The key problem to be addressed is the response of the climate system to changes
in forcing. In many cases there is a fairly direct and linear response and many
of the simulated changes fall into that class (Chapter 9).
These concern the large-scale general circulations of the atmosphere and ocean,
and they are in principle represented in current comprehensive coupled climate
models. Such possible large-scale dynamical feedbacks are also influenced by
small-scale processes within the climate system. The various feedbacks in the
climate system may amplify (positive feedbacks), or diminish (negative feedbacks)
the original response. We often approximate the response of a particular variable
to a small forcing by a scaling number. A prominent example of such a quantification
is the equilibrium global mean temperature increase per Wm-2 change
in the global mean atmospheric radiative forcing (see Chapter
9, Section 9.2.1).
While many aspects of the response of the climate system to greenhouse gas
forcing appear to be linear, regime or mode transitions cannot be quantified
by a simple number because responses do not scale with the amplitude of the
forcing: small perturbations can induce large changes in certain variables of
the climate system. This implies the existence of thresholds in the climate
system which can be crossed for a sufficiently large perturbation. While such
behaviour has long been known and studied in the context of simple models of
the climate system, such thresholds are now also found in the comprehensive
coupled climate models currently available. Within this framework, the possibility
for irreversible changes in the climate system exists. This insight, backed
by the palaeo-climatic record (see Chapter 2, Section
2.4), is a new challenge for global change science because now thresholds
have to be identified and their values need to be estimated using the entire
hierarchy of climate models.
To estimate the response properly, we must represent faithfully the physical
processes in models. Not only must the whole system model perform reasonably
well in comparison with observation (both spatial and temporal), but so too
must the component models and the processes that are involved in the models.
It is possible to tune a model so that some variable appears consistent with
that observed, but we must also ask whether it comes out that way for the right
reason and with the right variability. By examining how well individual processes
are known and can be modelled, we can comment on the capabilities and usefulness
of the models, and whether they are likely to be able to properly represent
possible non-linear responses of the climate system.
Examples of some processes in the climate system and its components that have
been dealt with in the Second Assessment Report (IPCC, 1996) (hereafter SAR)
and still are important topics of progress:
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