98 Modelling theclimate
distribution of salt in the ocean, which in turn affects the ocean density.
It is not surprising, therefore, to find that the ‘climate’ described by the
model is quite sensitive to the size and the distribution of water exchanges
at the interface.
Before the model can be used for prediction it has to be run for a
considerable time until it reaches a steady ‘climate’. The ‘climate’ of the
model, when it is run unperturbed by increasing greenhouse gases, should
be as close as possible to the current actual climate. If the exchanges are
not correctly described, this will not be the case. Getting the exchanges
right in the model has proved to be difficult. Until recently, many coupled
models introduced artificial adjustments to the fluxes (known as flux ad-
justments) so as to ensure that the model’s ‘climate’ is as identical as
possible to the current climate. However, the ocean component of the
model has been improved especially through introducing higher resolu-
tion (150 km or less), so the need for such adjustments has diminished
and many models are now able to provide an adequate description of the
climate with no such adjustments.
Before leaving the oceans, there is a particularfeedback which
should be mentioned between the hydrological cycle and the deep ocean
circulation (see box below). Changes in rainfall, by altering the ocean
salinity, can interact with the ocean circulation. This could affect the
climate, particularly of the North Atlantic region; it may also have
been responsible for some dramatic climate changes in the past (see
Chapter 4).
The most important feedbacks belong to the atmospheric and the
ocean components of the model. They are the largest components, and,
because they are both fluids and have to be dynamically coupled together,
their incorporation into the model is highly demanding. However, another
feedback to be modelled is the ice-albedo feedback, which arises from
the variations of sea-ice and of snow.
Sea-ice covers a large part of the polar regions in the winter. It is
moved about by both the surface wind and the ocean circulation. So that
the ice-albedofeedback can be properly described, the growth, decay and
dynamics of sea-ice have to be included in the model. Land ice is also
included, essentially as a boundary condition–afixedquantity – because
its coverage changes little from year to year. However, the model needs to
show whether there are likely to be changes in ice volume, even though
these are small, in order to find out their effect on sea level (Chapter 7
considers the impacts of sea-level change).
Interactions withthe land surface must also be adequately described.
The most important properties for the model are land surface wetness or,
more precisely, soil moisture content (which will determine the amount