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a negative feedback in their interaction with longwave radiation, and a positive
feedback when shortwave processes are evaluated.
To increase realism, consider possible changes in cloud distributions, specifi-
cally, in cloud-top altitudes. If lapse rates remain constant, cloud-top tempera-
tures decrease with increasing cloud top height (and vice versa). Because the
optical depth of clouds is large (see
chapter 4
), we can assume that the long-
wave emission to space from clouds, as well as the reflection of solar radiation,
occurs at cloud top. Thus, an increase in cloud-top altitudes makes the cloud
a more effective longwave trapping agent (enhanced longwave effect), but it
doesn't change how effectively the cloud cools climate (unchanged shortwave
effect). A feedback loop is sketched in
Figure 11.3a,
assuming that cloud-top
temperatures decrease as climate warms. This will be the case if a cloud layer is
displaced upward (with no change in lapse rate) or if the atmosphere becomes
more unstable in a way that deepens convection.
Another way to think about changes in cloud-top temperature in a global sense
is to consider the possibility that low or high clouds may be preferentially modi-
fied as climate changes. For example, if greenhouse gas-induced climate change
leads to a decrease in the coverage by low clouds with an increase in the coverage
by high clouds, then the overall effect on climate will be a positive feedback.
Cloud albedo is another potential contributor to climate change. Here, we
are concerned primarily with cloud microphysical properties, for example, the
liquid water content of a cloud, droplet size distributions, and features of a
cloud's ice particles. If the liquid water content of a cloud increases as climate
warms, for example, its shortwave albedo will increase and provide a negative
feedback on climate change (see
Fig. 11.3b)
.
Changes in atmospheric aerosols may also feed back to climate in significant
ways through their effects on clouds. This is the
indirect effect of aerosols
on
climate, in contrast with the direct effects produced when aerosols absorb and
scatter radiation. Natural and anthropogenic atmospheric aerosols serve as
cloud condensation nuclei
(CCN)—the basis for cloud seeding.
(a) Cloud top height
(b) Cloud albedo
CO
2
CO
2
+
+
T*
T*
-
+
-
+
Solar radiation
re
ected
Cloud liquid
water content
Cloud top
height
OLR
-
+
+
+
Cloud
albedo
Cloud top
temperature
Figure 11.3 Example cloud feedback loops related to (a) cloud top
height and (b) cloud brightness.
