The response of coastal wetlands to sea-level rise during the twenty-first century remains uncertain. Global-scale projections suggest that between 20 and 90 per cent (for low and high sea-level rise scenarios, respectively) of the present-day coastal wetland area will be lost, which will in turn result in the loss of biodiversity and highly valued ecosystem services. These projections do not necessarily take into account all essential geomorphological and socio-economic system feedbacks. Here we present an integrated global modelling approach that considers both the ability of coastal wetlands to build up vertically by sediment accretion, and the accommodation space, namely, the vertical and lateral space available for fine sediments to accumulate and be colonized by wetland vegetation. We use this approach to assess global-scale changes in coastal wetland area in response to global sea-level rise and anthropogenic coastal occupation during the twenty-first century. On the basis of our simulations, we find that, globally, rather than losses, wetland gains of up to 60 per cent of the current area are possible, if more than 37 per cent (our upper estimate for current accommodation space) of coastal wetlands have sufficient accommodation space, and sediment supply remains at present levels. In contrast to previous studies, we project that until 2100, the loss of global coastal wetland area will range between 0 and 30 per cent, assuming no further accommodation space in addition to current levels. Our simulations suggest that the resilience of global wetlands is primarily driven by the availability of accommodation space, which is strongly influenced by the building of anthropogenic infrastructure in the coastal zone and such infrastructure is expected to change over the twenty-first century. Rather than being an inevitable consequence of global sea-level rise, our findings indicate that large-scale loss of coastal wetlands might be avoidable, if sufficient additional accommodation space can be created through careful nature-based adaptation solutions to coastal management.

To Whom It May Concern We have extensively revised the manuscript 'Global coastal wetland change under sealevel rise and related stresses: the DIVA Wetland Change Model' by Spencer and coauthors, for further consideration for publication in Global and Planetary Change. We believe that we have addressed all the comments and queries raised by the reviewers in detail and in full. Our 'response to referees' indicates where on a manuscript the responses have been made. We believe that these responses have resulted in a significantly improved paper and we thank the referees and the editorial team for the opportunity to respond to the criticism of the original submission. We maintain the separation of the general narrative from a more specific set of technical issues raised in the supplementary material; we believe that this decision helps meet the journal's concern to present problems and results in a way that is suitable for a broad readership. However, for ease of review we include the Supplementary Material at the end of the revised manuscript. The manuscript has been prepared to conform to the instructions for contributors. This material has not been previously published elsewhere, nor is it under consideration for publication elsewhere. All the authors have approved this submission. There are no closely related manuscripts that have been submitted or are in press. As far as I am aware, there are no actual or potential conflicts of interest, of a financial, personal or other kind, with other people or organizations that could inappropriately influence, or be perceived to influence, this work. No funding source has had any involvement in the study design, collection, analysis and interpretation of the data, in the writing of the manuscript and in the decision to submit the paper for publication.

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This manuscript reviews the progresses made in the understanding of the dynamic interactions between coastal storms and salt marshes, including the dissipation of extreme water levels and wind waves across marsh surfaces, the geomorphic impact of storms on salt marshes, the preservation of hurricanes signals and deposits into the sedimentary records, and the importance of storms for the long term survival of salt marshes to sea level rise. A review of weaknesses, and strengths of coastal defences incorporating the use of salt marshes including natural, and hybrid infrastructures in comparison to standard built solutions is then presented. Salt marshes are effective in dissipating wave energy, and storm surges, especially when the marsh is highly elevated, and continuous. This buffering action reduces for storms lasting more than one day. Storm surge attenuation rates range from 1.7 to 25 cm/km depending on marsh and storms characteristics. In terms of vegetation properties, the more flexible stems tend to flatten during powerful storms, and to dissipate less energy but they are also more resilient to structural damage, and their flattening helps to protect the marsh surface from erosion, while stiff plants tend to break, and could increase the turbulence level and the scour. From a morphological point of view, salt marshes are generally able to withstand violent storms without collapsing, and violent storms are responsible for only a small portion of the long term marsh erosion. Our considerations highlight the necessity to focus on the indirect long term impact that large storms exerts on the whole marsh complex rather than on sole after-storm periods. The morphological consequences of storms, even if not dramatic, might in fact influence the response of the system to normal weather conditions during following inter-storm periods. For instance, storms can cause tidal flats deepening which in turn promotes wave energy propagation, and exerts a long term detrimental effect for marsh boundaries even during calm weather. On the other hand, when a violent storm causes substantial erosion but sediments are redistributed across nearby areas, the long term impact might not be as severe as if sediments were permanently lost from the system, and the salt marsh could easily recover to the initial state.

[1] Previous predictions on the ability of coastal salt marshes to adapt to future sea level rise (SLR) neglect the influence of changing storm activity that is expected in many regions of the world due to climate change. We present a new modeling approach to quantify this influence on the ability of salt marshes to survive projected SLR, namely, we investigate the separate influence of storm frequency and storm intensity. The model is applied to a salt marsh on the German island of Sylt and is run for a simulation period from 2010 to 2100 for a total of 13 storm scenarios and 48 SLR scenarios. The critical SLR rate for marsh survival, being the maximum rate at which the salt marsh survives until 2100, lies between 19 and 22 mm yr -1 . Model results indicate that an increase in storminess can increase the ability of the salt marsh to accrete with sea level rise by up to 3 mm yr -1 , if the increase in storminess is triggered by an increase in the number of storm events (storm frequency). Meanwhile, increasing storminess, triggered by an increase in the mean storm strength (storm intensity), is shown to increase the critical SLR rate for which the marsh survives until 2100 by up to 1 mm yr -1 only. On the basis of our results, we suggest that the relative importance of storm intensity and storm frequency for marsh survival strongly depends on the availability of erodible fine-grained material in the tidal area adjacent to the salt marsh.

Coastal tidal wetlands produce and accumulate significant amounts of organic carbon (C) that help to mitigate climate change. However, previous data limitations have prevented a robust evaluation of the global rates and mechanisms driving C accumulation. Here, we go beyond recent soil C stock estimates to reveal global tidal wetland C accumulation and predict changes under relative sea-level rise, temperature and precipitation. We use data from literature study sites and our new observations spanning wide latitudinal gradients and 20 countries. Globally, tidal wetlands accumulate 53.65 (95%CI: 48.52–59.01) Tg C yr−1, which is ∼30% of the organic C buried on the ocean floor. Modelling based on current climatic drivers and under projected emissions scenarios revealed a net increase in the global C accumulation by 2100. This rapid increase is driven by sea-level rise in tidal marshes, and higher temperature and precipitation in mangroves. Countries with large areas of coastal wetlands, like Indonesia and Mexico, are more susceptible to tidal wetland C losses under climate change, while regions such as Australia, Brazil, the USA and China will experience a significant C accumulation increase under all projected scenarios.

We reconstruct past accretion rates of a salt marsh on the island of Sylt, Germany, using measurements of the radioisotopes 210 Pb and 137 Cs, as well as historical aerial photographs. Results from three cores indicate accretion rates varying between 1 and 16 mm year −1 .Comparisons with tide gauge data show that high accretion rates during the 1980s and 1990s coincide with periods of increased storm activity. We identify a critical inundation height of 18 cm below which the strength of a storm seems to positively influence salt marsh accretion rates and above which the frequency of storms becomes the major factor. In addition to sea level rise, we conclude that in low marsh zones subject to higher inundation levels, mean storm strength is the major factor affecting marsh accretion, whereas in high marsh zones with lower inundation levels, it is storm frequency that impacts marsh accretion.

17Accretion rates, defined as the vertical growth of salt marshes measured in mm per 18 year, may be influenced by grazing livestock in two ways: directly, by increasing soil 19 compaction through trampling, and indirectly, by reducing aboveground biomass and thus 20 decreasing sediment deposition rates measured in g/m² per year . Although accretion rates 21 and the resulting surface elevation change largely determine the resilience of salt marshes to 22 sea-level rise (SLR), the effect of livestock grazing on accretion rates has been little studied. 23Therefore, this study aimed to investigate the effect of livestock grazing on salt-marsh 24 accretion rates. We hypothesise that accretion will be lower in grazed compared to ungrazed 25 M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT2 salt marshes. In four study sites along the mainland coast of the Wadden Sea (in the south-26 eastern North Sea), accretion rates, sediment deposition rates, and soil compaction of grazed 27 and ungrazed marshes were analysed using the 137 Cs radionuclide dating method. Accretion 28 rates were on average 11.6 mm yr -1 during recent decades and thus higher than current and 29 projected rates of SLR. Neither accretion nor sediment deposition rates were significantly 30 different between grazing treatments. Meanwhile, soil compaction was clearly affected by 31 grazing with significantly higher dry bulk density on grazed compared to ungrazed parts. 32Based on these results, we conclude that other factors influence whether grazing has an effect 33 on accretion and sediment deposition rates and that the effect of grazing on marsh growth Introduction 38Many coasts of the world show an enhanced rate of sea-level rise (SLR) over the past 39 century, and studies predict it to accelerate in the future (IPCC, 2007; Vermeer and 40 Rahmstorf, 2009). Global SLR was 3.1 mm yr -1 between 1993and 2003 (IPCC, 2007 unique flora and fauna (Schmidt et al., 2012). 54Given that lateral erosion is not occurring, the resilience of salt marshes to SLR is 55 largely determined by their ability to compensate higher water levels by increased vertical 56 accretion and/or reduced soil subsidence rates leading to increased surface elevation. Only if 57 accretion rates and the resulting increase in surface elevation are higher than rates of SLR, a 58 salt marsh will be able to keep pace with relative SLR. The surface elevation change in salt 59 marshes is the sum of sediment accretion, erosion, compaction processes, and possibleregional crustal movements (French, 1993 French et al., 2003). Many studies have investigated accretion rates in salt marshes (e. g. 74Cahoon and Turner, 1989; Dijkema, et al. 1990; Dijkema, 1997; Bellucci et al., 2007; 75 Baustian et al., 2012), and several models exists to predict the future development of salt 76 marshes (e.g. Allen, 1990;Temmerman et al., 2003; Bartholdy et al., 2004; French, 2006, 77 Schuerch et al., 2013). Yet, the question of whether accretion rates and the resulting surface 78 elevat...