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Marine Transgression in Modern Times
Abstract
Marine transgression associated with rising sea levels causes coastal erosion, landscape transitions, and displacement of human populations globally. This process takes two general forms. Along open-ocean coasts, active transgression occurs when sediment-delivery rates are unable to keep pace with accommodation creation, leading to wave-driven erosion and/or landward translation of coastal landforms. It is highly visible, rapid, and limited to narrow portions of the coast. In contrast, passive transgression is subtler and slower, and impacts broader areas. It occurs along low-energy, inland marine margins; follows existing upland contours; and is characterized predominantly by the landward translation of coastal ecosystems. The nature and relative rates of transgression along these competing margins lead to expansion and/or contraction of the coastal zone and—particularly under the influence of anthropogenic interventions—will dictate future coastal-ecosystem response to sea-level rise, as well as attendant, often inequitable, impacts on human populations.
Expected final online publication date for the Annual Review of Marine Science, Volume 16 is January 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... The impact of SWI on aquifers used for drinking water and irrigation has been studied for decades (Barlow & Reichard 2010, Werner et al. 2013, but an understanding of how shallow seawater intrusion affects coastal ecosystems is only beginning to emerge. SWI represents the leading edge of SLR, often preceding tidal inundation and active shoreline transgression and influencing landscapes far from the coast (Hein & Kirwan 2024, Tully et al. 2019a. Increasing salinity causes the stress and eventual mortality of a variety of coastal plants, with ghost forests and abandoned agricultural fields representing prominent visual indicators of ecosystem change (Kirwan & Gedan 2019) (Figure 1). ...
... However, assumptions of a static topography contrast with the general understanding that SLR results in dynamic changes to the topography of more seaward coastal landforms (e.g., beaches, deltas, and marshes), where surface elevations evolve according to the balance between erosion and sediment deposition (Anthony 2015, FitzGerald et al. 2008, Kirwan & Megonigal 2013. Geomorphic change is likely to be more subtle in landward portions of the coastal zone (i.e., the leading edge of SWI) and determined by processes that alter the balance between soil formation (e.g., plant productivity) and soil losses (e.g., organic matter decomposition) (Hein & Kirwan 2024). Feedbacks between geomorphology and SWI are potentially strong because elevation and topographic slope directly affect groundwater discharge and inundation frequency (Michael et al. 2013, Yu et al. 2016). ...
... Other processes may decrease the elevation of terrestrial soils during SWI, resulting in a positive feedback where geomorphic processes amplify the impacts of SWI on migrating ecosystems. Because the leading edge of SWI is typically located far inland from waves and tidal currents (Hein & Kirwan 2024, Hingst et al. 2023, Tully et al. 2019b), degradation of organic-rich soils is more likely to result in loss of elevation than physical erosion of sediment. Two processes are likely to contribute to losses of soil elevation (Figure 5c). ...
The impact of saltwater intrusion on coastal forests and farmland is typically understood as sea-level-driven inundation of a static terrestrial landscape, where ecosystems neither adapt to nor influence saltwater intrusion. Yet recent observations of tree mortality and reduced crop yields have inspired new process-based research into the hydrologic, geomorphic, biotic, and anthropogenic mechanisms involved. We review several negative feedbacks that help stabilize ecosystems in the early stages of salinity stress (e.g., reduced water use and resource competition in surviving trees, soil accretion, and farmland management). However, processes that reduce salinity are often accompanied by increases in hypoxia and other changes that may amplify saltwater intrusion and vegetation shifts after a threshold is exceeded (e.g., subsidence following tree root mortality). This conceptual framework helps explain observed rates of vegetation change that are less than predicted for a static landscape while recognizing the inevitability of large-scale change.
... As important, low-elevation and narrow barriers undergo a faster landward migration, or marine transgression, as more overwash events are able to transport sand from the beach to the backbarrier 7 , potentially exposing stored carbon-rich organic deposits from wetlands and coastal lagoons to high-energy waves at the nearshore 11 . Barrier migration can have a large impact on the size and characteristics of the coastal zone 12 and could potentially shift the carbon budget of the entire coastal system from a net carbon sink to a carbon source 13,14 . Barrier elevation thus offers a good description of the barrier state, in which case the formation and post-storm recovery of coastal dunes provide a crucial indication of barrier resilience and ulterior dynamical response to external drivers. ...
Barrier islands cover a large fraction of US coasts and support unique ecosystems and coastal infrastructure. The ‘barrier’ function of a barrier island depends on coastal dunes that can prevent storm flooding and widespread ecosystem loss. Furthermore, dune-less barriers are more susceptible to breaching and potential drowning under sea level rise. Here we study the transition from richly-vegetated barriers with mature dunes (‘high’ state) to dune-less barren barriers (‘barren’ state) using data from a representative set of barrier islands in Virginia, US. We find that these two states are possible stable solutions of a non-linear stochastic dynamics characterized by a tipping point at which barriers with elevation around beach berms experience a critical transition into a permanently barren state. Our results suggest that frequently-flooded dune-less barren islands are a natural endpoint of barrier’s evolution under sea level rise.
... Accelerating sea-level rise rates (Kemp et al. 2011) promote ecosystem transitions within low-lying coastal landscapes and fundamentally reorganize ecosystem structure (Hein and Kirwan 2024;Langston et al. in review). Coastal salt marshes are vital environments that provide ecosystem services such as critical habitat, storm surge protection, and long-term carbon storage (Chmura et al. 2003;Gedan et al. 2009;Möller et al. 2014). ...
Rising sea levels lead to the migration of salt marshes into coastal forests, thereby shifting both ecosystem composition and function. In this study, we investigate leaf litter decomposition, a critical component of forest carbon cycling, across the marsh-forest boundary with a focus on the potential influence of environmental gradients (i.e., temperature, light, moisture, salinity, and oxygen) on decomposition rates. To examine litter decomposition across these potentially competing co-occurring environmental gradients, we deployed litterbags within distinct forest health communities along the marsh-forest continuum and monitored decomposition rates over 6 months. Our results revealed that while the burial depth of litter enhanced decomposition within any individual forest zone by approximately 60% (decay rate = 0.272 ± 0.029 yr ⁻¹ (surface), 0.450 ± 0.039 yr ⁻¹ (buried)), we observed limited changes in decomposition rates across the marsh-forest boundary with only slightly enhanced decomposition in mid-forest soils that are being newly impacted by saltwater intrusion and shrub encroachment. The absence of linear changes in decomposition rates indicates non-linear interactions between the observed environmental gradients that maintain a consistent net rate of decomposition across the marsh-forest boundary. However, despite similar decomposition rates across the boundary, the accumulated soil litter layer disappears because leaf litter influx decreases from the absence of mature trees. Our finding that environmental gradients counteract expected decomposition trends could inform carbon-climate model projections and may be indicative of decomposition dynamics present in other transitioning ecosystem boundaries.
... Global processes, such as sea level rise, are responsible for landscape-scale shifts in coastal ecosystems extent (Hein & Kirwan, 2024;A. J. Smith & Goetz, 2021;E. ...
Sea level rise is leading to the rapid migration of marshes into coastal forests and other terrestrial ecosystems. Although complex biophysical interactions likely govern these ecosystem transitions, projections of sea level driven land conversion commonly rely on a simplified “threshold elevation” that represents the elevation of the marsh‐upland boundary based on tidal datums alone. To determine the influence of biophysical drivers on threshold elevations, and their implication for land conversion, we examined almost 100,000 high‐resolution marsh‐forest boundary elevation points, determined independently from tidal datums, alongside hydrologic, ecologic, and geomorphic data in the Chesapeake Bay, the largest estuary in the U.S. located along the mid‐Atlantic coast. We find five‐fold variations in threshold elevation across the entire estuary, driven not only by tidal range, but also salinity and slope. However, more than half of the variability is unexplained by these variables, which we attribute largely to uncaptured local factors including groundwater discharge, microtopography, and anthropogenic impacts. In the Chesapeake Bay, observed threshold elevations deviate from predicted elevations used to determine sea level driven land conversion by as much as the amount of projected regional sea level rise by 2050. These results suggest that local drivers strongly mediate coastal ecosystem transitions, and that predictions based on elevation and tidal datums alone may misrepresent future land conversion.
... For example, rapid SLR can exacerbate inundation stress and eventually lead to drowning of intertidal blue carbon coastal ecosystems, thereby reducing sequestration potential while also degrading soil C [7][8][9] . Additionally, SLR can lead to large C losses within the coastal zone by driving ecosystem transgression (for example, forest retreat, which prompts substantial aboveground biomass loss 10,11 ) and/or by driving erosion of C-rich sediments when exposed along open-ocean coasts 12,13 . Thus, coastal landscapes facing the combined threats of SLR and erosion risk a blue carbon stock that is both diminished and more fleeting. ...
Landward migration of coastal ecosystems in response to sea-level rise is altering coastal carbon dynamics. Although such landscapes rapidly accumulate soil carbon, barrier-island migration jeopardizes long-term storage through burial and exposure of organic-rich backbarrier deposits along the lower beach and shoreface. Here, we quantify the carbon flux associated with the seaside erosion of backbarrier lagoon and peat deposits along the Virginia Atlantic Coast. Barrier transgression leads to the release of approximately 26.1 Gg of organic carbon annually. Recent (1994–2017 C.E.) erosion rates exceed annual soil carbon accumulation rates (1984–2020) in adjacent backbarrier ecosystems by approximately 30%. Additionally, shoreface erosion of thick lagoon sediments accounts for >80% of total carbon losses, despite containing lower carbon densities than overlying salt marsh peat. Together, these results emphasize the impermanence of carbon stored in coastal environments and suggest that existing landscape-scale carbon budgets may overstate the magnitude of the coastal carbon sink.
Debidue Beach and North Inlet are a coupled mixed-energy inlet-barrier system along the South Atlantic Bight in South Carolina, USA. Long-term chronic erosion along much of Debidue throughout the 20th century is the result of a shoreline adjustment triggered by an avulsion in the main channel of North Inlet occurring between 1926 and 1934. We use a historical database of shoreline positions compiled by the United States Geological Survey (USGS) and the Analysis of Moving Boundaries Using R (AMBUR) package to quantify changes in the Debidue Beach shoreline over nearly 150 years from 1872 to 2011. This analysis documents relatively large-scale shoreline changes (and equivalent volumetric changes above local depth of closure “DOC”) following the shift in channel position, and a logarithmic decrease in erosion rates over the following decades. Mixed-energy ebb-dominant inlets have considerable effects on adjacent beaches due to their ability to retain and shed large quantities of sand relatively quickly. This study demonstrates that even systems exhibiting long-term stability — like North Inlet — are indeed migrational landforms and should be considered as such when formulating shoreline management plans along adjacent beaches.
Debidue Beach and North Inlet are a coupled mixed-energy inlet-barrier system along the South Atlantic Bight in South Carolina, USA. Long-term chronic erosion along much of Debidue throughout the 20 th century is the result of a shoreline adjustment triggered by an avulsion in the main channel of North Inlet occurring between 1926 and 1934. We use a historical database of shoreline positions compiled by the United States Geological Survey (USGS) and the Analysis of Moving Boundaries Using R (AMBUR) package to quantify changes in the Debidue Beach shoreline over nearly 150 years from 1872 to 2011. This analysis documents relatively large-scale shoreline changes (and equivalent volumetric changes above local depth of closure "DOC") following the shift in channel position, and a logarithmic decrease in erosion rates over the following decades. Mixed-energy ebb-dominant inlets have considerable effects on adjacent beaches due to their ability to retain and shed large quantities of sand relatively quickly. This study demonstrates that even systems exhibiting long-term stability-like North Inlet-are indeed migrational landforms and should be considered as such when formulating shoreline management plans along adjacent beaches.
Ecosystem connectivity tends to increase the resilience and function of ecosystems responding to stressors. Coastal ecosystems sequester disproportionately large amounts of carbon, but rapid exchange of water, nutrients, and sediment makes them vulnerable to sea level rise and coastal erosion. Individual components of the coastal landscape (i.e., marsh, forest, bay) have contrasting responses to sea level rise, making it difficult to forecast the response of the integrated coastal carbon sink. Here we couple a spatially-explicit geomorphic model with a point-based carbon accumulation model, and show that landscape connectivity, in-situ carbon accumulation rates, and the size of the landscape-scale coastal carbon stock all peak at intermediate sea level rise rates despite divergent responses of individual components. Progressive loss of forest biomass under increasing sea level rise leads to a shift from a system dominated by forest biomass carbon towards one dominated by marsh soil carbon that is maintained by substantial recycling of organic carbon between marshes and bays. These results suggest that climate change strengthens connectivity between adjacent coastal ecosystems, but with tradeoffs that include a shift towards more labile carbon, smaller marsh and forest extents, and the accumulation of carbon in portions of the landscape more vulnerable to sea level rise and erosion.
In the Pacific Basin, the El Niño/Southern Oscillation (ENSO) is the dominant mode of interannual climate variability, driving substantial changes in oceanographic forcing and impacting Pacific coastlines. Yet, how sandy coasts respond to these basin-scale changes has to date been limited to a few long-term beach monitoring sites, predominantly on developed coasts. Here we use 38 years of Landsat imagery to map shoreline variability around the Pacific Rim and identify coherent patterns of beach erosion and accretion controlled by ENSO. On the basis of more than 83,000 beach transects covering 8,300 km of sandy coastline, we find that approximately one-third of all transects experience significant erosion during El Niño phases. The Eastern Pacific is particularly vulnerable to widespread erosion, most notably during the large 1997/1998 El Niño event. By contrast, La Niña events coincide with significant accretion for approximately one-quarter of all transects, although substantial erosion is observed in southeast Australia and other localized regions. The observed regional variability in the coastal response to ENSO has important implications for coastal planning and adaptation measures across the Pacific, particularly in light of projected future changes in ENSO amplitude and flavour.
Coastal wetlands provide a wide array of ecosystem services, valued at trillions of dollars per year globally. Although accelerating sea level rise (SLR) poses the long-term threat of inundation to coastal areas, wetlands may be sustained in two ways: by positive net surface-elevation change (SEC) from sediment and organic matter buildup and by accumulation, or horizontal migration into refugia—low-lying, undeveloped upland areas that become inundated. Using a simple model together with high-resolution elevation data, we provide, across the contiguous United States, analysis of the local effects of SLR, maximum SEC rates, and coastal development on the long-term resilience of coastal wetlands. We find that protecting current refugia is a critical factor for retaining wetlands under accelerating SLR. If refugia are conserved under an optimistic scenario (a high universal maximum SEC rate of 8 mm/yr and low greenhouse gas emissions), wetlands may increase by 25.0% (29.4%–21.5%; 50th, 5th–95th percentiles of SLR) by the end of the century. However, if refugia are developed under a more pessimistic scenario (a moderate universal maximum SEC rate of 3 mm/yr, high greenhouse gas emissions, and projections incorporating high ice-sheet contributions to SLR), wetlands may decrease by −97.0% (−82.3%–99.9%). These median changes in wetland area could result in an annual gain of ∼732 billion in ecosystem services in the US alone. Focusing on key management options for sustaining wetlands, we highlight areas at risk of losing wetlands and identify the benefits possible from conserving refugia or managing SEC rates.
Coastal response to anthropogenic climate change is of central importance to the infrastructure and inhabitants in these areas. Despite being globally ubiquitous, the stability of rock coasts has been largely neglected, and the expected acceleration of cliff erosion following sea-level rise has not been tested with empirical data, until now. We have optimised a coastal evolution model to topographic and cosmogenic radionuclide data to quantify cliff retreat rates for the past 8000 years and forecast rates for the next century. Here we show that rates of cliff retreat will increase by up to an order of magnitude by 2100 according to current predictions of sea-level rise: an increase much greater than previously predicted. This study challenges conventional coastal management practices by revealing that even historically stable rock coasts are highly sensitive to sea-level rise and should be included in future planning for global climate change response. Results forecast that cliff retreat rates will increase by up to an order of magnitude by 2100 according to current predictions of sea-level rise, and reveal that even historically stable rock coasts are highly sensitive to sea-level rise.
Future sea-level rise poses an existential threat for many river deltas, yet quantifying the effect of sea-level changes on these coastal landforms remains a challenge. Sea-level changes have been slow compared to other coastal processes during the instrumental record, such that our knowledge comes primarily from models, experiments, and the geologic record. Here we review the current state of science on river delta response to sea-level change, including models and observations from the Holocene until 2300 CE. We report on improvements in the detection and modeling of past and future regional sea-level change, including a better understanding of the underlying processes and sources of uncertainty. We also see significant improvements in morphodynamic delta models. Still, substantial uncertainties remain, notably on present and future subsidence rates in and near deltas. Observations of delta submergence and land loss due to modern sea-level rise also remain elusive, posing major challenges to model validation.
Climate change, land subsidence, and coastal population growth are increasing coastal flood risks and changing land uses. Large-scale levee systems protect many urban areas from flooding, but much less is known about how rural coasts respond to sea level rise and increasing flood risks. This study examined previously unreported, small-scale earthen levees that have been constructed for centuries by individual landowners in rural, low-lying portions of the Chesapeake Bay region in the USA. Analysis of aerial imagery, digital elevation models, and field measurements revealed that levees are constructed from inorganic silt loam sediment consistent with adjacent terrestrial soils. They extend above Highest Astronomical Tide, and are today surrounded by marsh or low-lying terrestrial vegetation. Although preliminary measurements revealed inconsistent effects of levees on soil salinity or soil organic content, the landward side of levees are generally lower in elevation than the seaward side. They are also characterized by less flood tolerant vegetation and shallower organic-rich soils. These results suggest that small-scale levees may have historically impeded wetland development, though their effects today are unclear. This work highlights a historical approach to rural flood defense and suggests that, in some cases, the impacts of small levees are observable long after coastal retreat and levee abandonment. They are therefore part of the evolving human-landscape system for potential management into the future
Low‐elevation coastal areas are increasingly vulnerable to seawater flooding as sea levels rise and the frequency and intensity of large storms increase with climate change. Seawater flooding can lead to the salinization of fresh coastal aquifers by vertical saltwater intrusion (SWI). Vertical SWI is often overlooked in coastal zone threat assessments despite the risk it poses to critical freshwater resources and salt‐intolerant ecosystems that sustain coastal populations. This review synthesizes field and modeling approaches for investigating vertical SWI and the practical and theoretical understanding of salinization and flushing processes obtained from prior studies. The synthesis explores complex vertical SWI dynamics that are influenced by density‐dependent flow and oceanic, hydrologic, geologic, climatic, and anthropogenic forcings acting on coastal aquifers across spatial and temporal scales. Key knowledge gaps, management challenges, and research opportunities are identified to help advance our understanding of the vulnerability of fresh coastal groundwater. Past modeling studies often focus on idealized aquifer systems, and thus future work could consider more diverse geologic, climatic, and topographic environments. Concurrent field and modeling programs should be sustained over time to capture interactions between physical processes, repeated salinization and flushing events, and delayed aquifer responses. Finally, this review highlights the need for improved coordination and knowledge translation across disciplines (e.g., coastal engineering, hydrogeology, oceanography, social science) to gain a more holistic understanding of vertical SWI. There also needs to be more education of communities, policy makers, and managers to motivate societal action to address coastal groundwater vulnerability in a changing climate.
Coastal ecosystems represent a disproportionately large but vulnerable global carbon sink. Sea-level-driven tidal wetland degradation and upland forest mortality threaten coastal carbon pools, but responses of the broader coastal landscape to interacting facets of climate change remain poorly understood. Here, we use 36 years of satellite observations across the mid-Atlantic sea-level rise hotspot to show that climate change has actually increased the amount of carbon stored in the biomass of coastal ecosystems despite substantial areal loss. We find that sea-level-driven reductions in wetland and low-lying forest biomass were largely confined to areas less than 2 m above sea level, whereas the otherwise warmer and wetter climate led to an increase in the biomass of adjacent upland forests. Integrated across the entire coastal landscape, climate-driven upland greening offset sea-level-driven biomass losses, such that the net impact of climate change was to increase the amount of carbon stored in coastal vegetation. These results point to a fundamental decoupling between upland and wetland carbon trends that can only be understood by integrating observations across traditional ecosystem boundaries. This holistic approach may provide a template for quantifying carbon-climate feedbacks and other aspects of coastal change that extend beyond sea-level rise alone.
This paper presents a new empirical model, called the cumulative storm impact index (CSII), that quantifies the impact of coastal storms on sandy beaches. The new model utilizes user‐defined storm data to incorporate both individual storm magnitude and the cumulative effect of successive storms into an index, which is a proxy for beach erosion at a given time. Applying this model to long‐term water‐level data from a Virginia tide gauge showed that the greatest storm impact resulted not from the larger individual storms, such as the Ash Wednesday nor'easter of 1962, the “Perfect Storm” of 1991, or Hurricane Sandy of 2012, but rather from especially stormy winter seasons that occurred during the twenty‐first century. Additionally, the CSII model uncovered a trend—not detectable by single storm impact analyses—toward greater storm impacts, which began c. 1980 and continued to the present day. Finally, comparative analyses using wave power as a storm index shows CSII can capture decadal or seasonal scale storminess. We expect this model to have utility in many areas of the coastal sciences and engineering, including developing holistic response models, quantifying erosion potential at other locations, and managing coastal ecosystems.
Wave-dominated sandy beaches are highly valued by societies and are amongst the world’s most energetic and dynamic environments. On wave-dominated beaches with unlimited sand supply and limited influence of tide and geology, beach change has long been conceptualised in the morphodynamic framework of Wright and Short (1984). Such framework describes the occurrence of beach types based on wave conditions and sediment characteristics across the complete reflective–dissipative spectrum. Building on theoretical work, field/laboratory measurements and monitoring programmes, the physical mechanisms underpinning this morphodynamic framework have been progressively unravelled. Cross-shore morphological changes are primarily controlled by equilibrium and beach memory principles with below (above) average wave conditions driving down-state (up-state) transitions associated with onshore (offshore) sediment transport. Such cross-shore behaviour mostly reflects the imbalance between the onshore-directed sediment transport driven by wave nonlinearities and the offshore-directed sediment transport driven by the undertow. Self-organised morphological instabilities resulting from different positive feedback mechanisms are primarily responsible for alongshore morphological variability and the generation of rhythmic morphological features, such as crescentic bars, rip channels and beach cusps. Critically, wave climate and changes in wave regimes are key in driving the coupled cross-shore and longshore behaviour that ultimately explains modal beach state and frequency-response characteristics of beach morphological time series.
The response of coastal barrier islands to relative sea-level rise (SLR) is a long-debated issue. Over centennial and longer periods, regional barrier retreat is generally proportional to the rate of relative SLR. However, over multi-decadal timescales, this simplification does not hold. Field observations along the USA East Coast indicate that barrier retreat rate has at most increased by ~45% in the last ~100 years, despite a concurrent ≥200% increase in SLR rate. Using a coastal evolution model, we explain this observation by considering disequilibrium dynamics—the lag in barrier behaviour with respect to SLR. Here we show that modern barrier retreat rate is not controlled by recent SLR (last decades), but rather by the baseline SLR of the past centuries. The cumulative effect of the baseline SLR is to establish a potential retreat, which is then realized by storms and tidal processes in the following centuries. When SLR accelerates, the potential for retreat is first realized through removal of geomorphic capital. After several centuries, barrier retreat accelerates proportionally to the increase in SLR. As such, we predict a committed coastal response: even if SLR remains at present rates, barrier retreat in response to SLR will accelerate by ~50% within a century. The lag dynamics identified here are probably general, and should be included in predictions of barrier-system response to climate change. Coastal evolution simulations suggest that the modern retreat of coastal barrier islands is controlled by cumulative sea-level rise over the past several centuries and will accelerate by 50% within a century, even if sea-level rise remains at present rates.
Coastal wetlands are not only among the world's most valued ecosystems but also among the most threatened by high greenhouse gas emissions that lead to accelerated sea level rise. There is intense debate regarding the extent to which landward migration of wetlands might compensate for seaward wetland losses. By integrating data from 166 estuaries across the conterminous United States, we show that landward migration of coastal wetlands will transform coastlines but not counter seaward losses. Two-thirds of potential migration is expected to occur at the expense of coastal freshwater wetlands, while the remaining one-third is expected to occur at the expense of valuable uplands, including croplands, forests, pastures, and grasslands. Our analyses underscore the need to better prepare for coastal transformations and net wetland loss due to rising seas.
Observations of woody plant mortality in coastal ecosystems are globally widespread, but the overarching processes and underlying mechanisms are poorly understood. This knowledge deficiency, combined with rapidly changing water levels, storm surges, atmospheric CO2, and vapor pressure deficit, creates large predictive uncertainty regarding how coastal ecosystems will respond to global change. Here we synthesize the literature on the mechanisms that underlie coastal woody‐plant mortality, with the goal of producing a testable hypothesis framework. The key emergent mechanisms underlying mortality include hypoxic, osmotic, and ionic‐driven reductions in whole‐plant hydraulic conductance and photosynthesis that ultimately drive the coupled processes of hydraulic failure and carbon starvation. The relative importance of these processes in driving mortality, their order of progression, and their degree of coupling depends on the characteristics of the anomalous water exposure, on topographic effects, and on taxa‐specific variation in traits and trait acclimation. Greater inundation exposure could accelerate mortality globally; however, the interaction of changing inundation exposure with elevated CO2, drought, and rising vapor pressure deficit could influence mortality likelihood. Models of coastal forests that incorporate the frequency and duration of inundation, the role of climatic drivers, and the processes of hydraulic failure and carbon starvation can yield improved estimates of inundation‐induced woody‐plant mortality.
Arctic coastal erosion damages infrastructure, threatens coastal communities and releases organic carbon from permafrost. However, the magnitude, timing and sensitivity of coastal erosion increase to global warming remain unknown. Here we project the Arctic-mean erosion rate to increase and very likely exceed its historical range of variability before the end of the century in a wide range of emission scenarios. The sensitivity of erosion to warming roughly doubles, reaching 0.4–0.8 m yr−1 °C−1 and 2.3–4.2 TgC yr−1 °C−1 by the end of the century. We develop a simplified semi-empirical model to produce twenty-first-century pan-Arctic coastal erosion rate projections. Our results will inform policymakers on coastal conservation and socioeconomic planning, and organic carbon flux projections lay out the path for future work to investigate the impact of Arctic coastal erosion on the changing Arctic Ocean, its role as a global carbon sink, and the permafrost–carbon feedback. Coastal erosion in the Arctic is caused by permafrost thaw and wave abrasion enhanced by sea ice melt, both of which will increase under climate change. Projections of erosion rate across the Arctic indicate that mean erosion rates will rise beyond historical precedent over the twenty-first century.
The effects of sea level rise and coastal saltwater intrusion on wetland plants can extend well above the high-tide line due to drought, hurricanes, and groundwater intrusion. Research has examined how coastal salt marsh plant communities respond to increased flooding and salinity, but more inland coastal systems have received less attention. The aim of this study was to identify whether ground layer plants exhibit threshold responses to salinity exposure. We used two vegetation surveys throughout the Albemarle-Pamlico Peninsula (APP) of North Carolina, USA to assess vegetation in a low elevation landscape (≤ 3.8 m) experiencing high rates of sea level rise (3–4 mm/year). We examined the primary drivers of community composition change using Non-metric Multidimensional Scaling (NMDS) and used Threshold Indicator Taxa Analysis (TITAN) to detect thresholds of compositional change based on indicator taxa, in response to potential indicators of exposure to saltwater (Na, and the Σ Ca + Mg) and elevation. Salinity and elevation explained 64% of the variation in community composition, and we found two salinity thresholds for both soil Na⁺ (265 and 3843 g Na⁺/g) and Ca⁺ + Mg⁺ (42 and 126 µeq/g) where major changes in community composition occur on the APP. Similar sets of species showed sensitivity to these different metrics of salt exposure. Overall, our results showed that ground layer plants can be used as reliable indicators of salinity thresholds in coastal wetlands. These results can be used for monitoring salt exposure of ecosystems and for identifying areas at risk for undergoing future community shifts.
Sea-level rise (SLR) has been confirmed to be accelerating globally due to human-influence driven climate change. Multiple studies suggest many coastal communities will soon be inundated by SLR. Prior to inundation, habitable uplands above the high tide line first convert to uninhabitable wetlands, forcing human exodus. Habitability, not the land's presence above the low tide line, drives exodus. We determined the time left for uplands of the Town of Tangier of VA, USA to be converted to wetlands, analyzed local sea level rise data to determine the best local SLR scenario (low, mid, or high) fit, then compared upland conversion rate to the rate of population decline. The upland landmass constituting the Town of Tangier declined from 32.8 to 12.5 ha (1967–2019), accelerating over time, with complete conversion to wetlands predicted by 2051. The US Army Corps of Engineers (USACE) high SLR curve is the best fit to the local tide gauge's raw data (1967–2020), indicating local sea level rise has rapidly accelerated in recent decades, concomitant with the rate of wetland conversion. The Town's population, in decline since the 1930s, accelerated rapidly after 1980 and trended downward in tandem with the conversion of the Town's uplands to wetlands. We also estimated costs to relocate the Town as well as for a conceptual plan to provide long-term stability to the Town and Island of Tangier.
Coastal forested wetlands support many endemic species, sequester substantial carbon stocks, and have been reduced in extent due to historic drainage and agricultural expansion. Many of these unique coastal ecosystems have been drained, while those that remain are now threatened by saltwater intrusion and sea level rise in hydrologically modified coastal landscapes. Several recent studies have documented rapid and accelerating losses of coastal forested wetlands in small areas of the Atlantic and Gulf coasts of North America, but the full extent of loss across North America’s Coastal Plain (NACP) has not been quantified. We used classified satellite imagery to document a net loss of ~ 13,682 km2 (8%) of forested coastal wetlands across the NACP between 1996 and 2016. Most forests transitioned to scrub-shrub (53%) and marsh habitats (24%). Even within protected areas, we measured substantial rates of wetland deforestation and significant fragmentation of forested wetland habitats. Variation in the rate of sea level rise, the number of tropical storm landings, and the average elevation of coastal watersheds explained about 78% of the variation in coastal wetland deforestation extent along the South Atlantic and Gulf Coasts. The rate of coastal forest loss within the NACP (684 km2/y) exceeds the recent estimate of global losses of coastal mangroves (210 km2/y). At the current rate of deforestation, in the absence of widespread protection or restoration efforts, coastal forested wetlands may not persist into the next century.
The impacts of climate change on ecosystems are manifested in how organisms respond to episodic and continuous stressors. The conversion of coastal forests to salt marshes represents a prominent example of ecosystem state change, driven by the continuous stress of sea‐level rise (press), and episodic storms (pulse). Here, we measured the rooting dimension and fall direction of 143 windthrown eastern red cedar (Juniperus virginiana) trees in a rapidly retreating coastal forest in Chesapeake Bay (USA). We found that tree roots were distributed asymmetrically away from the leading edge of soil salinization and towards freshwater sources. The length, number, and circumference of roots were consistently higher in the upslope direction than downslope direction, suggesting an active morphological adaptation to sea‐level rise and salinity stress. Windthrown trees consistently fell in the upslope direction regardless of aspect and prevailing wind direction, suggesting that asymmetric rooting destabilized standing trees, and reduced their ability to withstand high winds. Together, these observations help explain curious observations of coastal forest resilience, and highlight an interesting nonadditive response to climate change, where adaptation to press stressors increases vulnerability to pulse stressors.
River deltas will likely experience significant land loss because of relative sea-level rise (RSLR), but predictions have not been tested against observations. Here, we use global data of RSLR and river sediment supply to build a model of delta response to RSLR for 6,402 deltas, representing 86% of global delta land. We validate this model against delta land area change observations from 1985–2015, and project future land area change for IPCC RSLR scenarios. For 2100, we find widely ranging delta scenarios, from +94 ± 125 (2 s.d.) km² yr⁻¹ for representative concentration pathway (RCP) 2.6 to −1,026 ± 281 km² yr⁻¹ for RCP8.5. River dams, subsidence, and sea-level rise have had a comparable influence on reduced delta growth over the past decades, but if we follow RCP8.5 to 2100, more than 85% of delta land loss will be caused by climate-change driven sea-level rise, resulting in a loss of ∼5% of global delta land.
Plain Language Summary
Forests along the upland edge of salt marshes are being killed by rising sea levels and replaced with salt marsh in a process called marsh migration. Marsh soils, unlike soils in forest, quickly accumulate carbon in their soils. This indicates that marsh migration could possibly increase carbon storage across the landscape. Here, we show the opposite, that sea level rise reduces coastal carbon stocks through the loss of woody aboveground biomass. This loss is partially offset with expanding organic soil in the newly formed marsh, but the amount of carbon lost from forest mortality is far greater than that gained by the growing marsh soils. Continued carbon accumulation in wetland soils may eventually compensate for forest carbon loss, but the time it would take for marshes to replace that lost carbon is at the scale of centuries, which is approximately the same amount of time predicted for marshes to drown from rising sea levels. This suggests that forest carbon may never be replaced and reveals a critical source of carbon under climate driven landscape changes.
Plain Language Summary
Barrier islands and spits tend to move landward in response to sea‐level rise and storms when sediment from the beach is washed into the barrier interior and beyond, but tall dunes at the front of a barrier can prevent this process from happening for all but the largest storms. Here, we explore the interactions between dunes, storms, and barrier migration with a new model (Barrier3D). Our model shows that barriers migrate when dunes are low but are stationary when dunes are tall. Over decades to centuries, repeated cycles of dune loss and regrowth lead to sporadic (or “discontinuous”) barrier retreat. Additionally, we find that sporadic behavior may become less common in the future in response to rising sea levels, while increasing storm intensity will change which environments are most likely to experience discontinuous retreat. These findings emphasize the importance of dune dynamics in controlling barrier evolution. Barrier migration over decadal timescales ‐ timescales relevant to coastal communities and managers ‐ is commonly modeled as a constant background process, but our results suggest that is not always the case.
Coastal salt marshes, which provide valuable ecosystem services such as flood mitigation and carbon sequestration, are threatened by rising sea level. In response, these ecosystems migrate landward, converting available upland into salt marsh. In the coastal-plain surrounding Chesapeake Bay, United States, conversion of coastal forest to salt marsh is well-documented and may offset salt marsh loss due to sea level rise, sediment deficits, and wave erosion. Land slope at the marsh-forest boundary is an important factor determining migration likelihood, however, the standard method of using field measurements to assess slope across the marsh-forest boundary is impractical on the scale of an estuary. Therefore, we developed a general slope quantification method that uses high resolution elevation data and a repurposed shoreline analysis tool to determine slope along the marsh-forest boundary for the entire Chesapeake Bay coastal-plain and find that less than 3% of transects have a slope value less than 1%; these low slope environments offer more favorable conditions for forest to marsh conversion. Then, we combine the bay-wide slope and elevation data with inundation modeling from Hurricane Isabel to determine likelihood of coastal forest conversion to salt marsh. This method can be applied to local and estuary-scale research to support management decisions regarding which upland forested areas are more critical to preserve as available space for marsh migration.
Climate change will necessitate evermore frequent and complex managed retreats in the future, and drafting policies that are equitable and just for those residents who are relocating will be essential. The USA’s first federally funded, community-scale, climate-driven resettlement is currently underway in coastal Louisiana. In January 2016, the U.S. Department of Housing and Urban Development (HUD) awarded the state of Louisiana $48.3 million to plan, design, and implement a structured, just, and scalable resettlement with former and current Isle de Jean Charles residents. Most Island households are multi-generational and directly descended from Jean Marie Naquin, after whose father the Island is named. Using interviews, ethnographic data, and policy documents, this paper will delineate and analyze the dimensions of sense of place, which, in this case, prompted policy changes dramatically different from standard relocation policies: assurance that the properties and land from which residents are departing will remain in their possession as long as the land remains. For most Island residents, this was non-negotiable. The intangible connection to place—feelings of belonging, lifestyle, family connections, and culture—plays a central role in many families’ decision to stay or go. The choice to relocate is rooted in this complex entanglement of identity, familial ties, land loss, historical and current marginalization, and a way of life passed on by multiple generations. In forthcoming community resettlements, continued access and ownership of the properties being left behind should be considered as a critical component for planning just retreats.
Climate change is driving ecological shifts in coastal regions of the world, where low topographic relief makes ecosystems particularly vulnerable to sea‐level rise, salinization, storm surge, and other effects of global climate change. The consequences of rising water tables and salinity can penetrate well inland, and lead to particularly dramatic changes in freshwater forested wetlands dominated by tree species with low salt tolerance. The resulting loss of coastal forests could have significant implications to the coastal carbon cycle. We quantified the rates of vegetation change including land loss, forest loss, and shrubland expansion in North Carolina’s largest coastal wildlife refuge over 35 yr. Despite its protected status, and in the absence of any active forest management, 32% (31,600 hectares) of the refuge area has changed landcover classification during the study period. A total of 1,151 hectares of land was lost to the sea and ~19,300 hectares of coastal forest habitat was converted to shrubland or marsh habitat. As much as 11% of all forested cover in the refuge transitioned to a unique land cover type—“ghost forest”—characterized by standing dead trees and fallen tree trunks. The formation of this ghost forest transition state peaked prominently between 2011 and 2012, following Hurricane Irene and a 5‐yr drought, with 4,500 ± 990 hectares of ghost forest forming during that year alone. This is the first attempt to map and quantify coastal ghost forests using remote sensing. Forest losses were greatest in the eastern portion of the refuge closest to the Croatan and Pamlico Sounds, but also occurred much further inland in low‐elevation areas and alongside major canals. These unprecedented rates of deforestation and land cover change due to climate change may become the status quo for coastal regions worldwide, with implications for wetland function, wildlife habitat, and global carbon cycling.
Plain Language Summary
The rapid, climate‐driven acceleration of global sea level threatens salt marshes and mangroves along low‐elevation shorelines. These coastal wetlands provide protection from storms along with other ecosystem services to vulnerable coastal communities, including several megacities. The question of how coastal wetlands will cope with future sea‐level rise is a subject of much debate, with recent research providing contradictory answers. Our analysis suggests that much of this can be attributed to the time window under consideration. Even coastal wetlands that are able to persist during the next few decades are likely to be much less resilient through the remainder of this century and beyond.
Sea level rise (SLR) poses a hazard to ecosystems and economies in low-lying coastal and estuarine areas. To better understand the potential impacts of SLR in estuaries, a comprehensive review of existing theory, literature, and assessment tools is undertaken. In addition, several conceptual models are introduced to assist in understanding interlinked estuarine processes and their complex responses to SLR. This review indicates that SLR impacts in estuaries should not be assessed via static (bathtub) approaches as they fail to consider important hydrodynamic effects such as tidal wave amplification, dampening, and reflection. Where hydrodynamic models are used, the existing literature provides a relatively detailed understanding of how SLR will affect estuarine hydrodynamics (e.g., tides and inundation regimes). With regards to the current understanding of, and ability to model, the connections between altered hydrodynamics (under SLR) and dependent geomorphic, ecological, and bio-geochemical processes, significant knowledge gaps remain. This is of particular concern as there is currently a paradigm shift towards more integrated and holistic management of estuaries. Estuarine management under accelerating SLR is likely to become increasingly complex, as decision-making will be undertaken with high uncertainty. As such, this review highlights that there is a fundamental requirement for more sophisticated and interdisciplinary studies that integrate physical, ecological, bio-geochemical, and geomorphic responses of estuaries to SLR.
Ecotones are responsive to environmental change and pave a path for succession as they move across the landscape. We investigated the biotic and abiotic filters to species establishment on opposite ends of a tidal marsh‐forest ecotone that is moving inland in response to sea level rise. We transplanted four plant species common to the ecotone to the leading or trailing edge of the migrating ecotone, with and without caging to protect them from ungulate herbivores. We found that species exhibited an individualistic response to abiotic and biotic pressures in this ecotone; three species performed better at the leading edge of the ecotone in the coastal forest, whereas one performed better at the trailing edge in the marsh. Specifically, grass species Phragmites australis and Panicum virgatum grew more in the low light and low salinity conditions of the leading edge of the ecotone (forest), whereas the shrub Iva frutescens grew better in the high light, high salinity conditions of the trailing edge of the ecotone (marsh). Furthermore, of the four species, only P. australis was affected by the biotic pressure of herbivory by an introduced ungulate, Cervus nippon, which greatly reduced its biomass and survival at the leading edge (forest). P. australis is an aggressive invasive species and has been observed to dominate in the wake of migrating marsh‐forest ecotones. Our findings detail the role of lower salinity stress to promote and herbivory pressure to inhibit the establishment of P. australis during shifts of this ecotone, and also highlight an interaction between two nonnative species, P. australis and C. nippon. Understanding migration of the marsh‐forest ecotone and the factors controlling P. australis establishment are critical for marsh conservation in the face of sea level rise. More generally, our findings support the conclusion that the abiotic and biotic filters of a migrating ecotone shape the resulting community.
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.
Climate change is intensifying tropical cyclones, accelerating sea-level rise, and increasing coastal flooding. River deltas are especially vulnerable to flooding because of their low elevations and densely populated cities. Yet, we do not know how many people live on deltas and their exposure to flooding. Using a new global dataset, we show that 339 million people lived on river deltas in 2017 and 89% of those people live in the same latitudinal zone as most tropical cyclone activity. We calculate that 41% (31 million) of the global population exposed to tropical cyclone flooding live on deltas, with 92% (28 million) in developing or least developed economies. Furthermore, 80% (25 million) live on sediment-starved deltas, which cannot naturally mitigate flooding through sediment deposition. Given that coastal flooding will only worsen, we must reframe this problem as one that will disproportionately impact people on river deltas, particularly in developing and least-developed economies.
Abstract Climate-driven changes in coastal flood risk have enormous consequences for coastal cities. These risks intersect with unequal patterns of environmental hazards exacerbating differential vulnerability of climate related flooding. Here we analyze differential vulnerability of coastal flooding in New York City, USA, as an environmental justice issue caused by shifts in flood risk due to increasing floodplain extents. These extents are represented by updates to the 100-year floodplain by the Federal Emergency Management Agency, and urban changes in land use, land value, and socio-economic characteristics of flood exposed populations. We focus on six local community districts containing disproportionately vulnerable communities. Across our study areas, we observed increases in the floodplain’s extent by 45.7%, total exposed population by 10.5%, and population living in vulnerable communities by 7.5%. Overall flood risk increases regardless of increases in the updated floodplain extent, as do floodplain property values. However, variability is high between community districts; in some cases, increases in exposure coincide with decreases in vulnerability due to shifts in racial demographics and increases in income (i.e. potential floodplain gentrification), while others experienced increases in exposure and vulnerability (i.e. double jeopardy). These findings highlight that the dominant drivers of coastal flood risk in NYC are ongoing real estate development and continued increases in sea level rise and storm severity, both of which have explicit implications for flood vulnerability. We describe the social processes governing development in the flood zone, namely zoning, resilience planning, and the determination of potential flooding severity and related insurance rates. We also discuss how these social drivers of risk intersect with social dimensions of vulnerability due to racist housing markets, and the distributions of public housing and toxic chemical hazards. We conclude with a framework for the analysis of contextual and outcome-based vulnerability to coastal flood hazards, and provide policy recommendations to reduce risks over the medium to long term.
The West African coast is vulnerable to natural hazards and human interventions. Although various measures have been taken at different scales, mostly at the local level, there is a need to improve management at the regional level. We examine these actions and possible solutions from different perspectives and provide conclusions and recommendations on the integration of solutions to improve coastal management. From North West Mauritania to across the Gulf of Guinea a system of coastal zoning that can be managed holistically is encouraged. The development of holistic planning is seen as a sustainable approach to management that seeks to link users/processes together rather than focus on a single particular issue and solution. Strengthening, monitoring, promoting the observation network and generalising open data centralisation and exchange for a better understanding of coastal dynamics and pressures is encouraged. There is a need for capacity building, expertise and federative actions. Furthermore, the need to identify and involve not only stakeholders, but also communities and scientists with multilevel inputs. All must agree on coordinated plans to achieve stakeholder objectives, using an approach adapted to the multi-spatial scale (e.g at the scale of sediment cells, integrating from the sources of sediment in river basins to their redistribution along the coast, perturbed by climate changes and anthropic stresses), so that only regional solutions are appropriate and will be effective. These must follow sustainable strategies with a multi-temporal sequenced solution and anticipate changes, or adaptive solutions using solutions in synergy with different time frames as well as managing natural and human systems responsibly. A plan that considers changes in coastal systems and anticipates impacts and adapts plans accordingly will be key.
Relict rocky shorelines are seemingly rarely preserved in the geological record, often underrepresented in the literature and the reasons for their scarcity, as well as the conditions needed for preservation are not well understood. On the “Wild Coast” shelf along the subtropical east coast of South Africa a series of submerged shorelines are present. High-resolution multibeam bathymetry, side scan sonar, ultra-high-resolution seismic profiling and legacy seismic reflection data show several fluvial channels incised into the shelf, associated with the Last Glacial Maximum (LGM) lowstand. Palaeo-shorelines are preserved at −85, −75, −60 and −40 m. The −85 and −75 m shorelines reflect sediment abundance and depositional features preserved as lagoonal depressions, lithified barrier-spits and coastal barrier dunes with limited bedrock influence. The shallower shorelines at −60 and −40 m occur on a steeper shelf gradient and reflect the development of rocky headlands, palaeo-cliffs and rock shore platforms. Their occurrence implies increasing bedrock control, erosion and sediment bypass during transgression.
The preservation of rocky shoreline signatures on the inner shelf is aided by competency of the bedrock lithologies, in addition to their depth relative to sea-level stillstands and rapid rises in sea level associated with meltwater pulses (MWPs) (notably the −60 m shorelines and MWP1B). MWPs may have helped facilitate the overstepping and eventual preservation of these shoreline landforms on the seabed. Preservation of the −40 m shoreline is considered to correlate with accelerated rates in sea- level rise prior to MWP1C. The preservation of the −85 and −75 m aeolianite shorelines are not linked to meltwater pulses, however, their substantial sediment volume precluded total loss through transgressive erosion and this, coupled with early cementation, promoted their partial preservation. Such deposits rarely survive subaerial exposure but could be preserved if covered by marine sediments. Conversely, the existence of well-preserved, Pleistocene-age rocky shorelines on the shelf points to their high potential for inclusion into the longer-term geological record under subsequent high or low sedimentation. The formation of distinctive rocky shorelines, however, may reflect alternating slow and fast rates of sea-level rise.
The rapid replacement of upland forest by encroaching marshland is a striking manifestation of global sea-level rise (SLR). Timely and high-resolution information on the location and extent of transition forest (the ecotone between upland forest and marsh where tree mortality due to seawater intrusion begins) is fundamental to understanding the processes and patterns of SLR-driven landscape reorganization. Despite its significance, accurate characterization of salt-impacted transition forest remains challenging due to the complexity of coastal environments, scarcity of ground-truth data, and the lack of effective mapping algorithms. Here we use the full archive of Landsat images between 1984 and 2021 to investigate the spectral, temporal, and phenological characteristics of transition forest, and develop a robust framework for monitoring coastal vegetation shifts in the mid-Atlantic U.S., a global SLR hotspot. We found that transition forest exhibits strong negative NDVI trends and a deviation of land surface phenology from marsh and upland forest that distinguishes itself from surrounding vegetation. By integrating temporal trends and land surface phenology, our results demonstrate superior discrimination between marsh and coastal forests to existing map products (e.g. NOAA Coastal Change Analysis Program, National Land Cover Database) that allows a reliable identification of the coastal treeline. We applied the approach to map regional land cover in 1985, 2000 and 2020 (overall classification accuracy >92%) and found that the area of coastal forest decreased by 22.0% from 1985 to 2020, the majority of which transitioned to marshland (92.3%, 5.3 × 10 3 ha). Based upon fine-scale patterns of coastal transgression, we created a practical workflow for spatially explicit quantification of forest retreat rates. Concurrent with rising sea level, coastal forests migrated upslope from 0.63 (± 0.27) m above sea level in 1985 to 0.78 (± 0.32) m above sea level in 2020, and horizontal forest retreat rates accelerated from 3.1 (range of 0-36) m yr − 1 during 1985-2000 to 4.7 (0-55) m yr − 1 during 2001-2020. As SLR continues to accelerate, our study may serve as a scalable solution for consistent tracking of coastal landscape evolution that is urgently needed for sustainable forest and wetland management.
The multiple hypotheses which exist to explain the initiation of transgressive aeolian sand sheets and dunefields, are reviewed and discussed. Direct evidence supporting many of these hypotheses is largely lacking. In South Australia, the Younghusband Peninsula coastal barrier extends ~180 km and predominantly comprises transgressive and parabolic dunefields. The 42 Mile Crossing area on the barrier is undergoing significant erosion at variable rates of 0.5 to 5.0 m/yr, and a new transgressive aeolian sand sheet has rapidly developed in ~1 year and is extending landwards at an average rate of 13 m/yr. This research provides unequivocal evidence that large‐scale shoreline and dunefield erosion does lead to the development of a new transgressive aeolian sand sheet (and eventual dunefield) phase thereby demonstrating an initiation mechanism that is likely linked to future sea level rise and climate change. We also show that the initiation process, and, in particular, the subsequent rate of sand sheet transgression occurs at an incredibly rapid rate (+100 m in 8 years).
Plain Language Summary
Coastal sand dunes border many of the world's coastlines and are highly adapted to local climate and conditions. How coastal dunes transition from predominantly vegetated and stable systems to wind‐blown sand sheets and dunefields transgressing prior terrain is a research area of pressing relevance due to forecasts of sea level rise and climate change. The factors or triggers that are considered to initiate transgressive aeolian sand sheets and dunefields are reviewed. The formation and evolution of a new transgressive aeolian sand sheet phase triggered by large‐scale shoreline erosion in South Australia is presented. According to the results from historical and satellite images, local shoreline erosion began in the late 1970s and has continued at highly variable rates. We show that once the foredune was removed, the high scarp created by wave erosion of the relict, vegetated transgressive dunefield destabilized and was then eroded by wind processes leading to the rapid development of a transgressive aeolian sand sheet. The initiation and evolution of the sand sheet provides an excellent example of how dunefields might respond to future sea level rise and climate change.
Tidal wetlands are expected to respond dynamically to global environmental change, but the extent to which wetland losses have been offset by gains remains poorly understood. We developed a global analysis of satellite data to simultaneously monitor change in three highly interconnected intertidal ecosystem types—tidal flats, tidal marshes, and mangroves—from 1999 to 2019. Globally, 13,700 square kilometers of tidal wetlands have been lost, but these have been substantially offset by gains of 9700 km ² , leading to a net change of −4000 km ² over two decades. We found that 27% of these losses and gains were associated with direct human activities such as conversion to agriculture and restoration of lost wetlands. All other changes were attributed to indirect drivers, including the effects of coastal processes and climate change.
The migration of salt marshes into forests along coastal regions is nowadays well documented. Sea level rise and storms threaten coastal forests by increasing groundwater levels and salinity. Salinization is the main cause of forest conversion to salt marsh in North America. In this paper we study groundwater levels and salinity in two wells installed at the border between forest and salt marsh in the lower Delmarva peninsula, USA. The upper well is located in the regenerative forest, where recruitment is still possible, while the lower well is located in the persistent forest, where only mature trees survive. Groundwater in the upper well is fresh at the root depth, while in the lower well the mean salinity is 8 ppt. Our data suggest that rainfall has an instantaneous effect on salinity and groundwater levels, but it does not affect salinity and groundwater levels on longer periods (weeks to months). Groundwater levels and salinity reflect the hydraulic gradient toward the marsh (a proxy for outgoing water fluxes), the uphill hydraulic gradient (a proxy for incoming water fluxes) and temperature (a proxy for evapotranspiration). Salinity increases when groundwater levels are high. To explain this result, we put forward the hypothesis that a high water table favors the flux of surficial, fresh water to the marsh, and loss of freshwater by evapotranspiration. These losses are likely replenished by saltier water moving at depth.
The development of coastal adaption pathways along sandy barrier-island coasts requires an understanding of multi-decadal barrier morphodynamics in response to forcings such as sea-level rise, sediment fluxes, and storminess. Long- and cross-shore sand exchanges among barrier-system compartments (shorefaces, beaches, dunes, tidal inlets, backbarrier lagoons) and between adjacent barrier islands can play a fundamental role in barrier-system morphology and resilience to sea-level rise. We explore these dynamics through investigation of historical areal changes within the 13 largely undeveloped Virginia Barrier Islands of the United States Mid-Atlantic Coast. We pair subaerial island areas mapped from historical surveys and modern aerial and satellite imagery with representative island thicknesses derived from stratigraphic and lidar-elevation data to estimate volumetric changes through time. Overall, the Virginia Barrier Islands lost an estimated 3.4% (16 × 10⁶ m³) of their 1887C.E. volume between 1887 and 2017C.E. The loss from three central islands (Parramore, Hog, and Cobb)—historically characterized by rotational behavior—was 43% (79 × 10⁶ m³). In particular, Parramore Island continuously decreased in volume over the 20th and 21st centuries. In contrast, growth of Assateague and northern Wallops islands (located at the northern, updrift end of the island chain) from 1887 to 2017C.E. resulted in a volume growth of ~59 × 10⁶ m³. Whereas the Virginia Barrier Islands as a whole modestly decreased in size (and thus volume) directly in response to storm impacts and accelerating relative sea-level rise since the mid-1800s, the differential behavior of islands south of Assateague suggests more regional controls on coastal geomorphic behavior. Specifically, we identify the roles of antecedent substrate; sand trapping and changing wave-refraction patterns associated with the growth of Assateague Island; and inter-island sand exchanges in response to changing updrift barrier-scale behaviors. These findings help to better understand the underlying mechanisms behind state changes of barrier islands; in particular, the mechanisms by which geomorphic change along one barrier island influences those located several to tens of kilometers downdrift. This highlights the need to consider coastal beach and sand management over long reaches, even in systems which are largely unaffected by development, nourishment, and shoreline hardening.
This chapter reviews the morphodynamics of open-ocean barrier systems, synthesizing classic studies, current scientific knowledge, and future research directions regarding 14 barrier provinces worldwide. Within a coastal tectonic framework, it addresses: (1) Amero-trailing-edge coasts (USA's New England coast, Mid-Atlantic Bight coast, North Carolina Outer Banks, Georgia Bight coast, Florida Atlantic coast; Brazil's Santa Catarina coast; Europe's German Bight coast; Australia's southern and western coasts); (2) marginal-sea coasts (USA's Florida Gulf Coast; Gulf Coast of Louisiana, Mississippi, and Alabama; Texas Gulf Coast; Australia's eastern coast); and (3) collision coasts (USA's Alaskan Pacific coast, Japan's and New Zealand's coasts). This chapter also provides a glossary, important conclusions, specific recommendations regarding future coastal research, and a robust, current set of references.
Saltmarsh area is decreasing globally from natural and anthropogenic stressors. Accelerating relative sea-level rise (SLR) is projected to exacerbate losses if not offset by upland saltmarsh migration (transgression). In the absence of coastal upland development, saltmarsh transgression rates increase with accelerating SLR and lower upland surface gradients. Storm wind and surge stress coastal upland forests causing defoliation, uprooting, and soil salinization, which makes upland areas more habitable for saltmarsh species and can promote transgression. This study aims to elucidate the contribution of storms to saltmarsh transgression by reconstructing transgression rates over the past 600 years during stormy and non-stormy conditions and fast and slow SLR rates. Our reconstructions are based on the stratigraphic record and historical aerial photography at three sites in North Carolina, U.S.A. where low-gradient pocosin upland grades into expansive saltmarsh. When sea level was rising <0.9 mm yr⁻¹, saltmarsh transgression rates at the two sites where saltmarsh is > 100 years were an average of 2 and 10 times faster during a paleo-stormy period (1400–1675 CE) than a subsequent non-stormy period. After 1865 CE when SLR accelerated to 2.4 mm yr⁻¹, transgression rates were an average of 7 times faster than the preceding slow SLR non-stormy period. The two sites where the historical record was not confounded by dredging show transgression was 7 times faster and saltmarsh areas increased an average of 28% during stormy decades than non-stormy decades; however, the rate of transgression only increased at the site with greatest surge during the stormy period characterized by strong northeast winds. Modeled transgression rates, using the paleo-upland slope and a sea level curve, do not match observed transgression rates for the paleo-stormy and rapid SLR periods. Furthermore, the thickness of saltmarsh peat younger than 1957 CE is greater than what would be predicted from independent records of SLR. Changes in the elevation of the upland surface, which is composed of peat, contributes to the disparity between predicted and observed transgression rates. The upland surface elevation can keep pace with some rates of SLR through vertical accretion; however, salinization and decomposition of upland vegetation from storm surge plus SLR decreases the elevation of the paleo-upland surface and increases accommodation and transgression rates. Along low-gradient coastlines with pocosin upland areas, SLR, subsidence, and storminess are coupled in modulating transgression rates and those processes need to be included in forecasts of saltmarsh response to climate change.
Human societies will transform to address climate change and other stressors. How they choose to transform will depend on what societal values they prioritize. Managed retreat can play a powerful role in expanding the range of possible futures that transformation could achieve and in articulating the values that shape those futures. Consideration of retreat raises tensions about what losses are unacceptable and what aspects of societies are maintained, purposefully altered, or allowed to change unaided. Here we integrate research on retreat, transformational adaptation, climate damages and losses, and design and decision support to chart a roadmap for strategic, managed retreat. At its core, this roadmap requires a fundamental reconceptualization of what it means for retreat to be strategic and managed. The questions raised are relevant to adaptation science and societies far beyond the remit of retreat.
Predicting change to shorelines globally presents an increasing challenge as sea level rise (SLR) accelerates. Many shoreline prediction models use the simplistic ‘Bruun rule’ for dealing with SLR profile translation, in-part due to alternative approaches being too complex and time-consuming to implement. To address this, we introduce ShoreTrans: a simple, rules-based, user-input driven, shoreface translation and sediment budgeting model, that applies the surveyed 2D-profile (not a parameterization), for estimating change to realistic coastlines, resulting from sea level rise and variations in sediment supply, while accounting for armouring, hard-rock cliffs and outcropping rocks. The tool can be applied to sand, gravel, rock and engineered coasts at a temporal scale of 10–100 years, accounting for shoreline trends as well as variability. The method accounts for: (1) dune encroachment/accretion; (2) barrier rollback; (3) non-erodible layers; (4) seawalls; (5) lower shoreface transport; (6) alongshore rotation; and (7) other sources and sinks. Uncertainty is accounted for using a probabilistic distribution for inputs and Monte Carlo simulations. We provide a first-pass assessment of two macrotidal UK embayments: Perranporth (sandy, dissipative, cross-shore dominant transport) and Start Bay (gravel, reflective, bi-directional alongshore dominant), then use idealised profiles to investigate the relative importance of forcing controls on shoreline recession and beach width. For the dissipative sandy site, the primary modes of coastal change are predicted to be short-term storm erosion and SLR translation while long-term trends may be important but are highly uncertain. For the reflective gravel site, the primary mode is multi-decadal longshore sediment flux, while short-term alongshore rotation and SLR translation are secondary. Relative to the ShoreTrans approach, the Bruun rule under-predicts shoreline recession in front of cliffs, seawalls and for low barriers that rollback, and over-predicts where large erodible dunes are present. ShoreTrans directly addresses change in beach width, with beaches in front of seawalls and cliffs predicted to shrink, such that narrow beaches (<50 m width) may disappear under 1-m SLR. As a standalone tool, ShoreTrans is transferable to many coast types and will provide coastal practitioners with a simple first-pass estimate of how the 2D appearance of a complex profile may change under SLR. A future benefit will be to combine this approach with existing hybrid modelling techniques to augment SLR translation predictions.
Low‐lying coastlines are vulnerable to sea‐level rise and storm surge salinization, threatening the sustainability of coastal farmland. Most crops are intolerant of salinity, and minimization of saltwater intrusion is critical to crop preservation. Coastal wetlands provide numerous ecosystem services, including attenuation of storm surges. However, most research studying coastal protection by marshes neglects consideration of subsurface salinization. Here, we use two‐dimensional, variable‐density, coupled surface‐subsurface hydrological models to explore how coastal wetlands affect surface and subsurface salinization due to storm surges. We evaluate how marsh width, surge height, and upland slope impact the magnitude of saltwater intrusion and the effect of marsh migration into farmland on crop yield. Results suggest that along topographically low coastlines subject to storm surges, marsh migration into agricultural fields prolongs the use of fields landward of the marsh while also protecting groundwater quality. Under a storm surge height of 3.0 m above mean sea level or higher and terrestrial slope of 0.1%, marsh migration of 200 and 400 m protects agricultural yield landward of the marsh‐farmland interface compared to scenarios without migration, despite the loss of arable land. Economic calculations show that the maintained yields with 200 m of marsh migration may benefit farmers financially. However, yields are not maintained with migration widths over 400 m or surge height under 3.0 m above mean sea level. Results highlight the environmental and economic benefits of marsh migration and the need for more robust compensation programs for landowners incorporating coastal wetland development as a management strategy.
Complexities of terrestrial boundaries with salt marshes in coastal lagoons affect salt marsh exposure to waves and sediments creating different potentials for marsh migration inland and seaward-edge erosion, and consequently, for marsh persistence. Between 2002 and 2017, migration and edge erosion were measured in three mainland geomorphic marsh types (headland, valley, hammock) and were used to assess the rate and spatial extent of marsh change for a Virginia coastal lagoon system. Treelines, shorelines, and marsh perimeters were delineated in ArcGIS at 1:600 resolution. All marsh types increased in spatial extent; increases were greatest for the valley type (0.58 ha ± 0.31 ha or + 0.32% per annum). Measured rates of migration (headland > valley > hammock) and erosion (headland > hammock > valley) for each geomorphic type were averaged and applied to obtain changes in these same marsh types at the regional scale. At this scale, valley marsh area increased (82.5 ha or 5.5 ha a−1) more than the other two marsh types combined. This analysis demonstrates the critical influence that geomorphic type has on lateral marsh responses to sea-level rise and that efforts to conserve or restore salt marshes are most likely to be successful when focused on valley marshes.
Coastal regions worldwide will be dramatically reshaped by the impacts of sea-level rise. Of particular concern are impacts on coastal wetlands, the loss of which would have consequences for both human and ecological communities. The future of many coastal wetlands will depend greatly on their capacities to migrate into uplands. Coastal resilience work within wetland sciences has increasingly focused on developing strategies to promote marsh migration into rural uplands; however, less attention has been given to the impacts that migrating marshes have on people in these landscapes. In this paper, we share rural perspectives and experiences with marsh migration through three case-studies from collaborative research with rural, low-lying communities on the Chesapeake Bay, USA. These case-studies demonstrate the complexities of the challenges facing rural communities as a result of marsh migration, and reveal important issues of equity and injustice that need attention in future coastal resilience work. We draw upon a socio-ecological systems (SES) approach to highlight potential human-ecological misalignments that emerge with marsh migration and to offer future research questions to inform socially-just and resilient wetland migration planning in rural coastal areas.
At seasonal to century timescales (mesoscale), the shoreface is a critical zone seaward of the surf zone and/or beachface, in which waves interact with the mobile seafloor to cause morphological change. This has important (and often unacknowledged) implications for adjacent shoreline form and behaviour both now and in the near-future. The shoreface has been relatively little studied from a mesoscale morphodynamic (morphological change over time) perspective and various definitions exist regarding its extent and morphodynamic subdivisions. To overcome the diversity and ambiguity of existing definitions we propose a standard terminology involving the external limits and subordinate zones of the shoreface.
In our definition, the landward limit of the shoreface coincides with the seaward limit of the fair weather surf zone, and where no surf zone is present, the base of the beachface. The shoreface itself is subdivided into upper and lower shorefaces, separated by the depth of closure (DoC) as defined by Hallermeier (1981). The seaward limit of the lower shoreface is defined by the limit of significant sediment transport, indicated by bed shear stress according to Valiente et al. (2019). All boundaries are temporally variable according to wave characteristics and timescale of study.
The upper shoreface is dynamic at seasonal to annual timescales and interacts with the adjacent surfzone via wave transformation and two-way sediment exchange. The lower shoreface is dynamic at decadal to millennial timescales and it interacts with the adjacent upper shoreface and inner shelf. The upper shoreface is strongly influenced by wave hydrodynamics whereas the lower shoreface is less dynamic and its shape is more heavily influenced by geological factors (nature and/or abundance of sediment, depth and erodibility of rock outcrop, etc.). Sediment exchange both within the shoreface and between shoreface and adjacent environments is strongly event-driven. Longshore, onshore and offshore transport mechanisms have been documented.
The shoreface profile influences, and is influenced by, wave transformation, however, the widely adopted shoreface equilibrium profile is not universally applicable. Instead, a diversity of shoreface morphologies exists in two and three dimensions. These are likely related to sediment supply and accommodation and we propose a spectrum of shoreface types based on these variables. Recent studies have shown that large-scale 3-D forms (e.g. shoreface-connected ridges and sorted bedforms) strongly influence shoreline behaviour, however, the dynamics of these shoreface bedforms requires further investigation. Each type of shoreface likely exhibits distinctive behaviour at the mesoscale (time scale of 10¹ to 10² years and a spatial scale of 10¹ to 10²km). This is proposed as a unifying model with which to integrate studies of shoreface dynamics at different spatial and temporal scales.
Sea level rise is reshaping the coasts, allowing coastal habitats such as tidal marshes to migrate inland. To predict where changes will occur, it is critical to understand the factors that influence land cover transition. Here, we test the influence of land cover type on land cover transition. We hypothesized that marsh migration may vary by upland land cover type, due to dominant plant species’ differences in salinity and inundation tolerance. Additionally, the response of people may make specific land cover types more likely to be protected from transition. We measured land cover change in high resolution aerial imagery over the relatively short period of 2009 to 2017 in coastal Somerset County, Maryland. In logistic models of land cover transition, we found that ‘agricultural land’ and ‘scrub shrub wetland / forested wetland’ cover classes were more likely to transition to ‘emergent wetland’ than ‘forest/scrub shrub’ or ‘urban or built-up land’ cover classes, after controlling for elevation and distance to shore, two well-known predictors of marsh migration. Over only 8 years, loss of upland area in the county totaled 6.1 km2, of which 5.7 km2 was agricultural land. This represents a loss of over 2% of the farmland in the county, the majority of which converted to emergent wetland during the study period.
Building on a small, yet growing body of scholarship focused on the political ecology of race and critical race studies of science and technology, this article follows the ways sediment, science, and race intersect on the grounds of environmental restoration in coastal Louisiana. Mobilizing ethnographic field work and historical research conducted with African-American communities and coastal scientists, I empirically expand upon geographer Kathryn Yusoff’s (2018) notion of the “geosocial registers” of the Anthropocene through an examination of the entwined histories of coastal engineering and racial inequality that situate contemporary debates about large scale coastal restoration projects along Louisiana’s disappearing coastline. In dialogue with critical work on the relationship between racism, science, and the constitution of the Anthropocene, I argue that coastal restoration is a geophysical and social process upon which racial inequality is forged and contested. The article concludes by considering how environmental restoration can participate in creating alternative forms of social and environmental repair by aligning the goals of coastal science with those of racial justice for communities of color living in changing coastal landscapes.
In a simple definition, beach rotation is the opposing movement of the shoreline along the two ends of an embayed beach, driven by longshore and/or cross-shore sediment transport in response to seasonal or periodic changes in wave direction and/or gradients in wave energy. However, when considered in detail, the mechanisms, drivers and timescales of beach rotation are complex, resulting from non-linear interactions of cross-shore and alongshore hydrodynamic forcing, sediment transport and morphological change, developed over single or combined timescales that range from storm events to decadal rotation driven by climate-forced changes in wave conditions. In the context of global change, morphodynamic complexity of beach rotation processes is further compounded by rising sea levels and changes in wave climate, and impacted by artificial modification of beach environments along increasingly engineered coastlines. The spatial and temporal complexity of beach rotation mechanisms creates significant challenges to morphodynamic modelling and management of embayed beaches.
Sandy coast changes on timescales from days to years and sometimes decades, primarily, result from the erosion–recovery (im)balance that is controlled by the respective contributions of storms and recovery conditions. Over the last decade, our understanding and predictive ability of storm-driven erosion and subsequent multi-day to multi-annual recovery has greatly improved, notably thanks to long-term and rapid-response coastal monitoring programmes. This chapter gives a broad overview of the definitions and processes that control storm-driven beach and dune erosion and subsequent (partial, complete or excess) recovery. Key concepts are illustrated using two well-documented case studies: response and (partial) multi-annual recovery from (1) over a severe winter period along the Atlantic coast of Europe characterised by unusually strong storm clustering episode; and (2) from a single severe storm with an anomalous wave direction along the southeast coastline of Australia. Finally, future perspectives and knowledge gaps in relation to impacts and recovery from extreme events are discussed.