Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 331

any significant increase in absorption by the atmosphere when the CO 2 concen-
tration is increased. Thus, with a CO 2 concentration of 280 ppm, absorption is
saturated between the vertical dashed lines labeled A and C in Figure 11.7 .
If the CO 2 concentration doubles (from the pre-industrial value) to 560 ppm in
the future, the absorption curve moves up by a factor of 2 on the log scale and the
saturated region expands to the region between vertical lines B and D, producing
a net heating effect. However, the additional heating effect (from absorption in the
regions between A and B and between C and D) is much smaller than the original
heating effect in going from 0 to 280 ppm (region between A and C). Thus, we see
that as increasingly more CO 2 is added to the atmosphere the heating effect
decreases per unit amount of CO 2 added. This is shown in Figure 11.6 . However,
the above description is overly simplistic. The actual effect of additional absorp-
tion by CO 2 is a change in the vertical profile of temperature through the
atmosphere, which results in a change in outward radiant emission from the
atmosphere (Lindzen et al., 1982; Lindzen, 1997, 2007).
A number of climatologists have addressed the problem of estimating future
global average temperature rise resulting from doubling of the CO 2 concentration
from the pre-industrial level of 280 to about 560 ppm sometime later in the 21st
century. The reasons for choosing the benchmark of 560 ppm for the future relate
to projections of future world population growth, increasing industrialization of
developing nations, and expected increase in the use of coal as the 21st century
progresses. The Inter-government Panel on Climate Change (IPCC) has developed
a number of alternative scenarios for future CO 2 emissions, depending on econom-
ics, technology, and policy changes. A widely used middle-of-the-road business-as-
usual scenario from the IPCC is shown in Figure 11.8 as ''IS92a''. In this scenario,
annual CO 2 emissions continue to increase through the 21st century. Two other
hypothetical future scenarios are shown in this figure. In one scenario, the CO 2
emission rate is held constant at the 2010 rate (estimated to be about 8Gt/yr of
carbon which is equivalent to 8 44/12 ¼ 29Gt/yr of CO 2 ) for the remainder of
the 21st century. 7 In the other scenario, there is a downward ramp to lower emis-
sion rates as the 21st century wears on. It should be noted that the latter two
scenarios require draconian modifications to the way that industrialized societies
produce and consume energy. These three scenarios lead to the buildup of CO 2 in
the atmosphere as shown in Figure 11.9 and assume that half of the CO 2 emitted
ends up in the atmosphere. 8 In the business-as-usual scenario, the CO 2 concentra-
7 The 8Gt/yr of carbon emissions consists of about 2Gt/yr from land clearing and about
6Gt/yr from fossil fuel burning and cement production. The expectation in the business-as-
usual scenario is that the land use figure will not change markedly but the fossil fuel
combustion will increase significantly in this scenario.
8 While this 50% assumption is representative of the past, it is not clear whether it will hold in
the future. One model predicts that the 50% distribution will continue through at least 2040:
F. T. Mackenzie, A. Lerman, and L. M. B. Ver, ''Recent past and future of the global carbon
cycle,'' in L. C. Gerhard, W. E. Harrison, and B. M. Hanson (Eds.), Geological Perspectives of
Global Climate Change, pp. 51-82.