This paper reviews impacts and consequences of sea-level rise (SLR) in Europe. This is not an easy task given the heterogeneity of the literature on this topic, so the authors should be congratulated for their effort. To do so, the authors have adopted a focus on the physical processes causing flooding erosion and salinization, rather than e.g. a focus on different types of coastal areas in Europe. The advantage is that it gives a useful reminder of the physical mechanism, the drawback is that it takes always some time before each section start addressing issues specific to Europe.

Besides this focus on impacts, some information is provided on consequences, e.g. on erosion based on previous publications, but overall the information on consequences is limited. To me this is not a surprise given the heterogeneity of information available on the consequences of SLR in Europe and the uncertainties, but may be it would be useful that the authors clearly say that we are lacking aggregated observations of consequences and – if they agree with me – the information on projections based on modelling are often difficult to assess and compare due to model assumptions and limitations.

The terminology used in the paper is clear overall, except sometimes when it comes to the terminology on hazards and adverse events (see ref below to the UNISDR and IPCC terminology). This should be clarified.

The main problem of the paper, from my perspective is a lack of clarity on its objectives : who is the target audience? What will they learn in this article? This is not quite clear in the current version and could be precised at the end of the introduction and reflected in the conclusion.

Below are specific comments

Specific comments (moderate)

1. Introduction

The introduction is interesting, but it would benefit from a bit of reorganisation and update. For example, the statements on exposure increase rely on relatively old references published in 2003. Meanwhile, as recognized by the authors line 65, setback zones have been setup, and this may have curbed trends in exposure, at least in some contexts. Furthermore, it would be good to remind that Europe has a long history of coastal protection compared to many other regions in the world already at this stage of the manuscript. Finally it is not clear whether the whole introduction is focused on present days, or both present days and the future. In the first case, it would be good to cover the topic of attribution of coastal impacts to sea-level rise. In the second case it would be good to separate clearly what applies to today, and what is projected to 2050, 2100 or beyond. For example migrations patterns could be different today and in 2100 when sea-level rise becomes a major driver of coastal migration. 

One way to clarify this could be to reorganize the introduction in a way that explains more clearly what is known with “high confidence” (e.g., flood risk will increase) from what is uncertain (e.g. the precise magnitude of flood or erosion impacts) or almost unknown (e.g., future migration responses to SLR). Another option would be to remind the target audience of the paper, their potential challenges and how this paper is going to help them. There may be other options. 

2. Summary of previous assessments

The review makes sense overall, and it is good to remind where coastal impacts can be found in AR6, but it is unclear to me how useful it is for a review focused on Europe: in practice, the authors could well say in the introduction that there has been previous assessments on climate change in Europe (IPCC), on specific coastal hazards (Eurosion…), on some relevant regions (Medecc), but that the Knowledge Hub is the first attempt to do what it does. So in summary I am not sure such a long summary is needed, may be a few sentences in the introduction would be sufficient.

Besides, there are several points that could require some precisions (see below, minor comments)

3. Source, Pathways, Receptors and consequences framework

The Source, Pathways, Receptors and Consequences framework section is very short. I wonder if it could be included in the introduction as this is framing the rest of the manuscript.

4. Coastal flooding and compounding flood events

This section has a focus on physical processes causing flooding. It is good to have a section on compound flooding, but it may be not clear to readers how increased precipitations due to climate change (higher atmospheric moisture due to increased temperatures) have been considered in the references cited. The discussion on pathways highlights the need to maintain defenses in order to maintain security levels, which is good. However, it does not mention the risk of coastal defenses being raised at the costs of coastal biodiversity losses (see e.g. Bednar-Friedl et al., 2022 p 1843). Another point which is missing is the projected change in terms of flooding modes: from overtopping to overflow, as shown in IPCC AR6 WGII CCP4 page 2245. This risk is important as areas currently exposed only to overtopping may not be prepared to future overflow and the associated risks to human life and larger economic damages. These aspects should at least be mentioned for completeness.

5. Coastal erosion

It is good (and not a surprise given the list of authors!) that the section identifies the topic of erosion/flooding interactions. There is also a summary of observed and projected erosion, which can be useful. However, it should be precised that these figures assume no geological constraints (to be confirmed by the authors – see Thiéblemont et al 2019 for this caveat) and no protection or nourishment.

This section is structured in a different way than the other ones on coastal flooding and salinization, which is not necessarily a problem, but raises the following questions: (1) why a section on monitoring for erosion and not flooding and salinizations? Why not an analysis on historical and future flooding and/or salinization?

6. Saltwater intrusion

I am less qualified on this topic and may not cover all subsections adequately. In general the section recognizes well the importance of aquifer recharge and water usages in driving current salinization in aquifers, but since the topic of this paper is on sea-level rise, a specific discussion on the impacts of sea-level rise or the vulnerability of different types of European aquifers could be useful. The Jouzel report in 2015 (in French) could provide some useful examples: https://www.ecologie.gouv.fr/sites/default/files/ONERC_Climat_France_XXI_Volume_5_VF_revisee_27fevrier2015.pdf  

Figure 12 seems good to me, but it is not well exploited in the text. Furthermore it would be useful to say clearly that the known case study are not exhaustive - e.g. salinization in the Loire estuary has been reported during the 2022 drought with concerns to drinking water supply, and is not reported here.  

Section 6.2 is clear, but I do not understand the focus on river salinization – the statements on reduced recharge apply to aquifers as well, do not they? In section 6.4, I am not sure that all messages apply to Europe (see detailed comments and the comment on sand mining in river beds). Finally may be one study to consider:  https://doi.org/10.1029/2023EF003581

7. Ecosystems and estuaries

The good point in this section is the reminder that coastal protection is often implemented at the cost of coastal ecosystem losses, as already reminded by the IPCC WGII report. That said, this section does not fit very well in the structure of this manuscript: the title suggests it may either focus on a particular type of receptor (ecosystems), or a particular type of geomorphological features (estuaries). In both cases it is not clear why this receptor or type of environment is not addressed in the previous sections. Besides, if a focus on ecosystems could be useful if it could give insight on how healthy ecosystems can reduce the hazards above, but this is not addressed here. I suggest reconsidering this section.

8. Conclusion

The conclusion could include a summary of gaps of knowledge / recommendations for future assessments.

Minor comments

- line 41: suggest: these threats "can be" reinforced by subsidence caused by human activities

- line 45: consider replacing "losses" by "risks" 

- line 47: Vousdoukas et al 2018b focus on extreme water levels. Therefore the statement on trends on exposure can not come from this paper. Suggest to either refer here to papers assessing impacts for various SSPs or to delete "with the relative importance of trends in exposure (related to coastward migration, urbanization and rising asset values) diminishing over time", as the next sentence is about the same topic. 

- line 49: The discussion on exposure in the introduction relies on papers published a long time ago (20 years). Meanwhile, setback zones have been defined in many countries to reduce exposure (e.g. Croatia, France...). This may have had an impact on exposure trends or not depending on the context. It would be good if the authors could either find a paper addressing this issue, or identify a gap of knowledge. 

- line 98 and earlier: the SYR builds on all AR6 reports, not only SROCC as suggested.

- line 116: please note this is expected annual damage, not annual damages. Furthermore these are projections based on flood models with coarse resolution and strong assumptions must be made on protection and exposure. Therefore these figures are indicative.

- Line 125: in Mediterranean ports the problem is also that waves are projected to propagate within ports with shallow bathymetry (see page 2244 in IPCC AR6 CCP4)

Line 134: It is true that one of the four key risks of Chapter 13 is the impacts of heat to ecosystems and that the marine heatwaves are causing massive mortalities, especially in the Mediterranean, as reported in CCP4, so the statement is certainly valid for coastal aquatic ecosystems. Yet in practice for coastal ecosystems another risk that is mentioned several time in the report is a massive use of hard engineering protection in response to coastal hazards and SLR, and therefore reducing habitats of coastal ecosystems (see page 1843 in Chapter 13). Coastal squeeze, not necessarily related to SLR, is also a major threat. To contextualize, references to IPBES here would be useful to remind the 5 major causes of ecosystem losses, which all apply to coastal ecosystems. 

Line 144: Hazard is a potential occurrence of an adverse event (see https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Annex-II.pdf) here the source describes the origin of the event (not the hazard)

Line 145: the “entity” could be named exposed element https://www.preventionweb.net/files/7817_UNISDRTerminologyEnglish.pdf

Line 168 – Fig 3 – the figures seems to assume overtopping only but there are other modes of flooding to consider (overflow, breaching)

Line 171 – “governing hazards” –confusion between adverse events and hazards (see above comment line 144)?

Line 209 – the fact that lower tidal range mean higher sensitivity to SLR is clear but why this precise threshold of 2m here?

Line 337 – some figures are available on www.eurosion.org (2004)

Line 342 – reference needed.

Line 463 – the sentence is not clear to me (?)

Line 498 – Figure 7 – on this figure it is not very clear what the pathway is.

Line 534 – “Earth Observation” may refer to satellite and in-situ monitoring – suggest to change to Earth observation from space or similar.

Line 545 and around: important to mention that erosion in www.eurosion.org can be assigned to coastal cliffs retreating at about 20cm a year in average, well above the detection threshold of Luijendijk et al 2018 – therefore the two datasets can not be compared.

Line 554 – Figure 8 –this type of representation has the problem that points are superimposed, so it is very difficult to visualize anything. Furthermore the precision of the dataset is low (0.5m/yr) – in some areas e.g. upper Normandy – rates of 0.2m/year are causing trouble for land use planning and human security. One way to simplify the figure and give justice to the dataset would be to plot only hotspots, that is areas with large erosion rates (e.g. larger than 1m/year)

Line 569 – I think this assumes no geological constraint – see Thieblemont et al 2019

Line 574 – this is without protection and without consideration of geological constraints (unerodible layers beneath the beach) – TBC

Line 632 – Figure 10 – suggest to add arrows to define precisely where is the source, the pathway and the receptor in this figure – note also that this assumes a porous, homogeneous and unconfined aquifer (e.g. not the case in many aquifers in southern france for example). Figure 11 shows some more complexity.

Line 758 – The study is in Vietnam. In Europe river bed sand mining is quite strictly regulated by e.g. the water framework directive, isn’t it? Is there any message applicable to Europe here?

Line 770 – Figure 10 and Figure 14 are quite similar – consider merging

Line 776 – it would be good to cite work in Europe as it exists (see references in IPCC reports, e.g. Cooper et al., 2016…)

I hope this review is useful

Gonéri Le Cozannet, BRGM, 29/01/2024

Bednar-Friedl, B., R. Biesbroek, D.N. Schmidt, P. Alexander, K.Y. Børsheim, J. Carnicer, E. Georgopoulou, M. Haasnoot, G. Le Cozannet, P. Lionello, O. Lipka, C. Möllmann, V. Muccione, T. Mustonen, D. Piepenburg, and L.Whitmarsh, 2022: Europe. In: Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 1817–1927, doi:10.1017/9781009325844.015.

Reimann L, Vafeidis AT, Honsel LE. Population development as a driver of coastal risk: Current trends and future pathways  ridge Prisms: Coastal Futures. 2023;1:e14. doi:10.1017/cft.2023.3

Thiéblemont, R., Le Cozannet, G., Toimil, A., Meyssignac, B. and Losada, I.J., 2019. Likely and high-end impacts of regional sea-level rise on the shoreline change of European sandy coasts under a high greenhouse gas emissions scenario. Water, 11(12), p.2607.

The paper “Sea level Rise in Europe: impacts and consequences” aims at providing an overview of the main type of impacts of SLR on the coast at the European scale. Three main processes are considered, i.e. flooding, erosion and saltwater intrusion, all having widespread impacts at the coast, also depending on the local setting. So it is not easy to have an overview at the continental scale. Moreover, the three processes are frequently overlapping, and this is addressed in the paper (such as in section 5.6). However, some repetition among chapters is present (see comments below) and might be avoided.

I indicate below, for each section, some comments. Additional minor suggestions are listed at the end of this document. Overall, I suggest minor revision.

Specific comments

The paper frequently refers to IPCC report and previous literature; in some cases, I found reported data not suitable to the case. This is the case of section 2, lines 129-130: “(…) it is suggested that by 2100 coastal retreat could reach approximately 100 m for a 4°C temperature increase”. This overall estimate is not appropriate, since coastal retreat largely depends on coastal slope and beach morphology (without considering sediment supply but “pure” SLR) so it should not be indicated as a constant value for any degree of global warming.

Again in section 2, at line 131 the possible threat to UNESCO World Heritage Sites is mentioned but no references are provided. A couple of papers dealing with WHS at risk for coastal flooding and SLR are, in case:

Reimann et al., 2018. Mediterranean UNESCO World Heritage at risk from coastal flooding and erosion due to sea-level rise. Nat Commun 9, 4161. https://doi.org/10.1038/s41467-018-06645-9

Romagnoli et al. 2022. Coastal Erosion and Flooding Threaten Low-Lying Coastal Tracts at Lipari (Aeolian Islands, Italy). Remote Sens.14(13), 2960; https://doi.org/10.3390/rs14132960

In section 3 the common approach applied for the analysis, namely the SPRC framework, is introduced. I expected a longer section than the present one (that is just 7 rows), citing other examples of coastal studies where this approach has been successfully applied, such as (for instance):

Villatoro et al. 2014. An approach to assess flooding and erosion risk for open beaches in a changing climate, Coastal Engineering, 87, 50-76, https://doi.org/10.1016/j.coastaleng.2013.11.009.

Also, the citation of “traditional exposure vulnerability approach” might be supported by some references.

In Figure 2, the sketch of should also report the term “Consequences” to be complete, otherwise delete it from the caption (as it is in figures 3, 7, 10).

Section 4 on coastal flooding is probably the more complete. I give below just some comments/suggestion.

In “4.1 Source”, among interannual SL variability (lines 185-187) it might be also cited the role of oceanographic processes at the basin/sub-basin scale. For the Mediterranean, these are recent references:

Menna et al., 2022. Climatic, Decadal and Interannual variability in the Upper Layer of the Mediterranean Sea Using Remotely Sensed and In-Situ Data. Remote Sens. 2022, 14(6), 1322; https://doi.org/10.3390/rs14061322.

Meli, M. et al. 2023. Sea-level trend variability in the Mediterranean during the 1993–2019 period. Frontiers in Marine Science, 10:1150488. https://doi.org/10.3389/fmars.2023.1150488

Some estimates referred to the text are not easily checked from figures. For instance:

-at lines 288-290 it is mentioned “the probability of compound flooding (….) is projected to robustly increase by the end of the 21^th century”. Based on figure 4b I understand that this larger probability increase is represented by D T of 40-80 % (orange to red colors), is it correct? It might be useful to quantify also in the text to help readers.

In Subsection 4.4, at lines 383-385 some estimates are given in the text for the increase in flooded area due to events with different recurrence times. I found not easy to compare these values with figure 6.

Subsection 4.5 on “initiative to develop flood-related climate services in Europe” is useful. Dealing with Adaptation, possible overlaps with related chapter of the Assessment Report might be checked and, in case, cross-reference is suggested.

Section 5 on coastal erosion from my point of view needs some clarification and possible reorganization due to some repetition.

The section mostly deals with processes affecting low-lying costs, i.e. the most threatened by SLR. A cliffed coast is instead represented in figure 7 (although erosion processes and impacts of SLR on this type of coast are not really accounted in the paper). I noted this discrepancy also in the Abstract (lines 28-29) where cliff failure is mentioned for coastal erosion - I suggest to simplify that sentence in "Coastal erosion leads also to damage and....(…)” and avoid mentioning cliffs.

Also, the concept of “negative coastal balance=erosion” given in “5.2 - Pathway for erosion” (line 500) is, in principle, correct for beaches. In cliffed costs, erosion and shoreline retreat are directly related to the balance between assailing force of waves and resisting force of the cliff material, when the former exceeds the latter. That is another reason to clarify with type of coasts the chapter is focusing on.

On the other side, at line 511, where it is mentioned “(….) factors that affect the erodibility of the beach” it might be added “and the cliff”.

A definition of coastal erosion is provided at line 459, in subsection 5.1. Other definitions might be confusing or not univocal, such as the above cited (at line 500) that “coastal erosion takes place when the sediment budget of a given area becomes negative”, for the reasons explained above. I suggest deleting this sentence since it only reflects part of the process, whose complexity is better explained in the text just below (lines 500-511). Another definition is given at line 514, in “Receptor and consequences of erosion” (“Coastal erosion is the process by which the land is worn away and is submerged in water”); this might be misleading. Submergence is also due to episodic flooding, not necessarily to coastal erosion. A further definition at lines 578-579 (“Erosion is a physical phenomenon where sand is removed from the shoreface and deposited elsewhere, usually offshore”) is also misleading because sediment transport and deposition are parts of cyclic processes modelling the emerged and submerged beach and should not be confused with (permanent) sediment loss that leads to erosion.

Among factors indicated as drivers of coastal erosion in subsections 5.1 and 5.2, some concepts and terms might be more precise and inclusive, trying at the same time to avoid repetitions.

For instance, instead than “terrestrial sediment supply” (line 471) the indication of "sedimentary balance of the coastal stretch" would be better used, because it includes also marine sediment supply, not only terrestrial one.

Factors influencing the sedimentary balance at the coast, both natural (climatic) and anthropogenic, should encompass a view to river catchment basins. This link should be better evidenced in a source-to-sink approach. An example of this is provided by references below:

Meli and Romagnoli, 2022. Evidence and implications of hydrological and climatic change in the Reno and Lamone river basins over the last century and in related coastal area, Emilia-Romagna, Northern Italy. Water, 14, 20650, doi: 10.3390/w14172650

Other natural factors with possible negative effects on the coastal budget are the occurrence of canyons' heads in the nearshore, that can subtract sediment from longshore drift. This potential effect could be mentioned, since this process is more frequent than commonly considered.

Among “human” factors (cited at line 507) it might be also mentioned the reduction of natural defence capability of the beach (its own resilience) due to alteration of natural coastal dynamics, and the stiffening of the coast caused by the construction of "hard" protection structures.

To conclude, subsections 5.1 and 5.2 are partially overlapping. The Authors might consider if there a way to avoid this.

Among the coastal monitoring programs mentioned in subsection 5.3 it might be included the long-term, regular monitoring carried out since the 1983 on the Emilia-Romagna coast (regional scale) on the emerged and submerged beach and on the shoreline (see public reports, https://www.arpae.it/it/temi-ambientali/mare, and adopted indicators on coastal erosion, https://webbook.arpae.it/erosione-costiera/index.html).

In Table 1 -Summary of the methods for monitoring, for what regards Drones, it might be added something like: “Marine drones such autonomous surface vehicles (ASVs) are usefully applied for monitoring the nearshore in shallow water and for testing the effects of mitigation strategies against erosion"Ref: Stanghellini et al., 2022; https://doi.org/10.3390/rs14225901

In subsection 5.4 “Historical shoreline change” it should be mentioned that many coastal areas are artificially stabilized, otherwise estimate given for erosion in Europe should be much worser.

In subsection 5.5 “Future shoreline change due to SLR” (I suggest adding this specific to the title), in the sentence regarding Mediterranean beaches (lines 557-558), I do not understand how the beach narrowness should depend on slope. Coarse-grained beach with higher slope may be narrow (but they are less vulnerable to SLR), while low-gradient, sandy beaches may be narrow or large for different reasons... and will be more exposed to SLR to due to their reduced height.

Furthermore, I question the projections reporting shoreline retreat (at lines 561-562): it should be specified (also in the caption of figure 9) that these estimates only represent effects due to SRL (according to a “bathtub approach” with respect to terrain model, I suppose), but they do not take into account the coastal morpho-dynamic evolution neither sedimentary budget, as it would be requested for estimating shoreline retreat due to erosion. These further aspects are well addressed in subsection 5.6, but not clarified here.

Section 6 on saltwater intrusion follows the same organization of previous sections (SPRC framework) and provides also some indications on possible adaptation measures. Very minor corrections to text and figures are indicated below.

Section 7: it is not clear to me the reason for this section dealing with impacts of SLR on specific environments (estuaries) and ecosystems. The impacts include all the three previously considered (flooding, erosion, saltwater intrusion) so it cannot be easily moved into one of the previous sections as it is. However, I found it disconnected to the rest of the paper.

The last paragraph (lines 793-796) is instead more general and partly overlaps with other sections of the chapter, also repeating some previous concepts; consider moving it or deleting it.

Minor comments and technical corrections:

-lines 56, 97: check parenthesis (too many open)

-lines 113-114 there is a repetition of what already reported at lines 107-108.

-line 298 in the sentence “(…) can also erode riverbanks and cause landslides, leading to further flooding” the Author would probably mean the "breaking of the embankments/levees".  This would be more correct than a general term "landslides".

-line 323: you could rewrite as “(….) and stabilization of beaches and dunes”

-line 330-331: “(….) a continued decline in the extent of natural systems (…)”, this reduction in the extent can be very well expressed with the concept of "coastal squeeze".

-line 467: I would delete citation "Romagnoli et al., 2022" from here (it is also mentioned just below as local-scale study).

-line 647: in the caption of Fig. 11 it might be specified “(b) current situation, with effects due to climate change and human activities”. Note that in the sketch c) the word “subsidence” should be corrected.

-line 693: “(…) such as due to subsidence” would sound better.

-Figure 13: this figure might be more extensively explained, it is very cryptic. SAR seems not mentioned in the text but only in the figure caption.

-line 755: a parenthesis is lacking at the end of the sentence “(Figure 14)”.

-Figure 14: correct in the caption “upconing”, it is the scientific term for the process indicated in figure.

-line 774: delete “other” from “On the one hand”.