Expert Review Comments on the IPCC WGI AR5 First Order Draft -- Chapter 8 Comment Chapter From From To To No Page Line Page Line Comment Response 8-1 8 0 In general very prudent and good descriptions of the knowledge base for solar radiative forcing changes. [Bo Noted Andersen, Norway] 8-2 8 0 NO2, NOx, etc should always be typed with a subscript. [Pieter Aucamp, South Africa] Accepted, this is taken into account 8-3 8 0 Put legend in box to make it clearer. [Pieter Aucamp, South Africa] Rejected, legends treated in standard IPCC formats 8-4 8 0 The term well-mixed GHGs is used instaed of Long-lived. I woudl recommend to use Long-lived since even Rejected, WMGHG will be used and explained long-lived gases are not necessarly well mixed at all times (e.g. close to large point sources). [Terje Berntsen, carefully Norway] 8-5 8 0 I welcome the evolution of the Radiative Forcing chapter since the TAR and AR4. It is nice to see new Noted concepts such a AF and GTP being endorsed by this Chapter. It is also appreciated that the authors have tried to make the chapter relevant to policymakers by estimating RF by species and by sectors. On the minus side, I feel that the wording is not quite as rigorous as it should be and I will try to point weaknesses in the text in my comments. [Olivier Boucher, France] 8-6 8 0 It would be nice to refer to particular sections or subsections of chapter 7 rather than Chapter 7 as a whole Accepted, taken into account when possible. [Olivier Boucher, France] 8-7 8 0 This is a well written section for this stage of the process. The text length in the various sections is Noted appropriate and the text is well supported by figures. A good feature is the inclusion of numberical results from previous assessment reports. [David Fahey, USA] 8-8 8 0 The more extensive treatment of CO2 equivalence metrics is an important policy relevant feature of this Taken into account, We agree that a more extensive chapter. Suggest that it be made clear if this topic is to be covered comprehensively in this chapter or if it will treatment of CO2 equivalence is an important policy be parsed between sections in the assessments of the 3 working groups as it has in past assessments. If its relevant feature. Re "parsed between sections in the assessment is to go beyond the physical science basis, suggest that this be made clear as it is a departure assessments of the 3 WGs": WGIII will also treat the from past assessments. [Haroon Kheshgi, United States of America] metric issue, but so far we don't know to which extent. Contact is established and our work will be coordinated. However, WGII will probably not write about metrics. That WGI goes beyond physical metrics is not a departure from previous assessments; economic metrics were to some extent discussed in AR4 WGI. 8-9 8 0 citations to Assessments are often too general (e.g., WMO, 2011); if made consistent with past reports they Taken into account, some of the citations are modified should be to the authors of the relevant chapters. [Stephen Montzka, USA] but some references to earlier WMO reports are kept if they are more general. 8-10 8 0 There is a lot of material in chapter 8 that belongs in chapters 2 and 7, and in many cases has already been Taken into account, agreement with Ch 2, 6 and 7 on presented there. The authors of chapter 8 should revise their text to reduce this duplication. [JOHN OGREN, cross-chapter issue. More references to previous USA] chapters have been implemented. 8-11 8 0 The chapter refers to "Radiative Forcing (RF)", but often just uses the word "forcing". Is "forcing" meant to be Taken into account - text revised and the following is "RF". I suggest checking for consistency throughout [Glen Peters, Norway] described 'We use the term 'forcing' in general discussions, and the specific terms RF or AF in cases where the distinction is important' 8-12 8 0 This chapter is greatly improved from the ZOD and the authors are to be commended for that. I do, however, Taken into account, more assessment is included in think that it is still closer to a review and not enough of an assessment. This is most apparent in the metrics the whole chapter and especially in the mentioned section and the discussions on atmospheric chemistry effects on RF. The greatly expanded sections on sections. chemical interactions are one of the largest changes from previous assessments. However, these sections suffer from being too general in tone and for not providing estimates for the magnitudes (on RF or metrics) of the effects discussed. Even if complicated processes have significant uncertainties, if the overall effect is likely to be small then this should be stressed instead of all of the uncertainties. Overall, I like the discussion of the new approaches to RF but I suggest some restructuring below and think you are overly harsh on the old RF Do not Cite, Quote or Distribute Page 1 of 119 Expert Review Comments on the IPCC WGI AR5 First Order Draft -- Chapter 8 Comment Chapter From From To To No Page Line Page Line Comment Response approach (as mentioned in several comment below). In my mind the AF is a natural evolution of the old RF approach and for most gases the two give similar results. For other agents (e.g., AIE), the primary benefit is computation simplicity compared with the old approach, as opposed to it being a fundamentally new thing (as it is stated in many places). [Robert Portmann, United States of America] 8-13 8 0 Chapter 8 is well laid out. The raionale for the observation scenarios is not always laid out. There needs to be Taken into account, more cross-referencing to Ch2 much more cross-referencing to Chapter 2 and an explanation of differences in the observations used. [John have been implemented. Remedios, United Kingdom of Great Britain & Northern Ireland] 8-14 8 0 Dear colleagues, The chapter is starting to shape up well. My comments are mostly focused on the sections Noted that I would have contributed to, if I had been able to continue as a Lead Author. Kind regards, Leon [Leon Rotstayn, Australia] 8-15 8 0 The aerosol forcing discussion in chapter 7 and 8 deserves a little more harmonization. It is a little confusing to Taken into account, aerosol forcing discussion in Ch8 find similar descriptions of eg the aerosol DRF again in chapter 8. [Michael Schulz, Norway] is reduced. 8-16 8 0 Is it possible to judge on the models capacity to reproduce observed aerosol concentration, deposition, AOD Rejected, details of validation of aerosols belong to and brightning trends at least for the last 30 years? This would add confidence in the models to reproduce the Chapter 7 forcing history. [Michael Schulz, Norway] 8-17 8 0 Some general discussion related to the global dimming and brightening could be considered in this chapter as Rejected, see comment 8-16 it is closely related to radiative forcing. If it is discussed elsewhere, it can be referenced. [Katsumasa Tanaka, Switzerland] 8-18 8 0 I have an impression that the focus on GTP in the current manuscript is unusually strong in considering the Taken into account, the text modified so it is more state of discussion concerning the question as to what should be the alternative metric?. In the current draft, it balance on the metric discussion in section 8.7 and may give a false impression that the report recommends GWP or GTP. I would suggest a paragraph at the ES. beginning of this chapter that states clearly and explicitly why, unlike the previous IPCC reports, GTP is frequently used in this new report. If I may propose, it could be something like as follows: "GWP and GTP are simultaneously used in this report to illustrate the complexity of the issues surrounding metrics. Since AR4, GTP has been more widely used in scientific research. However, other alternative metrics have also been proposed and under dispute (Section 8.1.2.6)." In general, I appreciate the multi-metric approach, a drastic change from AR4. This new approach should be useful to advance the discussion surrounding the metrics and make metric users aware of issues. [Katsumasa Tanaka, Switzerland] 8-19 8 0 The chapters is already in good shape. It reads very well. Well done. [Guus Velders, Netherlands] Noted 8-20 8 0 It was a pleasure to read Chapter 8 "Anthropogenic and Natural Radiative Forcing." The authors present an Rejected - one of the things we try to emphasize in authoritative and well-constructed overview of the subject matter. Obviously, a huge amount of work has gone AR5 is that choice of time horizon is arbitrary, and so into this effort and I applaud the result. I have one major comment and several minor comments. My major we show time evolution when practical. Short time comment is that the balance of the discussion is too focused on short- and very short-term effects. While it is horizons are clearly relevant in some cases, notably well understood that there is no single "correct" time horizon over which to assess radiative forcing impacts, I the response to volcanic eruptions which is largest for would argue that it is equally well understood that there are some "incorrect" time horizons (e.g., 1, 5, or 10 1-2 years following large eruptions years).