the United States could potentially offset as much as 2% of this country's emissions of CO₂.

[Bill Freedman Ph.D. ]

RESOURCES

BOOKS

Nowak, D.J., E.G. McPherson, and R.A. Rowntree, eds. Chicago's Urban Forest Ecosystem. Results of The Chicago Urban Forest Climate Project. General Technical Report NE-186, U.S.D.A. Forestry Service, Northeastern Forest Experiment Station, Radnor, PA, 1994.

Trexler, M.C., and C. Haugen. Keeping It Green: Tropical Forestry Opportunities for Mitigating Climate Change. Washington, D.C.: World Resources Institute, 1995.

PERIODICALS

Dixon, R.K., et al. "Forest Sector Carbon Offset Projects: Near-term Opportunities To Mitigate Greenhouse Gas Emissions." Water, Air, & Soil Pollution 70 (1993): 561-577.

Freedman, B., and T. Keith "Planting Trees For Carbon Credits. A Discussion Of Context, Issues, Feasibility, and Environmental Benefits, With Particular Attention To Canada." Environmental Review 4 (1996): 100-111.

Heisler, G.M. "Energy Savings With Trees." Journal of Arboric 12 (1986): 113-125.

Kinsman, J.D., and M.C. Trexler. "Terrestrial Carbon Management And Electric Utilities." Water, Air, Soil Pollution 70 (1993): 545-560.

McPherson, E.G. "Using Urban Forests For Energy Efficiency And Carbon Storage." Journal of Forestry 94 (1994): 36-41.

Sampson, R.N., et al. " Biomass Management and Energy." Water, Air, Soil Pollution. 70 (1993): 139-159.