Geoscience Reference
In-Depth Information
11
CLIMATE CHANGE PROCESSES
As discussed in the previous chapter and in chapter 4 , the immediate effect of
changes in atmospheric concentrations of greenhouse gases in the troposphere
is an increase in longwave radiation—the greenhouse effect, or direct radiative
forcing. Now, we consider the response of the climate system to that forcing.
11.1 CLIMATE SENSITIVITY
Defining a climate sensitivity parameter l,
2
T
2
*
F
,
l
/
(11.1)
allows us to compare and quantify the effects of various climate forcing factors
on climate. In Eq. 11.1, F is a radiative forcing (W/m 2 ) and T * is the globally
and annually averaged surface air temperature. T * is often used as a single-
parameter representation of the climate state, though it could be replaced with
any number of climate parameters, and F can represent various forcing factors.
Equation 11.1 is an idealized representation of a very complex system, but it is
useful for developing basic ideas of climate forcing and response.
As an example, consider the simplest representation of the climate system,
namely, the global-mean radiative balance at the top of the atmosphere. Solv-
ing Eq. 4.6 for T E provides an excruciatingly oversimplified model for the ra-
diative equilibrium temperature:
S
(
1
α
)
14
/
.
0
T
=
(11.2)
>
H
E
4
σ
The sensitivity of T E to changes in the solar constant is then
2
T
S
34
/
==
(
1
α
)
14
/
.
E
0
λ
(11.3)
<
F
S
2
S
44
σ
0
0
According to Eq. 11.3, T E is more sensitive to changes in the solar constant
for smaller values of S 0 and larger values of a. With a  0.31 and S 0  1368
W/m 2 ,
2 ==
T
E
2
λ
0.046 (
K/ Wm .
$
)
(11.4)
S
2
S
0
0
Keeping the same planetary albedo and reducing S 0 by 25% results in a
stronger sensitivity:
 
 
Search WWH ::




Custom Search