COMMENTS TO THE AUTHOR(S)
The manuscript uses a simple energy budget equation (as employed e.g. by Gregory et al 2004, 2008, Otto et al 2013) to test the consistency between three recent "assessments" of radiative forcing and climate sensitivity (not really equilibrium climate sensitivity in the case of observational studies).
The study finds significant differences between the three assessments and also finds that the independent assessments of forcing and climate sensitivity within AR5 are not consistent if one assumes the simple energy balance model to be a perfect description of reality.
The overall innovation of the manuscript is very low, as the calculations made to compare the three studies are already available within each of the sources, most directly in Otto et al.
The finding of differences between the three "assessments" and within the assessments (AR5), when assuming the energy balance model to be right, and compared to the CMIP5 models are reported as apparent inconsistencies.
The paper does not make any significant attempt at explaining or understanding the differences, it rather puts out a very simplistic negative message giving at least the implicit impression of "errors" being made within and between these assessments, e.g. by emphasising the overlap of authors on two of the three studies.
What a paper with this message should have done instead is recognising and explaining a series of "reasons" and "causes" for the differences.
- The comparison between observation based estimates of ECS and TCR (which would have been far more interesting and less impacted by the large uncertainty about the heat content change relative to the 19th century) and model based estimates is comparing apples and pears, as the models are calculating true global means, whereas the observations have limited coverage. This difference has been emphasised in a recent contribution by Kevin Cowtan, 2013.
- The differences in the forcing estimates used e.g. between Otto et al 2013 and AR5 are not some "unexplainable change of mind of the same group of authors" but are following different tow different logics, and also two different (if only slightly) methods of compiling aggregate uncertainties relative to the reference period, i.e. the Otto et al forcing is deliberately "adjusted" to represent more closely recent observations, whereas AR5 has not put so much weight on these satellite observations, due to still persisting potential problems with this new technology
- The IPCC process itself explains potential inconsistencies under the strict requirement of a simplistic energy balance: The different estimates for temperature, heat uptake, forcing, and ECS and TCR are made within different working groups, at slightly different points in time, and with potentially different emphasis on different data sources. The IPCC estimates of different quantities are not based on single data sources, nor on a fixed set of models, but by construction by expert based assessments based on a multitude of sources. Hence the expectation that all expert estimates are completely consistent within a simple energy balance model is unfounded from the beginning.
- Even more so, as the very application of the Kappa model (the simple energy balance model employed in this work, in Otto et al, and Gregory 2004) comes with a note of caution, as it is well known (and stated in all these studies) to underestimate ECS, compared to a model with more time-scales and potential non-linearities (hence again no wonder that CMIP5 doesn't fit the same ranges)
Summarising, the simplistic comparison of ranges from AR4, AR5, and Otto et al, combined with the statement they they are inconsistent is less then helpful, actually it is harmful as it opens the door for oversimplified claims of "errors" and worse from the climate sceptics media side.
One cannot and should not simply interpret the IPCCs ranges for AR4 or 5 as confidence intervals or pdfs and hence they are not directly comparable to observation based intervals (as e.g. in Otto et al).
In the same way that one cannot expect a nice fit between observational studies and the CMIP5 models.
A careful, constructive, and comprehensive analysis of what these ranges mean, and how they come to be different, and what underlying problems these comparisons bring would indeed be a valuable contribution to the debate.
I have rated the potential impact in the field as high, but I have to emphasise that this would be a strongly negative impact, as it does not clarify anything but puts up the (false) claim of some big inconsistency, where no consistency was to be expected in the first place.
And I can't see an honest attempt of constructive explanation in the manuscript.
Thus I would strongly advise rejecting the manuscript in its current form.