Environmental Engineering Reference
In-Depth Information
10.2.4.2 Cloud-Radiation Feedback
The feedback effect of clouds is complicated. Clouds can have both a negative and positive feedback
effect. The negative effect is due to reflection by clouds of the incoming solar radiation, contributing
to an increase of the earth's albedo and thus reducing the earth's surface temperature. The positive
effect is due to the clouds reflecting the outgoing earth's thermal radiation. The balance of the
two effects is dependent on cloud characteristics, their altitude, and droplet or crystal size. In
general, low, cumulus-type clouds reflect solar radiation: a cooling effect. High-altitude, cirrus-
type clouds reflect the earth's radiation: a warming effect. Modeling of cloud feedback effects is
highly uncertain. At this stage a best “guestimate” is that in the overall the positive feedback effect
wins (more high altitude clouds as a consequence of GHG caused surface water warming), so that
the cloud feedback effect will add another 20% (
β
≈
0.8) to the surface temperature increase due
to increasing GHG concentrations.
10.2.4.3 Aerosol Feedback
Similarly to clouds, natural and man-made small particles suspended in the air, called aerosols,
can interfere with the incoming solar and outgoing terrestrial radiation. The average composition
of aerosols is about one-third crustal material (fugitive dust from soil, sand, rocks, and shale),
one-third sulfate (mainly from sulfur emissions associated with fossil fuel combustion), and one-
third carbonaceous matter and nitrate (also from fossil fuel combustion). The diameter range of
the aerosols is much less than a micrometer. This diameter range is more effective in scattering
incoming solar radiation than in reflecting outgoing terrestrial radiation. Thus, the aerosol feedback
effect is thought to be negative, reducing the GHG effect by 10-15% (
1.1).
The aerosols may also have an indirect feedback effect. Aerosols serve as condensation nuclei
for clouds. The more aerosols in the air, the higher the probability of cloud formation. Depending on
the formed cloud height, and droplet or crystal size, clouds may reflect incoming solar radiation or
outgoing terrestrial radiation. This indirect effect is presently under intense study, and the resulting
feedback cannot yet be assessed.
β
≈
10.2.4.4 Ice-Albedo Feedback
As the earth's surface warms due to increased GHG concentrations, the fringes of the Arctic and
Antarctic ice caps may melt. Also, glaciers, which already are receding in this interglacial period,
may recede even faster. Because ice has a higher albedo (reflects more sunlight) than water and
land, the disappearance of ice will lead to a decrease in the earth's albedo [
in equations (10.1)
and (10.3)]. This will cause the earth's radiative temperature
T
E
, and concomitantly the surface
temperature
T
S
, to increase slightly. The ice-albedo effect may add 20% to the GHG-caused surface
warming (
α
0.8).
Altogether, the various feedback effects may double the GHG-caused global warming.
β
≈
10.2.4.5 Ocean Circulation Feedback
Another possible feedback mechanism is the alteration of the ocean circulation and currents. In
general, cold and highly saline water sinks to greater depths, and warm, less saline water rises.
The cold and larger-than-average saline waters are generated in the Arctics, as ice is formed at the
surface. These waters sink to the ocean bottom and move toward the equator. There, warmer and less
