Thursday, May 3, 2012


cc: D�ith� 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 <>

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.


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
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)?
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.
---------- Forwarded message ----------
From: <>
Date: 2008/8/13
Subject: Decision on Nature Geoscience manuscript NGS-2008-07-00710
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
PS Please use the link below to submit a revised paper:
*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):
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],
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
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

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
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

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