Figure 4: Simulating the
Earth's temperature variations, and comparing the results to measured changes,
can provide insight into the underlying causes of the major changes.
A climate model can be used to simulate the temperature changes that occur both
from natural and anthropogenic causes. The simulations represented by the band
in (a) were done with only natural forcings: solar variation and volcanic activity.
Those encompassed by the band in (b) were done with anthropogenic forcings: greenhouse
gases and an estimate of sulphate aerosols, and those encompassed by the band
in (c) were done with both natural and anthropogenic forcings included. From (b),
it can be seen that inclusion of anthropogenic forcings provides a plausible explanation
for a substantial part of the observed temperature changes over the past century,
but the best match with observations is obtained in (c) when both natural and
anthropogenic factors are included. These results show that the forcings included
are sufficient to explain the observed changes, but do not exclude the possibility
that other forcings may also have contributed. The bands of model results presented
here are for four runs from the same model. Similar results to those in (b) are
obtained with other models with anthropogenic forcing. [Based upon Chapter
12, Figure
12.7]
