Physical limits of sea-level rise adaptation in global river deltas

Domain: neuroscience
Published: 2026-02-14
Editorial outcome: major_revision

Abstract
Sea-level rise threatens deltas worldwide, requiring adaptation to flood risks. Delta adaptation is typically presented as a choice between five strategies: advance, protect-closed, protect-open, accommodate, and retreat. However, a full assessment of the physical feasibility of these strategies across deltas remains limited. We present a first-order assessment of the physical solution space for adaptation to sea-level rise for nearly 800 deltas globally. Here, we show that current technologies, resources, and space provide at least one physically feasible delta-wide strategy for every delta to adapt by 2100. This number may increase in the future through technical innovation or collaboration between deltas. The type and number of physically feasible strategies are mostly determined by delta’s physical characteristics, whereby large, urbanized, or frequently flooded deltas have fewer options than small, rural, or infrequently flooded deltas. Our analysis highlights the risk of resource limitations as global deltas will need to adapt simultaneously to future flood risks.

Introduction
Deltaic inhabitants are vulnerable to floods, droughts and land loss due to climate change1,2, with sea-level rise (SLR) expected to pose the largest challenge to these coastal areas3. Such impacts, combined with a rapidly expanding population, necessitate a long-term commitment to adaptation2,4,5. Several studies have assessed adaptation at the delta scale6–19. For example, in the Netherlands, a feasibility assessment showed that the construction lifetimes of barriers and pumping stations will pose significant challenges17,20 and in the United States, an analysis of location, damages, hazards, and socioeconomic and demographic characteristics informed voluntary buyouts of flood-prone properties18.

Global assessments of coastal adaptation are rare, but can aid local-scale assessments. To date, global studies include evaluations of flood protection costs and benefits21, timing of adaptation needs13,22 and currently implemented adaptation strategies23. The feasibility of adaptation has also been assessed across different dimensions, by identifying barriers to adaptation options in different geographical and socio-economic settings24. Yet, a physical feasibility assessment of adaptation strategies for global deltas under future flood risks remains limited.

In this context, mapping the physical solution space of deltas could inform the physical opportunities and constraints for delta adaptation at a global scale. The solution space is the ‘space within which opportunities and constraints determine why, how, when and who adapts to climate risks’25–27. In addition to the physical adaptation potential, resource limitations may also be triggered, as all deltas are vulnerable to the risks of SLR and may require synchronous adaptation in the future28–30. Resource coordination and innovation can therefore be guided and targeted through a global assessment of adaptation.

We categorise delta adaptation into five flood risk adaptation strategies. Following the IPCC framework, we distinguish protect, accommodate, advance and retreat2,31. We split the protection strategy into two, as, for deltas, a fundamental decision involves whether the river remains open to or closed off from the sea12,17, resulting in the protect-open and protect-closed strategies, respectively (Fig. 1; see Supplementary Text 1 for an extended description of strategies and examples in practice). We introduce a method to map the physical solutions space (PSS) of nearly 800 deltas to adapt to SLR. The PSS is mapped under three SLR scenarios following the IPCC’s SSP-RCP scenarios32 and across the aforementioned adaptation strategies, assuming the strategy is adopted across the entire delta (see ‘Methods’ and Supplementary Text 2 for a description of the delta areas). We map the PSS for the year 2100 to allow sufficient time for planning and implementation, given that SLR adaptation strategies typically have long lifespans and lasting impacts on society33. For each adaptation strategy, we create physical indicators of feasibility, which represent quantifiable requirements of key measures, namely: material requirements and availability for creating a new coastline (advance) or building coastal and/or riverine levees (protect), the river mouth width for constructing a storm surge barrier (protect-open), the river pump capacity and excess water storage capacity of retention areas (advance and protect-closed), the flood depth for elevating infrastructure and buildings (accommodate) and the available space for relocating urban areas (retreat) (see ‘Methods’). We assess the physical feasibility of adaptation strategies by applying three thresholds to each physical indicator (see ‘Methods’). If no thresholds are exceeded (i.e. there are no technological or physical limits), a strategy could be available to implement to reduce future flood risks. The thresholds chosen represent three scales of adaptation: (1) the current known measures, reflecting the largest or commonly used scales of measures in practice, (2) a low-resource (material) version of the current known measures and (3) an innovative version of the current known measures reflecting larger, more resource-demanding measures that would require technological advancements or collaboration to adopt (see Supplementary Dataset 1 for a database of existing examples of adaptation measures and supplementary Text 4 for rationale to support the thresholds). The low-resource measures are only physically feasible under conditions of restricted resources, which may limit the range of adaptation strategies available for implementation in deltas. In contrast, the innovative thresholds expand opportunities for adaptation and increase the range of physically feasible adaptation strategies, but only if technological advancements occur or if sufficient resources are available. Ultimately, whether a delta meets these thresholds determines the number and type of physically feasible adaptation options under future SLR.

In this work, we assess adaptation to SLR to reduce coastal flood risks while accounting for river flooding. We provide a first-order assessment of the PSS for nearly 800 deltas, showing that for every delta, at least one delta-wide adaptation strategy is physically feasible to implement following three SLR scenarios. However, the number and type of physically feasible adaptation strategies often differ across deltas based on the deltas’ physical characteristics and the scale of adaptation. By mapping the PSS of adaptation strategies, we provide insight into the technical innovation or coordination that may be necessary to address the impacts of future SLR in deltas.

Results
Following three SLR scenarios32 that span a wide range of flood-risk futures for 2100, we find that for every delta, physically feasible options exist with current space, technologies or resources, with at least one adaptation strategy available for SLR adaptation. Under an intermediate SSP2-4.5 global warming scenario specifically (median estimate of 0.47m SLR globally in 210034,35), we find that even under limited technical or resource conditions, at least one adaptation strategy is physically feasible for nearly every delta (except for the Rhine-Meuse; see Results for further elaboration), with all five strategies physically feasible for approximately 28% of deltas in their most low-resource form. Conversely, though, under conditions of increased technical innovation, the number of physically feasible strategies expands to at least two per delta, with approximately 68% of deltas having all five adaptation strategies physically feasible to implement.

The global fingerprint of delta adaptation shows that the number and type of physically feasible adaptation strategies varies between deltas and across all scales of adaptation potential (i.e. across all three thresholds; Fig. 2). Following an SSP2-4.5 scenario by 2100, the physical indicators reveal that several strategies are commonly physically feasible across deltas, with protect-open (99% of deltas), accommodate (84% of deltas) and retreat (98% of deltas) being physically feasible with currently known resource and technological capabilities and available space, requiring no innovation for delta-wide implementation (Fig. 2a). The remaining deltas face physical constraints that limit the implementation of these strategies (protect-open: 1%, accommodate: 12%, retreat: 2%) and require technological innovation or collaboration to make them physically feasible to implement. Innovation for these strategies includes storm surge barriers that exceed 9km in length, home raising of more than 1m, or planned relocations outside of the delta area due to a lack of space, respectively. We find that protect-closed and advance strategies face greater physical limits based on our indicators (Fig. 2b). While current pumping capacities are sufficient to manage mean river discharge for the majority of global deltas (91%), these capacities are exceeded during high river flows, necessitating larger pumping capacities or the storage of excess discharge in designated retention areas. Under a 99th percentile river flow event36,37, protect-closed is physically feasible for most deltas (75%) as the discharge can either be entirely pumped out based on current pump capacity capabilities, or excess discharge can be stored in retention areas. Alternatively, excess river water can be drained under gravity. However, future SLR may limit gravity drainage, which may increase pump capacity requirements beyond what is currently possible and further constrain the physical feasibility of protecting these deltas. Notably, advance is the least physically feasible strategy globally, with 27% of deltas lacking the necessary physical conditions for its implementation based on the indicators, even under assumed technical innovations (Fig. 2a). Moreover, despite having greater water retention capacities from coastal lagoons than protect-closed, advance is physically constrained by material limitations to aggrade offshore coasts, such as insufficient river sediment or offshore depths exceeding 30m (Fig. 2a). According to our indicators, advance is only physically feasible when both the pump capacity (based on the annual mean river discharge and the 99th percentile river discharge with sufficient retention areas) and the material availability (based on efficient river sediment collection or accessible offshore depth) thresholds are met (see ‘Methods’). However, a small river discharge and a large sediment discharge are not concurrent in deltas38 and deeper offshore depths physically constrain sand collection for coastline extensions.

Deltas with large urban areas generally have contracted PSSs, necessitating technical innovation to adapt to SLR (Fig. 5). This is especially evident in the accommodation strategy, where physical feasibility is based on the height required to elevate urban flooded areas. For example, in highly urbanised deltas, such as the Mississippi, the Ganges-Brahmaputra-Meghna and the Rhine-Meuse deltas (Fig. 4), innovation to elevate infrastructure by more than 1m and sometimes 2m may be required. For these deltas, protective strategies may be more preferable if the physical characteristics are suitable. These findings align with other studies, which show that urbanised deltas may opt for more protective measures if there is sufficient space and resources to construct them6,10,14,20.

In comparison, rural deltas with low population densities tend to have larger PSSs, supporting numerous physically feasible strategies in their lowest-resource forms (Fig. 5), as seen in the Flinders delta (Australia) and Taz delta (Russia) (Fig. 4). However, rural deltas may opt for a ‘living with water’ approach (accommodate) or a planned relocation to higher ground (retreat)6 instead of protective measures15.

The global pursuit of flood risk adaptation may be restricted by material needs. Following an advance strategy, we find that the material required to extend every delta’s coastline 8km seaward is estimated at ~3099km3, which greatly exceeds the available fluvial sediment supply across global rivers over 75 years (up to 2100). This points to the danger of resource limitations and potential competition for resources if this strategy becomes widespread among deltas4. Deltas may be able to meet their material requirements using their own resources, which is often observed in practice28,46–48. We find that many deltas (79%) are material-sufficient with currently existing nourishing technology, meaning they can advance their coastline using local material, either by collecting natural river sediment or from extracting offshore sand (Fig. 6). However, even if deltas can source sufficient material from their rivers or offshore coast, the pump capacity requirements may pose additional challenges. While effective river sediment collection may be sufficient to aggrade the new coastline, a large sediment discharge is typically simultaneous with a large river discharge, making pump capacity requirements unfeasible, thereby limiting the advance strategy’s physical feasibility38. The reverse situation is also possible, where deltas may meet the pump capacity requirements but are material-deficient and cannot collect sufficient material to extend their coastline, either due to large offshore depths (>60m) or low sediment retention rates (<20%).

Some deltas (8%) can only advance their coastline under innovative material collection conditions and the remaining deltas (17%) are material-deficient to pursue advance and will require import (Fig. 6). The total material required for material-deficient deltas amounts to approximately 2409km3, which is nearly 40 times larger than the annual global sediment budget from all dredging and land reclamation activities (9.8 Gt per year, 61.2 km3) in 201038 and far exceeds both the 50-year and the 75-year global river sediment supply (~416km3 and ~624km3, respectively) (Fig. 6). In these cases, alternative adaptation options or external materials, potentially sourced from other shallow seas, may be required. This highlights that adaptation efforts surpass local boundaries and collaboration between countries through knowledge and resource sharing may be necessary in the future4.

Protect strategies have substantially lower material requirements compared to the advance strategy, increasing their physical feasibility. The levee material requirements are ~6.81km3 for protect-closed and over double this, ~14.56km3, for protect-open (Fig. 6). These material requirements are considerably lower than the total quantity of sediment naturally delivered from rivers. As such, a portion of the sediment supply could be allocated to levee construction if the pump capacity or storm surge barrier width thresholds are also met, respectively.

Methods
We map global deltas’ physical solutions space (PSS) following three IPCC SSP-RCP scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5) by 2100. These methods are accompanied by the Supplementary Information, which includes information on the adaptation strategies assessed (see Supplementary Text 1), the spatial extent of the deltas (see Supplementary Text 2), the physical indicators’ equations and data sources per adaptation strategy (see Supplementary Text 3), additional support for the thresholds selected (see Supplementary Text 4), the flood risks for deltas and the differences between SLR scenarios (see Supplementary Text 5) and additional information on sensitivity analyses and model output comparisons to literature (see Supplementary Text 6).

The global delta dataset contains 769 delta locations represented by polygons that define the delta area as a four-point deltaic extent46,70 (see Supplementary Text 2 for a visualisation of the deltaic extent). These deltas represent 35% of 2174 global deltas71 but cover approximately 96% of the global delta geomorphic area (815,247km2 of 847,936km2). According to the 2022 global population, 348 million people were living on these deltas, which represented 4.3% of the population at the time72,73.

We assess adaptation to SLR to reduce coastal flood risks whilst accounting for river flooding, but excluding pluvial flooding. We create physical indicators of feasibility, which are quantifiable requirements that represent key measures associated with each adaptation strategy. For example, under a protect-closed strategy, a key measure is a pump to discharge the mean river discharge into the (elevated) sea, combined with retention areas to store excess water during a high river flow. As such, the physical indicators are pump capacity and the availability of water retention areas to store excess discharge that cannot be pumped out. Under an accommodate strategy, built-up areas are raised, so the physical indicator is the height necessary to raise these areas based on the flood depths in the delta. We use a combination of literature and interviews with experts in coastal engineering and flood resilience solutions to determine the types and proportions of indicators2,9,12,14,17,20,74 (Table 1). Each physical indicator is calculated using a simple geometric model (see Supplementary Text 3 for physical indicator equations). We run the model under three SLR scenarios following the IPCC’s SSP-RCP scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5), but do not find significant differences between the outputs and thus focus this study on an intermediate GHG emissions scenario (SSP2-4.5) (see Supplementary Text 5 and results section titled ‘Physical characteristics and adaptation thresholds versus climate change’).

We apply three thresholds to each physical indicator to assess the physical opportunities or constraints of the strategy (Table 2). Threshold 1 is the current known scale of adaptation measures, which is either the maximum known (e.g. the largest existing storm surge barrier) or most commonly used (e.g. home raising through stilts) magnitude of a measure in practice (see Supplementary Dataset 1 for a database of measures in practice and their magnitude). Threshold 2 is a low-resource version of the measure, which can be adopted under limited resource (material) conditions. Threshold 3 is a more advanced version of the current known measures and would require additional innovation, collaboration, or resources to adopt. We assume that such advanced or innovative measures provide more opportunities for delta adaptation; however, what is considered ‘innovative’ may differ between deltas (see Supplementary Text 4 for rationale on the thresholds).

To evaluate the model reliability, we conducted sensitivity analyses by changing the input parameters to values well beyond plausible ranges. This test confirmed that the model responds appropriately to extreme values, with large inputs contracting the PSS and smaller inputs expanding the PSS (see Supplementary Text 6). However, we also test parameter uncertainty by using plausible projected ranges for river and sediment discharge by 2100 from existing literature50–52. We evaluate how projected changes in these input parameters influence the PSS of global deltas and confirm that changes in individual deltas’ PSS do not alter the overall global adaptation trends. However, these changes may have implications for local-scale decision-making in 3–8% of deltas (see Supplementary Text 6 for examples of deltas impacted).

We further test how higher amounts of future SLR (2m and 4m) influence the PSS. We find that SLR beyond the likely range for this century contracts the PSS of many deltas, specifically decreasing the physical feasibility of the advance, accommodate and retreat strategies (see Supplementary Text 5 for further explanation and Supplementary Fig. 5 for a visualisation of this contraction).

The three thresholds applied (Table 2) provide insight into delta adaptation potential, while also testing the model sensitivity. As the threshold changes from low-resource to innovative, the PSS expands. If the innovation threshold is increased from a two fold to a ten fold change, the PSS expands for some deltas; however, the overall outcomes remain largely consistent. In most cases, strategies that are physically unfeasible under all three thresholds remain unfeasible even at higher thresholds (see Supplementary Text 6).

Finally, we compare the model outcomes with existing literature on known and future delta adaptation plans for 10 deltas. We find that the strategies we identify as physically feasible for certain deltas align with the currently implemented and planned future strategies (see Supplementary Text 6 and Supplementary Dataset 2).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript evaluates a range of possible solutions for adaptation to future sea-level rise conditions for a large number of river deltas globally. Specifically, the authors base their assessment on a first-order analysis of five different adaptation strategies, showing the number and type of physically feasible adaptation strategies across deltas. The results provide an overview on which deltas could adapt to future conditions using existing resources and technologies, and which require technical innovation or coordination with nearby territories to adapt to future flood risks.\nThis is the second time that I have to opportunity to review this manuscript, and I am glad to see that the authors have well addressed all my previous comments. The additional analyses have further improved the methodology and the robustness of results. The authors have also improved the desciption of the modelling steps and clarified the rationale behind some modelling assumptions.\nMy only remarck is that the manuscript seems not to have a data availability section. In case this is not requested at a later stage of the submission process, I would strongly encourage the authors to add this section.\nApart from this technical issue, in my opinion the manuscript can be published in its present form\nFrancesco Dottori\nIUSS Pavia, Italy\n(Remarks on code availability)\nI quickly examined the code and data provided by the authors but I did not try to install and run the code as I do not have time for a complete check.\nInput and output data seem to be available.\nI could not find README or similar instruction files explaining the structure of the code and input-output data. If this is the case, O would recommend adding these instructions.", "reviewer_2": "It is the second time I review this paper, following my earlier review of its prior version submitted to Nature Climate Change. This study delivers a valuable global-scale assessment of the physical solution space (PSS) for sea-level rise (SLR) adaptation across 769 river deltas, effectively addressing a critical gap in understanding the feasibility of five core adaptation strategies. The systematic quantification of material requirements, hydrodynamic constraints, and geomorphic thresholds—paired with the integration of global datasets like SWORD river networks and MERIT DEM—establishes a novel comparative framework. Key findings, such as the universality of at least one feasible strategy per delta, material limitations for the “advance” strategy, and the role of delta size and urbanization in constraining PSS, offer actionable insights for prioritizing adaptation resources. Compared with the prior version, I do observe specific improvements, including the revised calculation method for river widths of 369 deltas. However, the major concerns I raised earlier remain unresolved, and new issues have emerged with unexplained updates of parameters and results in the current manuscript, which require further attention.\nFirst, the definitions of the five adaptation strategies still lack refinement to align with real-world implementation. The distinction between “protect-open” and “protect-closed” remains overly narrow, as it focuses primarily on river-mouth structures (storm surge barriers versus dams) while overlooking broader components of practical flood defense systems. For example, “protect-closed” systems like the Dutch Delta Works typically integrate pumps with riverine levees to manage inland flooding, yet the current model excludes riverine levees from the protect-closed strategy—this omission likely underestimates total material requirements for levee construction and overestimates required pump capacity, given that riverine levees reduce the volume of water needing discharge. Conversely, a “true protect-open” strategy, common in practice (e.g., the Mississippi River’s IHNC Surge Barrier system), usually protects both coastlines and inland rivers with dikes while keeping the river mouth open, but the current model fails to explicitly incorporate river dikes into the protect-open strategy, weakening its alignment with real engineering solutions. For the “advance” strategy, the assumption of 100% pumped drainage of river flows (due to fully blocked outlets) is overly restrictive; real-world land reclamation projects often use partial closures or engineered channels to maintain natural river discharge, reducing pump demands to managing pluvial flooding rather than total river flow. This unrealistic assumption artificially limits the feasibility of the advance strategy, and revising these definitions and their associated indicators (such as adjusting pump capacity thresholds for advance to reflect partial drainage) is essential to improve the validity of the PSS assessment.\nSecond, the assumption of static river discharge (outlined in Suppl. Text SI3 of the current manuscript) remains problematic. Climate change is projected to alter precipitation and runoff patterns. For regionally vulnerable deltas—such as monsoonal deltas like the Ganges-Brahmaputra-Meghna—discharge variability could be much higher, directly impacting pump capacity requirements for protect-closed and advance strategies (higher discharge increases the need for larger pumps) and sediment supply for the advance strategy (altered discharge changes sediment transport and retention rates). The lack of sensitivity analyses incorporating projected discharge variability undermines the robustness of physical feasibility assessments; a stratified sensitivity analysis, for instance by delta climate zones (tropical, temperate, arid) using region-specific discharge projections, would better capture these uncertainties and strengthen the findings.\nThird, Figure 2a in the current manuscript suffers from poor visual discriminability, with thin lines and overlapping colors making trends difficult to decipher. A critical discrepancy further emerges between the figure and the text: the retreat strategy (represented by the outermost line) appears to be 100% feasible across all deltas, yet the text notes that 2% of deltas require innovation to implement retreat (e.g., relocations outside the delta). This inconsistency points to flawed graphical representation, and revisions are necessary.\nFourth, several key parameters and results have changed between the prior version and the current manuscript, with no explicit justification provided. Most notably, for the “advance” and “protect-closed” strategies, Threshold 3 (Innovative option) for pump capacity has been reduced from 12,000 m³/s (prior version) to 2,400 m³/s (current version)—an 80% reduction that directly alters the feasibility of these strategies. Yet, no rationale for this adjustment—such as literature citations of maximum operational pump capacities, technical limits of current pumping technology, or expert consensus—is provided in the main text, Supplementary Text SI4 (intended to support threshold selections with a database of existing measures), or associated datasets. Similarly, the percentage of deltas with all five strategies feasible in low-resource form under the SSP2-4.5 scenario has increased from 12% (prior version) to 38% (current version), with no explanation for this large discrepancy (e.g., whether it stems from revised data inputs, adjusted thresholds, or model improvements)—this lack of transparency erodes confidence in the result’s reliability. The authors must explicitly document the rationale for all parameter changes and cross-validate new thresholds with existing engineering practices or literature.\nIn summary, the study addresses an important and timely topic, but unresolved issues with strategy definitions, static discharge assumptions, and methodological transparency—paired with new concerns about unjustified parameter changes—limit its current impact. Addressing these comments will substantially improve the manuscript’s scientific rigor and alignment with real-world adaptation needs, ensuring its findings are both credible and actionable.", "reviewer_3": "This paper sets out a novel global assessment of the physical solution space (PSS) for adaptation of about 800 river deltas to sea level rise (SLR). It triangulates three sources of evidence: physical characteristics of deltas; SLR projections to 2100 (3 SSP scenarios); and three 'scales of adaptation' (assumptions about technical feasibility of adaptation options). It investigates 5 SLR adaptation strategies, including retreat. The results show that '...nearly every delta has at least one physically feasible adaptation strategy'. That seems to be an important result in an interesting and well-constructed paper. The point that this is a first-stage 'physical/engineering' assessment is clearly made and invites additional studies to understand the nature of financial, nature, political and cultural constraints of the proposed measures.\nThree main comments: 1. The paper presents results for SSP2-4.5 only, arguing that SLR variance is less important than physical characteristics (depth) of deltas to adaptation option feasibility. It would be good to have this more explicitly discussed in the paper, not just in supplementary material (it is an unexpected result). 2. The 'physical indicator thresholds' relate to soft adaptation limits and it would be useful if the values given in Table 2 could be justified. 3. Figure 2 shows that protect-open (levees and storm-surge barriers) is an option that is available more or less everywhere (97% of deltas). This also seems a surprising result, and it may be useful to provide some idea of the cost and time required to put these options in place. This is clearly not an economic feasibility study, but as the paper acknowledges, the finance is likely to be a constraint in many cases and an indication of the scale of this would be useful."}, "author_response": "Rebuttal: Physical limits of sea-level rise adaptation in global river deltas\nKiara G. Lasch, Jaap H. Nienhuis, Gundula Winter, Marjolijn Haasnoot\nReviewers Comments:\nReviewer #1 (Comments for the Author):\nThis manuscript evaluates a range of possible solutions for adaptation to future sea-level rise conditions\nfor a large number of river deltas globally. Specifically, the authors base their assessment on a first-order\nanalysis of five different adaptation strategies, showing the number and type of physically feasible\nadaptation strategies across deltas. The results provide an overview on which deltas could adapt to future\nconditions using existing resources and technologies, and which require technical innovation or\ncoordination with nearby territories to adapt to future flood risks.\nThis is the second time that I have to opportunity to review this manuscript, and I am glad to see that the\nauthors have well addressed all my previous comments. The additional analyses have further improved\nthe methodology and the robustness of results. The authors have also improved the desciption of the\nmodelling steps and clarified the rationale behind some modelling assumptions.\nMy only remarck is that the manuscript seems not to have a data availability section. In case this is not\nrequested at a later stage of the submission process, I would strongly encourage the authors to add this\nsection.\nWe thank Francesco for his positive review and are pleased to see he is satisfied with how we\naddressed his previous comments. We have now added a data availability section to the manuscript.\nApart from this technical issue, in my opinion the manuscript can be published in its present form.\nFrancesco Dottori\nIUSS Pavia, Italy\n(Remarks on code availability):\nI quickly examined the code and data provided by the authors but I did not try to install and run the code\nas I do not have time for a complete check.\nInput and output data seem to be available.\nI could not find README or similar instruction files explaining the structure of the code and input-\noutput data. If this is the case, O would recommend adding these instructions.\nAll the data and scripts are available through Code Ocean. We have now added a README.txt\nfile to explain the structure of the code and the IO data.\nReviewer #2 (Comments for the Author):\nIt is the second time I review this paper, following my earlier review of its prior version submitted to\nNature Climate Change. This study delivers a valuable global-scale assessment of the physical solution\nspace (PSS) for sea-level rise (SLR) adaptation across 769 river deltas, effectively addressing a critical\ngap in understanding the feasibility of five core adaptation strategies. The systematic quantification of\nmaterial requirements, hydrodynamic constraints, and geomorphic thresholds—paired with the integration\nof global datasets like SWORD river networks and MERIT DEM—establishes a novel comparative\nframework. Key findings, such as the universality of at least one feasible strategy per delta, material\nlimitations for the “advance” strategy, and the role of delta size and urbanization in constraining PSS,\noffer actionable insights for prioritizing adaptation resources. Compared with the prior version, I do\nobserve specific improvements, including the revised calculation method for river widths of 369 deltas.\nWe thank reviewer 2 for reviewing our paper for the second time. We appreciate their recognition\nof the paper’s contribution to addressing a gap in the knowledge of the physical feasibility of adaptation\nglobally, particularly highlighting the value of our systematic approach and novel comparative\nframework. We are pleased that the improvements in the revised version were noted and believe we have\nnow, with our extra analysis, addressed their comments comprehensively.\nHowever, the major concerns I raised earlier remain unresolved, and new issues have emerged with\nunexplained updates of parameters and results in the current manuscript, which require further attention.\nFirst, the definitions of the five adaptation strategies still lack refinement to align with real-world\nimplementation. The distinction between “protect-open” and “protect-closed” remains overly narrow, as it\nfocuses primarily on river-mouth structures (storm surge barriers versus dams) while overlooking broader\ncomponents of practical flood defense systems. For example, “protect-closed” systems like the Dutch\nDelta Works typically integrate pumps with riverine levees to manage inland flooding, yet the current\nmodel excludes riverine levees from the protect-closed strategy—this omission likely underestimates total\nmaterial requirements for levee construction and overestimates required pump capacity, given that\nriverine levees reduce the volume of water needing discharge.\nRegarding the definitions of the five adaptation strategies, we follow the IPCC framework\n(Glavovic et al. 2022). The separation between protect-open and protect-closed follows (Haasnoot et al.\n2020; van Alphen, Haasnoot & Diermanse 2022) and is intentionally implemented uniformly across the\nwhole delta. The protect-closed strategy reflects a fully closed connection between the river and the sea\nwhere discharge is pumped out and the entire coastline is protected. We acknowledge that, in practice,\nhybrid strategies can be implemented but then the solution space is unconstrained, and this does not\nanswer our main research question about the physical limits of any individual strategy.\nWe acknowledge that excluding riverine levees for “protect-closed” can lead to large pump\ncapacity requirements. However, we determine the pump requirements using the mean annual river\ndischarge per delta to determine whether at least the mean annual discharge can be pumped out nearly\nevery day of the year (much less than extreme discharges such as the annual maximum or a 1-in-100 year\ndischarge). This is a conservative assumption and could underestimate the pump capacity requirements\nduring extreme flows.\nTo address the reviewer comment, we have now included a global assessment of flood retention\ncapacity in the protect-closed strategy and in the advance strategy. We now evaluate whether deltas have\nsufficient inland retention capacities to store excess water that cannot be pumped during high-flow\nperiods. This assessment calculates the excess water following a 99th percentile river discharge (typical\nyearly flood) which is a modelled value from the Water Balance Model reanalysis between 1980 and 2012\n(Cohen et al. 2013), and uses spati\\al data to determine whether this water can be stored in existing lakes\nor wetlands. This additional analysis makes the assessment of the protect-closed strategy more robust by\nevaluating both mean river conditions as well as floods. We have now added the results from this analysis\nto the manuscript, the relevant equations to the Supplementary Text SI3, and the respective physical\nindicator to the methods section (see below and also highlighted in the manuscript).\nText added to results (Line 123) “While current pumping capacities are sufficient to manage mean river\ndischarge for majority of global deltas (91%), these capacities are exceeded during high river flows,\nnecessitating the storage of excess discharge in designated retention areas or larger pumping capacities.\nUnder a 99th percentile river flow event (Cohen et al. 2013; Nanditha & Mishra 2022), protect-closed is\nphysically feasible for most deltas (75%) as the discharge can either be entirely pumped out based on\ncurrent pump capacity capabilities, or excess discharge can be stored in retention areas. Alternatively,\nexcess river water can be drained under gravity. However, future SLR may limit gravity drainage, which\nmay increase pump capacity requirements beyond what is current possible and further constrain the\nphysical feasibility of protect-closed in these deltas.”\nText added to Supplementary Text SI3 (Line 151): “The required water retention capacity to store\nexcess river discharge during a flood event (maximum river discharge), assuming a 3-day flood event is\ncalculated as described above (Eqn. 4). However, retention areas in a protect-closed strategy only include\npermanent water bodies (A ) and wetland areas (A ) extracted from the global land use\nret,waterbodies ret,wetlands\ndataset (Buchhorn et al. 2019).”\nConversely, a “true protect-open” strategy, common in practice (e.g., the Mississippi River’s IHNC Surge\nBarrier system), usually protects both coastlines and inland rivers with dikes while keeping the river\nmouth open, but the current model fails to explicitly incorporate river dikes into the protect-open strategy,\nweakening its alignment with real engineering solutions.\nWe believe there may be a misunderstanding. We explicitly include river dikes and coastal dikes\nin protect-open. We also assess the feasibility of storm surge barriers that remain open under normal\nconditions and close during high water levels. We define protect-open in Figure 1 and specify the physical\nindicators in Table 1, and more information can be found in the Supplementary Text SI1 and in the\nequation in the Supplementary Text SI3.\nFor the “advance” strategy, the assumption of 100% pumped drainage of river flows (due to fully blocked\noutlets) is overly restrictive; real-world land reclamation projects often use partial closures or engineered\nchannels to maintain natural river discharge, reducing pump demands to managing pluvial flooding rather\nthan total river flow. This unrealistic assumption artificially limits the feasibility of the advance strategy,\nand revising these definitions and their associated indicators (such as adjusting pump capacity thresholds\nfor advance to reflect partial drainage) is essential to improve the validity of the PSS assessment.\nWe believe this comment arises from a misunderstanding of our calculations. The advance\nstrategy (also following the IPCC definition in Oppenheimer et al. (2019), Bongarts Lebbe et al. (2021)\nand Glavovic et al. (2022)) is defined as a sea-level rise adaptation strategy where the coastline is\nextended seaward, which creates new (reclaimed) land that is fully protected from the sea by levees, but\nalso protects existing inland areas (otherwise it would not be considered a flood-protection strategy). We\nagree that the advantage of this strategy is that it can provide space for flood water retention areas through\na coastal lagoon, as seen in the Netherlands for future adaptation strategies (van Alphen et al. 2022). Since\nthe new coastline forms a closed system with the retention lagoon, river drainage through pumps remains\nnecessary, similar to the protect-closed strategy.\nThat said, we acknowledge that the main flood protection benefit (increased space for flood\nretention) of the advance strategy was not incorporated in the original manuscript, restricting its\nrepresentation of real-world examples of advance. We have now incorporated flood water retention areas\nadjacent to the advanced coastline, which stores excess water during high river discharge events when\npumping alone is insufficient, and thereby increases the number of deltas that can implement the strategy.\nWe believe this analysis represents this strategy in a comprehensive way and appreciate this\nreviewer’s comment to bring this to our attention. Text describing this update has been added to the\nMethods section (additional physical indicator), the respective equation has been added to the\nSupplementary Text SI3, and corresponding results have been adjusted accordingly.\nText added to Supplementary Text SI3 (Line 113): “The required water retention capacity to store\nexcess river discharge during a flood event (maximum river discharge), assuming a 3-day flood event, is\ncalculated using:\n𝑇𝑓𝑙𝑜𝑜𝑑\n𝑉 = ∫ (𝑄 −𝑃𝐶 ) 𝑑𝑡\n𝑟𝑖𝑣𝑒𝑟,𝑒𝑥𝑐𝑒𝑠𝑠 𝑟,𝑚𝑎𝑥 𝑚𝑒𝑎𝑛\n0\n𝑉 = 𝑑(𝐴 +𝐴 +𝐴 )\n𝑟𝑒𝑡,𝑛𝑒𝑒𝑑𝑒𝑑 𝑟𝑒𝑡,𝑎𝑑𝑣𝑎𝑛𝑐𝑒 𝑟𝑒𝑡,𝑤𝑎𝑡𝑒𝑟𝑏𝑜𝑑𝑖𝑒𝑠 𝑟𝑒𝑡,𝑤𝑒𝑡𝑙𝑎𝑛𝑑𝑠\nWhere river discharge during a flood event (Q ) is the 99th percentile of discharges, as above (Cohen et\nr,rmax\nal. 2013) and where T is 3 days. We assess the storage capacity of potential retention areas (V ),\nflood ret,needed\nby summing the area in the advanced delta (A ), calculated as a product of offshore distance\nret,advance\n(2000m) and coastline length (c)(Fig. SI3.1), and permanent water bodies (A ) and wetland areas\nret,waterbodies\n( A ) extracted from the global land use dataset (Buchhorn et al. 2019). Each retention area is\nret,wetlands\nassumed to have a water storage capacity of d=1m depth.”\nRegarding the additional comment on pluvial flooding: Although we recognize this is a risk in\nmany deltas and other landforms, the focus of our study is sea-level rise adaptation while accounting for\nriver discharges. We therefore do not consider adaptation measures to prevent fluvial flooding. We now\nwrite this more clearly in the methods section.\nText added to methods (Line 419) “We assess adaptation to sea-level rise (SLR) to reduce\ncoastal flood risks whilst accounting for river flooding, but excluding pluvial flooding.”\nSecond, the assumption of static river discharge (outlined in Suppl. Text SI3 of the current manuscript)\nremains problematic. Climate change is projected to alter precipitation and runoff patterns. For regionally\nvulnerable deltas—such as monsoonal deltas like the Ganges-Brahmaputra-Meghna—discharge\nvariability could be much higher, directly impacting pump capacity requirements for protect-closed and\nadvance strategies (higher discharge increases the need for larger pumps) and sediment supply for the\nadvance strategy (altered discharge changes sediment transport and retention rates). The lack of\nsensitivity analyses incorporating projected discharge variability undermines the robustness of physical\nfeasibility assessments; a stratified sensitivity analysis, for instance by delta climate zones (tropical,\ntemperate, arid) using region-specific discharge projections, would better capture these uncertainties and\nstrengthen the findings.\nDuring the first round of reviews, we addressed reviewer 1 and 2’s concerns about river discharge\nvariability by conducting a sensitivity analysis that includes future changes to river and sediment\ndischarge, as outlined in Supplementary Text SI3 and SI6.\nIn this analysis, we use projections of future discharge (Van Vliet et al. 2013; Moragoda & Cohen\n2020). Specifically, we considered scenarios with river and sediment discharge increasing by up to 65%\nor decreasing by 23%, to consider the full range of variability from both basin and global scale\nprojections. The results of this analysis showed that the PSS of only a few of the global deltas (27-66\ndeltas) had expanded or contracted in response to altered river or sediment discharge values. Although\nthis change does not substantially alter the overall global assessment of the physical solution space, it has\nimportant implications for individual deltas.\nTo better reflect this, in the Supplementary Text SI6, we state how many deltas have contracted or\nexpanded PSSs following the projected changes in river and sediment discharge, respectively. Moreover,\nwe have now added, in the main text (results section “physical characteristics and adaptation thresholds\nversus climate change”), a small discussion on this projected change and regions that are expected to\nexperience increased or decreased discharges.\nText edited and examples added to Supplementary Text SI6 (Line 352): “Under increased river\ndischarge projections, protect-closed becomes physically constrained in 66 deltas that could previously\nadopt it under static conditions. Increased future river discharges raise pump capacity requirements that\nexceed current capabilities, and expand the required retention areas for excess discharge storage, thereby\nconstraining the PSS of these deltas. Examples include the Incomati (Mozambique), Wami (Tanzania) and\nNoatak (Alaska) deltas which are in regions (East Africa and the Arctic, respectively) that are projected to\nincrease according to previous studies (Van Vliet et al. 2013; Moragoda & Cohen 2020). Under these\nfuture conditions, innovation, hybrid or alternative adaptation strategies may be required to adapt to future\nflood risks. Under combined increased river and sediment discharge projections, the number of deltas that\ncan adopt advance decreases by 51 deltas compared to no sediment or river discharge change. Despite\nhigher sediment loads to aggrade the coastlines, increased river discharges constrain the pump capacity\ncapabilities and decrease the overall physical feasibility of this strategy. Examples include the Jiulong\nJiang (China) and Labuk (Malaysia) deltas, which are both located in tropical (monsoon) regions where\nriver and sediment discharges are projected to increase (Van Vliet et al. 2013; Moragoda & Cohen 2020).\nUnder decreased river discharge projections, we find that 40 additional deltas can adopt a protect-closed\nstrategy given lower pump capacity requirements and lower discharges during extreme events which\ndecreases the amount of space required to retain excess river water. For example, in regions where river\ndischarges are projected to decrease, such as Southern Europe and North America (Van Vliet et al. 2013;\nMoragoda & Cohen 2020), the PSS (for protect-closed) is projected to expand, as found in the Ebro\n(Spain) and the Altamha (USA) deltas, respectively. Under both decreased river and sediment discharge\nprojections, the advance strategy becomes physically feasible for an additional 27 deltas, primarily due to\nreduced pump capacity requirements. In these deltas, the pump capacity plays a larger role in determining\nphysical feasibility of advance than availability of river sediment to aggrade the coast, given that\nalternative material sources such as offshore sand remain accessible despite decreased sediment\ndischarges. Examples include the Altamha (USA) and Kikori (Papua New Guinea) deltas, where river and\nsediment discharges are projected to decrease, consistent with previous studies reporting these trends for\nNorth America and Australia, respectively (Van Vliet et al. 2013; Moragoda & Cohen 2020). While\nprojected increases or decreases of these parameters impacts several individual deltas, which has\nimplications for local scale decision-making, the general adaptation trends remain mostly consistent\nacross the global scale.”\nText added to manuscript results (Line 286) “Moreover, climate change is expected to alter future river\nand sediment discharge (Van Vliet et al. 2013; Dunn et al. 2019; Moragoda & Cohen 2020), which may\nimpact pump capacity and water retention requirements (protect-closed and advance) and sediment\navailability (advance). Our sensitivity analysis reveals that changes in these parameters, based on\nliterature (Van Vliet et al. 2013; Dunn et al. 2019; Moragoda & Cohen 2020), impact the PSS of several\ndeltas which influences local decision-making. Increased river discharge values typically constrain the\nPSS where protect-closed and advance become physically unfeasible without innovation, for example in\ntropical (monsoon) regions(Van Vliet et al. 2013). Conversely, decreased discharge projections typically\nexpand the PSS, making both advance and protect-closed physically feasible with current technological\ncapabilities and resources, for example, in North America (Van Vliet et al. 2013; Moragoda & Cohen\n2020) (see Methods and Supplementary Text SI6.1 for results from sensitivity analysis and examples of\ndeltas).”\nThird, Figure 2a in the current manuscript suffers from poor visual discriminability, with thin lines and\noverlapping colors making trends difficult to decipher. A critical discrepancy further emerges between the\nfigure and the text: the retreat strategy (represented by the outermost line) appears to be 100% feasible\nacross all deltas, yet the text notes that 2% of deltas require innovation to implement retreat (e.g.,\nrelocations outside the delta). This inconsistency points to flawed graphical representation, and revisions\nare necessary.\nThank you for your suggestion, we have improved visibility further by grouping deltas with\nsimilar physically feasible strategies together. Figure 2 has now been updated. Please see below the\nrevised figure (left) compared to the previous figure (right).\nThe retreat strategy appears 100% feasible for the darkest (innovation) threshold because it is. In\nour results, we state that 2% of deltas require innovation for retreat because this strategy is not feasible\nunder low-resource and current known thresholds, due to a lack of space to retreat to. This can be seen\ntowards the bottom of the figure where the darker shade of red is highlighted (innovation is required),\nbut\nthe lighter shades of red are not coloured in and the row transitions straight to green (Protect-open).\nSome\nexamples of deltas with this PSS have been highlighted in the figure.\nFourth, several key parameters and results have changed between the prior version and the current\nmanuscript, with no explicit justification provided. Most notably, for the “advance” and “protect-closed”\nstrategies, Threshold 3 (Innovative option) for pump capacity has been reduced from 12,000 m³/s (prior\nversion) to 2,400 m³/s (current version)—an 80% reduction that directly alters the feasibility of these\nstrategies. Yet, no rationale for this adjustment—such as literature citations of maximum operational\npump capacities, technical limits of current pumping technology, or expert consensus—is provided in the\nmain text, Supplementary Text SI4 (intended to support threshold selections with a database of existing\nmeasures), or associated datasets.\nDuring the first round of feedback, we changed the thresholds for low-resource and innovative\nconditions following a suggestion from reviewer 1. We do include literature citations of maximum\noperational pump capacities and technical limits of current pumping technology to support the use of the\ncurrent known thresholds on which the innovative threshold is based (see Supplementary Table SI4.1 and\nTableSI4.2 which supports the selection of our thresholds).\nIn response to reviewer 2’s comment, we have now expanded the literature overview with\nadditional sources on innovation over the last 100 years. We included years of construction and socio-\neconomic status of the countries, which gives us support for the pace of innovation (using past\nextrapolations) as well as low-resource options (using differences per economic status). In this analysis,\nwe found that some technological capabilities, such as material extracted from dredging or building\nheights have increased by more than 10-fold in the last 100 years. However, other technologies have\nchanged less (storm surge barriers, levee heights), by approximately 2-fold. A such, we believe a 2-fold\nincrease is a realistic pace of innovation to include, albeit conservative for certain measures. We have now\nexpanded on this in the Supplementary Text SI4.\nText added to Supplementary Text SI4 (Line 286): “Other examples of technological developments\nover longer timescales include innovation in infrastructural capabilities, which has led to an increase of\nover 10-fold in building height between 1995 and 2008 (Kayvani no date). Similarly, the volume of\nmaterial extracted from dredging has increased by nearly 10-fold from 1925 to 2015 (Cooper et al. 2018).\nThis reflects the substantial scale of technological innovations in the last century. However, the scales of\nother technologies, such as levee heights or storm surge barrier widths, have changed less over the last\n100 years. As such, assuming a 2-fold increase in technological capabilities by 2100 is a realistic pace of\ninnovation, albeit conservative for certain measures.”\nFinally, we still include an assessment of the previous 10-fold increase of thresholds and elaborate on its\ninfluence on the PSS in the Supplementary Text SI6.1 by stating: “Finally, we explore the influence of the\nchosen innovative threshold on the PSS. We increase the threshold by an order of magnitude, as opposed\nto a twofold increase, and find that substantial innovation in technological capabilities does not\nnecessarily imply more strategies are physically feasible. While certain adaptation measures, such as 10m\nstilts following an accommodate strategy, or 12,000m3.s-1 pumps following a protect-closed strategy,\nincrease the PSS for some deltas, the PSS of other deltas remain unchanged due to fundamental physical\ncharacteristics. This highlights that innovation alone does not provide more adaptation opportunities.\nGiven these findings, we maintain the assumption of a twofold increase in innovation capabilities for our\nanalysis since it is more realistic by 2100, and avoids overestimating adaptation opportunities.”\nSimilarly, the percentage of deltas with all five strategies feasible in low-resource form under the SSP2-\n4.5 scenario has increased from 12% (prior version) to 38% (current version), with no explanation for this\nlarge discrepancy (e.g., whether it stems from revised data inputs, adjusted thresholds, or model\nimprovements)—this lack of transparency erodes confidence in the result’s reliability. The authors must\nexplicitly document the rationale for all parameter changes and cross-validate new thresholds with\nexisting engineering practices or literature.\nWe apologize for the confusion. This change in percentage stems from the adjusted low-resource\nand innovation thresholds as described in the paragraphs above. We collected evidence on the current\nknown capabilities of adaptation measures which we used as the “current known” thresholds and created\na low-resource version based on this.\nWe have now added an analysis to assess the current known capabilities in countries with\ndifferent income levels (see Supplementary Text SI4). We find that data on land reclamations (Table\nSI4.1) reveal that higher income counties typically fill the same amount of space within a shorter time\nframe than lower-income counterparts. Moreover, the scale and height of home raising efforts is\nsubstantially more in higher income countries than lower income counterparts. Here, we assume that\nsome (lower income) countries would have lower resources than other (higher income) countries, and find\nthat the largest known scales of measures are in higher income countries (on which we base our “current\nknown” thresholds). As such, we believe that the 2-fold difference in thresholds between low-resource\nand current-known thresholds are conservative compared to the previous manuscript, but more accurate\nand validated against exiting practices. We have now added text to support this threshold selection (See\nSupplementary Text SI4).\nText added to Supplementary Text SI4 (Line 264): “The “low-resource” threshold is defined as half the\nvalue of the “current known” threshold (see Table 2 in manuscript). We use the relation between income\nlevel and adaptation capacity to support this threshold selection and apply all thresholds globally. We\nassume that some (lower income) countries would have lower resources than other (higher income)\ncountries, and find that the largest known scales of measures are in higher income countries (on which we\nbase our “current known” thresholds). Specifically, data from Table SI4.1 shows that home raising and\nland reclamation efforts in lower-income countries are generally smaller (number of homes and height\nraised) and less frequent (smaller areas of land over prolonged periods of time) than in higher-income\ncountries by approximately a factor of 2. As such, we believe that the 2-fold difference in thresholds\nbetween the low-resource and current-known thresholds are conservative, but validated against existing\nexamples.”\nIn summary, the study addresses an important and timely topic, but unresolved issues with strategy\ndefinitions, static discharge assumptions, and methodological transparency—paired with new concerns\nabout unjustified parameter changes—limit its current impact. Addressing these comments will\nsubstantially improve the manuscript’s scientific rigor and alignment with real-world adaptation needs,\nensuring its findings are both credible and actionable.\nThank you for all suggestions. We find that the additional analysis, evidence for thresholds\nselected, and explanations in our manuscript has much improved.\nReviewer #3 (Comments for the Author):\nThis paper sets out a novel global assessment of the physical solution space (PSS) for adaptation of about\n800 river deltas to sea level rise (SLR). It triangulates three sources of evidence: physical characteristics\nof deltas; SLR projections to 2100 (3 SSP scenarios); and three 'scales of adaptation' (assumptions about\ntechnical feasibility of adaptation options). It investigates 5 SLR adaptation strategies, including retreat.\nThe results show that '...nearly every delta has at least one physically feasible adaptation strategy'. That\nseems to be an important result in an interesting and well-constructed paper. The point that this is a first-\nstage 'physical/engineering' assessment is clearly made and invites additional studies to understand the\nnature of financial, nature, political and cultural constraints of the proposed measures.\nWe thank reviewer 3 for their positive feedback regarding the structure and main messages of this paper.\nThree main comments: 1. The paper presents results for SSP2-4.5 only, arguing that SLR variance is less\nimportant than physical characteristics (depth) of deltas to adaptation option feasibility. It would be good\nto have this more explicitly discussed in the paper, not just in supplementary material (it is an unexpected\nresult).\nIn the results section of our manuscript, we have two sections that elaborate on this. The first\nsection discusses how the adaptation thresholds play a stronger role in determining physical feasibility\nthan SLR. The second section is titled “physical characteristics versus climate change” and discusses how\nthe PSS is more sensitive to delta’s physical characteristics (for example, depth) than the projected SLR\nunder different scenarios. We have now merged these paragraphs into a section titled “Physical\ncharacteristics and adaptation thresholds versus climate change” to avoid confusion. In this section, we\nhave now elaborated on the sensitivity of the PSS constraints based on the delta characteristics and\nmeasure thresholds (the scale of adaptation) compared to climate change (SLR).\nText added to results section (Line 251): “We find that higher sea levels increase flood risks and shrink\nthe PSS, in agreement with previous studies (Oppenheimer et al. 2019; Haasnoot, Lawrence & Magnan\n2021; IPCC 2022). However, this change is relatively small compared to our modelled differences in\nphysical feasibility based on the scale of adaptation measures (i.e. thresholds). We find that the scale of\nthe adaptation measures has a greater influence on the physical feasibility of adaptation strategies than the\nSLR projections for 2100 (see Supplementary Text SI5 and Fig SI5.1 for a comparison between climate\nscenarios and thresholds). For instance, the scale of raising infrastructure following accommodate,\nnamely by 0.5m, 1m or 2m, exceeds the median projected global mean sea-level rise for 2100 under\nSSP1-2.6, SSP2-4.5 and SSP5-8.5 scenarios (0.4m, 0.47m, 0.63m, respectively)(Church et al. 2013).\nMoreover, the feasibility of offshore dredging to collect sand at depths of 15m, 30m, or 60m is more\nconstrained by technological capabilities to reach the sand than by a 0.5m sea-level rise.\nOur assessment further indicates that the current physical characteristics of the deltas (e.g. size, elevation,\nriver discharge as used in this study) are a stronger determinant of the physical feasibility of strategies by\n2100 than the projected SLR. For example, the physical feasibility of flood risk protection measures, such\nas storm surge barriers, depends more on the river width which varies from narrow (100m) to wide\n(10,000 m) across deltas, than on SLR following future climate scenarios by 2100. Thus, in deltas with\nwide rivers, protect-open is more physically constrained by the delta characteristics than SLR. Building\non this understanding …[rest of paragraph].\nUnder higher amounts of SLR beyond the likely range for this century or after 2100, SLR is expected to\nhave a larger influence on the PSS (see Supplementary Text SI5 and Fig SI5.2 for sensitivity analysis with\nhigher SLR). Moreover, climate change is expected to alter future river and sediment discharge (Van Vliet\net al. 2013; Dunn et al. 2019; Moragoda & Cohen 2020), which may impact pump capacity and water\nretention requirements (protect-closed and advance) and sediment availability (advance). … [rest of\nparagraph].”\nWe have now performed an additional sensitivity analysis to support the claim above that larger SLR will\ninfluence the PSS. We test how even greater SLR values (2m and 4m) impact the PSS of global deltas\n(see methods and findings in Supplementary Text SI5 and Fig. SI5.2).\nText added to methods section (Line 460) “We further test how higher amounts of future SLR (2m and\n4m) influences the PSS. We find that SLR beyond the likely range for this century contracts the PSS of\nmany deltas, specifically decreasing the physical feasibility of the advance, accommodate and retreat\nstrategies (see Supplementary Text SI5 for further explanation and visualisation of this contraction).”\nText added to Supplementary Text SI5 (Line 323): “In the future, high-end estimates of SLR find that\nsea-levels may rise to between 0.9m (by 2100) and 2.5m (by 2300) following an SSP1-2.6 scenario or\nbetween 1.6m (by 2100) and 10.4m (by 2300) following an SSP5-8.5 scenario (van de Wal et al. 2022).\nOur sensitivity analysis reveals that 2m and 4m of SLR constrain the PSS for many deltas, specifically for\nthe advance, retreat and accommodate strategies (Fig. SI5.2). Higher sea levels means larger offshore\ndepths for dredging and land aggrading (advance), less available non-flooded land (retreat), and greater\ninfrastructure elevation requirements following larger flood depths (accommodate) which constrains the\nphysical feasibility of implementing these strategies.”\n2. The 'physical indicator thresholds' relate to soft adaptation limits and it would be useful if the values\ngiven in Table 2 could be justified.\nWe agree that it is useful to highlight the sources in the manuscript instead of only in the\nsupplementary text. Thank you for the suggestion. The thresholds have now been justified with a “source”\ncolumn in Table 2 in the methods section of the manuscript.\n3. Figure 2 shows that protect-open (levees and storm-surge barriers) is an option that is available more or\nless everywhere (97% of deltas). This also seems a surprising result, and it may be useful to provide some\nidea of the cost and time required to put these options in place. This is clearly not an economic feasibility\nstudy, but as the paper acknowledges, the finance is likely to be a constraint in many cases and an\nindication of the scale of this would be useful.\nIndeed, while protect-open may be physically feasible, it is a costly strategy. We have now added\nmore on this to the discussion section.\nText added to discussion (Line 370): “For example, while protect-open is physically feasible across\nmajority of deltas to protect against coastal flooding (Fig. 2), it is a costly strategy in terms of both time\nand money. Given that economic capabilities differ between countries, those with a high gross domestic\nproduct (GDP) typically have high financial capacities to invest in risk-reducing technologies (Tessler et\nal. 2015) For example, the Netherlands, which adopts a partly protect-open strategy (with storm surge\nbarriers in the Eastern and Western Scheldt) manages the Delta Fund for climate adaptation, with an\nannual budget of approximately €1.25 billion. Over half of this budget is allocated for investing in new\nadaptation measures, while the remainder covers measure maintenance (Deltacommissaris 2021).\nHowever, countries with a lower economic capacity may be limited to low-capital strategies, highlighting\nhow economic barriers shape feasibility beyond the physical dimension.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "No further comments from my side, in my opinion the paper can be published as soon as the other referees agree", "reviewer_2": "This is the third time that I have the opportunity to review the paper, and I am glad to see that my previous comments have been well addressed. My only remaining recommendation is to remove the current Fig. 2b. While I acknowledge that this figure has been improved compared to the previous version, its readability remains insufficient. Moreover, it offers very little new information beyond what is already clearly presented in the well-structured Fig. 2a.", "reviewer_3": "This revision has taken seriously the comments and criticisms made by the three reviewers of the earlier draft and been substantially strengthened. This includes some reanalysis as well as clarification, with new sections added to the main text.\nI continue to believe this is an innovative paper that makes an important contribution to the debate about adaptation to climate change and SLR. The paper invites a further economic and ecological analysis of the consequences of the adaptation strategies tested in this technical analysis."}, "author_response": "Rebuttal: Physical limits of sea-level rise adaptation in global river deltas\nKiara G. Lasch, Jaap H. Nienhuis, Gundula Winter, Marjolijn Haasnoot\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nNo further comments from my side, in my opinion the paper can be published as soon as the other\nreferees agree.\nWe thank Reviewer 1 for their approval of our manuscript.\nReviewer #2 (Remarks to the Author):\nThis is the third time that I have the opportunity to review the paper, and I am glad to see that my\nprevious comments have been well addressed. My only remaining recommendation is to remove the\ncurrent Fig. 2b. While I acknowledge that this figure has been improved compared to the previous\nversion, its readability remains insufficient. Moreover, it offers very little new information beyond what is\nalready clearly presented in the well-structured Fig. 2a.\nWe are pleased to see that Reviewer 2 believes their comments have been well addressed, and we\nthank them for their time and feedback.\nRegarding Fig. 2b, we believe that this stacked histogram compliments Fig. 2a by showing the full\ndistribution of physical feasible strategies across deltas. This demonstrates the spread of feasible\nstrategies, showing similarities in the types and number of strategies across deltas which is not\ndistinguishable in Fig. 2a. As such, we opt to keep this sub-figure in our manuscript.\nReviewer #3 (Remarks to the Author):\nThis revision has taken seriously the comments and criticisms made by the three reviewers of the earlier\ndraft and been substantially strengthened. This includes some reanalysis as well as clarification, with new\nsections added to the main text.\nI continue to believe this is an innovative paper that makes an important contribution to the debate about\nadaptation to climate change and SLR. The paper invites a further economic and ecological analysis of\nthe consequences of the adaptation strategies tested in this technical analysis.\nWe are pleased to see Reviewer 3 is satisfied with the changes made. We thank them for their\nvaluable suggestions."}]

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