Geoscience Reference
In-Depth Information
increase in the planetary albedo causes a cooling in the globally and annually
averaged surface temperature of 3 K (3°C, or 6°F).
STEP 4. EVALUATE THE RESULTS
The final step, usually the most time consuming and interesting, is to evaluate
the results. Do they make sense? How do they compare with results from other
models and with our basic understanding of how climate works? What confi-
dence can be ascribed to the results? Was the climate sensitive to the forcing?
In this case, because a 10% increase in a caused a 3 K cooling of global climate
(as represented by the surface temperature), we would say that climate is sensi-
tive to changes in the planetary albedo of even a few percent.
12.2 SURFACE HEAT BALANCE
CLIMATE MODELS
The zero-dimensional climate model addresses the relationship between the
globally and annually averaged surface temperature and the average albedo,
and between surface temperature and the solar constant. But this model is
useless for studying the influence of increases in greenhouse gases on climate.
However, we can construct another analytical model based on the surface heat
balance discussed in
chapter 5
to study the response to the greenhouse effect,
or changes in the longwave back radiation due to increasing greenhouse gas
concentrations.
We used this model in section 11.1 to explore the climate sensitivity to direct
radiative forcing. Many unrealistic assumptions were needed to use Eq. 5.9
as a climate model, including that a,
H
S
, and
H
L
remain constant despite the
enhanced longwave radiative heating of the surface. We used this framework
to estimate the surface temperature response to direct radiative forcing in the
absence of climate feedbacks, so the calculation is seriously flawed as a climate
predictor. None of the important climate feedback mechanisms discussed in
chapter 11
were allowed to operate since none of the other surface heat flux
terms changed.
To refine this calculation, and better predict the system response, it is neces-
sary to take climate feedback processes into account. For example, the turbu-
lent heat fluxes,
H
S
and
H
L
, depend strongly on
T
S
(section 5.3); and
F
BACK
will change according to the water vapor-temperature feedback mechanism
when evaporation rates and, therefore, atmospheric water vapor rates in-
crease due to surface warming. When surface temperatures change the ice
albedo feedback will also operate, affecting the value of a. There also will
be complex changes in the properties and distribution of the world's clouds.
Changes in surface winds and in the hydrologic cycle will force changes in
the ocean circulation and in land surface conditions as well. These and many
other processes must be accounted for in a more accurate and complete pro-
jection of future climate.
