Estimating radiative forcing with a nonconstant feedback parameter and
linear response
Abstract
A new algorithm is proposed for estimating time-evolving global forcing
in climate models. The method is a further development of the work of
Forster et al. (2013), taking into account the non-constancy of the
global feedbacks. We assume that the non-constancy of this global
feedback can be explained as a time-scale dependence, associated with
linear temperature responses to the forcing on different time scales.
With this method we obtain stronger forcing estimates than previously
assumed for the representative concentration pathway experiments in the
Coupled Model Intercomparison Project Phase 5 (CMIP5). The reason for
the higher future forcing is that the global feedback parameter is more
negative at shorter time scales than at longer time scales, consistent
with the equilibrium climate sensitivity increasing with equilibration
time. Our definition of forcing provides a clean separation of forcing
and response, and we find that linear temperature response functions
estimated from experiments with abrupt quadrupling of CO$_2$ can be
used to predict responses also for future scenarios. In particular, we
demonstrate that for most models, the response to our new forcing
estimate applied on the 21st century scenarios provides a global surface
temperature up to year 2100 consistent with the output of coupled model
versions of the respective model.