gure 2-4: Simulating
the Earth's temperature variations and comparing the results to the 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 from
both 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 sulfate aerosols. Those encompassed by the band in (c)
were done with both natural and anthropogenic forcings included. From (b), it
can be seen that the 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. Similar results to
those in (b) are obtained with other models with anthropogenic forcing.
