Impact statement

Coasts are shaped by both processes of natural change and changes in patterns of use of space and resources by societies. Coasts are particularly sensitive to global change and continuously respond to these changes, often in ways that lead to new threats, or exacerbate existing threats, to the very supply of goods, services and space that make them so attractive to societies. Transformations for sustainable development at the coast present both a considerable set of challenges and opportunities. We argue that understanding the future of coastal environments requires a much wider, and more sophisticated, framing than simply considering the impacts of global sea level rise on rapidly growing coastal populations. In essence, the globalised view needs to be extended into arguments that stress: the importance of place-and people-based biophysical systems and social contexts; the need for technical innovation in restoring and rehabilitating degraded coastal environments; and the identification of just and equitable adaptive response pathways towards better, more robust coastal futures.

Coastal dynamics beyond the single metric of global sea-level rise

Quite different from a simple global signal, there are many different regional rates of relative sea-level (RSL) rise; some regions exhibited rates of RSL change five times the global mean between 1993 and 2003 (Bindoff et al., Reference Bindoff, Willebrand, Artale, Cazenave, Gregory, Gulev, Hanawa, Le Quéré, Levitus, Nojiri, Shum, Talley, Unnikrishnan, Solomon, Qin, Manning, Chen, Marquis, Averyt, Tignor and Miller2007). Inter-annual RSL variability is also likely to be significant, into decimetres in places, in the near future (e.g., Palmer et al., Reference Palmer, Gregory, Bagge, Calvert, Hagedoorn, Howard, Klemann, Lowe, Roberts, Slangen and Spada2020) and it may take decades for the signal of anthropogenically forced RSL change to be clearly detectable (Lyu et al., Reference Lyu, Zhang, Church, Slangen and Hu2014) and influence shorelines (D’Anna et al., Reference D’Anna, Idier, Castelle, Rohmer, Cagigal and Mendez2022). Micro-tidal coasts are likely to be more vulnerable to global change, with the effects being seen earlier, because the sea level rise signal becomes a larger proportion of the total water level (tide plus sea level rise) signal (Ponte et al., Reference Ponte, Carson, Cirano, Domingues, Jevrejeva, Marcos, Mitchum, van de Wal, Woodworth, Ablain, Ardhuin, Ballu, Becker, Benveniste, Birol, Bradshaw, Cazenave, De Mey-Frémaux, Durand, Ezer, Fu, Fukumori, Gordon, Gravelle, Griffies, Han, Hibbert, Hughes, Idier, Kourafalou, Little, Matthews, Melet, Merrifield, Meyssignac, Minobe, Penduff, Picot, Piecuch, Ray, Rickards, Santamaría-Gómez, Stammer, Staneva, Testut, Thompson, Thompson, Vignudelli, Williams, SDP, Wöppelmann, Zanna and Zhang2019). Furthermore, coastal change is driven not only by these slow onset, chronic dynamics (sea level rise and additionally ocean warming and ocean acidification) but also by acute shocks, from tropical and extratropical storms, associated storm surges, marine heat waves, coastal catchment wildfires, and freshwater flood inputs (e.g., Kirezci et al., Reference Kirezci, Young, Ranasinghe, Muis, Nicholls, Lincke and Hinkel2020; Warrick et al., Reference Warrick, East and Dow2023). All these impacts show high regional variability; thus, for example, while coasts on the western margins of the Pacific basin sit within the tropical cyclone belt and are at risk from storm surges, low-lying islands in more equatorial and eastern Pacific Ocean locations experience flooding from the combination of distant-source swell waves, king tides and sea level rise (e.g., Tuvalu and Kiribati: Hoeke et al., Reference Hoeke, Damlamian, Aucan and Wandres2021). Recent ‘heat domes’, catastrophic fluvial flooding, extensive droughts and large-scale coral bleaching events suggest that the climate change signal may well be first seen in these, until now rare, extreme events, with shortening recurrence intervals between them (Seneviratne et al., Reference Seneviratne, Zhang, Adnan, Badi, Dereczynski, Di Luca, Ghosh, Iskandar, Kossin, Lewis, Otto, Pinto, Satoh, Vicente-Serrano, Wehner, Zhou, Masson-Delmotte, Zhai, Pirani, Connors, Péan, Berger, Caud, Chen, Goldfarb, Gomis, Huang, Leitzell, Lonnoy, Matthews, Maycock, Waterfield, Yelekçi, Yu and Zhou2021). And compound extreme events (e.g., storm surge high water levels accompanied by freshwater runoff from heavy rains, or a sequence of tropical hurricanes making landfall) add further complexity to system response (Zscheischler et al., Reference Zscheischler, Westra, van den Hurk, Seneviratne, Ward, Pitman, AghaKouchak, Leonard, Wahl and Zhang2018). Concentrating solely on sea level rise drives a focus on a ‘future problem’ of slow vertical system response and neglects the other processes that shape our coastlines. These include settings where longshore processes, and thus lateral system response, as well as shorter-term processes, such as storms, are often dominant (e.g., for the under-assessment of extreme water levels compared to the focus on sea level rise see Wahl et al., Reference Wahl, Haigh, Nicholls, Arns, Dangendorf, Hinkel and Slangen2017). Furthermore, a sea level rise focus underplays those critical controls on system health that are only weakly linked to climate change, such as changes in fluvial and/or marine sediment supply.

Coastal systems as integrated biophysical systems

A significant proportion of coastal systems are vegetated (tidal wetlands, sand dunes, coastal cliffs) or are located behind biosedimentary systems (coral reefs, seagrass-floored lagoons) in inter-connected coastal ‘seascapes’ (Ogden et al., Reference Ogden, Nagelkerken, McIvor and Bortone2014). Knowledge about the entrainment, transport and deposition of coastal sediments and the nature of, and controls on, coastal ecology are well established, although research often reflects the ease of access (e.g., locations near field stations and institutions in developed economies, often in the summer months). Furthermore, physical science typically concentrates on monitoring (which prioritises the temporal element) whereas biological science largely focuses on sampling (which favours the spatial element). In addition, understanding the processes that shape and reshape coastal biophysical environments requires addressing the couplings between ecological and physical processes, in contrast to studying them separately (e.g., D’Alpaos, Reference D’Alpaos2011; Durán Vinent and Moore, Reference Durán Vinent and Moore2015). On many coasts, therefore, there is an urgent need to merge these different perspectives across disciplines and address coastal dynamics as an integrated biophysical system (Solan et al., Reference Solan, Spencer, Paterson, Unsworth, Christie, Blight, Brown, Brooks, Lichtman, Wei, Li, Thorne, Leyland, Godbold, Thompson, Williams, Plater, Moller and Amoudry2023). And as satellites open up the possibility of large spatial and temporal scale monitoring of a variety of coastal processes and ecosystem health monitoring, more holistic observational approaches become possible (e.g., Blount et al., Reference Blount, Carrasco, Cristina and Silvestri2022; Vitousek et al., Reference Vitousek, Buscombe, Vos, Barnard, Ritchie and Warrick2023). These new theoretical and observational approaches are particularly important given the growing interest in non-structurally engineered responses to coastal erosion and shoreline retreat (Van Zelst et al., Reference van Zelst, Dijkstra, van Wesenbeeck, Eilander, Morris, Winsemius, Ward and de Vries2021; Wedding et al., Reference Wedding, Reiter, Moritsch, Hartge, Reiblich, Gourlie and Guerry2022). Nature-based coastal protection offers the promise of long-term sustainability as, with adequate sediment supply, coasts have the potential to respond to environmental forcing, including the tracking of rising sea levels (Spalding et al., Reference Spalding, McIvor, Beck, Koch, Möller, Reed, Rubinoff, Spencer, Tolhurst, Wamsley, van Wesenbeeck, Wolanski and Woodroffe2014). However, nature-based approaches to enhancing resilience in the near future can in some cases reduce resilience over decadal timescales. Thus, for example, building artificially high barrier island dunes reduces storm overwash in the short term but promotes narrower and lower islands in the long term, so that when dunes are overtopped, sediment redistributions – and impacts on island infrastructure – are more immediate and severe (Magliocca et al., Reference Magliocca, McNamara and Murray2011). Understanding, and thus aiding, the trajectory of a coastal ecosystem’s ability to adapt and maintain functionality is fundamental to the long-term maintenance of natural capital and the delivery of ecosystem services. But what exactly do we know about the design rules and implementation practices to successfully work with natural processes in coastal risk management? (e.g., Orton et al. (Reference Orton, Ralston, van Prooijen, Secor, Ganju, Chen, Fernald, Brooks and Marcell2023) on the knowledge gaps behind the construction and operation of gated storm surge barriers). And given that risks and habitability (e.g., Horton et al., Reference Horton, Sherbinin, de Wrathall and Oppenheimer2021), over decadal timescales, change as the landscapes and ecosystems change, what practises will enable long-term, mutual resilience of coastal landscapes, ecosystems, and societies?

Coasts in the Anthropocene

On developed coasts, physical and ecological processes clearly impact human communities; the very processes that create and maintain coastal environments, including sea-level rise, storms, and ecological change, often represent hazards for people, structures and infrastructure. On the other hand, human decisions and actions have resulted in the removal, degradation and fragmentation of coastal environments and have lessened the ability of these systems to adapt to climate change and act as natural protective barriers for coastal populations (Simkin et al., Reference Simkin, Seto, McDonald and Jetz2022; De Dominicis et al., Reference De Dominicis, Wolf, van Hespen, Zheng and Hu2023). These impacts are not new; there have been centuries of modification, degradation and loss from the anthropogenic activities of land conversion (for agriculture, aquaculture, industry, housing and infrastructure) and misuse (dredging and canalisation, waste disposal and pollution) although one might argue that the scale of human impact – and its consequences – has greatly increased in the last 100 years. Thus, economic damages to coastal assets from tropical cyclones at 2100 are projected to increase by >300% on present values due solely to coastal development, a much larger effect than that projected for climate change impacts, even under high-end climate warming scenarios (Gettelman et al., Reference Gettelman, Bresch, Chen, Truesdale and Bacmeister2018).

It is clear that to understand how this complex socio-ecological-physical system works at the coast, and how it might evolve over decades as the coupled dance between human decisions and coastal environmental change unfolds, will require transdisciplinary approaches. Deep collaborations among stakeholders, practitioners, and researchers from a wide range of physical, biological and social sciences will be needed. Forging new, transdisciplinary science will produce the understandings that will inform decision-making, allowing communities and societies to evaluate the long-term outcomes of decisions with which they are faced, both now and to mid-century.

Transdisciplinary approaches will also be necessary to inform innovative restoration activities. Anthropogenic pressures are driving coastal ecosystem loss and coastal squeeze. However, restoration research is growing and there are important lessons to be learnt from those activities which analyse the ecological and social effects of restoration activities. A socio-environmental approach addresses the gap between governance, implementation and social commitment and allows for transfer of knowledge across the science-policy-practice continuum. Innovative approaches for water quality improvement, for hydrological reconnection and for restoring ecosystem services and societal benefits are priority areas for investigation. Inputs from indigenous knowledge systems and local communities are particularly important as these stakeholders are crucial actors in the implementation of ecosystem-based mitigation and adaptation strategies (Porri et al., Reference Porri, McConnachie, van der Walt, Wynberg and Pattrick2023).

For sustainable coastal futures the management, development, and use of the coast’s resources requires critical and reflective research across all disciplines to realise transformations to prevailing practices, institutional structures and processes, including consideration of the inter-connected ethical, cultural, political, social, economic, institutional, technological and behavioural dimensions of coastal development. There are many gaps in our knowledge and understanding about how to transform prevailing coastal thinking and practices. How can formal and informal coastal governance structures and processes be aligned to foster resilience, adaptive capacity and sustainability? What does experience in different coastal contexts reveal about limits, barriers and opportunities? What can be done to support and empower coastal communities that are most at risk in this era of global change – the exposed and vulnerable communities in low-lying deltaic regions (Shaw et al., Reference Shaw, Luo, Cheong, Abdul Halim, Chaturvedi, Hashizume, Insarov, Ishikawa, Jafari, Kitoh, Pulhin, Singh, Vasant, Zhang, Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022); sea level atoll islands (Duvat et al., Reference Duvat, Magnan, Perry, Spencer, Bell, Wabnitz, Webb, White, McInnes, Gattuso, Graham, Nunn and Le Cozannet2021; Cooley et al., Reference Cooley, Schoeman, Bopp, Boyd, Donner, Ghebrehiwet, Ito, Kiessling, Martinetto, Ojea, M-F, Rost, Skern-Mauritzen, Pörtner, Roberts, Tignor, Poloczanska, Mintenbeck, Alegría, Craig, Langsdorf, Löschke, Möller, Okem and Rama2022); and the circum-arctic region (Ford et al., Reference Ford, Pearce, Canosa and Harper2021)? How can conflicting beliefs, worldviews and practices be reconciled? What can be done to align interests and coastal equity and sustainability prospects across sectoral, institutional, geographic and temporal scales? What can be learned from past transformative coastal change? Such questions necessitate reframing how we understand and take actions in pursuit of ‘human progress’ with vitally important implications for how we reconcile individual, group and societal interests, rights and responsibilities within and between generations.