date: Mon, 17 Feb 2003 11:03:16 -0800 (PST) from: Eric Steig subject: review of Holocne paper by Masson-Delmotte et al. to: k.briffa@uea.ac.uk Dear Keith, Following is my review of Masson-Delmotte et al., "Common millennial-scale variability ....". My apologies for the delay. I would prefer anononimity for this review. I have reproduced the peer review form below. Sincerely, Eric Steig --------------- Paper title: Common millennial scale variability of Antarctic and Southern Ocean temperatures during the past 5000 years reconstructed from EPICA Dome C ice core Author(s): V. Masson-Delmotte, B. Stenni, and J. Jouzel Checklist: Title: Suitable. I suggest inserting "the" before "EPICA". Introduction: requires revision Discussion: sufficient Abstract: requires revision Methods: better description and error bars required for spectral analysis Conclusions: generally sound but spectral analysis overinterpreted Scope: international interest/interdisciplinary interest/general significance is apparent Length: appropriate Language and style: English requires work of copy-editor; generally well organized Referencing: some additional references required Figures: not all are essential; some revision required Recommendation: (1)/(2) Should be acceptable after modification and resubmission Importance Ratin: Major Contribution Detailed comments: The authors have made significant contributions to the interpretation of deuterium and deuterium excess from ice cores. Especially in the last few years they have demonstrated the utility of deuterium excess as a proxy for sea surface temperatures. The great advantage of their approach is that both local (ice sheet) temperatures and distant (sea surface) temperatures are obtained from a single ice core record, largely eliminating ambiguities about relative age. This paper provides new deuterium (dD) and deuterium excess (xs) data from the EPICA ice core at Dome C, Antarctica. An 2-D isotopic model is used to calculate linear functions relating dD and xs to site and source temperature, allowing conversion of the isotope ratios to useful climate variables. this approach has been used previously and shown to be very reasonble; it is probably even more reasonable for the Holocene than the last glacial period, since boundary conditions are changing less, especially on the millennial timescales emphasized in the paper. The paper is overall well organized, but there is too little description of the deuterium and dueterium excess, which some readers will not be familiar with. I suggest adding a short paragraph, prior to the one that begins "Here we focus..." which provides more reference to previous theoretical work on deuterium excess. Missing especially from the references is the Kavenaugh and Cuffey paper from the Greenland IGS meeting, and the Cuffey and Vimeux paper from Nature. Also, reference is made to the Stenni et al. paper from Science, but the reader has to guess what was in that paper. A brief description of that paper and its conclusions would be appropriate. The paper reaches three main conclusions. First, that the early Holocene optimum occurs early in Antarctica than at lower latitudes; second, that site and source temperatures co-vary after about 5000 years ago (which is tentatively attributed to an increase in ENSO-type variability); third, that there is significant temperature variability on timescales of ~800 years at the Antarctic site, but not at lower latitudes. Each of these conclusions is important, if correct, because each provides insights into how the climate system has evolved through the Holocene. I find particularly interesting the suggestion that the millennial-scale variability in the Antarctic is probably of regional origin, since it does not appear to occur at lower latitudes and is on a different timescale than the often-discussed 1500-year cycles of the North Atlantic. Overall, I think the conclusions are sound, but I am skeptical about some aspects. The interpretation of various "periodicities" in the data is overstated. In fact, no confidence intervals are shown in the spectral analysis plots, unless perhaps the dashed line shown is supposed to be the 95% confidence (?). The very different spectra obtained for the 0-3, 3-6, 6-9 and 9-12 ka intervals are interpreted as meaningful changes in the physical processes involved (e.g. "some periodicities only appear in the last thousdands of years"). A more conservative conclusion would be that none of these "periodicities" are actually significant, relative to red noise. Use of a more conservative spectral analysis routine, with a greater number of degrees of freedom (MTM notoriously uses too few degrees of freedom), would doubtless result in virtually no significant peaks being detected. I would advise against listing multiple "significant" periodicities (e.g. on page 8, it says "220, 176, 150, 110....") and focus instead on the much more interesting result that the deuterium is "redder" than the deuterium excess, which implies (as the authors state) that the millennial-scale power is of local rather than global origin. Related to this, Vimeuex, Masson-Delmotte and others reported a 900-year periodicity in deuterium excess from Taylor Dome. Why does this show up in xs (only) from Taylor Dome and dD (only) at Dome C? The supposed connection with solar variability during the last 2000 years is entirely unconvincing and I suggest deleting it. No statistical work is shown to suggest that this is significant. Since the paper should be revised, I will not comment on specific grammatical or typographical errors here. Overall, the English is fine but should be looked over by a good copy editor prior to publication. Two important things though: 1) "western" and "eastern Antarctica" is incorrect! Use the terms "West Antarctica" and "East Antarctica". These are place names, not geographic directions. 2) On page 10, the term "inversed" refers to the mathematical inversion, but as written it implies that the graphs are upside down. I would say "calculated" instead of "inversed." 3) Figures: Figure 1 should show other ice core sites discussed in the text (as well as well-known sites like Vostok). Figure 5, 8a: justification should be given for using the "reshaped" harmonic spectra; otherwise (preferably) these should not be used, as they are appropriate for electronic signal processing and are of dubious use in climate research where narrow band signals would be a major discovery! On both figures confidennce intervals should be shown and explained.