Abstract. With trillions of USD in assets facing climate-induced sea-level rise, oversimplified economic models might misinform responses. By capturing non-linearities in interconnected socio-economic and biogeophysical domains, and their local to global co-evolution, complexity science applied to sea-level rise uniquely enhances adaptation policies.

Despite growing evidence of a global coastal adaptation gap under accelerating sea-level rise, global economic evidence on when and how to close this gap remains scarce. Here, we identify economically optimal adaptation pathways for floodplains worldwide, enabling flexible timing and switching between adaptation options over time. We find that immediate adaptation is economically optimal for 28% of the global coastline, irrespective of the climate change scenario, reflecting that many floodplains are under-adapted today. By 2150, adaptation is economically warranted along 31% of the global coastline, with 20% optimally managed through retreat, 7% through coastal protection, less than 1% through accommodation, and 4% through combined retreat and inland protection. Relative future cost burdens of adaptation investments and flood damages are highest in the Global South, exacerbating inequities embedded in today’s adaptation gap.

Sea levels are increasing at an accelerated rate1,2 and this is expected to increase flooding along coastlines worldwide3. While cultural and natural heritage sites are among the assets most exposed to this threat4,5, a unified global assessment is currently lacking. Here we assess coastal flood exposure of all nearshore UNESCO World Heritage sites worldwide, under different global warming scenarios. We estimate that for a scenario with current climate mitigation policies and action, by the end of this century around one third of World Heritage Sites could be exposed to floods, corresponding to more than 1.1 million hectares of protected and preserved land. Limiting warming to the 1.5°C Paris agreement target would save 89 heritage sites from being exposed. Large countries are projected to face widespread exposure while smaller nations risk losing entire heritage systems. Combining our findings with the ND-GAIN index6 shows that 32 countries with low adaptive capacity are expected to experience high heritage exposure, especially Small Island Developing States. To safeguard these irreplaceable cultural and natural treasures, it is imperative to scale up heritage adaptation efforts and increase support for vulnerable regions.

Abstract Estuarine areas are currently at risk of compound flooding, the frequency and intensity of which is expected to increase with climate change. Even though efforts are made to adapt against single flood drivers using hard protection, potential subsequent changes in flood risk due to compound flooding are often overlooked in flood risk assessments. This is because risk assessments mostly focus on individual flood drivers and do not account for changes in risk from adaptation measures. We address this question and use hydrodynamic modeling to simulate compound flooding for two adaptation scenarios. We consider adaptation in terms of storm surge barriers, in two locations along the Trave estuary, namely Schlutup and Trave, at Lübeck, Germany. We assess the effectiveness of both storm surge barriers in reducing flooding by simulating individual‐driver, as well as low‐ and high‐magnitude compound flood scenarios. We find that while during low‐magnitude compound flooding both barriers reduce the overall flood extent by 25%–86%, high‐magnitude compound flooding leads to an increase of up to 100%, depending on the location of the barrier. Our results suggest that the river contribution is amplified by 52%–100% by the Schlutup Barrier. The Trave Barrier, however, only amplifies flood extents in the high‐magnitude compound flood scenarios. Our findings highlight the need to consider compound flooding in adaptation planning to avoid defense failure and unexpected increases in risk. However, as this study considers only two (low probability) extreme events, a more comprehensive approach is necessary to fully understand the overall impact on risk.

Recent model results in combination with observations have provided a first coherent picture of secular changes in ocean tides since 1993. Strengthening of ocean stratification has been identified as an important driver of the observed secular trends, where the barotropic tide is primarily affected through enhanced tidal conversion at topography. These changes are responsible for open-ocean trends in the order of 0.1 mm yr-1 for the barotropic M2 tide, increasing to magnitudes comparable to the tidal response to sea level change (0.2—0.4 mm yr-1) in several coastal regions. This has ramifications for global projections of future extreme sea levels, which either neglect changes in tides or consider them solely as a function of sea level rise. In this study, we employ a global high-resolution (1/12°) internal-tide permitting numerical ocean model to quantify future changes in ocean tides until 2100 as a result of upper-ocean warming and the concomitant increase in stratification. We simulate the evolution of leading tidal constituents in 5-year average time slices and use EC-Earth3P HighResMIP density data to constrain the model’s background stratification. As the Representative Concentration Pathway (RCP) in the EC-Earth3P simulation is a high greenhouse gas emission scenario (RCP8.5), we also consider data from a CM2.6 coupled global climate model, which is more closely aligned with a medium stabilisation scenario (RCP6.0).

Sea-level rise is an existential threat to coastal residents around the world as it progressively raises water levels, promoting increased flooding, erosion, salinization, and ecosystem degradation and loss. Unmitigated climate change will lead to rapid acceleration, with a meter of rise by 2,100 appearing likely and larger rises possible. Although some coastal areas are already being inundated, many others will increasingly suffer from rising sea level and more intense extreme water levels during high tides, winter storms, and hurricanes in the future. Small islands, deltas, and coastal cities are particularly threatened.

Over the past centuries, coastal marsh areas have been declining, mostly as a consequence of human impacts, including direct wetland conversion and land reclamation. More recently, accelerated global sea-level rise poses an additional challenge for the longevity of existing coastal marshes. This risk is further compounded by densely populated coastal zones, where coastal infrastructure inhibits the capacity of coastal marshes to migrate inland in response to rising sea levels (coastal squeeze). In the Mediterranean, coastal wetlands, incl. saltmarshes, are important contributors to the region’s high biodiversity, and provide a set of invaluable ecosystem services. Here, we present a study on the modelling of the future development of Mediterranean coastal marshes, taking into account both their ability to vertically adjust to increasing sea levels through sediment accumulation and their capacity to migrate inland in response to rising sea levels where sufficient inland migration space is available. In contrast to previous global studies, our preliminary results indicate an overall loss of coastal marshes by 2100 for all climate and management scenarios, even under low sea-level rise scenarios and when abundant inland migration space is available. Total losses are projected between 17% and 94% for RCP 2.6 with maximum available space for inland space; and RCP 8.5 with minimum space, respectively. A total loss of coastal marshes is projected for some Mediterranean countries by 2100. Nevertheless, the implementation of coastal management strategies facilitating the inland migration of coastal marshes as well restoration of catchment-to-coast sediment connectivity and enhancement of sediment trapping capacities can, to some degree, mitigate future coastal marsh losses.

The capacity of river mouths to reduce storm surge water levels upstream, referred to as along-estuary attenuation, has been assessed by several studies. The coastal protection function of semi-enclosed water bodies such as lagoons and channels with narrow inlets remains less explored and generalization is hampered by differences in morphology and hydrodynamic forcing. Here we use a hydrodynamic model to investigate surge attenuation along a microtidal channel with a narrow inlet at the Baltic Sea coast of Germany called The Schlei. We quantify the importance of wind and the contribution of the barrier spit system, which is constricting the inlet, to the reduction of water levels at the landward end of the channel. In addition, we explore the role of dikes in the region for the reduction of peak water levels and coastal flooding. We find effective along-channel attenuation inside The Schlei in its current state, which is mostly a result of the channel’s narrows. However, reduction rates decrease under simplified sea-level rise scenarios. Furthermore, along-channel attenuation is highly variable and can change to substantial amplification depending on hydrometeorological forcing. The barrier spit contributes to along-channel attenuation whereas the effect of existing dikes (or their removal) for along-channel attenuation is negligible.

Flood characteristics caused by extreme sea level (ESL) events depend largely on the magnitude of peak water levels (WLs) and their temporal evolution. However, coastal flood risk is generally assessed based on only a limited number of potential peak WLs and a selection of past events or a design hydrograph. We address this gap and systematically estimate (a) spatial annual and (b) event-based flood probabilities by comprehensively accounting for both a wide range of peak ESLs and their temporal evolution, herein referred to as hydrograph intensity. We simulate flooding at the German Baltic Sea coast with the hydrodynamic model Delft3D. We produce probabilistic flood maps, which detail flood exposed areas together with annual probability of flooding. Additionally, we show how the flood extent changes, when accounting for upper, median, and lower quantiles of hydrograph intensities. Our results demonstrate that the relevance of the intensity is site and ESL dependent. While flood extents of some ESLs of the upper and lower intensity bounds indicate no differences, others differ by up to 45%. Further, we consider two ESLs (2.24 and 2.55 m) and simulate 100 intensities for each. Compared to intensity quantiles, this results in flood extents of up to 60% difference. Hence, we find that quantiles of intensity do not cover the full range when addressing uncertainty due to hydrograph variability. We, therefore, recommend accounting for a wide range of hydrograph intensities in addition to using a wide range of ESL in future flood risk assessments. Implementing and maintaining coastal protection to mitigate damages arising from flooding is a costly endeavor. With many possible solutions, it is difficult to identify what measures offer the best outcomes and where to deploy them. To inform coastal managers and to promote efficient planning, risk-based assessments of flooding are typically prescribed. Coastal flood risk has generally been assessed based on the impacts of a few past storm surge events. However, this is not sufficient for capturing the full picture of possible flooding, as storms can vary in temporal evolution and duration. In this study, we assess flooding at the German Baltic Sea coast under a very large range of potential extreme sea levels (ESLs) varying in magnitude and duration. We provide maps which detail flood extents and their associated probabilities. In addition to highlighting areas exposed to flooding, our results show that the duration of a storm surge event can significantly affect flood extents in certain areas, depending on the magnitude of the event. To better understand and prepare for coastal flooding, we recommend that a wide range of ESL magnitudes and storm surges in terms of duration and evolution should be considered in future flood risk assessments. Probabilistic flood maps from extreme sea levels (ESLs) and hydrograph variability for a local case study provide relevant information for coastal plannersSensitivity of flood extent due to hydrograph variability depends on considered ESL height and topographyHydrograph intensity quantiles do not cover the full picture of flood extent sensitivity to storm surge variability

Improving our understanding of future ocean carbon uptake requires a nuanced understanding of the value of the annual ocean sink. Here, we combine an abatement cost-based approach and a climate damage-based approach to assess the value of the annual ocean sink. The former shows that the aggregate cost of national climate policies could increase by up to USD 80 billion if the ocean carbon sink weakens by 10 percent. As a complementary perspective, the damage-based approach shows that the annual ocean carbon sink contributes between USD 300 billion and USD 2,332 billion to countries' inclusive wealth. Despite the conceptual appeal of the damage-based approach for its potential insights into regional wealth redistribution, uncertainties in national social cost of carbon estimates make it less reliable than the abatement cost-based approach, which in turn provides more reliable estimates for a fiscal cost assessment of improved monitoring services of the ocean carbon sink. The annual ocean carbon sink contributes up to 2,332 billion US dollars to countries' inclusive wealth, and the cost of climate policies could increase by up to 80 billion US dollars if the carbon sink weakens by 10 percent, according to an analysis that uses cost-effective and damage-based approaches.

Information on urban land use, beyond the urban-rural dichotomy, can improve the assessment of potential impacts of coastal hazards by refining estimates of damages and supporting adaptation planning. However, the lack of a consistent definition of “urban” in previous studies has led to exposure estimates that vary considerably. Here, we explore the sensitivity of exposed population and built-up area in four settlement types, defined by four different built-up area datasets. We find large differences in the exposed population of up to 65% (127 million people) in the “Urban” class. The exposure estimates are highly sensitive to the density thresholds used to distinguish the settlement types, with a difference in exposed urban population of up to 53.5 million people when the threshold varies by 10%. We attribute the high sensitivity of the exposure estimates to the varying definitions of built-up area of the underlying datasets. We argue that the definition of urban land is crucial for coastal impact assessments and make recommendations for the use of the analyzed datasets.

Scenario analysis is a widely employed method for addressing uncertainties when assessing the physical and socio-economic impacts of climate change. Global scenarios have been extensively used in this context. However, these scenarios are in most cases not suitable for supporting local analyses. On the other hand, locally developed scenarios may lack the global context, thus having limited comparability with or transferability to other locations. The Shared Socioeconomic Pathways (SSP), which have been primarily developed for climate impact research, provide the possibility to extend the existing global narratives and adapt them to local characteristics in order to develop locally relevant scenarios. Here, we propose a methodological framework for producing harmonized scenarios across different case studies. This framework was developed in the EVOKED project and combines elements of top-down and bottom-up approaches to develop local scenarios for four regions in northern Europe. We employ the SSP as boundary conditions and, in cooperation with stakeholders from these four regions, develop local scenarios for a range of SSP. The developed sets of scenarios are consistently informed by global developments and are therefore comparable with other downscaled scenarios developed in different regions. At the same time, they have been based on local participatory processes, thus being locally credible and relevant to the needs of stakeholders. The local scenarios constitute a climate service per se as they can raise stakeholder awareness of the processes that will drive risk, exposure, and adaptive capacity in the future and inform discussions on mitigation strategies and adaptation pathways.

AbstractThe Agulhas Current system around South Africa combines the dynamics of strong ocean currents in the Indian Ocean with eddy–mean flow interactions. The system includes an associated interoceanic transport towards the Atlantic, Agulhas leakage, which varies on both interannual and decadal timescales. Agulhas leakage is subject to a general increase under increasing greenhouse gases, with higher leakage causing a warming and salinification of the upper ocean in the South Atlantic. The far-field consequences include the impact of the Agulhas Current on the Benguela Upwelling system, a major eastern boundary upwelling system that supports a lucrative fishing industry. Through sea surface temperatures and associated air–sea fluxes, the Agulhas Current system also influences regional climate in southern Africa, leading to a heterogeneous pattern of rainfall over southern Africa and to a reduction of precipitation in most areas under global warming conditions. Changes in the Agulhas Current system and the regional climate also cause changes in regional sea-level and wind-induced waves that deviate from global trends. Combining these oceanic changes with extreme precipitation events, global warming can considerably amplify flood impacts along the coast of South Africa if no adaptation measures are implemented.

<p>Future coastal risk will largely depend on where people build and settle, and not only on increases in extreme events or sea level rise as a result of global warming. In the past, hard engineering has been used to protect settlements in coastal lowlands. However, as this option becomes less viable and more expensive due to rapidly rising sea levels, coastal managers are increasingly turning to landuse planning interventions, such as setback zones or managed retreat. Although various studies show that one of the most effective approaches is to prevent urban expansion inside the coastal floodplain, limited research has been done to assess the potential of setback zones in minimizing future coastal exposure in Europe. This study enhances our understanding of the potential of coastal setback zones of different shapes in the EU by (1) assessing the avoided urban exposure resulting from the implementation of setback zones/retreat under different socioeconomic futures and (2) providing country-specific information on which type of setback zones is most beneficial in reducing urban exposure. For this purpose, we created spatially explicit projections of urban extent that consider different socio-economic futures and different types of setback zones to examine the effectiveness of these planning strategies in decreasing future urban exposure in Europe’s coastal lowlands. Our results show that by the year 2100, the majority of EU coastal countries can reduce the exposure of new urban land by at least 50% if coastal setback zones are established; and highlight that how we plan, build, and develop urban space in the EU coastal lowlands will be the defining factor on how exposed future urban areas are to sea-level rise.</p><p> </p>

Coastal areas are subject to hazards that can result in severe impacts due to the high concentration of people and assets in exposed locations. While climate-induced sea-level rise will exacerbate these hazards in the course of the 21st century, future dynamics in socioeconomic development will play an important role in driving impacts – as well as adaptation responses – in particular in countries with rapid population growth in low-lying coastal areas. Here, we synthesize the current state of knowledge related to current and future population development in coastal locations and the underlying trends in socioeconomic development affecting coastal impacts at continental to global scales. Currently, 2.15 billion people live in the near-coastal zone and 898 million in the low-elevation coastal zone globally. These numbers could increase to 2.9 billion and 1.2 billion, respectively, depending on the socioeconomic scenario (i.e., Shared Socioeconomic Pathway [SSP]) considered. Nevertheless, although these numbers indicate a rapid increase in exposure of population and assets to coastal hazards, they bear limited information about the actual impacts as they do not include information on the vulnerability of coastal population. Based on these insights, we stress the need to account for dynamics in socioeconomic development in coastal risk assessments, including exposure as well as vulnerability, and additionally exploring potential feedbacks due to adaptation responses and migration decisions. Last, we propose action points for future work that can inform long-term coastal planning for managing coastal risks.

Among the Baltic Sea littoral states, Germany is anticipated to endure considerable damage as a result of increased coastal flooding due to sea-level rise (SLR). Consequently, there is a growing demand for flood risk assessments, particularly at regional scales, which will improve the understanding of the impacts of SLR and assist adaptation planning. Existing studies on coastal flooding along the German Baltic Sea coast either use state-of-the-art hydrodynamic models but cover only a small fraction of the study region or assess potential flood extents for the entire region but rely on global topographic data sources and apply the simplified bathtub approach. In addition, the validation of produced flood extents is often not provided. Here we apply a fully validated hydrodynamic modelling framework covering the German Baltic Sea coast that includes the height of natural and anthropogenic coastal protection structures in the study region. Using this modelling framework, we extrapolate spatially explicit 200-year return water levels, which align with the design standard of state embankments in the region, and simulate associated coastal flooding. Specifically, we explore (1) how flood extents may change until 2100 if dike heights are not upgraded, by applying two high-end SLR scenarios (1 and 1.5 m); (2) hotspots of coastal flooding; and (3) the use of SAR imagery for validating the simulated flood extents. Our results confirm that the German Baltic coast is exposed to coastal flooding, with flood extent varying between 217 and 1016 km2 for the 200-year event and a 200-year event with 1.5 m SLR, respectively. Most of the flooding occurs in the federal state of Mecklenburg-Western Pomerania, while extreme water levels are generally higher in Schleswig-Holstein. Our results emphasise the importance of current plans to update coastal protection schemes along the German Baltic Sea coast over the 21st century in order to prevent large-scale damage in the future.

Abstract The ocean carbon sink annually removes about a third of anthropogenic CO2 emissions, reducing climate change damage and CO2 abatement costs. While land sinks have been integrated into climate policies, the ocean sink has not—for good reason, since the former stores carbon within the boundaries of a given country, while the latter removes carbon from the atmosphere as global commons. However, the question of the value of the oceanic carbon sink remains, and how it should be attributed when comparing a coastal country with a large exclusive economic zone (EEZ) to a landlocked country. Here, we demonstrate different approaches to valuing the ocean carbon sink, comparing a climate-change-damage-based approach with an abatement-based and market-based approach. We use a high-resolution carbon flux dataset (0.25x0.25 degrees) to estimate the oceanic carbon sinks and sources in coastal areas. We assign a net sink of 1.72 GtC proportional to countries with negative carbon fluxes in their EEZs. In our calculation, the annual value of the global ocean sink ranges from 66 B USD to 1432 B USD.

Climate change-induced sea-level rise will lead to an increase in internal migration, whose intensity and spatial patterns will depend on the amount of sea-level rise; future socioeconomic development; and adaptation strategies pursued to reduce exposure and vulnerability to sea-level rise. To explore spatial feedbacks between these drivers, we combine sea-level rise projections, socioeconomic projections, and assumptions on adaptation policies in a spatially-explicit model ('CONCLUDE'). Using the Mediterranean region as a case study, we find up to 20 million sea-level rise-related internal migrants by 2100 if no adaptation policies are implemented, with approximately three times higher migration in southern and eastern Mediterranean countries compared to northern Mediterranean countries. We show that adaptation policies can reduce the number of internal migrants by a factor of 1.4 to 9, depending on the type of strategies pursued; the implementation of hard protection measures may even lead to migration towards protected coastlines. Overall, spatial migration patterns are robust across all scenarios, with out-migration from a narrow coastal strip and in-migration widely spread across urban settings. However, the type of migration (e.g. proactive/reactive, managed/autonomous) depends on future socioeconomic developments that drive adaptive capacity, calling for decision-making that goes well beyond coastal issues.