The current rate of global fossil fuel use, and hence carbon dioxide emission from the major human source, is moderately well documented. But projections of future fossil fuel use depend sensitively on assumptions about population growth, economic activity, and policies toward fossil fuel use [see e.g., IPCC, 1992, 76-95]. Since much of the motivation for the current accelerated studies of climate change arises from a need to modify appropriately policies relating to population growth, economic activity, and fossil fuel use, it would be illogical to attempt to project these policies as the first step in the study, hence the use of scenarios covering a wide range of assumptions and the use of a no-policy-change (``business as usual'') scenario as a baseline for estimating the degree of policy modification needed to achieve desired results.
The atmospheric concentration of CO
is accurately
measured. This number, combined with emissions data, allow the
rate, but not the cause, of removal of excess CO
from the
atmosphere to be determined. Knowledge of ocean uptake of
excess carbon dioxide has slowly improved [ Siegenthaler and
Sarimento, 1993], and attention has increased on the role of
terrestrial biospheric uptake, in particular the role of CO
fertilization and nitrate fertilization in enhancing standing biomass
are under active study [ Schindler and Bayley, 1993].
Methane, the gas believed to be the second most important
in driving a human-induced climate change, is also released in
connection with fossil fuel use, but this gas also has major
biospheric sources, some of which are not well documented. Both
carbon dioxide and methane can enter into climate feedback
processes in which a warmer earth's surface can accelerate decay
processes in the soil that produce emissions of these gases, and
carbon dioxide enters directly into the photosynthetic process.
Improved understanding of these processes will probably require
an expanded program of field observations and experiments.
Because of the need for long term studies and the requirement for
open-top chambers and mixed species, field experiments on
fertilization will be difficult and expensive. Studies of CO
fertilization and related topics are reviewed in Bezzaz and Fejer
[1992] and Drake [1992]. See also Penner, et al., Tropospheric
Chemistry Research in the U.S.: 1991-1994, in this issue.