Drought and vegetation change in the central Rocky Mountains and western Great Plains: potential climatic mechanisms associated with megadrought conditions at 4200&thinsp;cal&thinsp;yr&thinsp;BP

Carter, Vachel A.; Shinker, Jacqueline J.; Preece, Jonathon

doi:https://doi.org/10.5194/cp-14-1195-2018

Articles | Volume 14, issue 8

https://doi.org/10.5194/cp-14-1195-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

“Global Challenges for our Common Future: a paleoscience perspective”...

https://doi.org/10.5194/cp-14-1195-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 8

Research article

|

15 Aug 2018

Research article |

| 15 Aug 2018

Drought and vegetation change in the central Rocky Mountains and western Great Plains: potential climatic mechanisms associated with megadrought conditions at 4200 cal yr BP

Vachel A. Carter, Jacqueline J. Shinker, and Jonathon Preece

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Final revised paper (published on 15 Aug 2018)
Preprint (discussion started on 29 Sep 2017)

Interactive discussion

Status: closed

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

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- Supplement

SC1: 'Data Review Team Comment', PAGES Data Review Team, 04 Nov 2017
- AC1: 'Research data input', Vachel Carter, 18 Nov 2017
- AC2: 'Research data input update', Vachel Carter, 29 Nov 2017
RC1: 'Discussion of method and statistical significance missing', Anonymous Referee #1, 17 Nov 2017
- AC3: 'Response to Reviewer 1', Vachel Carter, 17 Jan 2018
RC2: 'Referee Comments', Anonymous Referee #2, 21 Dec 2017
- AC4: 'Response to Reviewer 2', Vachel Carter, 17 Jan 2018

Peer-review completion

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

ED: Reconsider after major revisions (18 Feb 2018) by Heather Plumpton

AR by Vachel Kraklow on behalf of the Authors (31 Mar 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Apr 2018) by Heather Plumpton

RR by Anonymous Referee #3 (10 May 2018)

RR by Anonymous Referee #4 (27 May 2018)

Suggestions for revision or reasons for rejection

I have been asked to step in the editorial process of this article after the authors had provided the second version of their manuscript.

The question which seems to be the main source of disagreement between the authors and the reviewers is the following: Can we use modern extreme events as a guide for the description of mechanisms at work in long-lasting events such as megadroughts? We do not know, and there are reasons for it not be the case. For example, at centennial scales, slow ocean dynamics may generate dynamical patterns with no analog at the decadal or interannual scale.

This is of course a difficult question, and perhaps the article has been a missed opportunity to further document the analogue hypothesis before applying it. But it is a tricky issue and at this stage it would be unreasonable (and counter-productive) to request new analyses. Therefore, let me mainly comment on two points raised by reviewer #1.

Should the authors have used CMIP experiments? The "PMIP" section of CMIP does indeed include a few millennium runs which might contain some long droughts. At least, I expect that modern GCMs should generate some patterns of decadal variability and perhaps decade-long droughts. There would thus be some scope for testing, in the GCM universes, the similarity of mechanisms determining droughts at the interannual and decadal scales, or perhaps even centennial scale. Of course this would only be helpful if indeed GCMs are good simulators of the dynamics of megadroughts, which no-one can guarantee because they have not been calibrated for this purpose. Hence, inspecting megadroughts in GCMs (if there are, any) is certainly a very useful task, but I concede that it would involve some massive work falling out of the scope of the present article.

The authors have somehow answered the concerns of reviewer #1 bearing on the identification of patterns associated with dry years in the modern climate record. They should however still be more explicit about the number of degrees of freedom used in the Student t-test. Statistics of composite records generated from an autocorrelated time-series can be treacherous and it would be helpful to have a bit more details here. Short of this the reader cannot be sure that the approach is valid.

One the other hand, the justification and language around the "modern analogue technique" still requires more work. The paragraph added after the first review did not quite do the job. Forget about uniformitarianism. This is a historical notion, originally opposed to catastrophism, that was meant to express the assumption that Earth's processes have remained the same through time. In the modern parlance the concept of "uniformitarianism" has been diluted into the broader and vaguer principle that geological, palaeontological and palaeoclimatological observations are helpful to understand present and future Earth dynamics.
What is needed to motivate and justify further work is simple (plain) language justifying the objective of the approach, its limitations, and what should be done to consolidate the working hypotheses.

Other comments

- the column "Purpose of Climate Variable" in Table 1 is appallingly trivial.
- There is also a mistake in the numbering of sections and subsections starting at subsection 4.1.
- p. 12 l. 33: The citation " Using a complex numerical weather-predication model with data from May 1987 to May 1988, Palmer and Brankovic (1989) had significant skill in predicating an anomalous high pressure ridge over North America during the summer of 1988" is about a trained prediction model, and is irrelevant in the present context.
- Citation "By applying the modern climate analogue technique to paleoenvironnmental proxies from the mid-continent, Shinker et al. (2006) found that regional moisture influx and small-scale vertical motions in the atmosphere (i.e. subsidence or uplift) provide better information regarding precipitation than large-scale general circulation alone."
The wording is inaccurate. Shinker examined modern climatological data, to indeed observe that regional moisture influx and small-scale vertical motions in the atmosphere are a better predictor of precipitation than large-scale circulation in the modern climate. They then used this insight to help and contribute to the interpretation of palaeoclimate data. It is only at the latter stage that the hypothesis of an analogy applies.

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RR by Anonymous Referee #5 (08 Jun 2018)

Suggestions for revision or reasons for rejection

Carter et al’s manuscript entitled “Drought and vegetation changes in the central Rocky Mountains: Potential climatic mechanisms associated with drought conditions at 4200 cal yr BP,” uses instrumental, modeling, and paleoclimate evidence to provide a mechanism to explain dry conditions and the expansion of Aspen vegetation inferred from pollen in Long Lake (Carter et al 2013). Overall, the multiple lines of evidence used to explain the proxy data, and the strong review of background literature make a convincing case for their interpretations. I do have a few suggestions for the authors to consider.

1) This paper relies heavily on the pollen reconstruction in Carter et al 2013. I realize this data has already been published, but it seems very awkward that the pollen is not shown in a figure, as it is a major part of the manuscript. Perhaps the authors think this is redundant (as it is already published elsewhere), however to the reader (especially in the paleo community) it would make visualizing the changes at Long Lake through time much easier. I would encourage the authors to consider including the relevant pollen data from Long Lake either incorporated into an existing figure, or as a new figure itself.
2) The authors should include the reference “Oxygen isotope records of Holocene climate variability in the Pacific Northwest” (Steinman et al., 2016) reference in line 30 on page 8 and perhaps in some other sections in the paper. Steinman et al 2016 presents high-res oxygen isotopes records from three lakes that suggests the middle Holocene winters were wetter in western North America. This paper also provides model-based evidence and a robust review of the paleoclimate literature from the Pacific Northwest. This may be of use to the authors during revision.
3) Lines 14-15, page 9: Steinman et al 2016 suggest the a shift to more ENSO-like condition (i.e. drier winters) in the Pacific Northwest from approximately 3000-2000 yr BP. The references and discussion in this section needs to be updated to include other regional records.
4) I believe this was added to satisfy a reviewer, but why is the principle of uniformitarianism even stated. I feel that this is understood. I would defer to the authors discretion, but this is also awkward wording. I think something like “assuming similar conditions as modern” or something comparable would suffice.
5) Lines 6-9, page 3: Two uses of the word ‘analogue’ in the sentence. Confusing wording.
6) There is a typo at the end of Line 8 on page 2.

Steinman BA, Pompeani DP, Abbott MB, et al. (2016) Oxygen isotope records of Holocene climate variability in the Pacific Northwest. Quaternary Science Reviews 142: 40-60.

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ED: Reconsider after major revisions (10 Jun 2018) by Heather Plumpton

AR by Vachel Kraklow on behalf of the Authors (22 Jul 2018) Author's response Manuscript

ED: Publish subject to technical corrections (24 Jul 2018) by Heather Plumpton

AR by Vachel Kraklow on behalf of the Authors (25 Jul 2018) Author's response Manuscript

Short summary

Between 4200 and 4000 cal yr BP, paleoecological evidence suggests a megadrought occurred in the central Rocky Mountains and western Great Plains. Modern climate analogues were used to explore potential climate mechanisms responsible for the ecological changes. Analogues illustrate that warm and dry conditions persisted through the growing season as a result of anomalously higher-than-normal heights centred over the Great Plains which suppressed moisture transport to the region.