PCLM-D-21-00049

Projected novelty in the climate envelope of the California Current at multiple spatial-temporal scales

PLOS Climate

Dear Dr. Smith,

Thank you for submitting your manuscript on ocean climate novelty to PLOS Climate. After careful consideration, we feel that it has merit but does not fully meet PLOS Climate’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Both reviewers have provided a recommendation of "major revisions"; please see below. However, most of their questions seem to be fairly minor, and many simply request a justification rather than a revision. We look forward to receiving your revised manuscript with these justifications and clarifications.

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Erin Coughlan de Perez, PhD

Academic Editor

PLOS Climate

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Does this manuscript meet PLOS Climate’s publication criteria? Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe methodologically and ethically rigorous research with conclusions that are appropriately drawn based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: No

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This paper is an important and needed addition to the literature about environmental/climate novelty in marine ecosystems. It measures novelty in future environmental conditions in the regional case study of the California Current System. A very few studies in the literature have quantified novelty in marine ecosystems in general. This paper used two interesting new methods to compute novelty, hence presenting a major methodological contribution. It also applied multiple temporal and spatial scales that could be of high relevance for the management of ecosystem services in the region.

However, I do have some major concerns. In general, the study presently seems targeted to a niche audience, familiar with the region, and modelers of the area. This is despite the introduction ending with: "The expected value of this information is to identify areas of the CCS most likely to experience strong ecological stress and resulting social and economic impacts, and provide broad context for evaluating the suitability of end-of-century model-based ecological projections". In the introduction section, there is quite some jargon/abbreviations that can be avoided to improve the communication of the study contribution. Second, the data were not well explained and many abbreviations were used, not necessarily understood by marine/fisheries scientists, particularly those interested in species niches and management. This is a major issue, as it is difficult to understand fully the results and discussion. In addition, there is no description of the region and its currents for a broader audience, whether for climate modelers from other regions or for any marine scientist. Finally, there are a few missing discussion points such as acknowledging the limitations of the study and data and a lack of references. I would recommend major revisions.

Two general and minor comments: (1) Some words are written in British English and others in American English, please check for consistency. (2) Some sentences are very long (4-5 lines) and need to be reviewed throughout the text.

Here are my comments in detail by section:

Introduction

- Novelty has been defined in different terminologies and meanings in the literature. What is novelty or novel climate in this study?

- Lines 78-82: These are many abbreviations/acronyms, what do they stand for? To aim for an interdisciplinary audience (e.g., an audience interested in the species niches and fisheries as stated later in lines 94-96), the authors could simplify this information in the introduction and explain the acronyms in the method.

- Lines 88-90: What is the difference between the methods? The authors refer to Table 1, which explains how it is calculated. However, the difference between the methods and why they were specifically chosen for this study is not clear.

Minor:

- Line 77-78: “finer 0.1 horizontal resolution”: The authors have clarified this only in the discussion.

- Line 87: What is the second set? It has been mentioned also later in the method, but it is not explained.

- Line 95: “resulting social and economic impacts” I am not sure this was discussed later. I would recommend using a border expression like impacts on ecosystem services.

Method:

- Table 1: is a very insightful table and a nice abstract of the methodology. However, “how novelty was quantified” rather explains “how the percentage of novelty was extracted”. There is a missing line about “how novelty was quantified”. Secondly, what does 100% novelty stand for?

- Lines 152-156: Mahalanobis distance reduces correlations. Therefore, arguing that using more variables could “over-characterise each location, and lead to inflated, unmeaningful, or false novelty” may not be valid in the Mahalanobis distance case.

Why was the Mahalanobis not used for all variables (including the second set), then the most contributing variables could be used in the hypervolume calculations? And are the first and second sets of variables correlated? I am not suggesting here to change the analysis but more to strengthen the arguments.

The argument in lines 160-161 about variables of high importance for the ecological niches of the system of interest links well to the statements in the introduction. Could the authors provide references from the region to corroborate this argument?

- Lines 165-168: “the second set…” The computation of novelty is usually linked to the characteristics and the specific variables chosen for analysis. So, what does “compare” mean in this case and how was it done.

- Lines 171-180: Are these abbreviations common knowledge: “GFDL- ESM2M, HadGEM2- ES, and IPSL- CM5A- MR (abbreviated to GFDL, Hadley, and IPSL), which span the CMIP5”? It would be helpful to know what each of them means, what are the differences and the characteristics, and why were they particularly chosen. This would be necessary to understand the results and the discussion arguments.

Minor:

- Line 100-105: the description of the methods comes much later, which is a bit confusing.

- Line 217: Please add a reference for the 'drop one out' method/concept?

Results:

- Figure 3: For someone not familiar with the study area, what does the black line represent? Is it the EEZ?

- Is there a particular reason to show July in fig.4 and August in fig.2 and fig.3?

- Figure 4: “Jaccard similarity = 0.032 Sorensen similarity = 0.061 Unique fraction historical = 0.94 Unique fraction future = 0.93” please explain the context and meaning of these values. I do not recall seeing that in the method section.

Discussion:

- There are two clear separate sections of the discussion: "ecological implications, the when and where" and "method contributions and limitations". However, there are a few missing discussion points to the second section: What are the limitations of the models/data used in this study? For example, human pressure such as nutrient load (although you consider oxygen, there are some correlations) was not taken into account. Another limitation is that usually with future projections, uncertainties increase the further in time projections go. What are the risks and how was that accounted for at the end of the century projections? Finally, is it conservative to talk about specific years and do the models used include extreme events?

- Lines 377-379: "as the difference in warming between GFDL and the other two projections sometimes meant the difference between a mostly analog or mostly novel ocean climate." Unfortunately, since there is no information about the GFDL and the two others, it is hard to understand the arguments.

- Lines 384-396: very important and interesting, but please add references to support these arguments from the region or outside? Some suggestions: species geographical shift and novel communities (Antão et al., 2020; Bates et al., 2014; Pinsky et al., 2020). Plus, these are just potential consequences because this is not the focus of the study.

- Lines 401-405: Yes, there is a risk for fisheries in general and sardines and anchovies' fisheries in particular, but there is also a risk of eutrophication, which can affect all ecosystems services (recreation, cultural services, etc.).

- Lines 437-438: The differences in the methods are expected; what are the inconveniences of using the Mahalanobis distance with a small number of variables?

- Lines 466-470: these conclusions have already been made in previous studies (e.g., Beckage et al., 2011; Blenckner et al., 2021; Mora et al., 2013), please expend a bit on the literature used.

- The contribution of the paper is much more interesting and "novel" than the last sentence; The authors could maybe consider adding a small conclusion paragraph to highlight that.

Minor:

- Lines 371: "Our results are at a monthly and 0.1 (10 km) horizontal resolution, which represents sustained and meaningful 'on the ground' change for ecological and human communities." This is important information that should be stated in the intro or method

- Lines 376-377: nonlinear and threshold phenomena for species, for fisheries or for environmental conditions?

- Lines 411: I do not think reference 23 mentioned RCP6.5

Abstract:

Minor

- ”Even under high emissions”: Have other emission scenario than RCP8.5 been studied?

- “Consistent novelty”: weird expression

- What are “emerging fisheries”? fisheries worldwide are overexploited; it is better to use maintain/sustain fisheries or ecosystem services.

References:

Antão, L. H., Bates, A. E., Blowes, S. A., Waldock, C., Supp, S. R., Magurran, A. E., Dornelas, M., & Schipper, A. M. (2020). Temperature-related biodiversity change across temperate marine and terrestrial systems. Nature Ecology & Evolution, 4(7), 927–933. https://doi.org/10.1038/s41559-020-1185-7

Bates, A. E., Pecl, G. T., Frusher, S., Hobday, A. J., Wernberg, T., Smale, D. A., Sunday, J. M., Hill, N. A., Dulvy, N. K., Colwell, R. K., Holbrook, N. J., Fulton, E. A., Slawinski, D., Feng, M., Edgar, G. J., Radford, B. T., Thompson, P. A., & Watson, R. A. (2014). Defining and observing stages of climate-mediated range shifts in marine systems. Global Environmental Change, 26, 27–38. https://doi.org/10.1016/j.gloenvcha.2014.03.009

Beckage, B., Gross, L. J., & Kauffman, S. (2011). The limits to prediction in ecological systems. Ecosphere, 2(11), art125. https://doi.org/10.1890/ES11-00211.1

Blenckner, T., Ammar, Y., Müller-Karulis, B., Niiranen, S., Arneborg, L., & Li, Q. (2021). The Risk for Novel and Disappearing Environmental Conditions in the Baltic Sea. Frontiers in Marine Science, 8, 745722. https://doi.org/10.3389/fmars.2021.745722

Mora, C., Frazier, A. G., Longman, R. J., Dacks, R. S., Walton, M. M., Tong, E. J., Sanchez, J. J., Kaiser, L. R., Stender, Y. O., Anderson, J. M., Ambrosino, C. M., Fernandez-Silva, I., Giuseffi, L. M., & Giambelluca, T. W. (2013). The projected timing of climate departure from recent variability. Nature, 502(7470), 183–187. https://doi.org/10.1038/nature12540

Pinsky, M. L., Selden, R. L., & Kitchel, Z. J. (2020). Climate-Driven Shifts in Marine Species Ranges: Scaling from Organisms to Communities. Annual Review of Marine Science, 12(1), 153–179. https://doi.org/10.1146/annurev-marine-010419-010916

Reviewer #2: This work, “Projected novelty in the climate envelop of the California Current at multiple spatial-temporal scales,” is a regional assessment of climate novelty - the emergence of previously unseen combinations of environmental conditions. Using downscaled regional climate models, the authors assess how environmental conditios in the California Current System (CCS) change under a high-emissions climate scenario (RCP8.5). The authors use two methods of assessing local-scale novelty using hypervolumes and ExDet, highlighting areas of (dis)agreemet between the two. Main findings are that 40-50% of the CCS is novel by 2050 and nearly 100% by 2100. Novelty is primarily driven by changes in SST but variation in magnitude of warming across climate models suggests non-linear, “tipping point” thresholds may exist for novelty in the CCS. Local and regional assessments of climate novelty are important next steps for climate research, making this work timely. The manuscript is structured well but could benefit from improved clarity in the writing. However, I have strong reservations and questions regarding the decision to use a single emissions scenario, given that most high-level climate change studies are expected to analyze a range of climate scenarios. My recommendation is a “Major Revision”.

Major Issues:

I have one major issue regarding the manuscript, and that is the decision to only run the analysis for RCP8.5. Nearly all climate change studies, and particularly those focused on novelty, use at least two emissions scenarios. The authors don’t provide a justification for using a single scenario. If RCP8.5 is the only scenario with downscaled projections, that is fine and should be stated. However, if there are other scenarios with downscaled projections, the reasoning for excluding those needs to be presented and strongly argued for. If other climate scenarios do exist, I would need strong convincing that this could be published without the inclusion of at least one other climate change scenario.

Minor Issues:

Throughout the manuscript there are areas where clearer writing is needed. These are highlighted iny my specific comments.

Also, there are some issues with labeling and figure captions. You will also find these in the specific comments.

Specific comments:

L35: “climate projections” - this should be “climate models”. The study used three models and a single “projection”.

L70: also see Lotterhos et al. 2021 (Sci Reps) and Johnson & Watson 2021 (One Earth) for marine novelty assessments.

L78-82: It is unclear why RCP8.5 is the only scenario assessed. Given the norm of assessing a range of climate scenarios, this needs to be addressed. If it is data availability, that is fine, but regardless.

L102: Please explain why 1980-2009 was chosen as the period for historical conditions.

L118: Were the random dots used to statistically determine some feature of novelty? Unclear why they are necessary for the explanation of hypervolumes.

L124: Figure 1.d - what does the black polygon off the coast represent?

L171-180: Again, need to see language detailing why a single, high-emissions scenario was used and how the historical period was determined.

L251: Is “underestimated” the right word here? Given that both Mahalanobis distance and ExDet are attempting to detect novelty, it should state that the Mahalanobis distance “estimated less novelty.” Both methods are statistical measures of imperfect models, so they both approximate the truth, just on different ends of the spectrum.

L253: Please explain how and why you decided to dived the CCS into 14 areas? Was this based on oceanographic features? Fisheries management units?

L259: Why not show the average of all three ESMs in Fig 2a?

L281: Is there anything particularly noteworthy about 50% of the region being novel that made you highlight that threshold?

L285: How you determine “meaningful” novelty? If 20% is meaningful, maybe highlight that in Figure 2 instead of 50%. Or, add a transparent polygon that covers the range 20-60%.

L290-292: This seems very important and relevant to understanding the study. I’d like to see this mentioned earlier in the manuscript. Knowing this would aid the readers in interpreting the many results you have.

L303: re: Figure 3 - it does not show July and contradicts the figure caption. It shows April, Aug, and Dec.

L303: why were the regional-monthly and local scales combined here?

L323: Most “of the” difference between hypervolume “extents”...

L342: re: Figure 4 - do these overlap metrics speak to the average value across all combinations of variables or just a single pair of hypervolumes? Seems like the value should change depending on which pair you look at.

L368-371: It should read, “Even under high emissions,...” Second, “even” has several definitions, and here it could be misconstrued as stressing a comparison, which this study isn’t able to speak to. I’d consider rephrasing this sentence for clarity. Without knowing how novelty in the CCS plays out under other emissions scenarios, I don’t know if the finding is surprising or not.

L376-379: Great point! Maybe worth unpacking what that means?

L411: Reygondeau et al. assessed RCP2.6 and 8.5. There is also no RCP6.5.

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Reviewer #1: Yes: Yosr Ammar

Reviewer #2: No

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Projected novelty in the climate envelope of the California Current at multiple spatial-temporal scales

PCLM-D-21-00049R1

Dear Dr Smith,

We are pleased to inform you that your manuscript 'Projected novelty in the climate envelope of the California Current at multiple spatial-temporal scales' has been provisionally accepted for publication in PLOS Climate.

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Best regards,

Erin Coughlan de Perez, PhD

Academic Editor

PLOS Climate

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Thank you for the revised submission, and the detailed responses to the reviewers. We are pleased to accept the manuscript.

Reviewer Comments (if any, and for reference):