Intercomparison experiments (see Chapter 8, Section 8.5.4) have quantified
the inter-model differences in response to prescribed atmospheric forcing, and have demonstrated that the most significant outliers can be understood in terms of unrealistic physical approximations in their formulation, particularly the neglect of stomatal resistance.
The large spread
in cloud radiative feedbacks leads to the conclusion that
differences in cloud
response are the primary source of
inter-model differences in climate sensitivity (see discussion
in Section 8.6.3.2.2).
Based on the understanding of both the physical processes that control key climate feedbacks (see Section 8.6.3), and also the origin of
inter-model differences in the simulation of feedbacks (see Section 8.6.2), the following climate characteristics appear to be particularly important: (i) for the water vapour and lapse rate feedbacks, the
response of upper - tropospheric RH and lapse rate to interannual or decadal changes
in climate; (ii) for cloud feedbacks, the
response of boundary - layer clouds and anvil clouds to a change
in surface or atmospheric conditions and the change
in cloud radiative properties associated with a change
in extratropical synoptic weather systems; (iii) for snow albedo feedbacks, the relationship between surface air temperature and snow melt over northern land areas during spring and (iv) for sea ice feedbacks, the simulation of sea ice thickness.