Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 333

concentration in the atmosphere will likely reach 560 ppm before the end of the
21st century.
Thus, with the likelihood that the CO 2 concentration will rise to 560 ppm (or
more) during the 21st century, the question now becomes: What is the expected
global temperature rise due to the greenhouse effect from this change? Climatolo-
gists have attempted to answer this question by means of global climate models
(GCMs). In these models, scientists first estimate the greenhouse heating effect due
to doubling of the CO 2 concentration as an equivalent amount of downward
heat flow called a ''forcing''. Forcing units are measured in power per unit area
(W/m 2 ). Having estimated the forcing produced by doubling the pre-industrial
level of CO 2 , they next estimate the rise in global average temperature produced
by that forcing. This involves a complex computerized analysis of the coupled
atmosphere-ocean-landmass Earth system. In the following we denote the forcing
due to CO 2 as D F C and the change in global average temperature as D T C (the
additional heating due to CO 2 in Figure 11.6 ).
The relation between these quantities is
D T C ¼ D F C

where is the so-called ''climate sensitivity parameter''. We use the subscript C to

denote that this is entirely due to the carbon dioxide greenhouse effect, ignoring
the secondary effects due to this warming. According to the GCMs, the global
warming ( D T C ) produced by the CO 2 greenhouse effect increases evaporation
from the Earth's bodies of water and, hence, the global average humidity of the
atmosphere. Since water vapor is a powerful greenhouse gas (much more so than
CO 2 ) this increase in water vapor produces an additional forcing ( D F W ) which in
turn produces an additional temperature rise ( D T W ). In addition, as the humidity
of the atmosphere increases so does the degree of cloudiness of the Earth's atmo-
sphere resulting in a change in the Earth's albedo. This will produce yet another
change in temperature ( D T Cl ). Depending on the nature and distribution of clouds,
this term can in principle be positive or negative. Quite a few estimates have been
made of the cloud feedback factor by various investigators. In general, those of
the alarmist persuasion find that clouds amplify temperature changes, while
skeptics find the opposite. If cloud feedback is negative, it will reduce the tempera-
ture rise induced by increased water vapor. Thus, we see that D T W depends on
D T C but these quantities affect the D T Cl which provides the (probably negative)
feedback. The total temperature change due to CO 2 , water vapor, and changes in
cloudiness can be expressed as

D T tot ¼ g D F C
where g is the gain factor due to water vapor, cloudiness, and any other
climatological factors that change as CO 2 builds up.
Climate models have been used by a number of investigators to estimate D F C .
A value of about 3.7W/m 2 seems to be representative of a doubling of CO 2 from
the pre-industrial level of 280 ppm. Estimates of in the literature range from

about 0.3 to 0.8 C per W/m 2 , although some estimates exist outside this range.