chapter - Atmospheric and Oceanic Science

Global climate models
The last shows a trend much lower than the observed but also positive. This can be
explained because some studies suggest that that there were additional forcings like
solar and volcanic aerosols variability. The observed temperature anomaly along
the century was also plotted. As can be seen, in general, there is a reasonable adjustment
between the model and the observations with regard to long term trends, but
the model does represent in an adequate manner the interannual variability. This is
because of the intrinsically unpredictable character of the natural variability of the
climatic system and according to this, no matter how perfect the models could be,
the interannual variability in the long term will never be able to be well simulated.
Meanwhile it is to be noticed that the trend of increase represented by the model
follows the observed trend. Some further results of climate change simulation can
be seen in the Chapter 12 and 13.
Model intercomparison studies, such as those discussed above, provide valuable
information on the differences between GCM projections and the reasons for
these differences. However, the range of differences between GCM results is
unlikely to be indicative of the full range of uncertainties about future climate.
Three main sources of uncertainty can be identified:
1. Uncertainties in future greenhouse gas and aerosol emissions. The IS92 and provisional
SRES emissions scenarios described in Chapter 12 exemplify these
uncertainties, with each scenario implying different levels of atmospheric composition
and hence of radiative forcing.
2. Uncertainties in global climate sensitivity, due to differences in the way physical
processes and feedbacks are simulated in different models. This means that
some GCMs simulate greater mean global warming per unit of radiative forcing
than others.
3. Uncertainties in regional climate changes, which are apparent from the differences
in regional estimates of climate change by different GCMs for the same,
mean global warming.
While the results of GCM experiments probably capture a large part of the
uncertainty ranges in 2 and 3, they certainly cannot describe the range of uncertainty
due to emissions described in 1. Due to constraints of time and resources, only a
limited number of GCM experiments can be conducted. In addition, many experiments
have been specifically designed to be directly comparable with other models,
to aid model development, and their assumed forcing is very similar. Most early
GCM experiments were for a hypothetical 2 x CO2 equilibrium case. The AOGCM
outputs in the DDC are for transient forcing approximating to a 1% increase in
equivalent CO2 per year (close to the IS92a emissions scenario). One set of runs for
an IS92d emissions scenario (approximately a 0.5%/year increase) is also included
from the HadCM2 model. Some alternative emissions scenarios have also been
used in experiments with the GFDL model assuming 0.25, 0.5, 1, 2, and 4%/year
increases (Kattenberg et al. 1996), but these are not included in the DDC.
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