Evaluating the nature and extent of changes to climate sensitivity
between FGOALS-g2 and FGOALS-g3
Abstract
Equilibrium climate sensitivity (ECS) and its related feedbacks are
important metrics used to measure the global mean surface temperature
change in future climate projections. This paper uses the radiative
kernel approach and a simplified cloud feedback calculation (comparing
three different cloud feedback methods) to analyze the differences in
the ECS, as well as the feedbacks contributing to it, between two
versions of the Flexible Global Ocean-Atmosphere-Land System model
(i.e., FGOALS-g2 and FGOALS-g3). The results show that the ECS of
FGOALS-g3 is smaller than that of FGOALS-g2 (2.8 K versus 3.3 K). The
main feedbacks contributing to the ECS change in FGOALS-g3 are the
weaker surface albedo feedback and stronger negative shortwave cloud
feedback. The reduced surface albedo feedback in FGOALS-g3 is associated
mainly with its mean base state, which has a lower surface air
temperature and larger sea ice area compared with FGOALS-g2. The
enhanced negative shortwave cloud feedback in FGOALS-g3 is caused mainly
by the larger low-cloud area fraction and liquid water path.
Furthermore, the ECS change can be traced back to the different cloud
parameterization scheme, parameter tuning, ocean grid, and external
forcings used in FGOALS-g3, as these all affect the mean climate state
of the model.