Geoscience Reference
In-Depth Information
Figure 8.4
The earth-atmosphere-ocean system showing estimated equilibrium times, together with the wide time variations
involving the external solar, tectonic, geothermal and anthropogenic forcing mechanisms.
Source
: After Saltzman (1983).
is allowed to reach a new equilibrium and the result is
compared with a control experiment. A second approach
is to conduct a genuine climate change experiment
where, for example, the climate is allowed to evolve as
atmospheric trace gas concentrations are increased at
a specified annual rate (a transient experiment).
A key issue in assessments of greenhouse gas-
induced warming is the sensitivity of global climate
to CO
2
doubling which is projected to occur in the
mid-twenty-first century extrapolating current trends.
Atmospheric GCM simulations for equilibrium condi-
tion changes, with a simple ocean treatment, indicate an
increase in global mean surface air temperature of 2.5
to 5°C, comparing 1
CO
2
and 2
CO
2
concentrations
in the models. The range is in part the result of a
dependence of the temperature change on the temper-
ature level simulated for the base-state 1
part arises from the variations in the strength of feedback
mechanisms incorporated in the models, particularly
atmospheric water vapour, clouds, snow cover and
sea ice. Use of coupled atmosphere-ocean models,
however, suggest only a 1-2°C surface warming for
century-long transient or doubled CO
2
experiments
(see Chapter 13).
2 Simpler models
Because GCMs require massive computer resources,
other approaches to modelling climate have developed.
A variant of the GCM is the statistical-dynamical
model (SDM), in which only zonally averaged features
are analysed, and north-south energy and momentum
exchanges are not treated explicitly but are represented
statistically through parameterization. Simpler still
CO
2
, and in
