cc: "Nathan Gillett" , "Dáithí Stone" , "Phil Jones" , "Gabi Hegerl" , "Toru Nozawa" , "Michael Wehner" date: Sun, 24 Aug 2008 16:12:14 +0100 from: "Peter Stott" subject: Re: Decision on Nature Geoscience manuscript NGS-2008-07-00710 to: "Alexey Karpechko" Hi Nathan The two reviews certainly make for a contrasting set. I must admit I have a bit of sympathy for the first reviewer in the sense that we could really do with nailing better the processes that are causing the observed Antarctic temp changes because we do want to avoid an attribution result that is detecting a fingerprint derived from a cancellation of errors in the model. Given the strong expected effects of the SAM on peninsula warming and continental cooling, and the difficulties AR4 models have of simulating the SAM, I still feel we could make more of the SAM-removed residual trends. Maybe the story is that the model correctly captures the large scale anthropogenically forced SAM-removed residual warming whereas we have more work to do to disentangle the causes of the SAM part of Antarctic temperature changes ? The very enthusiasm of reviewer 2 makes me a bit nervous - we want to be careful of attributing grid-box trends if we haven't understand the influence of circulation changes. Hopefully though we can address this and get the paper through. Cheers, Peter On 8/14/08, Alexey Karpechko wrote: > Hi Nathan, > While the issue of trend significance may be the primary one, the reviewer > rises another issue: that models > may not accurately simulate the impact of anthropogenic forcing on > Antarctic surface temperatures. > Well, results of Monaghan et al. 2008b (which should be mentioned in the > text) suggest that models may overestimate the observed trends, likely due > to too strong water vapour feedback. > But, I guess, they do not overestimate the trends too strongly since D&A > analysis still finds observed trends consistent with modelled ones, correct? > Therefore, we have no reason so far to think that model shortcomings are > fatal for D&A analysis. > D&A analysis of station data contradicts somehow to results of Monaghan et > al. 2008b but I do not think this is the reason to withdraw the manuscript > since different > data set and different method is used. > Therefore, I think we could properly address this issue risen by the > reviewer if you decide to resubmit the manuscript, which is worth trying. > > Cheers > Alexey > > > > > ----- Original Message ----- From: "Nathan Gillett" > To: "Dáithí Stone" ; "Phil Jones" ; > "Gabi Hegerl" ; "Peter Stott" > ; "Toru Nozawa" ; "Alexey > Karpechko" ; "Michael Wehner" > Sent: Wednesday, August 13, 2008 7:59 PM > Subject: Fwd: Decision on Nature Geoscience manuscript NGS-2008-07-00710 > > > > > > 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: > > > > > http://mts-ngs.nature.com/cgi-bin/main.plex?el=A1Q3CGj2A3FlJ1J7A93rUshYh3SdI0gPrnGWsf3wZ > > > > > > > > *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 > (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. Nathan Gillett, > > Climatic Research Unit, > > School of Environmental Sciences. > > University of East Anglia. > > Email: n.gillett@uea.ac.uk > > http://www.cru.uea.ac.uk/~nathan/ > > > > Currently on sabbatical at: > > School of Earth and Ocean Sciences, > > University of Victoria, > > Gordon Head Complex, > > PO Box 3055, > > STN CSC, > > Victoria, BC, V8W 3P6, > > Canada. > > Tel: +1 250 472 4013 > > Fax: +1 250 472 4004 > > > > > **************************************************************************** > > > > > >