Assessment of warm-water coral reef tipping point thresholds

Pearce-Kelly, Paul; Altier, Andrew H.; Bruno, John F.; Cornwall, Christopher E.; McField, Melanie; Muñiz-Castillo, Aarón Israel; Rocha, Juan; Setter, Renee O.; Sheppard, Charles; Roman-Cuesta, Rosa Maria; Yesson, Chris

doi:https://doi.org/10.5194/esd-2023-35

Preprints

https://doi.org/10.5194/esd-2023-35

Preprints

02 Jan 2024

| 02 Jan 2024

Status: a revised version of this preprint was accepted for the journal ESD and is expected to appear here in due course.

Assessment of warm-water coral reef tipping point thresholds

Paul Pearce-Kelly, Andrew H. Altier, John F. Bruno, Christopher E. Cornwall, Melanie McField, Aarón Israel Muñiz-Castillo, Juan Rocha, Renee O. Setter, Charles Sheppard, Rosa Maria Roman-Cuesta, and Chris Yesson

Abstract. Warm-water coral reefs are facing unprecedented Anthropogenic driven threats to their continued existence as biodiverse, functional ecosystems upon which hundreds of millions of people rely. Determining the tipping point thresholds of coral reef ecosystems requires robust assessment of multiple stressors and their interactive effects. We draw upon a literature search and the recent Global Tipping Points Revision initiative to consider warm-water coral reef ecosystem tipping point threshold sensitivity. Considering observed and projected stressor impacts we recognise a global mean surface temperature (relative to pre-industrial) tipping point threshold of 1.2 °C (range 0.7–1.5 °C) and an atmospheric CO₂warming threshold of 350 ppm (range 326–400 ppm), whilst acknowledging that interacting stressors, ocean warming response time, overshoot and cascading impacts have yet to be sufficiently assessed but are likely to lower this threshold. These uncertainties around tipping point sensitivities for such a crucially important ecosystem underlines the imperative of robust assessment and, in the case of knowledge gaps, employing a precautionary principle favouring the lower range tipping point values.

Received: 03 Nov 2023 – Discussion started: 02 Jan 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Status: closed

RC1:
'Comment on esd-2023-3', Anonymous Referee #1, 23 Feb 2024
The paper is a comprehensive and reasonable review article focused on coral reefs under local as well as global environmental changes, which would be valuable for researchers and managers in this field.
However, ｗhile the paper describes the concept as a threshold characterized by non-linear hysteresis or cascading effects resulting from positive and negative feedback loops, it falls short in offering a systemic elucidation of why the outcome becomes synergistic rather than merely additional from the point of system science. To rectify this critical shortcoming, urgent measures should be taken to enhance the originality and academic contribution of this paper. Strongly recommended actions include incorporating more quantitative analyses or empirical evidence supporting the discussed concepts and delving deeper into the systemic dynamics driving the observed phenomena.
In conclusion, the lack of an effective explanation due to the absence of original data significantly undermines the academic value of this paper. Consequently, the paper, in its current state, is more suitable for publication in a general introduction book rather than an academic journal.
Other specific comments:

Lines 111-116 are mostly repeated in lines 124-129.

Chapters 8. Pollution, 9. Disruption, and 10. Disease should have their threshold values shown.

Chapter 11. Crown-of-thorns seastar is not an invasive species; it is common in coral reefs, but its problem lies in its outbreak.
Citation: https://doi.org/10.5194/esd-2023-35-RC1
- CC1: 'Reply on RC1', Paul Pearce-Kelly, 01 Mar 2024
  
  We thank the reviewer for the comments that this is a comprehensive and valuable review. It was our intention to create a review article that summarises our current knowledge of tipping points thresholds and dynamics for warm-water coral reefs. As such we are surprised at the request to incorporate novel quantitative analysis into the manuscript, we feel this is beyond the scope of this paper, would change the nature of the article and move it away from a focussed review. While we do try to supply a large number of high level references that provide information and insight on interactive dynamics, we do not seek to make any formal evaluation on the synergistic (or otherwise) nature of combined stressors. Our hope is to convey the diversity of contributing stressors beyond the major threat of warming and espouse the need to consider their direct and interactive impacts and significance for threat assessments. We will be sure to make the paper’s intent and scope clearer in the abstract and relevant text sections, and consider a potential title adjustment.
  
  We are also aware that since submitting this preprint some important new articles have come out that would be useful to include in this review, we are in the process of updating the manuscript, including making the edits to the sections helpfully highlighted by the reviewer, alongside other minor edits that have been identified by co-authors that were too late to make it into the primary submission.
  
  Citation: https://doi.org/10.5194/esd-2023-35-CC1
- AC2: 'Reply on RC1', Paul Pearce-Kelly, 17 May 2024
  
  Please see attachment for author responses to all reviewer comments.
  
  Citation: https://doi.org/10.5194/esd-2023-35-AC2
RC2:
'Comment on esd-2023-35', Anonymous Referee #2, 20 Mar 2024

Earth System Dynamic 0 2023-35
The paper reviews climate impacts on coral reefs to consider adjusting the tipping points for coral reefs. It is not clear until the conclusions that they are suggesting a further reduction in temperature thresholds from the original 1.5 estimates. There are many problems with this assessment that are briefly outlined below. Perhaps, the greatest is that it is a poor review of the literature in that it is out of date and selective citation of the literature. I only give a small number of papers that should be considered in a thorough and up-to-date review. The authors needs to do a systematic review of the recent literature and write a current status review that is more balanced, quantitative, and explains the decisions better.

There is also the problem of the tipping point definition and how it is measured and if they are rates or states, what is the time scale of the change, what are the specific irreversible changes, and what are the confidence intervals around estimates. There are two recent reviews that summarizes these problems well and would cause a reconsideration of the confidence in this new lower threshold.

Klein, S. G., C. Roch, and C. M. Duarte. 2024. Systematic review of the uncertainty of coral reef futures under climate change. Nature Communications 15:2224.

McClanahan , T. R. 2022. Coral responses to climate change exposure. Environmental Research Letters 17:073001.

Table 1 is the core finding of the paper but it still reads as poorly articulated draft. I would suggest the authors review the current literature, summarize it and present table 1 as a key element. Then, most of the text can focus on the evidence or lack of evidence for the conclusions in this table. The authors could conclude with what are the key unknowns required to better estimate the tipping points. Otherwise, the current draft is an uncritical, out of date, and selective citation review of a much richer, data intensive, and nuanced coral reef literature that should be used to make these important conclusions.

Abstract

The abstract is diffciult to distinguish from similar conclusions of past work. It is not that clear or adding much to past work from my reading as the context of the works is missing.

What a tipping point threshold is should be defined early, otherwise the text is mysterious. I assume this is an irreversible change but that often depends on the time scale of observation and what is being measured. Readers will want to know.

Usually, a sentences on methods for assessing tipping points is required to understand the results. It should be stated that this is a review.

The suggested tipping points seems similar to previous work, is it different and by how much? What would cause it to change.

What are the uncertainties. How were they addressed?

Introduction

It is important to distinguish states from rates. Rates determine states but this is not clear and confused as written. The states are also vague, is is coral cover, calcification, ecological state and integrety, biodiversity, etc?

There is a much larger literature than is being cited here and the introduction seems to out of date given the many recent meta-analyses and reviews. I would suggest the authors look closer at the literature in the past 5 years and mostly cite these large data compilation studies. Much is being learned and it is not always in line with the general consensus that is being expressed in this paper. These are just a few examples that come to mind.

Davis, K. L., A. P. Colefax, J. P. Tucker, B. P. Kelaher, and I. R. Santos. 2021. Global coral reef ecosystems exhibit declining calcification and increasing primary productivity. Communications Earth & Environment 2:105.

Klein, S. G., C. Roch, and C. M. Duarte. 2024. Systematic review of the uncertainty of coral reef futures under climate change. Nature Communications 15:2224.

McClanahan , T. R. 2022. Coral responses to climate change exposure. Environmental Research Letters 17:073001.

Tebbett, S. B., S. R. Connolly, and D. R. Bellwood. 2023. Benthic composition changes on coral reefs at global scales. Nature Ecology & Evolution 7:71-81.

Shlesinger, T., and R. van Woesik. 2023. Oceanic differences in coral-bleaching responses to marine heatwaves. Science of The Total Environment 871:162113.

Walker, A. S., C. A. Kratochwill, and R. van Woesik. 2024. Past disturbances and local conditions influence the recovery rates of coral reefs. Global Change Biology 30:e17112.

It is worth remembering that permanent reversal and large scale changes is not just due to climate but also overfishing on large scales. These have been shown to have tipping points and large scale changes. Here is a recent meta-analysis paper.

McClanahan, T. R., A. M. Friedlander, L. Wantiez, N. A. J. Graham, J. H. Bruggemann, P. Chabanet, and R. M. Oddenyo. 2022. Best‐practice fisheries management associated with reduced stocks and changes in life histories. Fish and Fisheries 23:422 - 444.

P103 – this threshold is not being well suported by recent studies. There are many large studies that do not find this to be very useful. Here is just one.

DeCarlo, T. M. 2020. Treating coral bleaching as weather: A framework to validate and optimize prediction skill. PeerJ 8:e9449.

If you are going to cite papers that use these threshold models, perhaps other approaches and findings should be cited as well. Low oxygen has been shown to be associated with higher coral cover, so the theory here is not supported empiricallly.

Vercammen, A., J. McGowan, A. T. Knight, S. Pardede, E. Muttaqin, J. Harris, G. Ahmadia, Estradivari., T. Dallison, E. Selig, and M. Beger. 2019. Evaluating the impact of accounting for coral cover in large‐scale marine conservation prioritizations. Diversity and Distributions 25:1564-1574.

McClanahan, T. R., and M. K. Azali. 2021. Environmental variability and threshold model’s predictions for coral reefs. Frontiers in Marine Science 8:1774.

There is a view that sea level rise will increase calcification but this was largely developed before the views of coral mortality. There are also the sediments and seabird studies that show increased nutrients improve reef condition and resilience.

Graham, N. A., S. K. Wilson, P. Carr, A. S. Hoey, S. Jennings, and M. A. MacNeil. 2018. Seabirds enhance coral reef productivity and functioning in the absence of invasive rats. Nature 559:250.

MacNeil, M. A., C. Mellin, S. Matthews, N. H. Wolff, T. R. McClanahan, M. Devlin, C. Drovandi, K. Mengersen, and N. A. J. Graham. 2019. Water quality mediates resilience on the Great Barrier Reef. Nature Ecology & Evolution 3:620.

L353 – the authors should give the reasoning for lowering the thresholds not just cite papers. The recent review by Klein et al 2024 indicate that lack of confidence intervals around thresholds.

Much of table 1 is out of date. However, they do cite Veron et al. 2029 to compensate for this problem. I look forward to reading it in 5 years. Authors should know the literature and how to organize and write a review and subsequently submit a more polished paper.

Citation: https://doi.org/10.5194/esd-2023-35-RC2
- AC1: 'Reply on RC2', Paul Pearce-Kelly, 21 Mar 2024
  
  We thank the referee for taking the time to write such a detailed response, there are many useful points in here that we will use to improve our manuscript.
  We are in the process of revising the manuscript, including incorporating the suggested references, and we will provide a detailed response to each point in due course.
  
  Citation: https://doi.org/10.5194/esd-2023-35-AC1

Status: closed

RC1:
'Comment on esd-2023-3', Anonymous Referee #1, 23 Feb 2024
The paper is a comprehensive and reasonable review article focused on coral reefs under local as well as global environmental changes, which would be valuable for researchers and managers in this field.
However, ｗhile the paper describes the concept as a threshold characterized by non-linear hysteresis or cascading effects resulting from positive and negative feedback loops, it falls short in offering a systemic elucidation of why the outcome becomes synergistic rather than merely additional from the point of system science. To rectify this critical shortcoming, urgent measures should be taken to enhance the originality and academic contribution of this paper. Strongly recommended actions include incorporating more quantitative analyses or empirical evidence supporting the discussed concepts and delving deeper into the systemic dynamics driving the observed phenomena.
In conclusion, the lack of an effective explanation due to the absence of original data significantly undermines the academic value of this paper. Consequently, the paper, in its current state, is more suitable for publication in a general introduction book rather than an academic journal.
Other specific comments:

Lines 111-116 are mostly repeated in lines 124-129.

Chapters 8. Pollution, 9. Disruption, and 10. Disease should have their threshold values shown.

Chapter 11. Crown-of-thorns seastar is not an invasive species; it is common in coral reefs, but its problem lies in its outbreak.
Citation: https://doi.org/10.5194/esd-2023-35-RC1
- CC1: 'Reply on RC1', Paul Pearce-Kelly, 01 Mar 2024
  
  We thank the reviewer for the comments that this is a comprehensive and valuable review. It was our intention to create a review article that summarises our current knowledge of tipping points thresholds and dynamics for warm-water coral reefs. As such we are surprised at the request to incorporate novel quantitative analysis into the manuscript, we feel this is beyond the scope of this paper, would change the nature of the article and move it away from a focussed review. While we do try to supply a large number of high level references that provide information and insight on interactive dynamics, we do not seek to make any formal evaluation on the synergistic (or otherwise) nature of combined stressors. Our hope is to convey the diversity of contributing stressors beyond the major threat of warming and espouse the need to consider their direct and interactive impacts and significance for threat assessments. We will be sure to make the paper’s intent and scope clearer in the abstract and relevant text sections, and consider a potential title adjustment.
  
  We are also aware that since submitting this preprint some important new articles have come out that would be useful to include in this review, we are in the process of updating the manuscript, including making the edits to the sections helpfully highlighted by the reviewer, alongside other minor edits that have been identified by co-authors that were too late to make it into the primary submission.
  
  Citation: https://doi.org/10.5194/esd-2023-35-CC1
- AC2: 'Reply on RC1', Paul Pearce-Kelly, 17 May 2024
  
  Please see attachment for author responses to all reviewer comments.
  
  Citation: https://doi.org/10.5194/esd-2023-35-AC2
RC2:
'Comment on esd-2023-35', Anonymous Referee #2, 20 Mar 2024

Earth System Dynamic 0 2023-35
The paper reviews climate impacts on coral reefs to consider adjusting the tipping points for coral reefs. It is not clear until the conclusions that they are suggesting a further reduction in temperature thresholds from the original 1.5 estimates. There are many problems with this assessment that are briefly outlined below. Perhaps, the greatest is that it is a poor review of the literature in that it is out of date and selective citation of the literature. I only give a small number of papers that should be considered in a thorough and up-to-date review. The authors needs to do a systematic review of the recent literature and write a current status review that is more balanced, quantitative, and explains the decisions better.

There is also the problem of the tipping point definition and how it is measured and if they are rates or states, what is the time scale of the change, what are the specific irreversible changes, and what are the confidence intervals around estimates. There are two recent reviews that summarizes these problems well and would cause a reconsideration of the confidence in this new lower threshold.

Klein, S. G., C. Roch, and C. M. Duarte. 2024. Systematic review of the uncertainty of coral reef futures under climate change. Nature Communications 15:2224.

McClanahan , T. R. 2022. Coral responses to climate change exposure. Environmental Research Letters 17:073001.

Table 1 is the core finding of the paper but it still reads as poorly articulated draft. I would suggest the authors review the current literature, summarize it and present table 1 as a key element. Then, most of the text can focus on the evidence or lack of evidence for the conclusions in this table. The authors could conclude with what are the key unknowns required to better estimate the tipping points. Otherwise, the current draft is an uncritical, out of date, and selective citation review of a much richer, data intensive, and nuanced coral reef literature that should be used to make these important conclusions.

Abstract

The abstract is diffciult to distinguish from similar conclusions of past work. It is not that clear or adding much to past work from my reading as the context of the works is missing.

What a tipping point threshold is should be defined early, otherwise the text is mysterious. I assume this is an irreversible change but that often depends on the time scale of observation and what is being measured. Readers will want to know.

Usually, a sentences on methods for assessing tipping points is required to understand the results. It should be stated that this is a review.

The suggested tipping points seems similar to previous work, is it different and by how much? What would cause it to change.

What are the uncertainties. How were they addressed?

Introduction

It is important to distinguish states from rates. Rates determine states but this is not clear and confused as written. The states are also vague, is is coral cover, calcification, ecological state and integrety, biodiversity, etc?

There is a much larger literature than is being cited here and the introduction seems to out of date given the many recent meta-analyses and reviews. I would suggest the authors look closer at the literature in the past 5 years and mostly cite these large data compilation studies. Much is being learned and it is not always in line with the general consensus that is being expressed in this paper. These are just a few examples that come to mind.

Davis, K. L., A. P. Colefax, J. P. Tucker, B. P. Kelaher, and I. R. Santos. 2021. Global coral reef ecosystems exhibit declining calcification and increasing primary productivity. Communications Earth & Environment 2:105.

Klein, S. G., C. Roch, and C. M. Duarte. 2024. Systematic review of the uncertainty of coral reef futures under climate change. Nature Communications 15:2224.

McClanahan , T. R. 2022. Coral responses to climate change exposure. Environmental Research Letters 17:073001.

Tebbett, S. B., S. R. Connolly, and D. R. Bellwood. 2023. Benthic composition changes on coral reefs at global scales. Nature Ecology & Evolution 7:71-81.

Shlesinger, T., and R. van Woesik. 2023. Oceanic differences in coral-bleaching responses to marine heatwaves. Science of The Total Environment 871:162113.

Walker, A. S., C. A. Kratochwill, and R. van Woesik. 2024. Past disturbances and local conditions influence the recovery rates of coral reefs. Global Change Biology 30:e17112.

It is worth remembering that permanent reversal and large scale changes is not just due to climate but also overfishing on large scales. These have been shown to have tipping points and large scale changes. Here is a recent meta-analysis paper.

McClanahan, T. R., A. M. Friedlander, L. Wantiez, N. A. J. Graham, J. H. Bruggemann, P. Chabanet, and R. M. Oddenyo. 2022. Best‐practice fisheries management associated with reduced stocks and changes in life histories. Fish and Fisheries 23:422 - 444.

P103 – this threshold is not being well suported by recent studies. There are many large studies that do not find this to be very useful. Here is just one.

DeCarlo, T. M. 2020. Treating coral bleaching as weather: A framework to validate and optimize prediction skill. PeerJ 8:e9449.

If you are going to cite papers that use these threshold models, perhaps other approaches and findings should be cited as well. Low oxygen has been shown to be associated with higher coral cover, so the theory here is not supported empiricallly.

Vercammen, A., J. McGowan, A. T. Knight, S. Pardede, E. Muttaqin, J. Harris, G. Ahmadia, Estradivari., T. Dallison, E. Selig, and M. Beger. 2019. Evaluating the impact of accounting for coral cover in large‐scale marine conservation prioritizations. Diversity and Distributions 25:1564-1574.

McClanahan, T. R., and M. K. Azali. 2021. Environmental variability and threshold model’s predictions for coral reefs. Frontiers in Marine Science 8:1774.

There is a view that sea level rise will increase calcification but this was largely developed before the views of coral mortality. There are also the sediments and seabird studies that show increased nutrients improve reef condition and resilience.

Graham, N. A., S. K. Wilson, P. Carr, A. S. Hoey, S. Jennings, and M. A. MacNeil. 2018. Seabirds enhance coral reef productivity and functioning in the absence of invasive rats. Nature 559:250.

MacNeil, M. A., C. Mellin, S. Matthews, N. H. Wolff, T. R. McClanahan, M. Devlin, C. Drovandi, K. Mengersen, and N. A. J. Graham. 2019. Water quality mediates resilience on the Great Barrier Reef. Nature Ecology & Evolution 3:620.

L353 – the authors should give the reasoning for lowering the thresholds not just cite papers. The recent review by Klein et al 2024 indicate that lack of confidence intervals around thresholds.

Much of table 1 is out of date. However, they do cite Veron et al. 2029 to compensate for this problem. I look forward to reading it in 5 years. Authors should know the literature and how to organize and write a review and subsequently submit a more polished paper.

Citation: https://doi.org/10.5194/esd-2023-35-RC2
- AC1: 'Reply on RC2', Paul Pearce-Kelly, 21 Mar 2024
  
  We thank the referee for taking the time to write such a detailed response, there are many useful points in here that we will use to improve our manuscript.
  We are in the process of revising the manuscript, including incorporating the suggested references, and we will provide a detailed response to each point in due course.
  
  Citation: https://doi.org/10.5194/esd-2023-35-AC1

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Short summary

Coral reefs face unprecedented threats from multiple stressors, many of which are linked to human activities. Some stressors have tipping points, that if exceeded, will cause coral collapse. These include temperatures rising 1.2 °C above pre-industrial levels and atmospheric CO₂ above 350 parts per million. Uncertainty remains for these thresholds as many stressors interact in ways we don’t understand. It is important to study these and employ a precautionary principle when planning our actions.


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Assessment of warm-water coral reef tipping point thresholds

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