Geoscience Reference
In-Depth Information
2
==
T
E
2
λ
0.058
K/ Wm
(
$
).
(11.5)
S
2
S
0
0
The preceding calculation addresses the sensitivity of the entire climate sys-
tem to changes in external solar forcing, but it is not helpful in evaluating the
system's response to increasing greenhouse gas levels. The sensitivity of the sur-
face climate to changes in greenhouse gas concentrations can be estimated using
the equilibrium surface heat balance equation (Eq. 5.9, with Eq. 5.10) combined
with a radiative model that accounts for changes in the longwave back radia-
tion at the surface,
F
BACK
, due to increasing greenhouse gases. Given that the
direct radiative effects of a doubling of tropospheric CO
2
is estimated to be an
increase in
F
BACK
of 4 W/m
2
(see exercise 10.1), and assuming that all the other
factors in the surface heat balance remain unchanged from the values given in
Figure 5.4,
the surface heat balance calculation gives the following results:
For today's climate,
=
−+−−
=
(1
α
)
S
F HH
14
/
SINC
BACK
S
L
T
289
K
.
(11.6)
>
H
S
εσ
For the doubled-CO
2
climate, increase
F
BACK
to 337 W/m
2
but hold all other
values fixed. Then,
T
S
290 K.
According to this simple calculation, a doubling of the atmospheric CO
2
concentration is estimated to cause a 1 K warming of surface temperature.
(This is in the absence of a climate system response, so stay tuned until the
following section which deals with the issues of climate feedback processes.)
By this rough estimate, the climate sensitivity to changes in the longwave back
radiation, l
LW
, is
1K
2
λ
/
=
0.25 W/m
K
/(
)
,
(11.7)
LW
2
4W/m
that is, for every 1 W/m
2
change in the longwave back radiation,
T
S
changes
by 0.25 K.
How accurate are these calculations? Would you imagine, for example, that
if the surface warmed even a little that the sensible and latent heat fluxes would
remain the same? Of course not, as a quick reference to
chapter 5
confirms.
In fact, small changes in the direct radiative forcing trigger other changes in
the climate system that greatly modify the climate's sensitivity to increasing
greenhouse gases.
11.2 CLIMATE FEEDBACK PROCESSES
How does climate sensitivity come about? One way to organize your thoughts
for investigating changes in the very complicated climate system is to study
individual climate feedback processes in isolation. Several examples of climate
feedbacks known to be important for determining climate are presented here.
Each example assumes that the feedback is triggered by an increase in the at-
mospheric CO
2
(or other greenhouse gas) concentration and a small increase
in the global-mean surface temperature,
T
*, in response to the direct radiative
