the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Mar 2025
Ideas and perspectives: Tipping into the unknown—the global consequences of biogeochemical system collapse
Abstract. With Earth's tipping points being approached and already exceeded, human-driven disruptions of carbon (C), nitrogen (N), and phosphorus (P) cycles are forcing Earth’s biogeochemical systems toward irreversible collapse. Potential functional extinction of key regulatory processes—such as carbonate precipitation, denitrification, phosphorus burial, and methane oxidation—could lead to cascading failures, spread beyond biogeochemical domains and further contribute to destabilize planetary homeostasis. This study examines scenarios how these disruptions may interact across biogeochemical cycles, and how they might create reinforcing feedback loops that amplify climate change, accelerate ecosystem degradation, and alter atmospheric and oceanic chemistry.
Our scenarios envision carbon cycle disruptions (ocean acidification, soil carbon loss) weaken CO2/CH4 sequestration and boost emissions. Eutrophication and oxygen depletion threaten the nitrogen cycle, increasing nitrates and N2O. Meanwhile, phosphorus release from hypoxic sediments sustains eutrophication and intensifies greenhouse gas emissions. Changing feedback loops may prevent the recovery of desirable biogeochemical conditions. A shift to anaerobic metabolism would favor sulfate reduction, methanogenesis, and ammonification, triggering biodiversity collapse, expanding anoxic zones, and allowing microbial extremophiles to dominate—echoing early anoxic Earth.
The loss of these regulatory functions would exacerbate global warming and push Earth’s ecosystems irreversibly into a “Hothouse Earth” regime. Immediate governance action is necessary to mitigate these risks by managing nutrient cycles, protecting carbon sinks, and incorporating biogeochemical feedbacks into climate policies. Without intervention, the accelerating extinction of key biogeochemical processes may render long-term climate protection unattainable.
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RC1: 'Comment on egusphere-2025-1153', Anonymous Referee #1, 09 Jun 2025
This manuscript addresses a highly relevant and timely topic: the potential collapse of Earth’s biogeochemical systems and the cascading consequences for planetary stability. The authors aim to present scenario-based insights into the consequences of functional extinctions in key processes related to the carbon (C), nitrogen (N), and phosphorus (P) cycles. While the overarching objective of the paper is compelling and important, several critical issues require clarification and development to enhance the paper’s rigor, coherence, and scientific impact.
The manuscript lacks a clear explanation of the methodology used to generate the proposed scenarios. It is unclear whether the scenarios are based on modeling efforts, expert elicitation, a systematic literature review, or conceptual synthesis. This ambiguity limits the credibility of the scenarios. At the very least, the authors should state explicitly whether this is a literature-based conceptual exploration and, if so, provide details on the scope of literature reviewed (e.g., number of articles, inclusion criteria, time frame). Including a brief explanation of how scenarios were constructed would add much-needed transparency.
The article would greatly benefit from grounding the scenarios in real-world examples or case studies. Adding a section that highlights observed ecosystem collapses linked to tipping points in biogeochemical cycles (e.g., eutrophication in coastal zones, desertification in drylands, or coral reef degradation due to ocean acidification) would improve the article’s credibility and pedagogical value. Associating each scenario with at least one known ecological process and a geographic example would make the argument more compelling and relatable.
Given the emphasis on irreversible change and tipping points, the manuscript should offer a more thorough conceptual framework on ecosystem resilience. The discussion would benefit from outlining the stages of resilience (resistance, threshold crossing, recovery, or collapse), and defining the specific tipping points the authors refer to. Currently, the term “tipping point” is used frequently but lacks operational definition or quantification. Including a schematic or table summarizing known or hypothesized thresholds for temperature, pH, precipitation, or redox potential that relate to key processes in C, N, and P cycles would strengthen the scientific foundation of the paper.
To better support the authors’ claims and enhance communication, I suggest the inclusion of:
- A figure showing the number of scientific publications (e.g., in the past decade) related to catastrophic ecological processes such as eutrophication, desertification, ocean acidification, and others.
- Diagrams for each major biogeochemical cycle (C, N, P) illustrating where tipping points may occur and what environmental thresholds might trigger functional collapse.
Several parts of the manuscript would benefit from structural revision to improve clarity and avoid redundancy. For example, the second and third paragraphs of the introduction are repetitive and could be merged into a single, more concise paragraph. Some sentences throughout the text are difficult to follow and should be rewritten for clarity.Minor edits and comments:
- L54: Delete "on zero" in “on zero in 2015.”
- L58: Add "ecosystems or Earth" after "regulatory."
- L59: The manuscript should specify which regulatory processes are being referred to.
- L76: Sentence is overly complex and hard to follow. Please revise for clarity.
- L78: Clarify what assumptions are made—are the scenarios based on collapse, worst-case projections, or something else?
- L82: Is this generalization applicable to all ecosystems? Please specify.
- L84: Clarify what “reinforce one another” refers to—presumably interactions among C, N, and P cycles?
- L90: Methodological explanation should be mentioned from the abstract onward, even if it is a conceptual/literature-based study.
- L115: Typo: “Desertificación” → “Desertification.”
- L123: Add “increase in CO₂ emissions.”
- L135: Did the authors mean “degradation of wetlands”?
- L141: Include subscript in “N₂O.”
- L167: This sentence should be expanded and better supported.
- L217: Clarify what feedbacks are being referenced.
- L225: Delete "these" before "interconnected." Add “of C, N, P” after “processes.”
- L630: Consider changing the subheading to:
6. Earth Catastrophic Scenarios, followed by subsections: - 6.1 Decline of Habitability
- 6.2 New Biochemical Regime
- 6.3 Unstable Reducing Atmospheric World
- L339: Clarify: do you mean changes in "atmospheric chemical composition"?
- L340: Is this an “extinction” or a transformation of biogeochemical processes?
- L350: Rephrase to: “These risks driven by changes in C, N, and P cycles.”
Citation: https://doi.org/10.5194/egusphere-2025-1153-RC1 -
AC1: 'Reply on RC1', Salvador Sánchez-Carrillo, 25 Aug 2025
We sincerely thank Reviewer #1 for their insightful and constructive comments on our manuscript “Tipping into the unknown—the global consequences of biogeochemical system collapse”. We greatly appreciate the recognition of the relevance and urgency of the topic. Below, we provide a point-by-point response outlining the actions we will take to address each comment.
Major Comments
- Lack of methodological clarity in scenario construction
Reviewer’s comment:
The manuscript lacks a clear explanation of the methodology used to generate the proposed scenarios.
Response:
We will revise both the abstract and the introduction to explicitly state that the manuscript is a conceptual essay. We will clarify that the scenarios are based on a synthesis of peer-reviewed literature and expert understanding, rather than modeling or systematic review protocols. Specifically, we will indicate that we reviewed 93 scientific articles published between 1999 and 2025, selected for their relevance to disruptions in the carbon, nitrogen, and phosphorus cycles. A new paragraph will be added to the introduction to describe the scope and rationale of our scenario construction.
- Need for real-world examples or case studies
Reviewer’s comment:
The article would benefit from grounding the scenarios in real-world examples of ecosystem collapse.
Response:
We will add a new subsection presenting real-world examples of ecosystem collapse linked to biogeochemical tipping points. These will include eutrophication in the Baltic Sea, desertification in the Sahel, and coral reef degradation in the Great Barrier Reef. Each example will be associated with a specific biogeochemical process and geographic context to enhance the manuscript’s credibility and pedagogical value.
- Conceptual framework on ecosystem resilience and tipping points
Reviewer’s comment:
The manuscript should offer a more thorough conceptual framework on ecosystem resilience and define tipping points operationally.
Response:
We will incorporate a new section outlining the stages of ecosystem resilience (resistance, threshold crossing, recovery, collapse) and provide operational definitions of tipping points. Additionally, we will include a table summarizing known or hypothesized environmental thresholds (e.g., temperature, pH, redox potential) relevant to key processes in the carbon, nitrogen, and phosphorus cycles.
- Suggested figures and diagrams
Reviewer’s comment:
Include a figure showing publication trends and diagrams of biogeochemical cycles with tipping points.
Response:
We will add two new figures:
A bibliometric figure illustrating the increase in scientific publications related to eutrophication, desertification, and ocean acidification over the past decade.
Conceptual diagrams of the carbon, nitrogen, and phosphorus cycles, highlighting potential tipping points and environmental thresholds.
- Structural and stylistic improvements
Reviewer’s comment:
Improve clarity and reduce redundancy, especially in the introduction.
Response:
We will revise the second and third paragraphs of the introduction to eliminate redundancy and improve logical flow. We will also simplify complex sentences throughout the manuscript to enhance clarity and readability.
Minor Comments
We will address all minor comments as suggested. Specific line edits will include:
Line 54: Remove “on zero”
Line 58: Add “ecosystems or Earth”
Line 59: Specify regulatory processes
Line 76: Revise sentence for clarity
Line 78: Clarify assumptions behind scenarios
Line 82: Specify ecosystem applicability
Line 84: Clarify feedback interactions among C, N, and P
Line 90: Mention methodology in abstract
Line 115: Correct “Desertificación” to “Desertification”
Line 123: Add “increase in CO₂ emissions”
Line 135: Clarify “degradation of wetlands”
Line 141: Correct to “N₂O” with subscript
Line 167: Expand and support the sentence
Line 217: Clarify referenced feedbacks
Line 225: Edit for clarity and precision
Line 630: Change subheading to “6. Earth Catastrophic Scenarios” with subsections 6.1–6.3
Citation: https://doi.org/10.5194/egusphere-2025-1153-AC1
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RC2: 'Comment on egusphere-2025-1153', Anonymous Referee #2, 09 Aug 2025
GENERAL
This paper:
(1) Discusses that the state of Earth's biogeochemical systems is at risk of crossing critical tipping points due to human activities disrupting the carbon, nitrogen, and phosphorus cycles. These disruptions threaten vital ecological processes and potentially lead to irreversible ecological collapse.
(2) Shows how climate change and ecosystem degradation are connected to "Hothouse Earth," emphasizing the need for environmental management. Each component of these cycles is important to mitigate ecological and atmospheric deterioration.
(3) Stresses that the governance actions are needed to mitigate risks by managing nutrient cycles, safeguarding carbon sinks, and incorporating biogeochemical feedbacks into climate policies. The loss of essential biogeochemical processes may make long-term climate protection impossible without intervention.The subject matter is compelling. With the recommended revisions below, the results have the potential to be successfully published.
SPECIFIC
(1) Introduction: There is no doubt that the human impact on Earth's ecosystems is a critical threat to our planet. We must act now to mitigate these risks and foster resilience. However, earlier studies in the field should be mentioned.Basic references:
--Journal of Atmospheric and Solar-Terrestrial Physics, (2016),150, 47-54.
--Ecological Modelling, (2017), 359, 69–79.(2) In the rest sub-paragraphs: The literature increasingly recognizes the possibility of functional extinctions in key biogeochemical processes, but there are major quantitative and mechanistic gaps, especially about thresholds, coupling across cycles, resilience timescales, and under-observed environments. Filling those gaps is urgent if we want reliable risk assessments and realistic mitigation strategies.
Based on this, briefly elaborate on the following research gaps:
2.1. When (and where) does “functional extinction” of carbon processes occur?
2.2. Magnitude and pathways of feedbacks from methanotroph collapse
2.3. Denitrification completeness and N₂O production under stress.
2,4. Long-term phosphorus burial dynamics and reversibility.
2.5. Cascading interactions and compound tipping points
2.6. Timescales of recovery and hysteresis.
2.7. Quantitative constraints on GHGs amplification from biogeochemical failures.
2.8. Observational and methodological limitations.Useful references:
--Biological Conservation 260 (2021): 109195.
--Science of the Total Environment 838 (2022): 156210.
--FEMS Microbiology Ecology 100, no. 3 (2024): fiae008.
--Global Change Biology 30, no. 1 (2024): e17082.
--Nature Reviews Microbiology, 23(4), pp.239-255.
--International Journal of Remote Sensing (2011), 32 (11), 3231-3238
--Biogeochemistry 168, no. 2 (2025): 1-20.RECOMMENDATION:
The paper requires significant revisions, taking into account all the aforementioned suggestions and comments.Citation: https://doi.org/10.5194/egusphere-2025-1153-RC2 -
AC2: 'Reply on RC2', Salvador Sánchez-Carrillo, 25 Aug 2025
We sincerely thank Reviewer #2 for their thoughtful and encouraging comments on our manuscript “Tipping into the unknown—the global consequences of biogeochemical system collapse”. We appreciate the recognition of the relevance and urgency of the topic and the constructive suggestions to strengthen the manuscript. Below, we outline the actions we will take to address each point raised.
General Comments
We acknowledge the reviewer’s summary of the manuscript and agree that the subject matter is compelling. We will incorporate the recommended revisions to enhance the scientific depth and clarity of the paper.
Specific Comments
- Introduction – Include earlier studies in the field
Reviewer’s comment:
Earlier studies in the field should be mentioned to contextualize the urgency of human impacts on biogeochemical systems.
Response:
We will revise the introduction to include references to foundational studies that have addressed biogeochemical tipping points and systemic collapse. Specifically, we will cite the suggested works from the Journal of Atmospheric and Solar-Terrestrial Physics (2016) and Ecological Modelling (2017), along with other relevant literature to provide historical context and strengthen the framing of our arguments.
- Research gaps – Elaborate on key unresolved questions
Reviewer’s comment:
The manuscript should briefly elaborate on major research gaps related to functional extinctions, feedbacks, thresholds, and resilience.
Response:
We fully agree with the reviewer’s assessment. We will add a new subsection that explicitly outlines the following research gaps:
1) Spatial and temporal patterns of functional extinction in carbon processes
2) Magnitude and pathways of feedbacks from methanotroph collapse
3) Denitrification completeness and N₂O production under stress
4) Long-term phosphorus burial dynamics and reversibility
5) Cascading interactions and compound tipping points
6) Timescales of recovery and hysteresis
7) Quantitative constraints on GHG amplification from biogeochemical failures
8) Observational and methodological limitations
We will also integrate the suggested references, including those from Biological Conservation (2021), Science of the Total Environment (2022), FEMS Microbiology Ecology (2024), Global Change Biology (2024), Nature Reviews Microbiology (2024), and Biogeochemistry (2025), to support and contextualize each gap.
Recommendation
We acknowledge the reviewer’s recommendation for significant revisions and will implement all suggested changes to improve the manuscript’s scientific rigor and relevance.
Citation: https://doi.org/10.5194/egusphere-2025-1153-AC2
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AC2: 'Reply on RC2', Salvador Sánchez-Carrillo, 25 Aug 2025
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