Spatio-temporal variability of Arctic summer temperatures over the past 2 millennia

Werner, Johannes P.; Divine, Dmitry V.; Charpentier Ljungqvist, Fredrik; Nilsen, Tine; Francus, Pierre

doi:https://doi.org/10.5194/cp-14-527-2018

Articles | Volume 14, issue 4

https://doi.org/10.5194/cp-14-527-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/cp-14-527-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 4

Research article

|

24 Apr 2018

Research article |

| 24 Apr 2018

Spatio-temporal variability of Arctic summer temperatures over the past 2 millennia

Johannes P. Werner, Dmitry V. Divine, Fredrik Charpentier Ljungqvist, Tine Nilsen, and Pierre Francus

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Final revised paper (published on 24 Apr 2018)
Preprint (discussion started on 30 Mar 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of "Spatio-temporal variability of Arctic Summer Temperatures over the past two Millennia: an overview of the last major climate anomalies"', Anonymous Referee #1, 17 Apr 2017
- AC2: 'Reply to the first reviewer', Johannes Werner, 21 Jun 2017
RC2: 'Review of: Spatio-temporal variability of Arctic Summer Temperatures over the past two Millennia: an overview of the last major climate anomalies', Nicholas McKay, 02 May 2017
- AC3: 'Reply to the review by N. McKay', Johannes Werner, 21 Jun 2017
- AC4: 'Trend analysis over Greenland', Johannes Werner, 08 Nov 2017
EC1: 'Data Review Team Comment', Darrell Kaufman, 22 May 2017
- AC1: 'Reply to the Data Review Comment', Johannes Werner, 13 Jun 2017
EC2: 'remaining critical issue', Darrell Kaufman, 30 Jun 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (09 Jul 2017) by Darrell Kaufman

AR by Anna Mirena Feist-Polner on behalf of the Authors (08 Dec 2017) Author's response

ED: Referee Nomination & Report Request started (15 Jan 2018) by Hugues Goosse

RR by Anonymous Referee #1 (11 Feb 2018)

Suggestions for revision or reasons for rejection

The manuscript is much improved from the previous submitted version. It presents a very thorough presentation of the reconstruction and a detailed analysis of several features of the reconstruction over the Common Era.

However, I think my main criticism from the first round of reviews still stands in that the article is definitely not hypothesis driven. Specifically, the article is not framed around answering a scientific question or hypothesis, rather it’s framed around the production and presentation of a reconstruction product. It’s true that the authors do many specific and quantitative analyses of the reconstruction and these do constitute assessing many hypotheses. But none of these analyses are moving the narrative of the article along a specific line of inquiry.

Perhaps my criticism here is off-base in that the presentation of a product is perfectly acceptable for publication in Climate of the Past. There’s nothing “wrong” with the science as far as I can see. But even if this article just remains primarily about the presentation of a product, I think it needs some more indications of the types of questions that one could answer with it. Why should I as a reader and potential user of the product get excited about the fact that it exists? What question(s) can I answer with it that I couldn’t have answered before? What benefit would I get in using a product like this that, more so than any other approach I think, explicitly accounts for many different types of uncertainty? More emphasis of reader-interest concerns like this could definitely be included in the abstract and conclusions without much more work for the authors.

One minor question. It is not clear to me what the authors mean in the last sentence of the abstract: "nor is it (from a statistical viewpoint) advisable to directly compare the reconstruction and instrumental data." This seems mis-phrased somehow. Doesn’t every reconstruction including this one verify their reconstructions against observation/instrumental data? And shouldn’t we expect that a reconstruction behave at least a little bit like instrumental products?

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RR by Nicholas McKay (14 Feb 2018)

Suggestions for revision or reasons for rejection

Werner et al., present a new climate field reconstruction for the terrestrial Arctic (>60N) based on ~50 annually-resolved temperature sensitive record. The reconstruction is created using an extended version of the age-uncertain BARCAST methodology presented in Werner and Tingley, 2015, which enables the authors to reconstruct the climate field while accounting for age uncertainty in the uncertain layer counted records (from ice cores and varved sediment). The result is a probabilistic CFR that extends back through the Common. This represents a major advance, both scientifically, as this is the longest and most data-rich CFR yet developed for the Arctic, and methodologically, as this is the first full-fledged CFR that I’m aware of that rigorously incorporates age uncertainty. These are both major scientific publications that warrant publication in Climate of the Past.

Although the focus of the article is the methodology and presentation of the reconstruction, the authors explore some of the features of the new reconstruction, including the predominant trends, and the relative warmth and coldness of decadal and century scale variability in past intervals.

I also reviewed the initial submission of the manuscript, and am happy to see that the authors have addressed my primary concerns with the original product, and the revised paper is considerably improved.

One item that I continue to be confused about is the temporal duration of the reconstruction (much to the authors’ frustration, I believe). The authors say that “the gridded reconstruction is limited to the time after 750 CE”, and otherwise imply that the field reconstruction begins in 750, however other times they simply say that “the analysis is limited to the time after 750 CE”, and they also show reconstructed trends, spatially, for the period from 1-1850 CE in figure 6a. After twice reviewing the manuscript, and two responses from the authors, my current suspicion is that spatial reconstruction covers the period from 1-2002, however is not robust before 750, and they urge caution when interpreting it, only examining it themselves when looking at long term trends (and urging caution). This is fine, it just needs to be well (and consistently explained). Also, if I’m correct, will the full 1-2002 interval be included in the reconstructed data fields? My opinion is that they should be, with caution urged anywhere the data are hosted.

I have a few specific comments that can be easily addressed below, but this is an exciting study and a well-written paper. After the minor issues discussed above are resolved, I suggest that it be published in Climate of the Past.

More specific questions:

1. Data: I was glad to see the data availability section said that all the input data and code would be available – does this mean the input proxy data, including age ensembles, as well as the instrumental target developed as part of the paper will be available? The link is not currently active, but I hope this is the case as those data, as well as the full ensemble output, are extremely valuable for the community.
2. Figure axis labels are inconsistently handled, in terms of the orientation, and the common variable (units) structure. Several of the axis labels have what code names rather than common names.
3. The definition of the MCA used several places in the manuscript (920–1060 CE) is narrower than normal, to coincide with the warmest 140 years in the reconstruction. Additionally, the authors refer to a “late” Roman Warm Period in the 4th and 5th centuries. I’d encourage the authors to not feel obliged to identify the observed warm periods with these more classical and general names, and explicitly discuss the difference between the observed warm periods and the more general concepts.
4. The abstract says 44 records, and the conclusion says 54 records.

I attached detailed comments in a marked version of the manuscript.

Sincerely,
Nick McKay

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (14 Feb 2018) by Hugues Goosse

AR by Johannes Werner on behalf of the Authors (23 Feb 2018) Author's response Manuscript

ED: Publish as is (01 Mar 2018) by Hugues Goosse

Short summary

We present a new gridded Arctic summer temperature reconstruction back to the first millennium CE. Our method respects the age uncertainties of the data, which results in a more precise reconstruction.

The spatial average shows a millennium-scale cooling trend which is reversed in the mid-19th century. While temperatures in the 10th century were probably as warm as in the 20th century, the spatial coherence of the recent warm episodes seems unprecedented.