cc: Dáithí Stone , Peter Stott , Toru Nozawa , Alexey Karpechko , Michael Wehner date: Thu Aug 14 14:26:16 2008 from: Phil Jones subject: Re: Fwd: Decision on Nature Geoscience manuscript NGS-2008-07-00710 to: Gabi Hegerl , Nathan Gillett Nathan, I'd pursue this. If you can get it into Nature then all the better, although the resubmission to Nature would have to be well argued in the accompanying letter as well as in the text. The spatial issue is important in the Antarctic and the region was the one missing from AR4. Cheers Phil At 13:17 14/08/2008, Gabi Hegerl wrote: Hi Nathan, sounds like you got the foot quite well into the door if we can address reviewer 1. I agree that the discounting of area results by lack of significance in individual stations is frustrating. Maybe though if one can give a bit more space to an explanation of why Antarctica does what it does would help (eg the SAM subtracted map)? Did the Monahan paper use the same kinds of models (with ozone forcing)? Gabi Nathan Gillett wrote: Hi all, We now have the reviews back on the polar temperature paper. Unfortunately it's rejected with a suggestion that we resubmit. One reviewer (reviewer 2) was very positive, and had few suggested changes. The other reviewer was unconvinced of the Antarctic analysis - his primary objection seemed to be that we shouldn't be able to detect anthro influence on Antarctic temperature if station temperature trends are not locally significant. However, he appeared not to consider that a large scale mean, or pattern of temperature trends may be significant even if individual station trends are not. Addressing these comments by calculating the significance of area mean temperature trends etc should be relatively straightforward - we've got to try to convince the non-specialist that the Antarctic trends are significant independently of the D&A analysis. I think it's worth revising and resubmitting to Nature Geoscience. Let me know what you think and suggestions for revising the paper. Cheers, Nathan ---------- Forwarded message ---------- From: Date: 2008/8/13 Subject: Decision on Nature Geoscience manuscript NGS-2008-07-00710 To: n.gillett@uea.ac.uk 13th Aug 2008 *Please ensure you delete the link to your author homepage in this e-mail if you wish to forward it to your co-authors. Dear Dr Gillett Your manuscript entitled "Attribution of polar warming to human influence" has now been seen by 2 referees, whose comments are attached. Although they find your work of some potential interest, referee 1 has raised concerns which in our view are sufficiently important to preclude publication of the work in Nature Geoscience, at least in its present form. If, after future work, you can provide compelling evidence for the statistical significance of your reported Antarctic temperature trends as well as for your attribution of those trends to natural and anthropogenic forcing, we will be pleased to consider a revised manuscript (unless, of course, something similar has by then been accepted at Nature Geoscience or appeared elsewhere). I should stress, however, that we would be reluctant to trouble our referees again unless we thought their comments had been addressed in full, and we would understand if you preferred instead to submit your manuscript elsewhere. In the meantime we hope that you find our referees' comments helpful. Yours sincerely, Alicia Newton Associate Editor Nature Geoscience Nature Publishing Group The Macmillan Building 4 Crinan Street London N1 9XW UK PS Please use the link below to submit a revised paper: [1]http://mts-ngs.nature.com/cgi-bin/main.plex?el=A1Q3CGj2A3FlJ1J7A93rUshYh3SdI0gPrnGWsf 3wZ *This url links to your confidential homepage and associated information about manuscripts you may have submitted or be reviewing for us. If you wish to forward this e-mail to co-authors, please delete this link to your homepage first. +44 20 7833 4000 Reviewers' comments: Reviewer #1 (Remarks to the Author): Summary: This paper attempts to formally attribute polar warming in both hemispheres to anthropogenic forcing. The approach of comparing GCM simulations forced by both natural (NAT) and natural + anthropogenic (ANT) has been used successfully in other attribution studies, but here the authors apply it to the polar regions where very little data is available. To isolate the difference between NAT and ANT, they employ an innovative detection and attribution technique. With regards to the Arctic, the model ANT trends appear to be reasonable compared to observations (Fig. 2). Due to the strong warming in the Arctic it would be hard to quarrel with the results for that region. With regards to the Antarctic, where there is less data and less warming than in the Arctic, the results are unconvincing. The authors fail to comment on a closely-related recent paper that has first-order consequences for their analysis (Monaghan et al. 2008b: 20th century Antarctic air temperature and snowfall simulations by IPCC climate models. Geophys. Res. Letts., 35, L07502, doi:10.1029/2007GL032630). That paper strongly suggests that the GCMs are too sensitive to anthropogenic forcing (more details below), and a key assumption of this study is that the GCMs are able to reasonably simulate anthropogenic influences on surface temperature. With this assumption in question, which is confounded by large uncertainty in the observed trends from the handful of available stations, and the fact that Antarctic warming is only likely to be statistically significant over a very small fraction of its surface area (<10%), the result that Antarctic warming is due to human influence is highly questionable. Detailed comments on the Antarctic analysis are given below. Without a convincing Antarctic analysis, I don't feel that this paper is suitable for publication in Nature Geoscience. Given the complexity of the topic, the authors might consider revising and submitting this important work to a high-profile journal that has room for a much more detailed analysis to be presented (Journal of Climate comes to mind). +++++++++++++++++++++++++++++++++++ Detailed Comments on the Antarctic analysis: The authors seem to ignore the fact that there has been no statistically significant warming over 90% of Antarctica. For example, Figure 3 is misleading, since it does not show statistical significance. According to the statistics on Gareth Marshall's Antarctic temperature website ([2]http://www.antarctica.ac.uk/met/gjma/), and confirmed in Turner et al. (2005), the long-term annual positive temperature trend in the Ross Sea at Scott Base is statistically insignificant [1958-2007 trend = +0.0172 {plus minus} 0.0230], as is the long-term annual positive temperature trend at Casey near 110 E [1958-2007 trend =+0.0118 {plus minus} 0.0212]. Therefore, only over the Antarctic Peninsula (<5% of Antarctic surface area) and at a single station on continental Antarctica (Novolazarevskaya), has long-term statistically significant warming been recorded. The spatial influence of Novolazarevskaya appears to be very limited, as the stations on either side of it have statistically insignificant temperature trends near zero. So, the stations with statistically significant warming likely represent a very small area of Antarctica (<10%), and additionally they are sandwiched within a sector that only spans 80 degrees of latitude, from -68 W to 12 E. The authors try to rectify this localized warming by noting that "positive trends predominate" over most of Antarctica according to the surface temperature synthesis of Monaghan et al. (2008). However, Monaghan et al. (2008) noted that the positive and negative trends over Antarctica were overwhelmingly statistically insignificant apart from the Peninsula and a small region around Novalazarevskaya. Chapman and Walsh (2007) also performed a gridded Antarctic surface temperature reconstruction like Monaghan (over a longer period) and got similar results. The point is, how can the authors attribute Antarctic surface warming to anthropogenic forcing, when there is so little evidence for warming to begin with? Why didn't the authors use the more spatially-comprehensive data of Chapman and Walsh (2007) or Monaghan et al. (2008) for their analysis, even if just for comparison with their station-based results? Considering the distribution of the Antarctic stations and their comparatively short records with high interannual variability, perhaps the only place on the continent where one could argue for a robust anthropogenic surface warming signal is over the Peninsula. Marshall et al. (2006) made a convincing case that summer warming on the east side of the Peninsula is due to increased foehn wind events resulting from a stronger SAM. In turn, the link between the SAM and anthropogenic influences, especially from stratospheric ozone depletion, has been established in previous modeling studies, some of which the author cites here. Therefore, if one infers from the existing literature that the small region of Antarctica that has warmed statistically significantly during the past 1/2 century has been mainly influenced by the SAM, then the results shown in this paper for Antarctica (attributing surface warming to human influence) are not particularly groundbreaking. One key assumption of this study is that the AR4 models are able to accurately simulate the impact of anthropogenic forcing on Antarctic surface temperatures. However, in a very closely related study that was not cited in this analysis (Monaghan et al. 2008b), the authors found that 5 AR4 models, two that were included in this study, had annual surface temperature trends that were substantially larger than observed during the past ~1/2 century. The statistically insignificant observed 1960-99 trend from Monaghan et al. (2008b) was 0.06 +/- 2.03 K, versus a highly significant GCM ensemble trend of 1.44 K +/- 0.34 K; all 5 GCM members had statistically significant positive trends (p<0.05). The authors, who also compared their results to the 100+ year Antarctic temperature record (1900-1999) of Schneider et al. (2006, GRL), found that the models results were much larger than observed over the past century as well. They examined why the GCM trends were so much more positive than observed and found (as the authors note in this paper) that the surface temperature sensitivity to the SAM is weaker than observed. More importantly, they concluded that in the GCMs, the influence of the SAM on surface temperatures appears to be overwhelmed by a spurious water vapor feedback. In turn, the water vapor feedback may be (wrongly) causing the much larger than observed GCM surface temperature trends over Antarctica. Their results indicate that the IPCC AR4 GCMs may not yet be able to fully simulate all of the impacts of anthropogenic forcing in Antarctica. If correct, their results signify that the key assumption of this study is not robust for Antarctica. Additionally, their study suggests that the Antarctic surface temperature datasets that are representative of surface temperature over the entire continent may yield a very different comparison with GCM results than is concluded from the comparison with the limited dataset used here. Reviewer #2 (Remarks to the Author): Overall this is an excellent manuscript and an important contribution to the detection and attribution debate. Detection and attribution studies require both models and observations, and this is often accomplished by comparing observations of actual changes to model-induced trends for models forced independently by natural, anthropogenic and combined forcings. This was first done, I think (Stott as an author will know for sure), by Stott, P., Tett, S., Jones, G., Allen, M., Mitchell, J. & Jenkins, G. (2000) Science 290, 2133-2137. Stott, P. (2003) Geophys. Res. Lett. 30, 1728. using four ensembles of a single model, and very strong evidence for global-scale detection and attribution was offered and was a key element in IPCC TAR--the authors of this submission might make this history a bit more prominent in a minor revision. Another study with more limited data coverage arguing that some regional skill was still evident in the same set of model runs was offered using observations of spring phenology of plants and animals as a proxy for spring temperature, and again a clear detection and "joint attribution" to anthropogenic causation--though a smaller fraction of variance explained--was also found in: Root, Terry L., Dena MacMynowski, Michael D. Mastrandrea, and Stephen H. Schneider, 2005: "Human-modified temperatures induce species changes: Joint attribution, " Proceedings of the National Academy of Sciences, 102, 21, 7465-7469 The latter used more sparse observational data and thus finding less variance explained than for global scale thermometer data in Stott et al papers was not surprising. BUt it did find skill at regional scales. In this submitted paper studying polar regions the authors aggregate four models, rather than one, and like earlier studies compare this for models driven by N and N&A forcings. Data in the polar regions is very sparse--more so than even the phenological ecological data sets--nevertheless the authors are admirably able to perform a heroic--and to me credible--effort to extract a signal of human-induced climate changes in this limited data set. My only suggestion to the authors is to consider framing their efforts in the context of earlier ones like mentioned above issues such as data coverage and show the evolution of D&A studies using N and N&A forced models and how all such studies at global to regional scales do agree that joint attribution is indeed a credible conclusion--and this latest study extends that to polar regions. In short, the authors should be congratulated on a fine addition to the literature. This email has been sent through the NPG Manuscript Tracking System NY-610A-NPG&MTS -- Dr Gabriele Hegerl School of GeoSciences The University of Edinburgh Grant Institute, The King's Buildings West Mains Road EDINBURGH EH9 3JW Phone: +44 (0) 131 6519092, FAX: +44 (0) 131 668 3184 Email: Gabi.Hegerl@ed.ac.uk The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. Prof. Phil Jones Climatic Research Unit Telephone +44 (0) 1603 592090 School of Environmental Sciences Fax +44 (0) 1603 507784 University of East Anglia Norwich Email p.jones@uea.ac.uk NR4 7TJ UK ----------------------------------------------------------------------------