Geoscience Reference
In-Depth Information
bounds of its usefulness. For the simplest climate model, these assumptions are
as follows:
(i) The earth system is in equilibrium with the sun, so there is no temperature
trend and energy input energy output.
(ii) Stefan's law holds, that is, the earth system can be treated as a blackbody
(emissivity 1).
(iii) The relationship between
T
E
and
T
S
is the same for various climate states.
The first assumption indicates that this climate model cannot provide infor-
mation about transient climate states, that is, the evolution from one climate
state to the next. It can provide information only about different equilibrium,
or steady-state, climates.
Similar to the discussion of climate sensitivity in
chapter 11,
an analyti-
cal calculation using the simplest climate model is used to illustrate the basic
methodology of climate model simulation. We use the model to simulate the
consequences of a 10% increase in planetary albedo.
STEP 1. UNPERTURBED CLIMATE STATE
The first step in any climate model simulation is to produce a
control climate
,
which is a representation of the unperturbed, or present day, climate. With
present-day values for the albedo (a 0.310) and solar constant (S
0
1368
W/m
2
), the simultaneous solution of Eqs. 12.1 and 12.2 gives
T
E
254 K and
T
S
289 K (or 16°C , or 61° F). This is the zero-dimensional model's control
climate state.
STEP 2. MODEL EVALUATION
The second step is to evaluate the accuracy of the control simulation of the
observed climate. For the zero-dimensional model, this involves comparing the
observed globally and annually averaged
T
E
and
T
S
climatologies with those
produced by the model. For this simple climate model, the circular nature of this
validation is readily apparent, since one of the governing equations (Eq. 12.2)
is empirically derived. Like most climate models, the simplest climate model is a
blend of theoretical governing equations (Eq. 12.1) and empirical constants (Eq.
12.2). In more complex models, discussed below, the agreement with observa-
tions is not perfect and the influence of observed values is much weaker.
STEP 3. PERFORM THE EXPERIMENT
To address the question at hand—What are the consequences of a 10% in-
crease in planetary albedo?—the value of a is increased by 10%, to 0.341. All
other values are held fixed to isolate the effects of the albedo change. Solving
Eqs. 12.1 and 12.2 again, but with the changed albedo, we get
T
E
251 K and
T
S
286 K. Thus, the answer to the question posed to this model is: A 10%
