Terrestrial and marine plastic pollution outlook in the Mediterranean region: a box-model approach based on OECD policy scenarios

Segur, Théo; Sonke, Jeroen E.

doi:https://doi.org/10.5194/egusphere-2024-3031

Preprints

https://doi.org/10.5194/egusphere-2024-3031

Preprints

21 Nov 2024

| 21 Nov 2024

Terrestrial and marine plastic pollution outlook in the Mediterranean region: a box-model approach based on OECD policy scenarios

Théo Segur and Jeroen E. Sonke

Abstract. Plastic pollution in the Mediterranean region and Sea raises serious concerns for ecosystem and human health. Plastic dispersal from Mediterranean watersheds in Southern Europe, Northern Africa and Middle-East, and Nile basin is complex due to the different (mis-)managed waste streams, population dynamics and climate. In this study, an environmental plastics mass budget and box-model is proposed for the Mediterranean region based on recent observations. We use this model to explore plastics dispersal under different OECD plastic production and waste management policy scenarios toward the end of the 21st century. We find that the current Mediterranean marine plastic stock (sea surface, water column, sandy beach and sediments) of 7 million metric tons (Mt, median, IQR 3–15 Mt) in 2015 constrains continental plastic runoff to 0.31 Mt y^-1 (median, IQR 0.14–0.57 Mt y^-1). The total marine plastics stock would increase 4-fold by 2060 under a business-as-usual scenario, reaching 26 Mt (median, IQR 13–48 Mt). Implementation of the OECD Global Ambition policy scenario, that targets near-zero new plastics waste leakage, would not significantly lower this stock (25 Mt, median, IQR 12–44 Mt) by 2060. This is because marine litter remote sensing observations attribute most, 0.27 Mt y^-1 (88 %), of recent plastic runoff to Southern Europe, where high rainfall will continue to mobilize legacy plastic waste from land to sea, regardless of low leakage targets. About 1.5 % of all Mediterranean legacy plastic waste reached the marine environment, meaning that most plastic waste still resides on land (361 Mt, 76 %). Moreover, in the marine environment, 83 % of plastic mass resides in shelf sediments (median 6 Mt, IQR 2–14 Mt), which are fragile ecosystems that host most of the Mediterranean Sea biodiversity, and are not easy to clean up. This underlines the necessity to address upstream legacy plastic waste on land. Land-based remediation scenarios modelled here show that total plastic runoff from land to sea can be reduced 2-fold (0.22 Mt y^-1, IQR 0.16–0.27 Mt y^-1) compared to the business-as-usual scenario in 2060, and significantly reduce the total plastic stock in the marine environment.

Received: 27 Sep 2024 – Discussion started: 21 Nov 2024

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Journal article(s) based on this preprint

30 Sep 2025

Terrestrial and marine plastic pollution outlook in the Mediterranean region: a box-model approach based on OECD policy scenarios

Théo Segur and Jeroen E. Sonke

Biogeosciences, 22, 5139–5156, https://doi.org/10.5194/bg-22-5139-2025,https://doi.org/10.5194/bg-22-5139-2025, 2025

Short summary

Théo Segur and Jeroen E. Sonke

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3031', Anonymous Referee #1, 25 Apr 2025

This article presents a model of plastic pollution in the Mediterranean area. A mass budget for the year 2015 was proposed, and a box-model was calibrated to evaluate different OECD policy scenarios up to 2060. All steps and assumptions of the method are well-justified, and the results and conclusions are thoroughly developed. The authors critically examine the limitations of the current model and provide suggestions for improvement, although some of these potential improvements strongly depend on further studies for determining microplastics in the various environmental compartments considered. In particular, the lack of information on the concentration of nanoplastics in these compartments leads to a portion of the plastic pollution being 'missed,' an issue that must be addressed in future research. This type of study is crucial for understanding the potential impact of plastic waste reduction policies and for emphasizing the lack of ambition in current proposals. It is key to fostering the development of more robust and effective policies. This article is recommended for publication after minor revisions. The comments are provided below.
Abstract
Line 4 : « based on recent observations. » be more specific, From what year is the literature used in this study?
Introduction :
Line 30 : Introduce the idea that plastic waste fragments in the environment, which in turn influences its mobility.
Line 51 : Please specify what is referred to as plastic litter in this context. Later in the text, it appears to include macroplastics, but does it also encompass microplastics?
Lines 50-55 : What can explain the variability between studies, and how does your study address these uncertainties?
Methods
Lines 90-91 : « small microplastics (SMP) defined as 1 μm ≤ L<0.3 mm ». "Why this category? It is not a standardized size classification in the literature."
Lines 96-97 : « Fragmentation is caused by (photo-)oxidation, structural fragilization and mechanical abrasion at a rate of approximately 3% per year (Chamas et al., 2020; Lebreton et al., 2019; Sonke et al., 2022, 2024) ». This information should already be presented in the introduction, see coment below
Tables S1 and S2 : How do you explain that some of your sd are 0.0?
Lines 191-195 : SMP extrapolation from global data. Could you please explain in more detail how you performed this extrapolation? Why is this the best option? Could an extrapolation from P and LMP in the Mediterranean be considered as an alternative?
Lines 209-210 : It is more detailed, rather than being contradictory.
Results and Discussion
-line 321 : It is important to note here that « The Ocean Cleanup » is a non-profit organization

Citation: https://doi.org/10.5194/egusphere-2024-3031-RC1
- AC1: 'Reply on RC1', Théo Segur, 23 May 2025
  
  Dear reviewer, Dear Editor,
  Please find attached our response to the questions raised by RC1 during the first review round.
  As we understand, the revised manuscript will be uploaded latter.
  We would like to thank RC1 for their constructive comments and suggestions during the review of this manuscript, and for their support for publication.
  Sincerely,
  Théo SEGUR and Jeroen E. SONKE
  
  Citation: https://doi.org/10.5194/egusphere-2024-3031-AC1
RC2:
'Comment on egusphere-2024-3031', Anonymous Referee #2, 02 May 2025
This study explored the fate of plastic waste generated in the Mediterranean Sea catchment and authors proposed a box-model to evaluate different OECD policy scenarios toward 2060. This is a highly forward-looking study, essential for the development of public policy. The model is well presented and results are interesting. I recommend this paper for publication after major revision.
Major comments and other comments
I recommend authors stating right from the introduction that this is a prospective, exploratory exercise, that it tries to take into account a maximum of plastic pollution (large, small, etc.), but that it cannot consider all categories for all the compartments but this model tries to take as much as he can. I appreciated the limitation of the model section, which really help to understand what the model proposes or not.
My first and solely concern is the concept of runoff, which still unclear from my side. L70. of plastic runoff from land to sea. What do you mean by runoff form land-to-sea? Remobilization of all plastic litter undependably of the connection with river first and then Sea? Or direct runoff to the Sea? It’s also parametrized to consider the intensity of the rain? A major conclusion of this study is the important of runoff – what I would call river discharges during high flow periods or during flooded but not definitely runoff. From my point of view, this concept, or what runoff mean, should be better explained.
Implementation of the OECD Global Ambition policy scenario, that targets near-zero new plastics waste leakage, would not significantly lower this stock (25 Mt, median, IQR 12-44 Mt) by 2060. Totally agree with this conclusion. Very important as regard the OCED recommendation and the objectives. Behind this policy scenario, the main idea is to significantly reduce the plastic consumption and not necessary the plastic pollution in the environmental compartment. One other important idea is the “legacy stock” of plastic.
L15. his underlines the necessity to address upstream legacy plastic waste on land. How upstream? On land? Or for the consumption?
L24. Plastic items are also very mobile due to their relatively low density and buoyancy, and can travel long distances by rivers and ocean currents. Some studies suggest that significant amount of plastic litter can be also trapped along the river banks and flooded pain aera.
L69. This study investigates the fate of plastic waste in the Mediterranean Sea catchment across various environmental compartments 70 (terrestrial, sea surface and water column, shelf and deep sediments, beaches and atmosphere). Until here, we can not determine if the study will focus on plastic litter only or include microplastic. If atmosphere is included, probably microplastic is considered. If yes, why? It’s clearer L90-95. L280. Results on the atmospheric SMP cycle are not presented here, as they are not yet constrained enough in the Mediterranean. I suggest to remote this section to the core manuscript. It can be mentioned in model structure.
region to provide reasonable levels of uncertainty
Figure 2. Which data are used to build this figure?
L112. Plastic mass transport between boxes is approximated to be first order, where the plastic flux FA→B [Mt y-1] from box A to box B is proportional to the plastic mass MA [Mt] in box A. How can we justify this assumption? Which implication would have a different one? It’s here a question of the assumption sensibility.
Table 1. For each study, in addition to the stock considered, which compartments are considered? This is important since in your model you consider all compartements.
2.4.6 Runoff to sea surface. Here what is the concept of runoff? It’s the discharges by river during high flows/flooded?
The geographical distribution of plastic runoff was found to be dominated by S. Europe (87.9%), followed by N. Africa & M. East (12.0%) and the Nile basin (0.1%). Do this observation is linked to the crossed explanation, population density and high flows of river? In contradiction with other studies (as underlined by authors). This has a very strong implications for policy. I would suggest to explain more why you found these contradictory results and how our approach is relevant in regards to others studies.

L395. his means that most of MMPW is still in terrestrial areas, which are not detailed yet in the model. Totally agree.
Results section. Do the results/conclusions will be different by considering only the large fraction of plastic litter (> 5 mm)?
Minor comments
L4. for the Mediterranean region. Please clarify the aera.
L54. Simon-Sánchez et al. (2022) reviewed and reported concentrations in sediments (300 items kg-1) and beaches (60 item kg-1), insisting on the high uncertainties and 55 variability between studies. You discussed here about microplastics. What is the link with plastic litter mentioned?
L60. Sea litter by Cózar et al. (2024) highlighted the close relationship between marine litter occurrence and heavy rainfall events, pointing at Southern Europe as the largest macroplastic source to the Mediterranean Sea. What is their hypothesis?
L142. The year 2015 is chosen as reference for calibration. Why?
Table 2. What is the unit of the first reported concentration? 5.6 10-3
L289. we calculare. Calculate
Citation: https://doi.org/10.5194/egusphere-2024-3031-RC2
- AC2: 'Reply on RC2', Théo Segur, 23 May 2025
  
  Dear reviewer, Dear Editor,
  Please find attached our response to the questions raised by RC1 during the first review round.
  As we understand, the revised manuscript will be uploaded latter.
  We would like to thank RC2 for their constructive comments and suggestions during the review of this manuscript, and for their support for publication.
  Sincerely,
  Théo SEGUR and Jeroen E. SONKE
  
  Citation: https://doi.org/10.5194/egusphere-2024-3031-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3031', Anonymous Referee #1, 25 Apr 2025

This article presents a model of plastic pollution in the Mediterranean area. A mass budget for the year 2015 was proposed, and a box-model was calibrated to evaluate different OECD policy scenarios up to 2060. All steps and assumptions of the method are well-justified, and the results and conclusions are thoroughly developed. The authors critically examine the limitations of the current model and provide suggestions for improvement, although some of these potential improvements strongly depend on further studies for determining microplastics in the various environmental compartments considered. In particular, the lack of information on the concentration of nanoplastics in these compartments leads to a portion of the plastic pollution being 'missed,' an issue that must be addressed in future research. This type of study is crucial for understanding the potential impact of plastic waste reduction policies and for emphasizing the lack of ambition in current proposals. It is key to fostering the development of more robust and effective policies. This article is recommended for publication after minor revisions. The comments are provided below.
Abstract
Line 4 : « based on recent observations. » be more specific, From what year is the literature used in this study?
Introduction :
Line 30 : Introduce the idea that plastic waste fragments in the environment, which in turn influences its mobility.
Line 51 : Please specify what is referred to as plastic litter in this context. Later in the text, it appears to include macroplastics, but does it also encompass microplastics?
Lines 50-55 : What can explain the variability between studies, and how does your study address these uncertainties?
Methods
Lines 90-91 : « small microplastics (SMP) defined as 1 μm ≤ L<0.3 mm ». "Why this category? It is not a standardized size classification in the literature."
Lines 96-97 : « Fragmentation is caused by (photo-)oxidation, structural fragilization and mechanical abrasion at a rate of approximately 3% per year (Chamas et al., 2020; Lebreton et al., 2019; Sonke et al., 2022, 2024) ». This information should already be presented in the introduction, see coment below
Tables S1 and S2 : How do you explain that some of your sd are 0.0?
Lines 191-195 : SMP extrapolation from global data. Could you please explain in more detail how you performed this extrapolation? Why is this the best option? Could an extrapolation from P and LMP in the Mediterranean be considered as an alternative?
Lines 209-210 : It is more detailed, rather than being contradictory.
Results and Discussion
-line 321 : It is important to note here that « The Ocean Cleanup » is a non-profit organization

Citation: https://doi.org/10.5194/egusphere-2024-3031-RC1
- AC1: 'Reply on RC1', Théo Segur, 23 May 2025
  
  Dear reviewer, Dear Editor,
  Please find attached our response to the questions raised by RC1 during the first review round.
  As we understand, the revised manuscript will be uploaded latter.
  We would like to thank RC1 for their constructive comments and suggestions during the review of this manuscript, and for their support for publication.
  Sincerely,
  Théo SEGUR and Jeroen E. SONKE
  
  Citation: https://doi.org/10.5194/egusphere-2024-3031-AC1
RC2:
'Comment on egusphere-2024-3031', Anonymous Referee #2, 02 May 2025
This study explored the fate of plastic waste generated in the Mediterranean Sea catchment and authors proposed a box-model to evaluate different OECD policy scenarios toward 2060. This is a highly forward-looking study, essential for the development of public policy. The model is well presented and results are interesting. I recommend this paper for publication after major revision.
Major comments and other comments
I recommend authors stating right from the introduction that this is a prospective, exploratory exercise, that it tries to take into account a maximum of plastic pollution (large, small, etc.), but that it cannot consider all categories for all the compartments but this model tries to take as much as he can. I appreciated the limitation of the model section, which really help to understand what the model proposes or not.
My first and solely concern is the concept of runoff, which still unclear from my side. L70. of plastic runoff from land to sea. What do you mean by runoff form land-to-sea? Remobilization of all plastic litter undependably of the connection with river first and then Sea? Or direct runoff to the Sea? It’s also parametrized to consider the intensity of the rain? A major conclusion of this study is the important of runoff – what I would call river discharges during high flow periods or during flooded but not definitely runoff. From my point of view, this concept, or what runoff mean, should be better explained.
Implementation of the OECD Global Ambition policy scenario, that targets near-zero new plastics waste leakage, would not significantly lower this stock (25 Mt, median, IQR 12-44 Mt) by 2060. Totally agree with this conclusion. Very important as regard the OCED recommendation and the objectives. Behind this policy scenario, the main idea is to significantly reduce the plastic consumption and not necessary the plastic pollution in the environmental compartment. One other important idea is the “legacy stock” of plastic.
L15. his underlines the necessity to address upstream legacy plastic waste on land. How upstream? On land? Or for the consumption?
L24. Plastic items are also very mobile due to their relatively low density and buoyancy, and can travel long distances by rivers and ocean currents. Some studies suggest that significant amount of plastic litter can be also trapped along the river banks and flooded pain aera.
L69. This study investigates the fate of plastic waste in the Mediterranean Sea catchment across various environmental compartments 70 (terrestrial, sea surface and water column, shelf and deep sediments, beaches and atmosphere). Until here, we can not determine if the study will focus on plastic litter only or include microplastic. If atmosphere is included, probably microplastic is considered. If yes, why? It’s clearer L90-95. L280. Results on the atmospheric SMP cycle are not presented here, as they are not yet constrained enough in the Mediterranean. I suggest to remote this section to the core manuscript. It can be mentioned in model structure.
region to provide reasonable levels of uncertainty
Figure 2. Which data are used to build this figure?
L112. Plastic mass transport between boxes is approximated to be first order, where the plastic flux FA→B [Mt y-1] from box A to box B is proportional to the plastic mass MA [Mt] in box A. How can we justify this assumption? Which implication would have a different one? It’s here a question of the assumption sensibility.
Table 1. For each study, in addition to the stock considered, which compartments are considered? This is important since in your model you consider all compartements.
2.4.6 Runoff to sea surface. Here what is the concept of runoff? It’s the discharges by river during high flows/flooded?
The geographical distribution of plastic runoff was found to be dominated by S. Europe (87.9%), followed by N. Africa & M. East (12.0%) and the Nile basin (0.1%). Do this observation is linked to the crossed explanation, population density and high flows of river? In contradiction with other studies (as underlined by authors). This has a very strong implications for policy. I would suggest to explain more why you found these contradictory results and how our approach is relevant in regards to others studies.

L395. his means that most of MMPW is still in terrestrial areas, which are not detailed yet in the model. Totally agree.
Results section. Do the results/conclusions will be different by considering only the large fraction of plastic litter (> 5 mm)?
Minor comments
L4. for the Mediterranean region. Please clarify the aera.
L54. Simon-Sánchez et al. (2022) reviewed and reported concentrations in sediments (300 items kg-1) and beaches (60 item kg-1), insisting on the high uncertainties and 55 variability between studies. You discussed here about microplastics. What is the link with plastic litter mentioned?
L60. Sea litter by Cózar et al. (2024) highlighted the close relationship between marine litter occurrence and heavy rainfall events, pointing at Southern Europe as the largest macroplastic source to the Mediterranean Sea. What is their hypothesis?
L142. The year 2015 is chosen as reference for calibration. Why?
Table 2. What is the unit of the first reported concentration? 5.6 10-3
L289. we calculare. Calculate
Citation: https://doi.org/10.5194/egusphere-2024-3031-RC2
- AC2: 'Reply on RC2', Théo Segur, 23 May 2025
  
  Dear reviewer, Dear Editor,
  Please find attached our response to the questions raised by RC1 during the first review round.
  As we understand, the revised manuscript will be uploaded latter.
  We would like to thank RC2 for their constructive comments and suggestions during the review of this manuscript, and for their support for publication.
  Sincerely,
  Théo SEGUR and Jeroen E. SONKE
  
  Citation: https://doi.org/10.5194/egusphere-2024-3031-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (30 May 2025) by Edouard Metzger

AR by Théo Segur on behalf of the Authors (04 Jun 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (11 Jun 2025) by Edouard Metzger

RR by Anonymous Referee #1 (20 Jun 2025)

ED: Publish as is (06 Jul 2025) by Edouard Metzger

AR by Théo Segur on behalf of the Authors (15 Jul 2025) Author's response Manuscript

Journal article(s) based on this preprint

30 Sep 2025

Terrestrial and marine plastic pollution outlook in the Mediterranean region: a box-model approach based on OECD policy scenarios

Théo Segur and Jeroen E. Sonke

Biogeosciences, 22, 5139–5156, https://doi.org/10.5194/bg-22-5139-2025,https://doi.org/10.5194/bg-22-5139-2025, 2025

Short summary

Théo Segur and Jeroen E. Sonke

Supplement

https://doi.org/10.5194/egusphere-2024-3031-supplement

Model code and software

Mediterranean Plastic Box-Model code Théo Segur and Jeroen Sonke https://github.com/TheoSEGUR/Med_Plastic_BoxModel.git

Théo Segur and Jeroen E. Sonke

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Our paper provides a quantification of plastic pollution in the Mediterranean region, and several policy scenario projections based on OECD data toward 2100. We estimate a 4-fold increase of Mediterranean marine plastic stock by 2060 and that the implementation of terrestrial plastic cleanup can significantly help to reduce plastic pollution transfer from land to sea. Our results provide insight for policy makers, which is needed at the regional scale in a context of the UNEP plastic treaty.


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Terrestrial and marine plastic pollution outlook in the Mediterranean region: a box-model approach based on OECD policy scenarios

Journal article(s) based on this preprint

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Interactive discussion

Peer review completion

Journal article(s) based on this preprint

Supplement

Model code and software

Viewed

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Cited

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