Global Warming

Comparison withobservations 103

Figure 5.21The

predicted and observed

changes in global land and

ocean surface air

temperature after the

eruption of Mount

Pinatubo, in terms of

three-month running

averages from April to June

1991 to March to May

1995.

An example of such simulations is shown in Figure 5.22, where the
observed record of global average surface air temperature is compared
with model simulations taking into account in turn natural forcings, an-
thropogenic forcings (i.e. the increase in greenhouse gases and aerosols)
and the combination of natural and anthropogenic forcings. Although
the simulations in Figure 5.22 are based on one model, similar results
have been obtained from many models.
Three interesting features of Figure 5.22 can be noted. Firstly, that the
inclusion of anthropogenic forcings provides a plausible explanation for
a substantial part of the observed temperature changes over the last cen-
tury (especially for the latter part of the century), but that the best match
with observations occurs when both natural and anthropogenic factors
are included. In particular it is likely that changes in solar output and
the comparative absence of volcanic activity were important variations
in natural forcing factors during the first part of the twentieth century.
Secondly, the model simulations show variability up to a tenth of a degree
Celsius or more over periods of a few years up to decades. This variabil-
ity is due to internal exchanges in the model between different parts of
the climate system, and is not dissimilar to that which appears in the ob-
served record. Thirdly, due to the slowing effect of the oceans on climate
change, the warming observed or modelled so far is less than would be
expected if the climate system were in equilibrium under the amount
of radiative forcing due to the current increase in greenhouse gases and
aerosols.
There remains, however, a large amount of natural variability in both
the observations and the simulations and much debate has taken place
over the last decade or more about the strength of the evidence that
global warming due to the increase in greenhouse gases has actually