Geoscience Reference
In-Depth Information
however, some processes are more important than others. Rather than attempt-
ing to include all the details of climate system interactions, which may be all but
impossible, the thoughtful study of climate and climate change seeks to identify
the most important processes and to include them accurately in models.
Projections of future climate are most useful for impacts analysis and de-
cision making when they are accompanied by an evaluation of uncertainty.
Comparisons of historical model runs with observations lend confidence to the
ability of a given model to predict future climate. However, an accurate simula-
tion of today's climate does not guarantee an accurate prediction of the future
because different forcing factors may be dominant in the future and some em-
pirically derived factors in model parameterizations may not be applicable to
future climate.
Model confidence is also developed through the use of
ensemble
simula-
tion techniques. A simulation may be rerun several times with initial condi-
tions adjusted slightly to introduce noise, or with different versions of physical
parameterizations to test the model's sensitivity. The model projections from
each ensemble member can then be averaged to produce a prediction, and the
spread among the ensemble members provides a measure of uncertainty.
Another way of building confidence in climate model predictions is to com-
pare many different models. Although GCMs are all governed by the same
basic equations (Box 12.1), each model employs different numerical integra-
tion schemes, smoothing and, especially, physical parameterizations. In fact,
the most important reason for disparity among AGCM simulations has been
attributed to the parameterization of clouds and convection. If the model pre-
dictions agree with one another despite the differences in parameterizations
and other factors, it suggests that these “discretionary” model features are not
dominating the projected climate change.
12.7 REFERENCE AND ADDITIONAL READING
IPPC, 2007: Contribution of Working Group I to the Fourth Assessment Re-
port of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin,
M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller
(eds.). Cambridge University Press, Cambridge and New York.
12.8 EXERCISES
12.1. Consider the zero-dimensional climate model.
(a) Derive an expression for the climate sensitivity parameter
2
T
E
2
λ
/
α
α
that quantifies the sensitivity of
T
E
to a.
(b) Answer the following questions based on the results from (a):
What is the sign of l
a
, and what does the sign imply about the
relationship between changes in
T
E
and a?
