|I am sorry to say that I still have reservations about the paper in|
its present form. I am afraid that there are some rather fundamental
disagreements between the authors and the reviewer so that my comments
are rather general.
My major point is that the paper is still missing a clear message,
going beyond what is already known.
Yes, in a doubled CO2 climate, the direct radiative forcing of CO2 will
lead to a cooling in the upper stratosphere, but this is certainly
well known (e.g. WMO 2018).
The cooling in the upper stratosphere, by affecting upper
stratospheric ozone chemistry (but which ozone chemistry exactly, see
below), leads to an increase in ozone (referred to in the abstract as
"ozone feedback", which leads to an increase in short-wave heating
(i.e. yields a radiative feedback that generally mitigates this
cooling) -- I agree, but again, what is new here?
The abstract states that the "temperature response due to dynamical
feedbacks is small in global average", but I find it difficult to
understand which dynamical feedbacks are meant here -- perhaps
enhanced tropical upwelling in a 2xCO2 run?
Further, it is stated that the "temperature change in the lower
stratosphere is influenced by the water vapour feedback", but again,
the processes in question here remain unclear. I suggest to state at
least what the sign of the temperature change is (increase or
decrease?) and what "water vapour feedback" means (increase or
decrease of water vapour? chemical impact of water vapour on ozone or
radiative effects of water vapour?).
What remains is an evaluation of CFRAM as a tool to study climate
change. There could be new developments here, but if the focus were
on CFRAM than the nature of the paper would be much more
methodological that it is in its present form.
I see also remaining disagreements between the authors and the reviewer
judging from the reply to my comments.
The authors state "We are not speaking here about the changes in
O3-concentration due to the ozone hole, but rather changes in ozone
concentration that are resulting from changes in the CO2
I did not talk about the "ozone hole" either, what I am talking about
is the upper stratospheric ozone loss, which is also chlorine
driven. And enhanced levels of stratospheric chlorine are around for
many decades to come. From reading the manuscript, I assume that Cl
was set to zero, or perhaps 0.6 ppb -- but I am not sure (see also
below). I think this issue could be at least briefly addressed (if
chlorine is not relevant for the present study it should be stated in
the paper). Other chemical effects could be due to changing N2O
levels. I think it is not a good idea to remove the reference to the
WMO ozone assessment (WMO 2018) entirely.
I stated in my previous review that "it should be clearly said that
the middle atmosphere is *not* in radiative equilibrium" -- in
response the authors changed the discussion, which clearly improved
the presentation of this aspect. However I find the sentence "In the
absence of eddy motions the zonal-mean temperature would relax to a
radiatively determined state" still misleading. This sounds a bit as
this were still a possible state of the atmosphere; note that
without "eddy motions", i.e. without atmospheric waves, in a radiative
equilibrium there are no heating or cooling terms, i.e. no transport
across isentropic surfaces, in other words no Brewer-Dobson
circulation. (And I think that throughout most of the troposphere,
outside of convection, the troposphere is not dynamically unstable).
A few details:
l 214: WACCMs chemical model is associated here with CO2 changes?
What is the CO2 chemistry in WACCM? Or is it transport and changing
CO2 emissions that are relevant here?
l. 224: I think you need more documentation here on the WACCM run. I
see pre-industrial CO2 and doubled CO2 (also SSTs are
mentioned), but a lot of other fields are unclear;
e.g. stratospheric chlorine, N2O, CH4. Aerosol loading? Are
these compounds (and the entire setup) based on
"pre-industrial"? Could tropospheric ozone be relevant
(different between the runs). Perhaps you could use a
citation, where all the assumptions of the WACCM run are
described? Note that when you use pre-industrial with (just)
CO2 doubled, these doubled CO2 runs do not describe a future
l 745-748: You state "Ozone plays a major role in the chemical and
radiative budget of the middle atmosphere. The ozone
distribution in the mesosphere is maintained by a balance
between transport processes and various catalytic cycles
involving nitrogen oxides, HOx and Clx radicals." Which
transport processes with an impact on ozone are you referring