FUTURE CLIMATE CHANGE - The Climate Casino - page 42

absorb more of the sunlight and become very hot. Adding CO 2 to the
atmosphere is like having an invisible team of trolls slowly painting
your car darker and darker. This analogy is also useful because it allows
for the possibility that you might actually like a dark-colored car, for
example, if you live in a wintry climate. However, if you live in a hot
region like Arizona or India, you might fi nd the idea of your car getting
darker and hotter quite unattractive. 6
PROJECTIONS OF FUTURE CLIMATE CHANGE
The previous section provided the basic intuition behind climate-
change science. From a practical point of view, we need to know the
magnitude and timing of warming along with other effects such as pre-
cipitation and sea-level rise. For a starting point, let us consider the im-
pacts of doubling atmospheric CO 2 . This question has been studied by
climate scientists for more than a century and is a standard calculation.
As it turns out, because of the complexities of the science, our current
understanding is still incomplete.
Figure 7 shows a sketch of the estimated climate sensitivity found
in recent climate model comparisons. 7 While the models are continu-
ally improved and refi ned, the calculated sensitivity of climate to CO 2
increases has changed little over the last three decades. 8 In a standard
model comparison, such as that shown in this fi gure, several climate
models were run with identical scenarios. They fi rst ran the models
with no increase of atmospheric CO 2 . They then ran a scenario in which
atmospheric CO 2 increases smoothly and doubles over seventy years,
then holds steady at that doubled level for the indefi nite future. This is
an artifi cial situation, but it is useful for comparing models.
The models make two important calculations. They fi rst estimate a
“transient response,” which is the temperature increase after seventy
years, or at the time of CO 2 doubling. The curve to the left shows the
distribution of transient responses, whose average was 1.8°C.
The models also calculated an “equilibrium response,” which is the
long-run temperature increase once all adjustments have taken place.
The equilibrium results are shown by the distribution to the right in
Figure 7. The average equilibrium or long-run temperature increase for