Relative Sea-level Rise Research Articles

Impact of coseismic uplifting on relative sea level change in the Southern Adriatic during the past 4.500 years – New evidence from Dubrovnik epicentral area based on analysis of algal rims and tidal notches

The Southern Adriatic is recognized as one of the most seismically active regions of the Central Mediterranean. It hosted the strongest historically known and instrumentally recorded earthquakes in Croatia and Montenegro. We conducted a detailed study of algal rims and tidal notches along the Dubrovnik–Konavle coastal area, aimed to contribute in reconstruction of relative sea level (RSL) change, palaeoearthquakes and neotectonics during the last 4500 years. The RSL reconstruction based on high-resolution geochronology showed that the spatial variability of RSL change is largely controlled by local tectonics, leading generally to an alternation of periods of RSL rise and rapid falls caused by coseismic uplift events. During the interseismic periods RSL rose at rates from 1.03 to 0.7 mm/yr before AD, followed by a slowdown to ~0.43 mm/yr, and reaching ~1.3 mm/yr after 1850s. The only exception relates to a period of slow RSL drop probably related to the 3.2 ka cold event. Rapid RSL falls caused by coseismic uplift indicate the occurrence of several large-scale events during the 4th–6th centuries AD, the 800–1360 cal AD period, and in 1395, with hints of earlier events between 1395 cal BC and 565 cal AD. These instances correspond to either unknown or insufficiently documented earthquakes. Conversely, the earthquakes in 1520 and 1667 have been clearly identified. The most compelling evidence of coseismic uplift is associated with the 1667 Dubrovnik earthquake, with an estimated uplift of 40–60 ± 15 cm documented along 40 km of coastline. Analyses of multiple sites with the same high-resolution marker show that Holocene tectonic deformations vary along different fault segments with minimum uplift rates ranging from ~0.3 to ~0.9 mm/yr over ~1.5 ka. The main seismogenic source responsible for coseismic uplifts is attributed to the Dalmatian unit basal thrust and its NE-dipping splays at the Adria-Dinarides boundary.

Assessing the impact of hurricane Fiona on the coast of PEI National Park and implications for the effectiveness of beach-dune management policies

The impact of waves, storm surge, and aeolian transport associated with Post-tropical Storm Fiona (offshore significant wave height ∽ 8 m, storm surge up to 2 m) on the sandy beaches and foredunes of the north shore of Prince Edward Island National Park (PEINP), Canada, are assessed. Management policies and practices, as they apply to sandy beach systems within PEINP, are reviewed in the context of the shoreline changes attributed to Fiona. The effectiveness of these policies and practices are evaluated to inform the potential performance of beach-foredune systems as natural protection measures that mitigate the impacts of large-magnitude storms and relative sea-level rise (RSLR) on shoreline change. The analyses utilise survey data, ground photography, and unoccupied aerial vehicle (UAV) imagery collected before (October 2021 to July 2022) and after (October 2022 and May 2023) Fiona. In general, the largest dunes were characterised by erosion of the stoss slope, with landward retreat of the dune toe by < 6 m and minimal impact on crest height and position. Small foredunes (< 5 m in height) generally showed significantly greater erosion in terms of dune profiles, with dune breaching occurring at some locations. Foredunes perched on bedrock and till, which were typically smallest in size, were subject to complete erosion, thereby exposing the hard underlying surface. Overall, the impact of Fiona on sandy beach systems in PEINP was relatively modest in many locations, reflecting the success of existing management policies and practices that protect and maintain the integrity of foredunes by minimizing human impacts and avoiding ‘coastal squeeze’.

Coupled Short-Ig Wave Dunamics Over A Shallow Barrier Reef

Reef barriers play a major role many coral islands, by sheltering the lagoon from the ocean wave energy and then creating a unique habitat for many species. This filtering action becomes increasingly crucial for ecosystems health and shoreline protection in the context of climate change and related sea level rise, degradation of coral systems and modification of wave conditions. A strong research effort has therefore been engaged by the coastal oceanographers community for the last two decades to improve our knowledge and prediction skills of wave dynamics over coral reef systems. A widely reported observation is the importance of infragravity waves (IG) over wave-driven reef systems, whether fringing or barrier reefs. IG are primarily forced by groups in the incoming short-wave (SW) field, either by the release of bound waves or the breakpoint oscillations (Bertin et al. 2018). IG period typically ranges between 30 and 200s, which makes them prone to excite or interact with natural seiching modes in reef-lagoon systems often ranging in the Very Low Frequency (VLF) band. Further research efforts are now necessary to better understand the interaction between long IG/VLF oscillations and SW field. In particular, long waves are expected to play a dynamic depth-filtering role on SW energy, acting as long carrier wave able to promote the propagation of larger SW groups by IG/VLF crests. More generally, the spectral energy transfers over the reef crest-flat system and their relative importance w.r.t. frictional and breaking dissipation are not fully understood over the complete range of surface waves. The aim of the present study is to analyse and to discuss a series of field observations performed on the barrier reef of Maupiti Island, French Polynesia. A particular focus is placed on the interaction between SW and IG wave fields across the reef crest-flat system.

Vulnerability of tidal morphologies to relative sea-level rise in the Venice Lagoon

The rise in sea level and land subsidence are seriously threatening the diversity of tidal morphologies that have made the Venice Lagoon such a distinctive landscape. Here, we assess the vulnerability of tidal morphologies to relative sea-level rise based on a new conceptual framework that accounts for both above- and below-sea-level zones, sedimentary architecture, and surface morphology. Around 80 % of the lagoon area will face moderate to severe vulnerability by 2050, doubling compared to the 1990s. While the subtidal zone may be relatively less threatened compared to past conditions, the drastic decline in intertidal morphologies is alarming. This contributes to the flattening and deepening of the lagoon topography and thus to the loss of lagoon landscape diversity, likely leading to a decrease in the ecosystem services the tidal morphologies provide. The interconnection of intertidal and subtidal morphologies is crucial for maintaining the overall health and functionality of the lagoon's ecosystem. Any disruption to one aspect can have ripple effects throughout the entire system.

Autogenic shrinkage and channel destabilization of an overexpanded downstream alluvial system under steady rise of relative sea level: An experimental study

The dynamics of downstream alluvial systems undergoing relative sea level (RSL) rise exhibit significant variations depending on spatial size. According to autostratigraphy theory, such systems cannot sustain deltaic sedimentation under a steady RSL rise (rate Rslr), and are prone to non-deltaic transgression once their plain area A exceeds the critical limit Acrt. During this transgression, A decreases asymptotically toward Acrt, while the overall alluvial aggradation rate Ragg_overall increases toward Rslr. In this study, we investigated the behavior and morphodynamics of an overexpanded system through physical modeling, with a focus on the inherent area scale of the depositional system. In the early stages of non-deltaic transgression (A >> Acrt, Ragg_overall << Rslr), the alluvial channels experience minimal aggradation rate Ragg_channel (< Rslr), leading to stabilization and inundation. As the transgression progresses, the channels aggrade more rapidly, increasing the likelihood of lateral migration and avulsion. Eventually, a state of morphodynamic equilibrium is reached, characterized by A ∼ Acrt, Ragg_overall ∼ Rslr, and Ragg_channel ∼ constant > Rslr despite sustained non-deltaic transgression. During this equilibrium stage, the channels undergo rapid aggradation, resulting in complete destabilization with continuous lateral migration and frequent avulsion, forming system-wide subaqueous steps. The implications of overexpansion and autogenic shrinkage in response to RSL rise extend to stratigraphic convergence during RSL cycles and provide insights into Holocene non-deltaic transgression. Moreover, when combined with decelerating RSL rise, this process could have facilitated marine delta development. However, projected accelerated RSL may lead many modern marine deltas to transition into non-deltaic transgressive systems. Additionally, natural alluvial channels may exhibit varying behavior depending on the spatial size of the non-deltaic transgressive system. Furthermore, the autogenic shrinkage model may offer an explanation for the stepped surfaces observed on alluvial fans and deltas on Mars.

Vegetation dieback in the Mississippi River Delta triggered by acute drought and chronic relative sea-level rise

Vegetation dieback and recovery may be dependent on the interplay between infrequent acute disturbances and underlying chronic stresses. Coastal wetlands are vulnerable to the chronic stress of sea-level rise, which may affect their susceptibility to acute disturbance events. Here, we show that a large-scale vegetation dieback in the Mississippi River Delta was precipitated by salt-water incursion during an extreme drought in the summer of 2012 and was most severe in areas exposed to greater flooding. Using 16 years of data (2007–2022) from a coastwide network of monitoring stations, we show that the impacts of the dieback lasted five years and that recovery was only partial in areas exposed to greater inundation. Dieback marshes experienced an increase in percent time flooded from 43% in 2007 to 75% in 2022 and a decline in vegetation cover and species richness over the same period. Thus, while drought-induced high salinities and soil saturation triggered a significant dieback event, the chronic increase in inundation is causing a longer-term decline in cover, more widespread losses, and reduced capacity to recover from acute stressors. Overall, our findings point to the importance of mitigating the underlying stresses to foster resilience to both acute and persistent causes of vegetation loss.

Holocene Foraminifera, Climate, and Decelerating Rise in Sea Level on the Mud Patch, Southern New England Continental Shelf

Abstract We examined Holocene benthic foraminiferal biofacies, % planktonic foraminifera, and lithofacies changes from New England mud patch cores and present a relative sea-level (RSL) record to evaluate evolution of these rapidly deposited (30–79 cm/kyr) muds. Sandy lower Holocene sections are dominated by Bulimina marginata. The mud patch developed from 11–9 ka as RSL rise slowed from 10 to 7 mm/yr; mud deposition began when the cores (69 to 91 m modern) were inundated below storm wave base. An Elphidium-B. marginata fauna developed at ca. 7–6 ka as RSL rise slowed from approximately 7 to 2 mm/yr. A Globobulimina fauna developed at 3 ka as RSL rise slowed to 1 mm/yr, reflecting lower O2 conditions. Single specimen δ18O analyses of Globobulimina show ∼1‰ variations over the past 3 kyr, reflecting a shelf bottom water seasonal cycle of 4–5°C, and a temperature minimum during the Little Ice Age with warming since.

Persistence of Holocene ice cap in northeast Svalbard aided by glacio-isostatic rebound

The deglaciation of the Svalbard-Barents Sea Ice Sheet was driven by relative sea-level rise, the incursion of North Atlantic waters around Spitsbergen, and increasing summer insolation. However, ice retreat was interrupted by asynchronous re-advances that occurred into high relative seas, during a period associated with warm regional waters and elevated summer temperatures. Better understanding of this complex style of deglaciation and the dynamic response to a warming climate can serve as an important analogue for modern warming and today's ice sheets. We present evidence from northern Svalbard of glacier re-advances during the Late Glacial-Early Holocene in hand with relative sea-level history and the occurrence of thermophilous molluscs. We argue that glacio-isostatic adjustment during the transition into the Holocene influenced ice marginal dynamics and as a result, the southern region of the Åsgardfonna ice cap persisted through the Holocene Thermal Maximum.

Holocene Sea-level impacts on Venice Lagoon's coastal wetlands

Venice Lagoon (Italy), the largest wetland in the Mediterranean basin, is extremely vulnerable to variations in relative sea level (RSL) which is locally defined by an average rising rate of about 2.5mm per year, resulting from both sea-level change and vertical land movements. The environmental pressures stemming from projected higher RSL rising rates will have a profound impact on Venetian coastal ecosystems with a significant loss of wetlands partly due to a drastic reduction of salt marsh habitats. To understand how changes in marine influence could create such ecological upheaval in the near future, and fully remodel these coastal salt marshes, we reconstructed 5650 years of RSL rise history and land subsidence impacts on ecosystem dynamics during the Holocene transgression of Venice Lagoon. We show that the evolution of ecosystems mainly mirrors the gradual intrusion of salt water that progressively reshaped the coastal vegetation by turning the area into salt lagoon habitats. Before marine influence became dominant, the area was mainly fed by substantial freshwater supplies allowing the development of a diversified alluvial vegetation. Environmental pressures increased markedly about 6800–6600 years ago when seawater began to significantly influence the area, affecting marsh-swamp ecosystems. These marine inputs promoted the spread of halophytic and salt-tolerant vegetation types which laid the foundations for what would become the current salt marsh habitats. Venice Lagoon serves as a stark reminder of how rising sea levels, accompanied by increased saltwater intrusion into freshwater habitats and adjacent lands, can drastically alter and reshape pre-existing ecosystems. The lagoon's long-term ecological record indicates that contemporary fluctuations in RSL pose a substantial ecological threat, potentially culminating in a major upheaval of aquatic habitats in the near future.

A deep-learning model for rapid spatiotemporal prediction of coastal water levels

With the increasing impact of climate change and relative sea level rise, low-lying coastal communities face growing risks from recurrent nuisance flooding and storm tides. Thus, timely and reliable predictions of coastal water levels are critical to resilience in vulnerable coastal areas. Over the past decade, there has been increasing interest in utilizing machine learning (ML) based models for emulation and prediction of coastal water levels. However, flood advisory systems still rely on running computationally demanding hydrodynamic models. To alleviate the computational burden, these physics-based models are either run at small scales with high resolution or at large scales with low resolution. While ML-based models are very fast, they face challenges in terms of ensuring reliability and ability to capture any surge levels. In this paper, we develop a deep neural network for spatiotemporal prediction of water levels in coastal areas of the Chesapeake Bay in the U.S. Our model relies on data from numerical weather prediction models as the atmospheric input and astronomical tide levels, while its outputs are time series of predicted water levels at several tide gauge locations across the Chesapeake Bay. We utilized a CNN-LSTM setting as the architecture of the model. The CNN part extracts the features from a sequence of gridded wind fields and fuses its output to several independent LSTM units. The LSTM units concatenate the atmospheric features with respective astronomical tide levels and produce water level time series. The novel contribution of the present work is in spatiotemporality and in prioritization of the physical relationships in the model to maintain a high analogy to hydrodynamic modeling, either in the network architecture or in the selection of predictors and predictands. The results show that this setting yields a strong performance in predicting coastal water levels that cause flooding from minor to major levels. We also show that the model stands up successfully to the rigorous comparison with a high-fidelity ADCIRC model, yielding mean RMSE and correlation coefficient of 14.3 cm and 0.94, respectively, in two extreme cases, versus 12.30 cm and 0.96 for the ADCIRC model. The results highlight the practical feasibility of employing fast yet inexpensive data-driven models for resilient coastal management.

Disappearing cities on US coasts

The sea level along the US coastlines is projected to rise by 0.25–0.3 m by 2050, increasing the probability of more destructive flooding and inundation in major cities1–3. However, these impacts may be exacerbated by coastal subsidence—the sinking of coastal land areas4—a factor that is often underrepresented in coastal-management policies and long-term urban planning2,5. In this study, we combine high-resolution vertical land motion (that is, raising or lowering of land) and elevation datasets with projections of sea-level rise to quantify the potential inundated areas in 32 major US coastal cities. Here we show that, even when considering the current coastal-defence structures, further land area of between 1,006 and 1,389 km2 is threatened by relative sea-level rise by 2050, posing a threat to a population of 55,000–273,000 people and 31,000–171,000 properties. Our analysis shows that not accounting for spatially variable land subsidence within the cities may lead to inaccurate projections of expected exposure. These potential consequences show the scale of the adaptation challenge, which is not appreciated in most US coastal cities.

Rising sea levels and the increase of shoreline wave energy at American Samoa

American Samoa is experiencing rapid relative sea level rise due to increases in global sea level and significant post-2009 earthquake land subsidence, endangering homes and critical infrastructure. Wave and water-level observations collected over a fringing reef at Faga‘itua Bay, American Samoa, in 2017 reveal depth-limited shoreline sea-swell wave heights over the range of conditions sampled. Using field data to calibrate a one-dimensional, phase-resolving nonhydrostatic wave model (SWASH), we examine the influence of water level on wave heights over the reef for a range of current and future sea levels. Assuming a fixed reef bathymetry, model results predict rising sea levels will escalate nearshore extreme water levels that are dominated by an increase in nearshore sea-swell wave heights. Model results provide insight into how and at what reef depths rising sea levels reduce reef capacity to dissipate wave energy, compounding shoreline threats. This study aims to bring increased attention to the immediate threats to American Samoa’s way of life, and to demonstrate the utility of SWASH for extrapolating wave transformation to future sea level.

The contribution of coastal land subsidence to potential sea-level rise impact in data-sparse settings: The case of Ghana’s Volta delta

Deltas are highly valuable environmental systems, ensuring various livelihoods through their ecosystem services. However, human impact and climate change stressors are impacting deltas immensely. Consequently, many deltas, including Ghana's Volta Delta, are facing increasing risks, especially as hazards are increasing in magnitude and impacting coastal livelihoods. To provide a better understanding of coastal hazards in the Volta Delta, this study assessed the Delta's subsidence regime and its consequences for the potential impact of sea-level rise (SLR). Using the Interferometric Synthetic Aperture Radar (InSAR) technique and Global Navigation Satellite System (GNSS) surveys, vertical land motion (VLM) was documented. Interferograms of Sentinel-1 data from 2016 to 2020 indicated subsiding rates of up to −9.2 mm/yr. By combining local VLM information with recent SLR projections and elevation data, this study updates those projections and provides local assessments of potential Relative SLR (rSLR) impact. According to these locally improved scenarios, up to ∼45 % of the Delta will fall below local sea level by 2100, of which close to 10 % is explained by the integration of local VLM data alone. Depending on the climate change scenarios used, land subsidence will increase the deltaic area at risk by 4.31 % (96.27 km2) to 10.18 % (227.64 km2) and consequently exacerbate its exposure to coastal inundation. To avert the projections, the study recommends robust monitoring regimes; alternative freshwater sources to groundwater; reduced sediment trapping and river obstruction; and the need to stall ongoing oil and gas prospecting and subsequent extraction in the Voltain Basin.

Impact of land-use change on salt marsh accretion

Salt marsh vertical accretion that keeps pace with relative sea-level rise (RSLR) promotes habitat resilience and a continuously growing carbon stock. Vertical accretion, however, hinges on various salt marsh biogeomorphic feedback loops that respond to changing sediment supply, storms events, accelerating RSLR, elevation, and vegetation density. During the latter half of the 20th century, a large proportion of watersheds on the lower coastal plain of North America experienced land cover change. Many salt marshes show slow rates of vertical accretion over that period suggesting degradation and destabilization attributed to reduced upstream sediment loads and changing proportions of mineral sediment to organic matter. Here, we investigated a potential dichotomy observed at many sites in coastal North America between previously published accretion rates of the subtidal bay bottoms and adjacent salt marshes that were exceeding and lagging behind RSLR since 1950 CE, respectively. In 12 small coastal watersheds that formed within tributary and coastal prism incised valleys, we coupled 210Pb-derived accumulation rates of fringing salt marshes and previously published data from adjacent tidal creek bay bottoms with geospatial analysis of watershed land cover change since 1959, inundation duration, and vegetation density. Accumulation rates were higher at marsh sites within tributary incised valleys with lower relief watersheds, smaller tidal ranges, shorter inundation times, higher vegetation densities, and where land cover changes were dominated by increased cleared forest area. Additionally, mass accumulation rates (MAR) accelerated at 8 of the 12 marsh sites after a Major Land Cover Change (MLCC). Despite this acceleration in MAR, only 2 marsh sites recorded average sediment accumulation rates (SAR) greater than RSLR after a MLCC, which suggests that most of the marsh sites are drowning or being converted to subtidal habitat. This may be misleading, however, because modern marsh platform elevations at all but one marsh site were above local mean sea level, so the marshes could be accommodation limited. This work supports the disconnect found in previous research suggesting that estuaries are accumulating sediment at sustainable rates while their fringing salt marshes are not, which illuminates the complexities associated with sedimentation regimes in small local watersheds and their fringing salt marshes.

Vertical accretion trends project doughnut-like fragmentation of saltmarshes

Coastal saltmarshes keep pace with sea-level rise through in-situ production of organic material and incorporation of allochthonous inorganic sediment. Here we report rates of vertical accretion of 16 new sediment cores collected proximal to platform edges within saltmarshes located behind four barrier islands along the southeast United States coast. All but two of these exceed the contemporaneous rate of relative sea-level rise, often by a factor of 1.5 or more. Comparison with 80 additional measurements compiled across the Georgia Bight reveals that marshes situated closer to inlets and large bays generally accrete faster than those adjacent to small creeks or within platform interiors. These results demonstrate a spatial dichotomy in the resilience of backbarrier saltmarshes: marsh interiors are near a tipping point, but allochthonous mineral sediment fluxes allow enhanced local resilience along well-exposed and platform-edge marshes. Together, this suggests that backbarrier marshes are trending towards rapid, doughnut-like fragmentation.

Elevation Changes in Restored Marshes at Poplar Island, Chesapeake Bay, MD: I. Trends and Drivers of Spatial Variability

Tidal marshes provide numerous ecosystem services, but are threatened by recent increases in global sea level rise (SLR). Marsh restoration and creation are important strategies for mitigating marsh loss, restoring ecosystem services, increasing coastal community resilience, and providing much needed habitat for threatened species. Dredged material resulting from navigation channel maintenance can provide a substrate for these restoration projects. Few studies, however, have addressed the sustainability of these marshes. The Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island, where fine-grained, nutrient-rich dredged material from upper Chesapeake Bay is being used to create > 302 ha of tidal marshes, provides a case study. The low supply of inorganic sediment is counteracted by abundant nutrients, stimulating high rates of organic matter production and elevation change. Using > 10 years of data from 39 surface elevation tables, we found that the mean low marsh rate of elevation change (7.7 ± 3.21 mm year−1) was double the mean high marsh rate (3.6 ± 0.47 mm year−1) and exceeded the natural reference marsh (3.0 ± 2.28 mm year−1) and relative SLR (5.7 mm year−1). By stimulating organic matter production, the high nutrient substrate appears to offset the low inorganic sediment inputs in mid-Chesapeake Bay. Spatial variability was correlated with initial elevation, but was also influenced by local factors that may affect sediment redistribution within the marshes.

The Middle to Late Holocene in the Agro Pontino and the Fondi basin (Lazio, Italy): A palaeogeographical overview

The Agro Pontino, a vast plain located south of Rome (southern Lazio, Italy), has served as a valuable resource for central Italian palaeogeographical, vegetational and archaeological studies for decades. This paper reviews the regional development of the Agro Pontino and adjacent Fondi basin from the Würmian (Marine Isotope Stages 2–5d) until the end of the protohistoric, with an emphasis on the period around the Early Bronze Age (EBA) date of the Avellino eruption. Following postglacial relative sea level rise, a dissected Pleistocene landscape drowned and was filled with lagoonal, fluvial, alluvial, lacustrine and marsh deposits. In the Agro Pontino, two lakes emerged divided by an alluvial fan. The unique regionally preserved stack of Middle to Late Holocene sediments, including multiple macroscopically visible tephra layers within a depositional environment, allowed for a detailed regional palaeogeographical, tephrochronological and vegetational reconstruction. This study presents the latest palaeogeographical findings from the Agro Pontino and the Fondi basin for the Middle Holocene and the EBA, for which tephra identification and dating forms the solid basis. It contributes to the reconstruction of the Pontine plain and the Fondi basin during this period and furthers our palaeoenvironmental knowledge of the conditions for human subsistence in this region during the later phase of the EBA, around the time of the Avellino eruption (c. 1900 calBC).

Relative Sea Level Rise in the Winyah Bay-Waccamaw River Tidal System Over the Last Thirteen Years

Prediction of sea level rise (SLR) in response to climate change has been the focus of worldwide research, most focusing on the impact by human development. The research has been limited to estuaries and tidal rivers near harbors dealing with the hydrodynamics of reversing tidal flows. This article focuses on the Waccamaw River National Wildlife Refuge in coastal South Carolina where freshwater unidirectional flow is common. We examined the record of water levels in the Waccamaw and Pee Dee Rivers over the period 2007–2019 and the length of record of the United States Geographical Survey (USGS) gauge at Pawleys Island on the Waccamaw River. The Atlantic Ocean, off the southeastern coast of the US, has experienced accelerated SLR since 2000. National Oceanic and Atmosphere Administration (NOAA) tide gauges from Fort Pulaski on Cockspur Island in Georgia to Beaufort, North Carolina, show significant increase in long-term SLR since then with an average since 2007 of approximately 10 mm y-1. Since the study period was less than the 18.6-year cycle of lunar precession, tidal ranges were expanding for much of the study period resulting in the rate of rise of Mean Higher High Water (MHHW; the average of the highest tide levels during each day) being greater than the rate of increase of Mean Lower Low Water (MLLW; the average of the lowest tide levels during each day) in all ocean stations. We examined water levels at NOAA and USGS gauges from Oyster Creek, in North Inlet to Conway on the Waccamaw River and Near Bucksport on the Pee Dee River. We found mean water levels increased more rapidly with distance from the ocean with an apparent SLR &gt; 40 mm y-1 at Conway on the Waccamaw and Bucksport on the Pee Dee. In contrast to the ocean NOAA gauges, the estuary/river gauges showed more rapid increase of daily minimum water level (an approximation of MLLW) than daily maximum water level (an approximation of MHHW) with an extreme of apparent rise of minimum water levels of 58 mm y-1 at Bucksport on the Pee Dee. Nearly 50% of the increase in apparent SLR was due to an increase in the annual average freshwater flow of the Pee Dee and Waccamaw Rivers. Over the past 13 years the Waccamaw National Wildlife Refuge has experienced an apparent SLR that was more than double that observed at the edge of the ocean. The rise has been greater in the height of daily low water than in the height of daily high water. The increase was driven by both tidal hydrodynamics and an increase in the rate of flow in the Pee Dee and Waccamaw Rivers. These findings have important implications for land managers, policymakers, and homeowners in the region as people in the middle to upper estuaries need to plan for rates of relative SLR rise much greater than the frequently discussed rates in the ocean.

Geomorphic Response of the Georgia Bight Coastal Zone to Accelerating Sea Level Rise, Southeastern USA

Synthesis of geologic and chronologic data generated from Holocene sedimentary sequences recovered along the inner continental shelf, shoreface, and modern coastal zone of the Georgia Bight reveal a synchronous sequence of paleoenvironmental events that occurred in response to rate of sea level rise tipping points. During the early Holocene (11.7–8.2 cal kyr BP), the paleoshoreline was overstepped and submerged by rapidly rising seas that averaged ~5 mm yr−1. Rates of rise during the middle Holocene (8.2–4.2 cal kyr BP) averaged ~2 mm yr−1 and this deceleration resulted in the formation of coastal environments and sedimentary sequences that were subsequently reworked as the shoreface continued its landward and upward migration. The modern coastal zone emerged commensurate with the late Holocene (4.2–0 cal kyr BP), when the rate of sea level rise averaged &lt;1 mm yr−1. Analysis of water level data collected at six NOAA tide gauge stations located along the Georgia Bight coast indicates the rate of relative sea level rise has increased from a historical average of 3.6 ± 0.2 mm yr−1 (&lt;1972 to 2022) to 6.6 ± 0.8 (1993 to 2022) and during the 21st century it has averaged 9.8 ± 0.3 mm yr−1 (2003 to 2022). The current rate of sea level rise is nearly double the early Holocene rate of rise. Based upon a novel application of the principle of uniformitarianism (i.e., the past is the key to the future), the likely geomorphic trajectory of the Georgia Bight coastal zone under conditions of 21st century accelerating sea level rise will be one of increasing instability (e.g., coastal erosion) and flooding (e.g., overwash, breaching). Evidence of an emerging instability within the coastal zone has been previously reported throughout the region and supports the trajectory of geomorphic change proposed herein. This will ultimately result in the submergence of existing landscapes and replacement by estuarine and marine environments, which may hasten in pace and scale given the current rate of sea level rise is expected to continue accelerating throughout this century. These findings have not been previously reported and should be considered by coastal practitioners responsible for conceptualizing risk, as well as the formulation and implementation of adaptation action plans designed to mitigate threats to the built and natural environment induced by climate change.

Carrying capacity for tree biomass of a subtropical mangrove along a river in Japan inferred from forest structural features

AbstractA subtropical mangrove along the Miyara River in Ishigaki Island, Japan was studied for evaluating the carrying capacity for biomass of the mangrove stands. The stem diameters D, tree height H, and fine roots mass were measured, while aboveground biomass (AGB) and belowground coarse root biomass (BGBcoarse) were estimated. The AGB, BGBcoarse, and fine root mass were estimated as 130, 31, and 13 Mg ha−1 in the Rhizophora stylosa; 271, 94, and 11 Mg ha−1 in the downstream Bruguiera gymnorrhiza; and, 228, 81, and 6.4 Mg ha−1 in the upstream B. gymnorrhiza stand, respectively. The AGB and BGBcoarse in the R. stylosa stand were significantly lower than B. gymnorrhiza stands, and fine root mass was significantly higher than upstream B. gymnorrhiza stand. Significantly lower mean individual phytomass wt specific to tree density ρ of R. stylosa stand than B. gymnorrhiza stand in the ρ − wt relationship denoted the lower carrying capacity for AGB of R. stylosa than that of B. gymnorrhiza. The results showed that high soil pore water salinity and low pH at the downstream did not limit biomass and potential canopy height Hmax of mangrove along a river gradient but AGB and BGBcoarse differed between different species at the same edaphic environment. Analysis of aboveground and belowground biomass variations between stands of two mangrove species along environmental gradients from upstream to downstream could be useful in assessing the consequences of sea level rise in relation to climate change on the evolution of blue carbon dynamics.