Winter–spring warming in the North Atlantic during the last 2000 years: evidence from southwest Iceland

Richter, Nora; Russell, James M.; Garfinkel, Johanna; Huang, Yongsong

doi:https://doi.org/10.5194/cp-17-1363-2021

Articles | Volume 17, issue 3

https://doi.org/10.5194/cp-17-1363-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/cp-17-1363-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 3

Research article

|

22 Jun 2021

Research article |

| 22 Jun 2021

Winter–spring warming in the North Atlantic during the last 2000 years: evidence from southwest Iceland

Nora Richter, James M. Russell, Johanna Garfinkel, and Yongsong Huang

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Final revised paper (published on 22 Jun 2021)
Preprint (discussion started on 26 Jun 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review cp-2020-84', Anonymous Referee #1, 13 Aug 2020
- AC1: 'Response to Reviewer #1', Nora Richter, 04 Nov 2020
SC1: 'Comment on seasonality and influence of landscape disturbances', David Harning, 20 Aug 2020
- AC2: 'Response to David Harning', Nora Richter, 04 Nov 2020
RC2: 'Review of Richter et al. "Cold season warming in the North Atlantic during the last 2000 years: Evidence from Southwest Iceland"', Anonymous Referee #2, 29 Sep 2020
- AC3: 'Response to Reviewer #2', Nora Richter, 04 Nov 2020

Peer-review completion

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

ED: Reconsider after major revisions (05 Nov 2020) by Bjørg Risebrobakken

AR by Nora Richter on behalf of the Authors (17 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Jan 2021) by Bjørg Risebrobakken

RR by Anonymous Referee #1 (14 Feb 2021)

Suggestions for revision or reasons for rejection

Dear authors,

I have now read your rebuttal to my comments and those made by another reviewer. In light of these, I have additional suggestions that ought to be reflected before publication in my opinion:

RIK38E: my previous comment concerning the use of this index may have been mis-read. I did not suggest to use RIK38E to calibrate your samples, but merely to (better) distinguish between Group I and 2 producers (see Longo et al. 2016/2018). As I said in my initial comments, separating them is quite critical for robust temperature inferences, and using RIK38E values may significantly refine this distinction.

Ice-off dates: both me and the other reviewer of this manuscript encourage the authors to assess their model by using free and easily available observational data to assess the potential impact of variable ice-off dates. Yet, no steps have been undertaken in the revised manuscript to do so, while the authors do use observational data to justify their calibration. I strongly encourage the authors to take this comment to heart, especially in light of the high-amplitude temperature shifts that they infer.

Group I studies: both me, but especially the other reviewer, ask for a bit more regional context about the regional application of the UK37 proxy. But instead of adding certain relevant recent studies, a number of papers have been left out. As some of these are mentioned in the rebuttal, I suspect something as gone awry here. I urge you to have a look at this: the most striking examples (previously mentioned, but now somehow missing) include 1) Harning et al. 2020 GRL, and 2) van der Bilt et al. 2020 GRL.

Calibration: you argue that most alkenones in your site derive from Group I producers, remove samples that (also) contain Group II, and yet opt for a Group III temperature calibration because these values are “more reasonable”. I don`t find this a particularly strong line of argumentation, and think you need to discuss this decision in more detail. Key element: available Group I calibrations now derive from many NH lakes and produce calibration slopes that are rather similar, so why would VGHV be so different?

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RR by Anonymous Referee #2 (19 Feb 2021)

Suggestions for revision or reasons for rejection

Richter et al. present a revised and nicely improved manuscript, with the major accomplishments of the manuscript in place from the initial submission and many of my primary concerns from the original manuscript addressed. In particular, I find the modified discussion around the seasonality of the alkenone proxy to be very clear and convincing, given the available calibration data.

While I would suggest some further revision for clarity, my criticisms of the current manuscript are mostly minor and I recommend the manuscript for publication:

-One of the primary conclusions reads as that increasing winter-spring insolation over the common era drove significant cold-spring warming at VGHV. However, the lake model they generate shows very little ice-off date/water temperature response to the amount of winter-spring insolation change over the common era. While this model does show a strong response to temperature, they haven’t really explained how that change in insolation could drive that kind of temperature change (do they attribute the trend then to local feedbacks? Other climate process? driven by insolation). In my opinion, it would improve the manuscript if the authors are able to explain this aspect of their interpretation in very clear way. On this point, I find the discussion around drivers of climate around Iceland and the NH (section 4) somewhat meandering. It was a little bit of a challenge for me to tease out the important take-aways, and I think the section could be improved with a little bit of restructuring for clarity and flow.

-Given the qualitative nature of the proxy, I am somewhat unconvinced that relatively small trends outside of the long-term warming trend are climatically meaningful. The authors dedicate a good amount of text to discussing multi-decadal to centennial scale trends (section 4.2) but do not put these into the context of error on the proxy (are these changes larger than the error on any of the calibration data? Even if they are, can we trust that error range in a region that has no local calibration data?). It strikes me as an unnecessary and under supported component of the paper. It seems to me like it would be a better use of available space to clearly explain why, if the primary driver is insolation, from 0-400 CE does not seem to actually track with winter-spring insolation.

And finally, a note to the authors to check the caption on Fig. 5 prior to final publication.

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ED: Reconsider after major revisions (03 Mar 2021) by Bjørg Risebrobakken

Dear Nora Richter et al.,

Thank for very much for submitting your revised “Winter-spring warming in the North Atlantic during the last 2,000 years: Evidence from Southwest Iceland” manuscript. I have now received two evaluations of the revisions done. One still recommends major revisions, and both raise questions that I recommend you to address carefully. In part these comments reflect issues raised by both reviewers in the first review round, but not taken into account. There are also questions raised with respect to the interpretation relative to the strength of the suggested forcing.

I would also like to ask for more background information to be provided with respect to the model setup. According to Dee et al., 2018, lake specific tuning of parameters is necessary, in addition to the user defined initial conditions. Please provide information on choices made for local tuning of the model, as well as specification of individual input variables and sources of these. And how well does your control simulation represent the known lake conditions? As far as I can see you show no validation of the model set up for your lake? In part this links back to one of the main concerns of the reviewer, related to assessment of the model.

I will ask for a revision of the manuscript, taking into account the concerns still raised. I will look forward to seeing the revised version and your responses to the reviewer’s comments. Please provide responses to each comment raised and a version of the revised manuscript including track changes highlighting how the changes have been implemented.

Thank you very much for submitting your work to Climate of the Past.

Best regards,
Bjørg Risebrobakken
Editor, Climate of the Past

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AR by Nora Richter on behalf of the Authors (14 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (15 Apr 2021) by Bjørg Risebrobakken

RR by Anonymous Referee #1 (18 May 2021)

ED: Publish as is (18 May 2021) by Bjørg Risebrobakken

Short summary

We present a reconstruction of winter–spring temperatures developed using organic proxies preserved in well-dated lake sediments from southwest Iceland to assess seasonal temperature changes in the North Atlantic region over the last 2000 years. The gradual warming trend observed in our record is likely influenced by sea surface temperatures, which are sensitive to changes in ocean circulation and seasonal insolation, during the winter and spring season.