Coastal flood damage and adaptation costs under 21st century sea-level rise are assessed on a global scale taking into account a wide range of uncertainties in continental topography data, population data, protection strategies, socioeconomic development and sea-level rise. Uncertainty in global mean and regional sea level was derived from four different climate models from the Coupled Model Intercomparison Project Phase 5, each combined with three land-ice scenarios based on the published range of contributions from ice sheets and glaciers. Without adaptation, 0.2-4.6% of global population is expected to be flooded annually in 2100 under 25-123 cm of global mean sea-level rise, with expected annual losses of 0.3-9.3% of global gross domestic product. Damages of this magnitude are very unlikely to be tolerated by society and adaptation will be widespread. The global costs of protecting the coast with dikes are significant with annual investment and maintenance costs of US$ 12-71 billion in 2100, but much smaller than the global cost of avoided damages even without accounting for indirect costs of damage to regional production supply. Flood damages by the end of this century are much more sensitive to the applied protection strategy than to variations in climate and socioeconomic scenarios as well as in physical data sources (topography and climate model). Our results emphasize the central role of long-term coastal adaptation strategies. These should also take into account that protecting large parts of the developed coast increases the risk of catastrophic consequences in the case of defense failure. coastal flooding | climate change impact | loss and damage A lthough increased coastal flood damage and corresponding adaptation may be one of the most costly aspects of climate change (1), few studies have assessed this impact globally. The first of these studies considered flood risk to people under a 1-m sea-level rise and adaptation via dikes, but without socioeconomic change (2). Follow-up studies refined this analysis in several directions: (i) adding a range of sea-level scenarios and a single socioeconomic scenario (3, 4), (ii) applying a range of socioeconomic scenarios (5), (iii) extending the resolution of the coastal zone to subnational levels (6, 7), and (iv) including regional patterns of climate-induced sea-level rise (6). These studies further differ in the digital elevation model (DEM) and spatial population datasets used, as well as the adaptation strategies applied. No study has, however, explored all of these dimensions together.This paper addresses this gap and assesses the impacts of increased coastal flooding on population and assets by comparing results attained using various available data sources and adaptation strategies under a comprehensive sample of state-of-theart socioeconomic and sea-level rise scenarios. Flood risk is considered in terms of expected annual damage to assets, expected annual number of people flooded, and adaptation costs in terms of dike investment and additional main...

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.

Global models of tide, storm surge, and wave setup are used to obtain projections of episodic coastal flooding over the coming century. The models are extensively validated against tide gauge data and the impact of uncertainties and assumptions on projections estimated in detail. Global "hotspots" where there is projected to be a significant change in episodic flooding by the end of the century are identified and found to be mostly concentrated in north western Europe and Asia. Results show that for the case of, no coastal protection or adaptation, and a mean RCP8.5 scenario, there will be an increase of 48% of the world's land area, 52% of the global population and 46% of global assets at risk of flooding by 2100. A total of 68% of the global coastal area flooded will be caused by tide and storm events with 32% due to projected regional sea level rise. Sea level rise is a well-accepted consequence of climate change 1-3. Although the focus of the general public often tends to be on the rate and magnitude of increase in mean sea level, the major threats of coastal flooding and erosion are significantly impacted by episodic storm surge and wave setup (the temporary increase in mean water level due to the presence of breaking waves) as well as their time of occurrence in relation to astronomical tide 4. As approximately 600 million people live in low elevation coastal zones [i.e. LECZs-coastal regions less than 10 m above mean sea level (MSL)] which generate approximately US$1 trillion of global wealth 2,5-7 , both the environmental and socioeconomic impacts associated with episodic coastal flooding can be massive. Both national and global assessments of projected coastal flooding due to the combination of extreme events and sea level rise are critical in informing policy directions, as detailed in a number of IPCC reports. Such largescale assessments can also identify regional "hot-spots" where more detailed modelling is required. The time and space scales involved in such large-scale assessment are challenging. The time scales vary from the duration of individual storms (of order hours) to projections over the coming century (of order decades). The physical scales are also demanding, varying from the bathymetry of individual beaches (10s of m) to basin-scale storms to global comparisons of potential impacts (1000s of km). As a result, to form tractable solutions and guide such policy development, a variety of simplifying assumptions must be made. This study aims to undertake such an analysis. It assembles extensive model and measured datasets at coastlines around the world and combines these to provide projections of global extreme sea level and coastal flooding by 2100. The required simplifications may result in local errors but comparisons with recorded tide gauge data indicates that, to first-order, the simplified model adopted reproduces extreme sea levels to reasonable accuracy at national and global-scale. The Discussion

In the version of this Article originally published, in the sentence beginning "Here, we quantify global-mean relative sea-level rise... " in the Abstract, the value '2.5 mm yr -1 ' should have been '2.6 mm yr -1 ' . Furthermore, the sentence "In 2015, this floodplain population is approximately 235 million people. " should have made clear that the value of the floodplain population was for the scenario without subsidence and climate-induced sea-level rise (SLR); it has now been amended to "Without subsidence and climate-induced SLR, the global floodplain population in 2015 would have been approximately 235 million people. " Also, the beginning of the subsequent sentence "Assuming no subsidence and no climate-induced SLR... " has been amended to "Still assuming no subsidence and no climate-induced SLR... " The online versions of the Article have been corrected.

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.