Mean Sea Level Rise Research Articles

Enhancing Sea Level Rise Estimation and Uncertainty Assessment from Satellite Altimetry through Spatiotemporal Noise Modeling

The expected acceleration in sea level rise (SLR) throughout this century poses significant threats to coastal cities and low-lying regions. Since the early 1990s, high-precision multi-mission satellite altimetry (SA) has enabled the routine measurement of sea levels, providing a continuous 30-year record from which the mean sea level rise (global and regional) and its variability can be computed. The latest reprocessed product from CMEMS span the period from 1993 to 2020, and have enabled the acquisition of accurate sea level data within the coastal range of 0–20 km. In order to fully utilize this new dataset, we establish a global virtual network consisting of 184 virtual SA stations. We evaluate the impact of different stochastic noises on the estimation of the velocity of the sea surface height (SSH) time series using BIC_tp information criterion. In the second step, the principal component analysis (PCA) allows the common mode noise in the SSH time series to be mitigated. Finally, we analyzed the spatiotemporal characteristics and accuracy of sea level change derived from SA. Our results suggest that the stochasticity of the SSH time series is not well described by a combination of random, flicker, and white noise, but is best described by an ARFIM/ARMA/GGM process. After removing the common mode noise with PCA, about 96.7% of the times series’ RMS decreased, and most of the uncertainty associated with the computed SLR decreased. We confirm that the spatiotemporal correlations should be accounted for to yield trustworthy trends and reliable uncertainties. Our estimated SLR is 2.75 ± 0.89 mm/yr, which aligns closely with recent studies, emphasizing the robustness and consistency of our method using virtual SA stations. We additionally introduce open-source software (SA_Tool V1.0) to process the SA data and reduce noise in surface height time series to the community.

Circulation and Stratification Changes in a Hypersaline Estuary Due to Mean Sea Level Rise

Hypersaline Hamelin Pool, with mean salinity >65, is located in Shark Bay, Western Australia. The high salinity has reduced its biodiversity, but it is home to a diverse assemblage of modern marine stromatolites. The limited exchange of water between Hamelin Pool and the rest of Shark Bay, due to the presence of the shallow Faure Sill together with high evaporation and low rainfall-runoff have resulted in a hypersaline environment. With climate-change-induced mean sea level rise (MSLR), hydrodynamic processes that maintain the hypersaline environment may be affected and are the focus of this paper. Oceanographic observations, together with hydrodynamic model results, were used to examine the hydrodynamic processes under present and future MSLR scenarios. A large attenuation in the tidal range, changes in the tidal characteristics, and current speeds together with a strong salinity gradient were observed across the Faure Sill under present-day conditions. Under an MSLR scenario of 1 m, the tidal amplitude decreased by up to 10% to the north, whilst to the south, the tidal range increased by up to 15%. Regions of strong vertical stratification were present on both sides of the Faure Sill. The simulations indicated that, under MSLR scenarios, these regions expanded in area and exhibited higher levels of vertical stratification. The salt flux across the Faure Sill was maintained as a diffusive process under MSLR scenarios.

Verification of tidal trends in Cananeia (SP) and Santos (SP)

Recent studies indicate an imminent global rise in mean sea level. However, comprehending variations in tidal patterns is imperative before analyzing the scenario of extreme events, typically associated with the combination of high tides and intense meteorological influences. This study presents local trends in sea level, amplitude, and phase of the two main tidal components in Cananeia (SP) and Santos (SP), utilizing tide gauge data from records of sufficient length to avoid the need for nodal correction in the tide components. Using more up-to-date tide gauge data allowed the investigation of observed trends. Long-term tidal data series revealed an upward trend in mean level of approximately 4,324 mm/year in Cananeia and 0,538 mm/year in Santos. Both locations were affected by human activities along their estuaries, with Santos experiencing significant variations in the amplitude and phase values of the tidal constituents annually.

Effects of sea level rise and tidal flat growth on tidal dynamics and geometry of the Elbe estuary

Abstract. Future global mean sea level rise (SLR) will affect coastlines and estuaries in the North Sea and therefore also coastal protection structures, unique local ecosystems and important waterways. SLR will not only raise water levels but also influence tidal dynamics and morphodynamics, which is why the tidal flats of the Wadden Sea can grow to a certain extent with SLR. Investigations on the effects of climate-change-induced SLR and the related potential bathymetric changes inside of estuaries form an important basis for identifying vulnerabilities and developing appropriate adaptation strategies. To analyse the influence of potential SLR and tidal flat elevation scenarios on the tidal dynamics in the Elbe estuary, we used a highly resolved hydrodynamic numerical model of the German Bight. The analysis results show increasing tidal range in the Elbe estuary solely due to SLR. They also reveal strongly varying changes with different tidal flat growth scenarios: while tidal flat elevation up to the mouth of the estuary can cause tidal range to decrease relative to SLR alone, tidal flat elevation in the entire estuary can lead to an increase in tidal range relative to SLR alone. Further analyses show how the geometric parameters of the Elbe estuary are changing due to SLR and tidal flat elevation. We discuss how these changes in estuarine geometry can provide an explanation for the changes in tidal range.

Future changes in intense tropical cyclone hazards in the Pearl River Delta region: an air-wave-ocean coupled model study

The Pearl River Delta (PRD) region is highly vulnerable to tropical cyclone (TC)-caused coastal hazards due to its long and meandering shoreline and well-developed economy. With global warming expected to continue or worsen in the rest of the twenty-first century, this study examines the TC impact on the PRD coastal regions by reproducing three intense landfalling TCs, namely Vicente (2012), Hato (2017), Mangkhut (2018), using a sophisticated air-wave-ocean coupled model of high spatial resolution (1-km atmosphere and 500-m wave and ocean). The simulations are conducted using present-day reanalysis data and the same TCs occurring in a pseudo-global warming scenario projected for the 2090s. Results indicate that the coupled model accurately reproduces the air-wave-ocean status during the TC episodes. The 2090s thermodynamic status effectively increases the intensity of intense TCs, leading to more severe coastal hazards including gale, rainstorm, and storm surges and waves. On average, the maximum surface wind speed within 50–200 km to the right of the TC center can increase by 4.3 m/s (+22%). The 99th and the 99.9th percentile of accumulated rainfall will increase from 405 to 475 mm (+17.3%), and from 619 to 735 mm (+18.6%), respectively. The maximum significant wave height at the ocean is lifted by an average of 57 cm (+13.8%), and the coastline typically faces a 40–80 cm increase. The maximum storm surges are lifted by 30–80 cm over the open sea but aggravate much higher along the coastline, especially for narrowing estuaries. For Typhoon Vicente (2012), there is more than a 200 cm wave height increase observed both at open sea and along the coastline. In the 2090s context, a combination of mean sea level rise, storm surge, and wave height can reach more than 300 cm increase in total water level at certain hot-spot coastlines, without considering the superposition of spring tides.

Storm surge changes around the UK under a weakened Atlantic meridional overturning circulation

Climate model projections of future North Atlantic storm track changes under global warming are very uncertain, with models showing a variety of responses. Atmospheric storms force storm surges which are a major contributor to coastal flooding hazard in the UK, and so it is important to know how this process might be influenced by climate change—not only what future is probable, but what is possible? As a contribution to answering that question, we drive a simplified model of the north-west European coastal shelf waters with atmospheric forcing taken from climate simulations with HadGEM3-GC3-MM (1/4 degree ocean, approx. 60 km atmosphere in mid-latitudes) which exhibit a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC). The first is a ‘hosing’ simulation in which a rapid shut-down of the AMOC is induced by modelling the addition of freshwater to the North Atlantic. The second is the HadGEM3 GC3.05 perturbed parameter ensemble simulation under Representative Concentration Pathway 8.5 (RCP 8.5) which was used to inform the UK Climate Projections 2018 (UKCP18). This model has a high climate sensitivity and exhibits substantial weakening of the AMOC. We find substantial simulated increases at some sites: up to about 25% increase in the expected annual maximum meteorological component of the storm surge. In both the hosing simulation and the ensemble simulation, the greatest projected increases are seen at some west coast sites, consistent with strengthening of the strongest westerly winds. On the south-east coast, projected changes are smaller in the hosing simulation and generally negative in the ensemble simulation. The ensemble simulation shows a decrease in the strongest northerly winds as well as the growth in the westerlies. Overall, these low-likelihood increases over the 21st century associated with storminess are smaller than the likely contribution from mean sea-level rise over the same period, but, importantly, larger than the so-called “high-end” changes associated with storminess that were reported in UKCP18.

Acoustic observations of individual bubble release events from melting glacier ice in an arctic fjord

Melting below the waterline of marine-terminating glaciers and ice sheets has the potential to remove ice that is holding back land-based ice upstream, and increase the rate of global mean sea level rise. It is difficult to observe this melting directly, and the physical processes involved remain an active area of study. As glacier ice melts, the air bubbles trapped in the ice are released into the water. The air bubbles are often at greater pressure than the surrounding hydrostatic pressure in the water, causing them to be released explosively and generate sound through resulting monopole oscillations. Studying this sound at the level of individual bubbles provides an opportunity to learn more about the role of the bubbles in affecting heat flux across the ice-water boundary layer. It is also essential for proposed efforts to use acoustic emissions as a tool to quantify glacier-scale submarine melting. We will present and discuss observations made at the ice-water interface of floating pieces of glacier ice in an Arctic fjord. These observations included measurements of the water thermal structure, velocity, and salinity, as well as ablation of the ice face, made concurrently with acoustic measurements of bubble release pulses from a 2-element hydrophone array.

Sea level variability in Gulf of Guinea from satellite altimetry

Coastal zones with dense populations, low elevations and/or inadequate adaptive capacity are on the frontline of unprecedented impacts from climate change. The Gulf of Guinea (GoG), stretching from Liberia to Gabon, is in particular vulnerable to coastal flooding caused by local and/or climate-induced sea level rise. In this region, interannual to decadal coastal sea level changes remain poorly understood, mainly due to a lack of tide gauge stations. Here we use nearly three decades (1993–2021) of satellite altimetry data to study the link between the Equatorial Atlantic and coastal GoG sea level variability. The rate of mean sea level rise increased from 3.47 to 3.89 ± 0.10 mm/yr from the Equatorial oceanic domain to the GoG coastal area, with an acceleration of 0.094 ± 0.050 mm/yr2. This corresponds to a mean sea level rise of about 8.9 cm over the entire altimetry period, 1993–2021. We focus on the (extreme) warm/cold events that occur in both the GoG during Atlantic Niños, and along the Angola-Namibia coast during Benguela Niños. Both events are driven by remote forcing via equatorial Kelvin waves and local forcing by local winds, freshwater fluxes and currents intensifications. Analysis of altimetry-based sea level, sea surface temperature anomalies, 20 °C isotherm based PIRATA moorings, and the Argo-based steric and thermometric sea level allows us to follow the coastal trapped waves (CTWs) along the GoG, and its link with major events observed along the strong Equatorial Atlantic warmings in 2010, 2012, 2019 and 2021. Both 2019 and 2021 warming have been identified as the warmest event ever reported in this region during the last 40 years. A lag of 1 month is observed between equatorial and West African coastal trapped wave propagation. This observation may help to better anticipate and manage the effects of extreme events on local ecosystems, fisheries, and socio-economic activities along the affected coastlines. In order to enable informed decision-making and guarantee the resilience of coastal communities in the face of climate change, it emphasises the significance of ongoing study in this field.

Timing of Cordilleran-Laurentide ice-sheet separation: Implications for sea-level rise

During the last deglaciation, collapse of the saddle between the North American Cordilleran and Laurentide ice sheets led to rapid ice-sheet mass loss and separation, with meltwater discharge contributing to deglacial sea-level rise. We directly date ice-sheet separation at the end of the saddle collapse using 64 10Be exposure ages along an ∼1200-km transect of the ice-sheet suture zone. Collapse began in the south by 15.4 ± 0.4 ka and ended by 13.8 ± 0.1 ka at ∼56°N. Ice-sheet model simulations consistent with the 10Be ages find that the saddle collapse contributed 6.2–7.2 m to global mean sea-level rise from ∼15.5 ka to ∼14.0 ka, or approximately one third of global mean sea-level rise over this period. We determine 3.1–3.6 m of the saddle collapse meltwater was released during Meltwater Pulse 1A ∼14.6-14.3 ka, constituting 20–40% of this meltwater pulse's volume. Because the separation of the Cordilleran and Laurentide ice sheets occurred over 1–2 millennia, the associated release of meltwater during the saddle collapse supplied a smaller contribution to the magnitude of Meltwater Pulse 1A than has been recently proposed.

Globally consistent estimates of high-resolution Antarctic ice mass balance and spatially resolved glacial isostatic adjustment

Abstract. A detailed understanding of how the Antarctic ice sheet (AIS) responds to a warming climate is needed because it will most likely increase the rate of global mean sea level rise. Time-variable satellite gravimetry, realized by the Gravity Recovery and Climate Experiment (GRACE) and Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) missions, is directly sensitive to AIS mass changes. However, gravimetric mass balances are subject to two major limitations. First, the usual correction of the glacial isostatic adjustment (GIA) effect by modelling results is a dominant source of uncertainty. Second, satellite gravimetry allows for a resolution of a few hundred kilometres only, which is insufficient to thoroughly explore causes of AIS imbalance. We have overcome both limitations by the first global inversion of data from GRACE and GRACE-FO, satellite altimetry (CryoSat-2), regional climate modelling (RACMO2), and firn densification modelling (IMAU-FDM). The inversion spatially resolves GIA in Antarctica independently from GIA modelling jointly with changes of ice mass and firn air content at 50 km resolution. We find an AIS mass balance of −144 ± 27 Gt a−1 from January 2011 to December 2020. This estimate is the same, within uncertainties, as the statistical analysis of 23 different mass balances evaluated in the Ice sheet Mass Balance Inter-comparison Exercise (IMBIE; Otosaka et al., 2023b). The co-estimated GIA corresponds to an integrated mass effect of 86 ± 21 Gt a−1 over Antarctica, and it fits better with global navigation satellite system (GNSS) results than other GIA predictions. From propagating covariances to integrals, we find a correlation coefficient of −0.97 between the AIS mass balance and the GIA estimate. Sensitivity tests with alternative input data sets lead to results within assessed uncertainties.

Disentangling the drivers of future Antarctic ice loss with a historically calibrated ice-sheet model

Abstract. We use an observationally calibrated ice-sheet model to investigate the future trajectory of the Antarctic ice sheet related to uncertainties in the future balance between sub-shelf melting and ice discharge, on the one hand, and the surface mass balance, on the other. Our ensemble of simulations, forced by a panel of climate models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), suggests that the ocean will be the primary driver of short-term Antarctic mass loss, initiating ice loss in West Antarctica already during this century. The atmosphere initially plays a mitigating role through increased snowfall, leading to an Antarctic contribution to global mean sea-level rise by 2100 of 6 (−8 to 15) cm under a low-emission scenario and 5.5 (−10 to 16) cm under a very high-emission scenario. However, under the very high-emission pathway, the influence of the atmosphere shifts beyond the end of the century, becoming an amplifying driver of mass loss as the ice sheet's surface mass balance decreases. We show that this transition occurs when Antarctic near-surface warming exceeds a critical threshold of +7.5 ∘C, at which the increase in surface runoff outweighs the increase in snow accumulation, a signal that is amplified by the melt–elevation feedback. Therefore, under the very high-emission scenario, oceanic and atmospheric drivers are projected to result in a complete collapse of the West Antarctic ice sheet along with significant grounding-line retreat in the marine basins of the East Antarctic ice sheet, leading to a median global mean sea-level rise of 2.75 (6.95) m by 2300 (3000). Under a more sustainable socio-economic pathway, we find that the Antarctic ice sheet may still contribute to a median global mean sea-level rise of 0.62 (1.85) m by 2300 (3000). However, the rate of sea-level rise is significantly reduced as mass loss is likely to remain confined to the Amundsen Sea Embayment, where present-day climate conditions seem sufficient to commit to a continuous retreat of Thwaites Glacier.

Impacts of sea level rise on the cultural heritage of oceanic islands: Modeling twenty-first century scenarios in the Canary archipelago

Oceanic islands constitute 0.05% of emerged land masses but account for 2% of UNESCO World Heritage Sites. Physical constraints have determined that the history of these remote territories is marked by globally unique forms of colonization and human cultures. A significant part of this heritage, located in coastal areas, is threatened by the mean sea level rise (SLR) due to anthropogenic climate change. This paper presents a regional approach to the risk estimation of SLR in the cultural heritage of oceanic islands, taking the Canary Islands (Spain) as an example. The Canary Islands is one of the most prominent oceanic archipelagos in the world (∼1500 km of coastline), where cultural heritage, preferentially located in the coastal areas (up to 50% in the first 1 km), nicely represents the two major stages of human occupation of islands (aboriginal and European). Through process-based modeling, high resolution flood maps were obtained for 20 climate scenarios and complete coverage of the archipelago. These maps were combined with detailed planimetries of 343 officially designated cultural heritage sites, applying damage functions to relate flood depth with the type of construction and the historical period. The results indicate potential damage to 55 cultural heritage sites of the Canary Islands which could be significant in 15 of them. For these cases, it is recommended to study specific adaptation measures aimed at mitigating the flood risk, such as moving the heritage assets away from the hazard areas or reinforcing and protecting the structures to reduce their vulnerability.

A Review of the Contribution of Satellite Altimetry and Tide Gauge Data to Evaluate Sea Level Trends in the Adriatic Sea within a Mediterranean and Global Context

The relatively new sea level satellite altimetry and secular coastal tide gauge data made the reconstruction of sea levels on regional and global scales possible about one century back. Due to better estimations of the Earth’s crustal, glacial, tectonic, and other possible motion biases in tide gauge data, some additional improvements can be expected in sea level reconstructions, analysis, and predictions. A more detailed review of published sea level-related results was conducted for the Eastern Adriatic coast, including the operation of the tide gauge network and data processing, crustal movement estimations, and the establishment of a new reference height system in Croatia, based on five tide gauge sea level data. It was shown that sea level variation and trend-related indicators are spatially homogeneous, especially on a sub-Adriatic scale. The regional Adriatic Sea mean sea level rise rate of +2.6 mm/year for the satellite altimetry era (1993–2019) is less than the global mean sea level (GMSL) rise rate of +3.3 mm/year for the period of 1993–2022. Several empirical methods for GMSL projections and expected IPCC (Intergovernmental Panel on Climate Change) assessments until the end of the 21st century are considered.

Assessment of coastal vulnerability index (CVI) and its application along the Labuhan Maringgai coast, east Lampung Indonesia

The Coastal Vulnerability Index (CVI) is one of the simplest and commonly used methods to assess coastal vulnerability. In this way, it is a common tool contributing to the decision-making process in long-term coastal planning and management. This study aims to assess the level of coastal vulnerability according to coastal typology of Labuhan Maringgai Coast section. The variable formulation of CVI is divided into hydrodynamic factors (mean significant wave heights, mean tide range, and mean sea level rise) and morpho dynamics factors, i.e. geomorphology, coastal slope, and average width of emerged beach. The morpho dynamics data were obtained through observation, direct measurements in the field by systematic sampling, extraction from aerial photographs and digital surface model (DSM). The results of modelling and predictions by several agencies are derived to obtain the hydrodynamics data. The coast of Labuhan Maringgai was divided into 12 transects and the coastal vulnerability index were classified into three categories such as low, moderate, and high. The results of CVI method showed that the most vulnerable transect is located at Desa Bandar Negeri characterized by mud flats and damaged mangroves.

The Need for Multi‐Century Projections of Sea Level Rise

AbstractThe latest assessment report of the Intergovernmental Panel on Climate Change (IPCC) provided scenario‐based local sea level projections to 2150 and characterized the long‐term committed global mean sea level rise on 2,000‐ and 10,000‐year time horizons associated with peak surface warming levels. Turner et al. build on the scientific assessment of the IPCC to provide time‐continuous projections of future sea level rise to 2500. These projections fill an important knowledge gap to help inform coastal decision‐making processes and more fully quantify the benefits of mitigation actions in terms of limiting future sea level rise. However, limited understanding of ice instability processes remains a key scientific challenge and improved observational and modeling capability are critical to reducing uncertainties and monitoring the trajectory of observed change.

The impact of mean sea level rise on the oceanic water exchange of a back-reef lagoon

The potential impact of the future mean sea level rise (SLR) on residence times in back-reef lagoons remains uncertain. This study employs a hydrodynamical model and a tracer model to investigate the repercussions of SLR on residence times. Two scenarios are considered: one with a growing fringing reef, maintaining a semi-closed lagoon, and the other where the fringing reef overflows due to SLR, facilitating direct cross-reef ocean water exchange. The results reveal a nuanced picture of how SLR-induced changes in fringing reef dynamics influence residence times. In the growing reef scenario, where the lagoon remains semi-closed, a decrease in residence time of less than 10% is projected under SLR scenarios. However, in the non-growing scenario with overflow, allowing direct cross-reef exchange, a substantial 20%–40% decrease in residence time compared to the present state is observed. The probability density distribution of residence times throughout the year exhibits significant differences between the two scenarios. Particularly, the non-growing reef case displays a higher probability of residence time less than 10 h. Seasonal variability in cross-reef exchange contributions is attributed to coastal monsoon winds. The onset of the summer monsoon introduces an inter-annual variability in the contribution of cross-reef exchange to lagoon-ocean water exchange. This finding has broader implications for similar coastal back-reef lagoon systems, offering insights into estimating future global changes in residence times. In summary, this study emphasises the pivotal role of SLR-induced reef changes in shaping the dynamics of lagoon-ocean water exchange. The intricate interplay between fringing reef growth and overflow under SLR scenarios significantly influences residence times, with potential implications for the broader understanding of coastal back-reef lagoon systems. The findings contribute valuable insights into the complex interactions between climate-induced sea level changes and coastal ecosystems, guiding future assessments of global residence time variations.

Small-scale vertical distribution of macrofauna on a shallow tropical coral reef

Around 15% of small vagile macrofauna could be radically disturbed and even become locally extinct in a scenario of coral reef drowning in the coming decades. Benthic organisms distribute themselves in zones of their preference, regulated by physical and biological factors, at a broad or small scale. Studies on small-scale vertical distribution are expected to grow in terms of relevance due to the likely consequences of potential rises in sea level for ecosystem biodiversity. Due to the scarcity of knowledge on this topic, this study aims to characterize the small-scale vertical distribution and zonation of the macrobenthic community in the midlittoral and shallow infralittoral zones of a coral reef environment. For the first time, we tested the hypothesis that there are important differences in the vertical distribution patterns of macrofauna in the reef environment using Artificial Substrate Units (ASU). Community structure showed significant differences even within a few centimeters. The midlittoral showed greater heterogeneity than the infralittoral, which may be threatened by the rise in mean sea level. In the horizontal plane, differences in community structure were observed, probably due to physical factors such as current flow and wave energy. The paucity of this type of research suggests the need for experimental work aimed at investigating the risks of sea level rise regarding habitat loss, biodiversity, and the modification of macrobenthic communities of coastal reefs.

Empirical projection of global sea level in 2050 driven by Antarctic and Greenland ice mass variations

Global mean sea level rise, driven by ice mass loss in Antarctic and Greenland Ice Sheets (AIS and GrIS), is a significant consequence of global warming. Although various ice sheet models have attempted to predict the ice mass balance and subsequent sea level changes, non-trivial disagreements between models exist. In this study, we employ an empirical approach to estimate the future (2050) ice mass changes for both ice sheets, assuming their historical patterns of ice dynamics would persist in the coming decades. To achieve this, we estimate decadal-scale ice discharge variations by subtracting the surface mass balance (SMB) from the observed ice mass changes and extrapolate linear trend and acceleration components of ice discharges up to 2050. We also consider future SMB data from Coupled Model Intercomparison Project phase 6 models to estimate net ice mass balance. Our estimates suggest that from 2021 to 2050, the global sea level rise due to AIS and GrIS ranges between 6–19 mm and 15–31 mm, respectively. Additionally, we investigate regional sea level variability resulting from geoid changes induced by ice mass changes in both regions, highlighting that heterogeneous sea level changes may cause more pronounced sea level rises in lower latitude regions, where major cities are located.

Causes of accelerated High-Tide Flooding in the U.S. since 1950

The U.S. coastlines have experienced rapid increases in occurrences of High Tide Flooding (HTF) during recent decades. While it is generally accepted that relative mean sea level (RMSL) rise is the dominant cause for this, an attribution to individual components is still lacking. Here, we use local sea-level budgets to attribute past changes in HTF days to RMSL and its individual contributions. We find that while RMSL rise generally explains > 84% of long-term increases in HTF days locally, spatial patterns in HTF changes also depend on differences in flooding thresholds and water level characteristics. Vertical land motion dominates long-term increases in HTF, particularly in the northeast, while sterodynamic sea level (SDSL) is most important elsewhere and on shorter temporal scales. We also show that the recent SDSL acceleration in the Gulf of Mexico has led to an increase of 220% in the frequency of HTF events over the last decade.

A Variational LSTM Emulator of Sea Level Contribution From the Antarctic Ice Sheet

AbstractThe Antarctic ice sheet (AIS) will be a dominant contributor to global mean sea level rise in the 21st century but remains a major source of uncertainty. The Ice Sheet Model Intercomparison for CMIP6 (ISMIP6) is an ensemble of continental‐scale models for studying the evolution of the AIS and projecting its future contribution to sea level. Due to their complexity and computational cost, ISMIP6 simulations are sparse and generated infrequently. Emulators are smaller‐scale models that approximate ISMs and enable experimentation and exploration into the drivers of sea level change. We introduce a neural network (NN) emulator to approximate the ISMIP6 ensemble, using a variational Long Short‐Term Memory (LSTM) with Monte Carlo dropout to quantify single‐projection uncertainty. The proposed NN emulator is compared to a Gaussian Process (GP) emulator on four criteria: accuracy of point estimates and predictive distributions of individual model projections, approximation of the ensemble projections, and model training time. The NN predicts more accurately on single projections, with a mean absolute error of 0.46 mm Sea Level Equivalent (SLE) versus 0.73 mm SLE for the GP, and has more accurate uncertainty estimates. The NN emulator also better approximates the ensemble distribution of ISMIP6 model projections, with a Kullback‐Leibler divergence of 18.26 versus 199.14 for GP at the projection year 2100. The NN enables more accurate experimentation with a reduced runtime, offering a new tool for understanding the important role of regional precipitation, ice sheet drainage systems, and interannual and longer timescale dynamics.