In order to accurately simulate the historical increase of atmospheric
carbon dioxide it is necessary for the model to accurately simulate
the historical temperature record. In order to achieve this another
experiment was run with the climate carbon cycle model, very similar
to that described above, but this time including other factors which
influence climate. These other factors are the effect of sulphate
aerosols in the atmosphere (which affect both the Earth's radiation
balance and also cloud formation), variations in the solar cycle
(which affects how much heat reaches Earth from the Sun), and the
effect of volcanoes (which have a cooling effect on the surface
of the Earth, by ejecting ash and gases which block the Sun's radiation).
| The results of this experiment
showed that the model could accurately simulate both the temperature
and carbon dioxide records from pre-industrial times up to the
present day. The figure (right) shows the global mean temperature
simulated by the experiment with extra forcing factors compared
with observations of global mean temperature. For comparison
the temperature from the first experiment is shown too. It is
clear that the second run is much closer to the observed values.
The experiment correctly simulates the warming early in the
20th Century, the flattening in the middle and the warming towards
the end. The first experiment predicted too large a temperature
rise by the end of the century.
|  Global mean
temperatures from the experiment with sulphate and natural forcing
factors (green line) compared with observed temperatures from
the UEA-MOHSST data set (black line). Temperature from the first
carbon cycle experiment are shown in red for comparison.
|
| The top panel (left) shows a similar graph,
but this time of atmospheric carbon dioxide concentration. Once
again the run with extra forcing factors fits the observed values
much more closely. The lower panel shows the same data, but
extended through to 2100. Despite the slight differences at
2000 the two experiments have very similar responses by 2100.
The reduction of soil carbon, and die back of the Amazon forest
dominate both experiments, and the accelerated climate change
which results is very similar.
Atmospheric carbon dioxide concentrations from the experiment
with sulphate and natural forcing factors (green line) compared
with observed concentrations from the IS92a scenario (black
line). Concentrations from the first carbon cycle experiment
are shown in red for comparison. Panel (a) (top) focuses
on the period up to present day. Panel (b) (bottom)
shows the results from the whole experiment up to 2100.
| |
