Future emissionsof carbon dioxide 39is more vegetation growth in the northern hemisphere than the south-
ern, a minimum in the annual cycle of carbon dioxide in the atmosphere
occurs in the northern summer. Estimates from carbon cycle models of
the uptake by the land biosphere are constrained by these observations
of the difference between the hemispheres.^6
The carbon dioxide fertilisation effect is an example of a biological
feedback process. It is a negative feedback because, as carbon dioxide
increases, it tends to increase the take-up of carbon dioxide by plants
and therefore reduce the amount in the atmosphere, decreasing the rate
of global warming. Positive feedback processes, which would tend to
accelerate the rate of global warming, also exist; in fact there are more
potentially positive processes than negative ones (see box on page 40).
Although scientific knowledge cannot yet put precise figures on them,
there are strong indications that some of the positive feedbacks could
be large, especially if carbon dioxide were to continue to increase, with
its associated global warming, through the twenty-first century into the
twenty-second. Carbon dioxide provides the largest single contribution to
anthropogenic radiative forcing. Its radiative forcing from pre-industrial
times to the present is shown in Figure 3.8. A useful formulafor the
radiative forcingRfrom atmospheric carbon dioxide when its atmos-
pheric concentration isCppm is:R= 5 .3ln(C/C 0 ) whereC 0 is its
pre-industrial concentration of 280 ppm.
Future emissions of carbon dioxide
To obtain information about future climate we need to estimate the future
atmospheric concentrations of carbon dioxide, which depend on future
anthropogenic emissions. In these estimates, the long time constants
associated with the response of atmospheric carbon dioxide to change
have important implications. Suppose, for instance, that all emissions
into the atmosphere from human activities were suddenly halted. No
sudden change would occur in the atmospheric concentration, which
would decline only slowly. We could not expect it to approach its pre-
industrial value for several hundred years.
But emissions of carbon dioxide are not halting, nor are they slowing;
their increase is, in fact, becoming larger each year. The atmospheric
concentration of carbon dioxide will therefore also increase more rapidly.
Later chapters (especially Chapter 6) will present estimates of climate
change during the twenty-first century due to the increase in greenhouse
gases. A prerequisite for such estimates is the knowledge of what changes
in carbon dioxide emissions there are likely to be. Estimating what will
happen in the future is, of course, not easy. Because nearly everything
we do has an influence on the emissions of carbon dioxide, it means