Expert Review Comments on the IPCC WGI AR5 First Order Draft -- Chapter 5 Comment Chapter From From To To No Page Line Page Line Comment Response 5-1 5 0 0 0 0 The coverage of the chapter seems a bit too limited and selective although I understand the number of pages Taken into account. Covered in new section 5.5 are limited. For example, why abrupt climate change in the Holocene is not covered only those in the glacial? (Holocene regional changes) within space limitations. Some of the topics probably are dealt in earlier reports, but because of its importance for future implications coverage of the Holocene climate change should be increased. [Takuro Kobashi, Japan] 5-2 5 0 Too many references in the text will distract the attention of the readers [Muhammad Amjad, Pakistan] Noted 5-3 5 0 This chapter is generally difficult to read for non specialists. The style has to be corrected as well as the Taken into account. Sections reorganised and logics numerous typos and spelling mistakes. The content needs to be "simplified" and better organised. To my better explained. opinion, there is still some work to do on this chapter. [CATHERINE BELTRAN, France] 5-4 5 0 In general it is difficult to get the main message. It would be easier if the main results were highlighted and if Noted "what's left to do" was clearly assigned. [CATHERINE BELTRAN, France] 5-5 5 0 general comment. I found the material included in this chapter quite convincing. The overall organisation of the Noted; Revised introduction should give better chapter is fine, except that the exact place of some pieces of information should be reconsidered. [PASCALE guidance BRACONNOT, France] 5-6 5 0 The chapter covers a wide range of topics. In its present form, several sections still resemble most to an Taken into account. Text is revised to better stress the interesting syntheses from which it is not always easy to isolate the key points that are important with regards robust findings, provide a better coverage of the range to the IPCC assessement. it also gives the feeling that very little is known and useful, just because there is too of views, and the key uncertainties. many places were it is said that things are not well understood, not well known, which is true, but tone down too much what is really known. I would suggest that there is a place were knowledge gap is highlited, but that the wording better reflect what is known and what are the progresses since last report that are of use to understand future changes. Similary it is important to tell about the uncertainties, but maybe withough making excessive use of this word by telling what is meant by uncertainty in the different parts. [PASCALE BRACONNOT, France] 5-7 5 0 I would like to see some more discussion about the evolution of the climate in the first millennium CE prior to Taken into account to the extent possible given space the Medieval Climate Anomaly, especially with the explicit mentioning of the Roman Warm Period and the constraints. New section 5.5 (Holocene regional Dark Age Cold Period. Since the publication of the IPCC Fourth Assessment Report many more long proxy changes) briefly refers to this time period. records with high to medium temporal resolution – reflecting either changes in temperature, precipitation or drought – have been published making it potentially feasible to place the modern global warming into a much longer time perspective than was possible at the time of the IPCC Fourth Assessment Report. This is especially relevant since during the second millennium CE large volcanic eruptions and solar minimums have tended to coincide, making it hard to separate the influence of solar and volcanic forcing, whereas they are better separated during the first millennium CE. A better understanding of the regional to global climate during the first millennium CE is thus important in order to better understand the relative influence of volcanic and solar forcing, respectively, on decadal and longer time-scales. [Fredrik Charpentier Ljungqvist, Sweden] 5-8 5 0 Related to the comment above is the relative lack of discussion of natural multi-centennial (quasi)oscillations Taken into account together with comments 5-1 and (e.g., the Bond cycles) in the climate system and their possible relationship to long-term changes in solar 5-7. forcing. An improved understanding of natural multi-centennial climate (quasi)oscillations is important in order to better predict the direction of future natural climate evolution and for investigating if the cause of natural climate variability is likely to reinforce or counteract the anthropogenic global warming. A discussion of natural multi-centennial climate oscillations also places large-scale climate changes, as the Medieval Climate Anomaly and Little Ice Age, in a larger context. At the very least, the limitations of predicting climate from past multi-centennial climate (quasi)oscillations ought to be outlined. [Fredrik Charpentier Ljungqvist, Sweden] 5-9 5 0 Several recent studies discuss climate cycles, and the following can be mentioned as examples: Wanner, H., Taken into account together with comments 5-1 and Solomina, O., Grosjean, M., Ritz, S. P., and Jetel, M.: Structure and origin of Holocene cold events, 5-7. Quaternary Sci. Rev., 30, 3109–3123, 2011; Humlum, O., Solheim, J., and Stordahl, K.: Identifying natural contributions to late Holocene climate change, Glob. Planet. Change, 79, 145–156, 2011; Breitenmoser, P., Beer, J., Brönnimann, S., Frank, D., Steinhilber, F., and Wanner, H.: Solar and volcanic fingerprints in tree-ring chronologies over the past 2000 years. Palaeogeogr. Palaeocl., 313–314, 127–139, 2012. Breitenmoser et al. (2012) provides a good presentation of the detection of the DeVries cycle of solar activity in tree-ring records. Do not Cite, Quote or Distribute Page 1 of 157 Expert Review Comments on the IPCC WGI AR5 First Order Draft -- Chapter 5 Comment Chapter From From To To No Page Line Page Line Comment Response [Fredrik Charpentier Ljungqvist, Sweden] 5-10 5 0 I would like to see a somewhat longer section discussing the Holocene Thermal Maximum. Our knowledge of Taken into account. Covered in new section 5.5 the Holocene Thermal Maximum has increased substantially since the publication of the IPCC Fourth (Holocene regional changes) within space limitations. Assessment Report. The Holocene Thermal Maximum is rather important for understanding non-linear feedbacks in the climate system. The direct results of the orbital changes during the mid-Holocene should have been a large warming in the summer in the Northern Hemisphere and a slight cooling during the winter, whereas the Southern Hemisphere would have experienced somewhat cooler summers and warmer winters. But much evidence nevertheless points to a substantial warming during all season in most of the extra-tropical Northern Hemisphere and also in large parts of the Southern Hemisphere. This means that the enhanced seasonal forcing resulted in strong positive feedbacks in the climate system, and likely large-scale reorganization of the latitudinal heat transport, that are still poorly understood and not fully captured in the climate models. In proxy data and model comparisons it is quite clear that the proxy records usually show larger changes in annual mean temperature than the majority of the models for most regions. [Fredrik Charpentier Ljungqvist, Sweden] 5-11 5 0 One important reference that I would like to see mentioned in this context of the Holocene Thermal Maximum Noted. is Shakun and Carlson (2010) that shows, after assessing numerous proxy records, that the warmest conditions during the Holocene occurred in the Northern Hemisphere 8±3.2 ka and in the Southern Hemisphere 7.4±3.7 ka. It could also be of interest to refer to the borehole temperature estimates by Huang et al. (2008) that point to that the earth experienced multi-centennial periods with global mean temperatures at least 1°C above the pre-industrial temperatures or even more. The full reference to Huang et al. (2008) is: Huang, S. P., Pollack, H. N., and Shen, P.-Y.: A late Quaternary climate reconstruction based on borehole heat flux data, borehole temperature data, and the instrumental record, Geophys. Res. Lett., 35, L13703, doi:10.1029/2008GL034187, 2008. [Fredrik Charpentier Ljungqvist, Sweden] 5-12 5 0 The concept of "climate sensitivity" is used frequently in this chapter, yet does not adequately define this Taken into account. 5.1 and revised box 5.1 (Earth concept (and for example, Charney vs. earth-system) at least until the very end in Box 5.3 A recommended system feedbacks, now first box) address the focus on the differences between these concepts and precisely how they compare to each other in the differences between climate and Earth system beginning of the chapter. [Chris Colose, United States] sensivitiy. 5-13 5 0 Perhaps prefer the wording "astronomical forcing" to the wording "orbital forcing" ? [Bernard De Saedeleer, Noted Belgium] 5-14 5 0 chapter 5 has redundancies with chapter 6, 8 and 13. I would suggest to keep all paleoaspects (past CO2, Taken into account. Overlaps clarified. Chapter 5 CH4, N2O, past RF forcing, greenhouse gases, solar, volcanic) within chapter 5 and refer to chapter 5 in does not address mechanisms of the carbon cycle chapter 6 and chapter 8. The past sea level discussion I would keep in chapter 13 and refer to chapter 13 in (this is in chapter 6). Chapter 5 still describes proxy- chapter 5. [Hubertus Fischer, Switzerland] based information on sea level which is synthetized in Chapter 13. 5-15 5 0 I am having a major difficulty with this chapter with its current structure.