Regional Sea Level Changes Research Articles

A CNN-BiGRU sea level height prediction model combined with bayesian optimization algorithm

Traditional linear models are insufficient for capturing the complex dynamics of sea level changes. This paper aims to predict sea level time series using neural network models adapted for nonlinear data. Currently, few researchers use a combination of convolutional neural networks and bidirectional gated recurrent units (CNN-BiGRU) optimized with hyperparameter tuning for sea level prediction. There is also a lack of detailed discussion on the randomness of neural network initialization in prediction domains. Therefore, this study employs the bayesian optimization algorithm (BO) to optimize the CNN-BiGRU model, resulting in the BO-CNN-BiGRU model. Experiments initially compared the BO-CNN-BiGRU model with five other models using data from ten tidal stations in the US, showing that the model outperformed the others. To address initialization randomness, we used ten random seeds for statistical analysis, which demonstrated that the BO-CNN-BiGRU model performed well in terms of predictive performance and robustness. Finally, the BO-CNN-BiGRU model was applied to satellite altimetry grid data from the Bohai and Yellow Seas in China, yielding a linear trend of 3.92 ± 0.63 mm/a from 1993 to 2023, consistent with the China Sea Level Bulletin, further validating the model's effectiveness. This model can be used to predict regional sea level change.

Optically dating of a Paleolithic site in coastal regions of South China and its correlation with the late Pleistocene environment evolution

The coastal regions of South China are crucial for the emergence and migration of early Austronesian hominids to Taiwan Island and the Pacific islands, but most Paleolithic sites in this area remain unexplored wilderness areas without clear archaeological stratigraphy or formal excavation. The research on archaeological chronology faces challenges due to the absence of direct evidence for determining site ages; instead, it relies on indirect methods such as comparing lithology, knapping techniques, and other usage characteristics with regional stratigraphic ages based on recovered stone tools. This presents a challenge when studying prehistoric human-land relationships in the region. However, the Kuahaicun site located on coastal islands along the southeastern coast offers valuable materials for investigating activities of Paleolithic populations along both South China’s coast and its islands since the late Pleistocene period. This study employs a comprehensive investigation and sampling approach, utilizing optically stimulated luminescence dating to determine burial age. Furthermore, by integrating regional aeolian landform characteristics and trends in sea level changes, we examine the influence of geomorphic environmental evolution during the transition from Paleolithic to Neolithic periods on ancient human migration and diffusion patterns. 1) The findings suggest that the stratigraphy primarily consists of sedimentary layers dating back to 25–10 ka, representing the transition from Late Paleolithic to Early Neolithic. The luminescence sensitivity of quartz grains indicates diverse sediment sources in this area, including aeolian deposits with efficient long-distance transportation due to thorough and repeated bleaching, as well as alluvium and colluvium deposits with limited bleaching. 2) By comparing the developmental process of coastal aeolian sand with regional sea level changes, it becomes evident that the main period of Austronesian ancestor activity in South China, as evidenced by this site, significantly aligns with a crucial phase of aeolian deposition. Moreover, this correlation coincides with three low sea level phases during the late Pleistocene Fujian marine transgression. 3) Commencing from the Middle Paleolithic period onwards, Austronesian ancestors gradually migrated downstream along the Minjiang River and Jiulong River towards their respective estuaries. The presence of a transient land bridge during the Late Pleistocene Fujian marine transgression (18–15 ka) potentially facilitated the migratory movements and territorial expansions of ancient populations along China’s southern coastline, enabling their transition from mainland coastal regions to nearby islands.

Characteristics and driving mechanisms of early Holocene weakened monsoon events: Evidence from northern Bohai Bay, China

The characteristics and driving mechanisms of early Holocene extreme climatic events can provide a basis for the prediction and assessment of such events under ongoing global warming. Coastal areas, with their unique geographical location and fragile ecological environment, are highly sensitive to climate change, making the study of extreme climatic events in these regions potentially providing a different understanding of the processes and mechanisms involved. We studied the Holocene sediments of core BXZK04 from northern Bohai Bay, China, combining 9 AMS 14C dates with high-resolution analyses of pollen, algal remains, and grain size, together with modern pollen databases, to quantitatively reconstruct mean annual precipitation. Our principal findings are: (1) The climate in northern Bohai Bay during 10,900–8600 cal yr BP showed a gradual trend towards increased humidity, with fluctuations. We propose that long-term climate change in the study area was dominated by Northern Hemisphere insolation and regional sea-level changes. (2) Two significant weak monsoon events occurred in the study area, during 9600–9300 cal yr BP and 9060–8950 cal yr BP, corresponding to the 9.4 ka and 8.9 ka climatic events, respectively. During these events, forest vegetation decreased, herbaceous vegetation expanded, and there was a significant reduction in monsoon precipitation (a decrease of ∼200 mm). These events may have been controlled by the weakening of solar activity and reduced Atlantic Meridional Overturning Circulation (AMOC) caused by freshwater influx to the North Atlantic, leading to the southward shift of the Intertropical Convergence Zone (ITCZ) and more frequent El Niño events.

Perspective on Regional Sea-level Change and Coastal Impacts

Perspective on Regional Sea-level Change and Coastal Impacts

Regional Mean Sea Level Variability Due to Tropical Cyclones: Insights from August Typhoons

This study investigates the interannual variations in regional mean sea levels (MSLs) of the northeast Asian marginal seas (NEAMS) during August, focusing on the role of typhoon activity from 1993 to 2019. The NEAMS are connected to the Pacific through the East China Sea (ECS) and narrow, shallow straits in the east, where inflow from the southern boundary (ECS), unless balanced by eastern outflow, leads to significant convergence or divergence, as well as subsequent changes in regional MSLs. Satellite altimetry and tide-gauge data reveal that typhoon-induced Ekman transport plays a key role in MSL variability, with increased inflow raising MSLs during active typhoon seasons. In contrast, weak typhoon activity reduces inflow, resulting in lower MSLs. This study’s findings have significant implications for coastal management, as the projected changes in tropical cyclone frequency and intensity due to climate change could exacerbate sea level rise and flooding risks. Coastal communities in the NEAMS region will need to prioritize enhanced flood defenses, early warning systems, and adaptive land use strategies to mitigate these risks. This is the first study to link typhoon frequency directly to NEAMS MSL variability, highlighting the critical role of wind-driven processes in regional sea level changes.

Expansion of C4 plants in the tropical Leizhou Peninsula during the Last Glacial Maximum: Modulating effect of regional sea-level change

Due to the lack of relatively long-term, high-resolution terrestrial records in tropical southern China, there is limited published research on terrestrial vegetation changes and their responses to regional and/or global climate forcings since the last glacial period. In this study, a 170-cm-long peat core (covering the interval from ~44.1 to 9.3 cal kyr BP) recovered from the Xialu peatland in Leizhou Peninsula, South China, was analyzed for organic carbon isotope (δ13Corg), along with total organic carbon, total nitrogen, and bulk dry density, to investigate past vegetation and hydroclimatic changes. Our results showed that C4 plants dominated the study region during Marine Isotope Stage (MIS) 2 (29–14 cal kyr BP), indicating generally cooler and drier conditions during MIS 2 relative to late MIS 3 (~ 44.1–29 cal kyr BP) and early MIS 1 (14–9.3 cal kyr BP). In particular, the driest conditions occurred during the Last Glacial Maximum (~ 25–19 cal kyr BP) when sea level was at its lowest. In addition, several millennial-scale climatic events associated with the expansion of C4 plants were clearly identified. Our record is sensitive to a variety of glacial-interglacial forcings, including regional processes and global forcing, among which the inundation history of Beibu Gulf due to sea-level change during the late Quaternary, which has been neglected in previous studies, may have played an important role in modulating paleo-hydroclimatic changes in tropical southern China.

Simulated Impact of Time‐Varying River Runoff and Greenland Freshwater Discharge on Sea Level Variability in the Beaufort Gyre Over 2005–2018

AbstractGlobal mean sea level has been rising at a rate of 3.25 ± 0.4 mm yr−1 over 1993–2018. Yet several regions are increasing at a much faster rate, such as the Beaufort Gyre in the Arctic Ocean at a rate of 9.3 ± 7.0 mm yr−1 over 2003–2014. At interannual to decadal time scales, the Beaufort Gyre sea level is controlled by salinity changes due to sea ice melt and wind‐driven lateral Ekman convergence–divergence of freshwater. This study uses recent Greenland discharge and river runoff estimates to isolate and quantify the sea level response to freshwater fluxes variability over the period 1980–2018. It relies on sensitivity experiments based on a global ocean model including sea‐ice and icebergs. These sensitivity experiments only differ by the freshwater fluxes temporal variability of Greenland and global rivers which are either seasonal climatologies or fully time varying, revealing the individual and combined impact of these freshwater sources fluctuations. Fully varying Greenland discharge and river runoff produce an opposite impact on sea level trends over 2005–2018 in the Beaufort Gyre region, the former driving an increase, while the latter, a decrease. Their combined impact leads to a fairly weak sea level trend. The sea level response is primarily driven by salinity variations in the upper 300 m, which are mainly caused by salinity advection involving complex compensations between passive, active, and nonlinear advection. This study shows that including the temporal variability of freshwater fluxes in forced global ocean models results in a better representation of regional sea level change.

Quality assessment of the nominal attitude model of TOPEX/Poseidon using quaternion data

The altimetry mission TOPEX/Poseidon was operational between 1992 and 2005 and provided innovative insight into global and regional sea level changes and their variability. The correct modelling of the spacecraft orbit in precise orbit determination requires detailed information on the attitude of the spacecraft body and the solar array. Since the availability of attitude observations in form of quaternion data for this mission is limited, the nominal orientation model has to be used mainly. In this study, we compare the nominal and quaternion-based approaches of the TOPEX/Poseidon attitude realisation and validate the nominal model w.r.t. the observed attitude. We found good agreement between both approaches in the spacecraft-related roll, pitch, yaw, and solar array angles with slight differences of up to 0.31° on average. The best accordance is obtained in the yaw angles. A general angular offset of 0.12° and -0.19° on average is found in the pitch and solar array angle differences, respectively. Parameters estimated in the orbit computation process also reflect the good agreement between both orientation models with slightly better results when using the observed attitude. However, in some intervals, the nominal model lacks detailed information about the correct attitude. These can be adjusted using the information from the quaternion data. Such intervals indicate off-nominal spacecraft attitude events like drifts or offsets. Purely geometrically analysed, mispointing in the roll and pitch components of about 0.25° results in a change of the altimeter phase centre position in the radial direction of about 7 mm, which directly impacts the sea-level heights determined from altimetry measurements.

The Role of Anthropogenic Forcings on Historical Sea‐Level Change in the Indo‐Pacific Warm Pool Region

AbstractDetecting and attributing sea‐level rise over different spatiotemporal scales is essential for low‐lying and highly populated coastal regions. Using the Detection and Attribution Model Intercomparison Project (DAMIP) from the Coupled Model Intercomparison Project Phase 6, we evaluate the role of anthropogenic forcing in sea‐level change in the historical (1950–2014) period in the Indo‐Pacific warm pool region. We use three models that have at least 10 ensemble members, corresponding to different DAMIP simulations. We determined the changes in regional sea level from both natural and anthropogenic forcings. Our results demonstrate: (a) the emergence of an anthropogenic footprint on regional sterodynamic sea‐level change has a large spatiotemporal diversity over the Indo‐Pacific warm pool region with the earliest emergence in the western Indian Ocean; (b) a significant rise in dynamic sea level (DSL) (up to 25 mm) and thermosteric (up to 40 mm) sea level over the western Indian Ocean due to greenhouse gas forcing; (c) a positive Indian Ocean Dipole‐like pattern in the DSL changes over the tropical Indian Ocean; (d) a significant increase in the halosteric contribution to sea‐level rise in the Indo‐Pacific warm pool region, and (e) a pronounced rise of manometric sea level (up to 20 mm) over shallow oceans and coastal regions in recent decades. These results provide a comprehensive spatiotemporal analysis of the attribution of anthropogenic factors to sea‐level changes in the Indo‐Pacific warm pool region.

Spatial–Temporal Variations in Regional Sea Level Change in the South China Sea over the Altimeter Era

This study utilizes 27 years of sea level anomaly (SLA) data obtained from satellite altimetry to investigate spatial–temporal variations in the South China Sea (SCS). The local mean decomposition (LMD) method is applied to decompose the sea level data into three components: high-frequency, low-frequency, and trend components. By removing the influence of high-frequency components, multiple time series of regular sea level changes with significant physical significance are obtained. The results indicate that the average multi-year SLA is 50.16 mm, with a linear trend of 3.91 ± 0.12 mm/a. The wavelet analysis method was employed to examine the significant annual and 1.5-year periodic signals in the SCS SLA series. At the seasonal scale, the sea level rise in coastal areas during autumn and winter surpasses that of spring and summer. Moreover, there are generally opposing spatial distributions between spring and autumn, as well as between summer and winter. The linear trends in multi-year SLA for the four seasons are 3.70 ± 0.13 mm/a, 3.66 ± 0.16 mm/a, 3.49 ± 0.16 mm/a, and 3.74 ± 0.33 mm/a, respectively. The causes of SCS sea level change are examined in relation to phenomena such as monsoons, the Kuroshio Current, and El Niño–Southern Oscillation (ENSO). Based on the empirical orthogonal function (EOF) analysis of SCS SLA, the contributions of the first three modes of variance are determined to be 34.09%, 28.84%, and 8.40%, respectively. The temporal coefficients and spatial distribution characteristics of these modes confirm their associations with ENSO, monsoons, and the double-gyre structure of SCS sea surface temperature. For instance, ENSO impacts SCS sea level change through atmospheric circulation, predominantly affecting the region between 116° E and 120° E longitude, and 14° N and 20° N latitude.

Impact of Mass Redistribution on Regional Sea Level Changes Over the South China Sea Shelves

AbstractThis study investigates long‐term sea level changes in the South China Sea (SCS) using a validated high‐resolution regional ocean model simulation for the Maritime Continent. The contributions of ocean mass redistribution and steric sea level are examined to understand the sea level variations. The ocean bottom pressure (OBP) serves as an indicator of sea level variations linked to alterations in ocean mass flux. The OBP accounts for over 80% of the total sea level change over the shelves, while the steric sea level emerges as the dominant factor, contributing over 50% to the sea level change in the deep SCS. Luzon Strait transport shows a weakening trend in the last six decades, resulting in higher heat accumulation and larger steric expansion in the deep SCS. The ocean mass redistribution acts as a mechanism to balance the contrasting steric induced sea level changes over the deep SCS and shallow continental shelves.

Impact of the solid Earth mass adjustment by the 2011 Tohoku–Oki earthquake on the regional sea level and hydrological mass change recovery from GRACE

SUMMARY For more than a decade, GRACE data have provided global mass redistribution measurements due to water cycles, climate change and giant earthquake events. Large earthquakes can yield gravity changes over thousands of kilometres from the epicentre for years to decades, and those solid Earth deformation signals can introduce significant biases in the estimate of regional-scale water and ice mass changes around the epicentres. We suggest a modelling scheme to understand their contribution to the estimates of water and ice mass changes and to remove the earthquake-related solid mass signals from GRACE data. This approach is composed of physics-based earthquake modelling, GRACE data correction and high-resolution surface mass change recovery. In this study, we examined the case of the 2011 Tohoku–Oki earthquake to better estimate the regional sea level and hydrological mass changes in the East Asia. The co- and post-seismic changes from GRACE observations were used to constrain the earthquake model parameters to obtain optimal self-consistent models for the earthquake source and the asthenosphere rheology. The result demonstrated that our earthquake correction model significantly reduced the mass change signals by solid Earth deformation from the time-series of regional surface mass changes on both land and oceans. For example, the apparent climate-related ocean mass increase over the East Sea was 1.59 ± 0.11 mm yr−1 for 2003–2016, significantly lower than the global mean ocean mass trend (2.04 ± 0.10 mm yr−1) due to contamination of the earthquake signals. After accounting for the solid mass changes by the earthquake, the estimate was revised to 1.87 ± 0.11 mm yr−1, that is increased by 20 per cent and insignificantly different from the global estimate.

Sedimentary System Evolution and Petroleum Geological Significance of the Quaternary Ledong Formation in Qiongdongnan Basin

Quaternary Ledong formation in Qiongdongnan Basin is rich in hydrate, it is import to clarify the evolution and distribution rule of the sedimentary system of Ledong formation in this area. Based on 3D seismic data and drilling data, combined with the characteristics of regional sea level change, and guided by the principle of sequence stratigraphy, this paper established a high-precision sequence stratigraphic framework of Ledong Formation, identified five third-order sequences, and pointed out that the S14 interface was the conversion surface of progradational continental slope to accretive continental slope. The Ledong Formation was divided into the upper member of Ledong Formation and the lower member of Ledong Formation, which controls the development of submarine fans. The results show that the lower member of Ledong Formation has developed a large channelized submarine fan of the western provenance central canyon system, and the upper member of Ledong Formation has developed a small scale submarine fan, mass flow and sheet sand complex sedimentary body outside the northern provenance canyon system. The sedimentary type and temporal and spatial evolution characteristics are defined, which provides geological basis for the prediction of favorable reservoir distribution of hydrate and shallow gas in this area.

Orbital obliquity evolution during the late Paleozoic ice age across the northeastern gondwana: Implications for regional sea-level change trigger and reservoir quality assessment

Orbitally-induced cycles build stratigraphic sequences on time spans ranging from several thousand to several million years by altering depositional conditions. Previous works have shown that the gas-bearing Faraghan Formation (Cisuralian) has been impacted by orbitally-forced climatic change, resulting in the development of sedimentary sequences on astronomical cycle timescales. Obliquity evolution has been observed in the Faraghan Formation, with ∼1.2 Ma and ∼173 ka modulation cyclicities recorded. We attribute a strong sedimentary noise observed in the Faraghan Formation, which is distinct from the other noises, to a transregional sea-level rise, most likely correspond to MFS P10. The prolific gas zones (PGZ) in the Faraghan Formation are in consistent relationship with obliquity maxima and depositional noise events, which are associated with high obliquity power. This constant offers the hypothesis that obliquity altered the sedimentary regime in such a way that favorable conditions for reservoir quality development occurred. The ∼173 ka obliquity modulation cycle is recorded from the Late Paleozoic. It is postulated that this periodicity was the causal mechanism for the creating of the Faraghan Formation's fifth-order sequences. The reconstructed sea-level patterns from sedimentary noise modeling (DYNOT) confirm the three third-order sequences proposed for the Faraghan Formation by displaying three different sedimentary noises. The detected ∼1.2 Ma obliquity modulation cycles match the ∼1.2 Ma periodicities filtered from the DYNOT median values, indicating that the ∼1.2 Ma orbital cycle is one of the primary drivers of regional sea level. ∼1.2 Ma obliquity modulation cycles are correlated with third-order sequences, implying that the aforementioned cycle was one of the key mechanisms causing the creation of these third-order sequences.

Sensitivity of MPI-ESM Sea Level Projections to Its Ocean Spatial Resolution

Abstract The dependence of future regional sea level changes on ocean model resolution is investigated based on Max Planck Institute Earth System Model (MPI-ESM) simulations with varying spatial resolution, ranging from low resolution (LR), high resolution (HR), to eddy-rich (ER) resolution. Each run was driven by the shared socioeconomic pathway (SSP) 5-8.5 (fossil-fueled development) forcing. For each run the dynamic sea level (DSL) changes are evaluated by comparing the time mean of the SSP5-8.5 climate change scenario for the years 2080–99 to the time mean of the historical simulation for the years 1995–2014. Respective results indicate that each run reproduces previously identified large-scale DSL change patterns. However, substantial sensitivity of the projected DSL changes can be found on a regional to local scale with respect to model resolution. In comparison to models with parameterized eddies (HR and LR), enhanced sea level changes are found in the North Atlantic subtropical region, the Kuroshio region, and the Arctic Ocean in the model version capturing mesoscale processes (ER). Smaller yet still significant sea level changes can be found in the Southern Ocean and the North Atlantic subpolar region. These sea level changes are associated with changes in the regional circulation. Our study suggests that low-resolution sea level projections should be interpreted with care in regions where major differences are revealed here, particularly in eddy active regions such as the Kuroshio, Antarctic Circumpolar Current, Gulf Stream, and East Australian Current. Significance Statement Sea level change is expected to be more realistic when mesoscale processes are explicitly resolved in climate models. However, century-long simulations with eddy-resolving models are computationally expensive. Therefore, current sea level projections are based on climate models in which ocean eddies are parameterized. The representation of sea level by these models considerably differs from actual observations, particularly in the eddy-rich regions such as the Southern Ocean and the western boundary currents, implying erroneous ocean circulation that affects the sea level projections. Taking this into account, we review the sea level change pattern in a climate model with featuring an eddy-rich ocean model and compare the results to state-of-the-art coarser-resolution versions of the same model. We found substantial DSL differences in the global ocean between the different resolutions. Relatively small-scale ocean eddies can hence have profound large-scale effects on the projected sea level which may affect our understanding of future sea level change as well as the planning of future investments to adapt to climate change around the world.

Regionalizing the sea-level budget with machine learning techniques

Abstract. Attribution of sea-level change to its different drivers is typically done using a sea-level budget approach. While the global mean sea-level budget is considered closed, closing the budget on a finer spatial scale is more complicated due to, for instance, limitations in our observational system and the spatial processes contributing to regional sea-level change. Consequently, the regional budget has been mainly analysed on a basin-wide scale. Here we investigate the sea-level budget at sub-basin scales, using two machine learning techniques to extract domains of coherent sea-level variability: a neural network approach (self-organizing map, SOM) and a network detection approach (δ-MAPS). The extracted domains provide more spatial detail within the ocean basins and indicate how sea-level variability is connected among different regions. Using these domains we can close, within 1σ uncertainty, the sub-basin regional sea-level budget from 1993–2016 in 100 % and 76 % of the SOM and δ-MAPS regions, respectively. Steric variations dominate the temporal sea-level variability and determine a significant part of the total regional change. Sea-level change due to mass exchange between ocean and land has a relatively homogeneous contribution to all regions. In highly dynamic regions (e.g. the Gulf Stream region) the dynamic mass redistribution is significant. Regions where the budget cannot be closed highlight processes that are affecting sea level but are not well captured by the observations, such as the influence of western boundary currents. The use of the budget approach in combination with machine learning techniques leads to new insights into regional sea-level variability and its drivers.

Disentangling Holocene Climate Change and Human Impact from Palaeoenvironmental Records from the Scottish West Coast

Phases of rapid climate change throughout the early to mid Holocene coincide with regional human population expansion in Scotland and North-West Europe. Palaeoenvironmental signals of climate and anthropogenically driven vegetation changes can therefore be difficult to separate. To identify whether it is possible to distinguish potential signatures of anthropogenic clearance and agricultural activities from climatic drivers of landscape change in the early to mid Holocene in the region, two topographically contrasting sites on the Isle of Skye and the Isle of Bute were investigated. A multiproxy approach including pollen, spore, microcharcoal, loss on ignition and particle size analyses was adopted to investigate changes in vegetation and climate. There are subtle indications that the 8200 cal BP climate event had an effect on the vegetation composition at both sites. Signals of anthropogenic woodland clearance are apparent early in the sequence at Peat Hill (Bute), indicated by a peak in Poaceae (grass) cereal-type (7–14%) at 8592–8793 cal BP, alongside a decrease in arboreal pollen, which could not be associated with a regional episode of climate change. Early to mid Holocene vegetation changes at Lyndale House (Skye) occur alongside regional changes in precipitation and sea level and therefore cannot be readily separated. Continuous declines in arboreal pollen from ca. 5000 cal BP at Lyndale House indicates the onset of widespread clearance on Skye via felling and sustained grazing pressures.

Mid- to late Holocene vegetation response to relative sea-level fluctuations recorded by multi-proxy evidence in the Subei Plain, eastern China

Reconstructing Holocene changes in regional sea level is useful for understanding future sea-level scenarios. In this paper we examined the sedimentology, pollen, foraminifera, and organic matter properties of a well-dated profile from the Subei Basin that spans the last 6.7 ka. By integrating our findings with the archaeological record, we investigated mid- to late Holocene vegetation responses to relative sea-level fluctuations and considered the impact of climatic variations on cultural development. In the middle Holocene, the rapid and abrupt expansion of salt marsh vegetation (Chenopodiaceae, Cyperaceae, and Poaceae) suggests a relative rise in sea level. From 5.7 to 4.2 cal ka BP, lowland swamps were settled by Neolithic communities, as indicated by peak levels of microscopic charcoal and a significant rise in herbaceous pollen species, such as Cyperaceae, Typha, Myriophyllum, and Poaceae. From 4.2 to 3.7 cal ka BP, a major increase in Chenopodiaceae abundance and a δ13C excursion were observed along with a decrease in Cyperaceae and Poaceae levels, which revealed that human activity significantly decreased following the 4.2 ka climate event. Evidence suggests that the area under investigation experienced a relatively high sea level between 6.7 and 6.0 ka, a relative decrease in sea level between 6.0 and 5.7 ka, a steady sea-level condition between 5.7 and 4.2 ka, and gradual replacement by a freshwater marsh between 5.7 and 4.2 ka. The pollen and δ13 C records demonstrate that the relative sea level rose quickly from 4.2 to 3.7 ka, which could correspond to the 4.2 ka event. Enhanced El Niño-Southern Oscillation (ENSO) variance and intense climatic events in eastern China were the driving forces behind the mid-late Holocene relative sea-level shift. The rise in relative sea level eventually led to the collapse of the Qingdun and Jiangzhuang sites in the Subei Basin. Our results provide independent vegetation evidence of the simultaneous changes in relative sea level and variations in climate.

Regional sea-level highstand triggered Holocene ice sheet thinning across coastal Dronning Maud Land, East Antarctica

The East Antarctic Ice Sheet stores a vast amount of freshwater, which makes it the single largest potential contributor to future sea-level rise. However, the lack of well-constrained geological records of past ice sheet changes impedes model validation, hampers mass balance estimates, and inhibits examination of ice loss mechanisms. Here we identify rapid ice-sheet thinning in coastal Dronning Maud Land from Early to Middle Holocene (9000–5000 years ago) using a deglacial chronology based on in situ cosmogenic nuclide surface exposure dates from central Dronning Maud Land, in concert with numerical simulations of regional and continental ice-sheet evolution. Regional sea-level changes reproduced from our refined ice-load history show a highstand at 9000–8000 years ago. We propose that sea-level rise and a concomitant influx of warmer Circumpolar Deep Water triggered ice shelf breakup via the marine ice sheet instability mechanism, which led to rapid thinning of upstream coastal ice sheet sectors.

River Deltas and 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. ▪ There are large differences in the initiation time and subsequent delta progradation during the Holocene, likely from different sea-level and sediment supply histories. ▪ Modern deltas are larger and will face faster sea-level rise than during their Holocene growth, making them susceptible to forced transgression. ▪ Regional sea-level projections have been much improved in the past decade and now also isolate dominant sources of uncertainty, such as the Antarctic ice sheet. ▪ Vertical land motion in deltas can be the dominant source of relative sea-level change and the dominant source of uncertainty; limited observations complicate projections. ▪ River deltas globally might lose 5% (∼35,000 km2) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario.