human activities at the present time. Thus, for any scenario of future anthro-
pogenic CO 2 emissions, there is at least as great an uncertainty over what pro-
portion will remain in the atmosphere as exists for current emissions. In all
probability the uncertainty is even greater, since climatological and other global
changes, whether human-induced or natural, are likely to alter the rates at which
the various environmental reservoirs take up and release CO 2.
Estimating the amount of CO 2 that will be emitted by human activities over
the next 100 years is probably even less certain than calculating how it will par-
tition between the air, ocean and land. Although the factors that determine the
amounts of anthropogenic emissions can be identified, their quantification can
only be guessed at. The size of the human population is a very important factor.
We know it is rising and will almost certainly continue to do so (at an unknown
rate). Similarly the standard of living of many people from less developed coun-
tries is rising and this will lead to greater use of energy in those parts of the world.
How this energy is generated will have a profound bearing on how much CO 2 is
emitted. The future of CO 2 emissions is crucial for policy decisions. For example,
will it be necessary to curb future fossil fuel combustion and, if so, when and by
how much, in order to prevent or at least ameliorate undesirable alteration in
global climate?
Despite these difficulties some attempts have been made to predict atmos-
pheric CO 2 levels into the next century. The results of one such study are shown
in Fig. 7.11. The different curves correspond to different scenarios of population256 Chapter Seven
COconcentration (ppm) 23004005006007008009001000Year2000 2020 2040 2060 2080 2100Fig. 7.11Atmospheric CO 2 concentrations calculated for various emission scenarios. From
IPCC (2001). With permission of the Intergovernmental Panel on Climate Change.