Environmental Engineering Reference
In-Depth Information
14
Stratosphere
12
10
8
Increased
6
Present
Troposphere
4
Mean
radiating
height
2
0
Surface
200
220
240
260
280
300
320
T
E
=
255
T
S
=
288
Temperature (K)
Figure 10.4
Effect of increased GHG concentrations on surface temperature and vertical temperature
profile of the atmosphere.
10.2.4
Feedback Effects
In addition to radiative forcing due to enhanced concentration of GHG, the modeling of global
warming is further complicated because a rising surface temperature inevitably will cause secondary
effects, called
feedback effects
. This can be expressed by the proportionality
T
S
∝
Q
/β
(10.4)
where
T
S
is the rise of surface temperature,
Q
is the radiative forcing due to GHG alone, and
β
is a factor accounting for the feedback effect. If
1, the feedback is positive, which will increase
the surface temperature even more than by increasing greenhouse gas concentrations alone. If
β>
β<
1, the feedback is negative, which will cause a smaller surface temperature increase. There
are several feedback effects possible: water vapor, cloud, aerosol, ice-albedo, and ocean circulation
feedbacks.
10.2.4.1 Water Vapor Feedback
This mechanism may be the most important feedback effect. Water vapor is a strong infrared
absorbing gas at wavelengths between 5 and 7
m. As the average earth's
surface temperature rises because of increasing GHG concentrations, more evaporation will occur
from the vast ocean surfaces, hence the atmosphere will become laden with more water vapor;
that is, the humidity will increase. This would result in stronger absorption of the outgoing far-IR
radiation, thus a positive feedback. Radiative models predict that the water feedback effect may
increase the GHG caused global warming by about 60% (
µ
m and above 10
µ
β
≈
0.6).
