This is at least in part due to the fact that the HadCM2
effective climate sensitivity increases with time (see Section 9.3.4.1).
But what I actually wrote was that in GISS - E2 - R
effective climate sensitivity increases with time since the forcing was applied, as it does in many GCMs.
Although the model's ECS is underestimated, that is only because
its effective climate sensitivity increases over time.
It is possible that
effective climate sensitivity increases over time (ignoring, as for equilibrium sensitivity, ice sheet and other slow feedbacks), but there is currently no model - independent reason to think that it does so.
Not exact matches
They conclude, based on study of CMIP5 model output, that equilibrium
climate sensitivity (ECS) is not a fixed quantity — as temperatures
increase, the response is nonlinear, with a smaller
effective ECS in the first decades of the experiments,
increasing over time.
Hansen's paper adds in the possibility of ice sheet response in the relatively near term (centuries, if not decades), which leads to an
effective doubling of the
sensitivity of
climate to our CO2
increase.
This will clearly result in more papers trying to explain this fact on subjects such as
climate sensitivity, as well as whether or not CFCs caused more warming than originally thought, radiation of heat into space as earth's
effective temperature
increases, and whether the saturation of absorption of EMR by CO2 in the atmosphere actually fits the logrithmic curve.
They conclude, based on study of CMIP5 model output, that equilibrium
climate sensitivity (ECS) is not a fixed quantity — as temperatures
increase, the response is nonlinear, with a smaller
effective ECS in the first decades of the experiments,
increasing over time.
The
effective climate sensitivity and ocean heat uptake are compared by Raper et al. (2001b) using the CMIP2 data set (1 % / yr CO2
increase to doubling).