Title: Feedback analysis of climate change simulations
Abstract: The climate sensitivity parameter that describes the change in surface temperature due to
a unit change in radiative forcing has long been assumed to be constant. However, recent
studies found that the climate sensitivity parameter varies, not only amongst models for
the same forcing but also within the same model where it may strongly depend on the
strength and on the type of the applied radiative forcing.
By means of the “Partial Radiative Perturbation”-method (PRP-method), a complete
feedback analysis of CO2 driven climate change simulations is performed to identify the
individual feedback processes which are responsible for the variation in climate sensitivity
parameter. To include all components of the feedback analysis, the stratospheric temperature
feedback is introduced in this work. It describes the stratospheric temperature
change due to a radiative forcing. This feedback is found to be weakly positive. The
combination of the stratospheric temperature feedback and the instantaneous radiative
forcing allows to approximate the stratosphere adjusted radiative forcing which is known
to be a better climate predictor than the instantaneous forcing.
In a set of CO2 driven equilibrium climate change simulations, the water vapour, the
cloud and the stratospheric temperature feedback are found to vary the most under increasing
radiative forcing. Hence, the interplay between these three feedback processes
causes an increase of the climate sensitivity parameter when the atmospheric carbon dioxide
concentration is quadrupled in comparison to a doubling of the CO2 concentration.
For climate change simulations with a small CO2 radiative forcing, it was not possible
to identify the feedback processes which are responsible for a varying climate sensitivity
parameter. Thus, forcings must be sufficiently large to establish significant differences of
feedbacks that are interpretable to explain differences in climate sensitivities.
Feedbacks of CO2 driven simulations with and without interactively coupled atmospheric
chemistry are also compared. Only the stratospheric temperature feedback differs
significantly among these simulation experiments. For the simulation without interactively
coupled chemistry, the stratospheric temperature feedback is considerably larger
than for the simulation with interactively coupled chemistry where the trace gases could
adjust to the radiative perturbation. The change in ozone is found to be responsible
for the difference between these simulations. Ozone changes to a CO2 radiative forcing
causes a negative feedback, which reduces the stratospheric temperature feedback, when
the reaction of the atmospheric chemistry to the CO2 perturbation is included.
Moreover, the strengths and weaknesses of the PRP-method are investigated. This
method is only suitable for calculating independent feedbacks and to yield a balance
of radiative forcing and feedbacks, if the forward and backward PRP calculations are
combined. If only the forward or the backward PRP calculation is considered, interactions
between feedbacks occur, which render the separation of the climate response into individual feedbacks as unpracticable. In particular, the water vapour and the lapse rate
feedback as well as the water vapour and the cloud feedback show large overlapping effects.
These overlapping effects are completely erased when forward and backward PRP
calculations are combined.
Publication Year: 2014
Publication Date: 2014-06-01
Language: en
Type: article
Access and Citation
AI Researcher Chatbot
Get quick answers to your questions about the article from our AI researcher chatbot