CARBON BUDGET OF UNITED STATES FORESTS

Issue

Carbon is a national policy issue because gases containing carbon are increasing in the atmosphere. These Greenhouse gases may effect climate change.

The following is the first section of an early draft of the paper: Heath, L.S., and L.A. Joyce. (In press.) Carbon sequestration in forests as a national policy issue. In: The National Silviculture Workshop, May 19-21, 1997, Warren, PA. U. S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station, General Technical Report.

Problem

Cumulative effects of human activities through time and over broad regions are significantly influencing natural processes at global to local scales. Atmospheric composition has been altered noticeably by the release of greenhouse gases such as carbon dioxide, methane, and nitrous oxide, from fossil fuel emissions, heating buildings, and land-based activities such as land-clearing for urban use. These gases are expected to warm the earth by allowing sunlight to reach the Earth's surface, but by blocking generated heat from escaping the atmosphere; some of the gases also react to thin the ozone layer which shields the Earth from harmful solar radiation. Figure 1 illustrates that atmospheric carbon dioxide concentration has increased from a pre-industrial 280 ppm to about 350 ppm currently (IPCC, 1990). It is projected to range from approximately 500-700 ppm in the year 2070 (Wigley and Raper, 1992) for an optimistic scenario and high fossil fuel emission scenario respectively, with the moderate scenario (shown in Figure 1) falling in the middle. Other greenhouse gases show similar trends.

INSERT FIGURE 1

Figure 1. Historical annual estimates and moderate scenario projection of atmospheric CO2 concentration. Estimates through 1955 based on measurements on air trapped in ice, from mid-1950 to present by direct atmospheric measure at Mauna Loa, Hawaii (IPCC, 1990). Projection based on moderate expected growth with existing policies (Wigley and Raper, 1992).

Scientists generally agree that the increases in greenhouse gas concentration will effect climate change, particularly warmer temperatures, on a global scale. Although weather has featured a general warming trend (see Figure 2), scientists are not able to conclusively state that climate change is occurring because of the high variability of weather and the long time-frame inherent in the definition of climate. However, general scientific consensus is that under existing policies scenario, global mean temperature will increase above the present value by about 1 degree C by 2025 and 3 degrees C by 2100. There is little confidence about how temperature may change on a regional or smaller scale. Much effort is currently going into climate model development to relax assumptions and improve projections.

INSERT FIGURE 2

Figure 2. Land air temperatures averaged over the Northern Hemisphere, expressed as deviations relative to 1951-1980. Curve of values from Hansen and Lebedeff (1987), annual values from PD Jones (1988, 1986), after IPCC (1990).

Policy context

Because of the possible dire consequences of climate change, nations are examining ways to control greenhouse gas emissions in the face of economic and population growth pressures. Strategies in the United States focus on various energy-related sectors of the economy such as transportation, manufacturing, and forestry. The forestry sector is currently sequestering more carbon than it emits, and is considered an area to provide opportunities to mitigate fossil fuel emissions in the near-term until ways to reduce fossil fuel emissions can be developed. Generally, activities that increase biomass on a site, such as tree planting, increase carbon sequestration, and activities that decrease biomass such as prescribed burning release carbon to the atmosphere.

Literature cited

Hansen, J., and S. Lebedeff. 1987. Global trends of measured surface air temperature. Journal of Geophysical Research 92: 13345-13372.

Intergovernmental Panel on Climate Change. 1990. Climate Change: the IPCC scientific assessment, World Meteorological Organization/United Nations Environmental Program, Cambridge University Press, Cambridge, MA. 365 p.

Jones, P. D. 1988. Hemispheric surface air temperature variations: recent trends and an update to 1987. Journal of Climate 1: 654-660.

Jones, P.D., S.C.B. Raper, R.S. Bradley, H.F. Diaz, P.M. Kelly and T.M.L. Wigley. 1986. Northern hemisphere surface air temperature variations, 1851- 1984. J Clim Appl Met 25: 161-179.

Leverenz, J.W., and D.J. Lev. 1987. Effects of carbon dioxide-induced climate changes on the natural ranges of six major commercial tree species in the western United States. In: Shands, W. E. and J. S. Hoffman, eds. The greenhouse effect, climate change, and U.S. forests, Conservation Foundation, Washington, DC, p. 123-156.

Wigley, T.M.L., and S.C.B. Raper. 1992. Implications for climate and sea level of revised IPCC emissions scenarios. Nature 357: 293-300.