CARBON BUDGET OF UNITED STATES FORESTS
Carbon is a national policy issue because gases containing carbon
are increasing in the atmosphere. These Greenhouse gases may effect
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
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
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).
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.
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.