Vertical land motion is a key component of relative sea-level changes in coastal areas. Rates of vertical land motion can vary temporally owing to anthropogenic and natural processes. Yet, such nonlinear behaviour has not been fully accounted for in twentieth-century sea-level budgets or projections because long-term observations at relevant spatial scales are scarce. Here we infer vertical land motion at a global set of tide gauge stations by comparing their records with a probabilistic reconstruction of climate-related sea level (1900-2021) that integrates model-based estimates of sterodynamic, barystatic and inverse-barometer contributions. Differences between climate-related sea-level and tide gauge records primarily reflect vertical land motion and reveal previously unreported temporal variations linked to subsurface fluid withdrawal, as well as seismic and volcanic processes. We show that decadal fluctuations in regional relative sea-level trends can exceed those driven by climate-related processes by an order of magnitude. Consequently, vertical land motion projections based on linear extrapolations introduce systematic median sea-level projection errors of typically up to 7.6 mm yr-1 at sites influenced by seismic or volcanic activity and 5.6 mm yr-1 at the other sites. Our time-varying vertical land motion estimates constrain geophysical models of anthropogenic and volcano-tectonic crustal processes and pave the way for more robust sea-level projections at tide gauges.

Abstract. Interferometric Synthetic Aperture Radar (InSAR) is widely used to monitor surface-elevation change in subsiding coastal regions, but inconsistencies between studies hinder understanding of the processes driving vertical land motion (VLM). Here we compare two recent InSAR datasets from the central U.S. Gulf Coast which yield similar mean rates (−2.8 ± 2.8 and −3.3 ± 1.8 mm yr−1) but show negligible spatial correlation (R2=0.05), except in medium to highly developed urban areas (R2>0.5). Using 41 Global Navigation Satellite System records from adjacent Pleistocene uplands with minimal shallow subsidence and sediment accretion, we find a median VLM of −1.2 mm yr−1, largely driven by glacial isostatic adjustment that is higher than previously believed. InSAR data exhibit larger uncertainties and are presently unable to capture this rate. Given the struggles of InSAR in vegetated landscapes, we recommend that vertical velocities below 5 mm yr−1 are interpreted with utmost caution.

With global temperatures poised to exceed the 1.5 °C Paris Agreement threshold—a level that triggered substantial ice-sheet collapse during the Last Interglacial—low-elevation coastal zones face sea-level commitments far beyond current planning horizons. With this geological frame of reference, we examine the impact of sea-level rise on what may be the most physically vulnerable coastal zone in the world using prehistoric and contemporary patterns of human mobility. We highlight the positive aspects of the recently commenced out-migration in this region and argue that the fate of communities landwards of this coastal zone will be decided in the next few decades. Climate change-induced sea-level rise poses a significant societal challenge, but the extent of potential inundation remains uncertain. Looking at the recent geologic past and at prehistoric human adaptation provides valuable insights into possible new coastlines and strategies to help inform the future of modern coastal communities.

The rapid transformation of the ocean–atmosphere–cryosphere system during the last deglaciation holds clues that may be useful to understand climate change during this century and beyond. Within this context, sea-level change connects the climate system components, but the limited temporal resolution and spatial distribution of relative sea-level records has hindered progress towards closing the ice sheet–sea-level budget since the Last Glacial Maximum. Here we present a relative sea-level record, compiled using radiocarbon-dated basal peat, from the Mississippi Delta stretching back to ~10,000 years ago and combine it with the best available relative sea-level data worldwide for the final episode of the last deglaciation (9,000–7,000 years ago). Geophysical modelling shows that these precise data constraints favour approximately 14 m of ice melt in North America through this interval, 4–10 m greater than previously estimated, and at least three times greater than the Antarctic contribution. Our results call for a major revision of the deglacial ice history with implications for, among others, collapse of the saddle connecting two ice domes over Hudson Bay, the associated abrupt cooling ~8,200 years ago and the sensitivity of the Atlantic Meridional Overturning Circulation to freshwater forcing. The melting of the last remnants of the North American ice sheets in the early Holocene led to 14 m of global sea-level rise, higher than prior estimates, according to proxy constraints from the Mississippi Delta and other localities.

With an acceleration of global sea-level rise during the satellite altimetry era (since 1993) firmly established, it is now appropriate to examine sea-level projections made around the onset of this time period. Here we show that the mid-range projection from the Second Assessment Report of the IPCC (1995/1996) was strikingly close to what transpired over the next 30 years, with the magnitude of sea-level rise underestimated by only ∼1 cm. Projections of contributions from individual components were more variable, with a notable underestimation of dynamic mass loss from ice sheets. Nevertheless—and in view of the comparatively limited process understanding, modeling capabilities, and computational resources available three decades ago—these early attempts should inspire confidence in presently available global sea-level projections. Such multidecadal evaluations of past climate projections, as presented here for sea-level change, offer useful tests of past climate forecasts, and highlight the essential importance of continued climate monitoring.

Predicting climate impacts is challenging and has to date relied on indirect methods, notably modeling. Here we examine coastal ecosystem change during 13 years of unusually rapid, albeit likely temporary, sea-level rise ( > 10 mm yr −1 ) in the Gulf of Mexico. Such rates, which may become a persistent feature in the future due to anthropogenic climate change, drove rising water levels of similar magnitude in Louisiana’s coastal wetlands. Measurements of surface-elevation change at 253 monitoring sites show that 87% of these sites are unable to keep up with rising water levels. We find no evidence for enhanced wetland elevation gain through ecogeomorphic feedbacks, where more frequent inundation would lead to enhanced biomass accumulation that could counterbalance rising water levels. We attribute this to the exceptionally rapid sea-level rise during this time period. Under the current climate trajectory (SSP2-4.5), drowning of ~75% of Louisiana’s coastal wetlands is a plausible outcome by 2070.

Abstract The rapid transformation of the ocean-atmosphere-cryosphere system during the last deglaciation serves as a potential model for climate change during this century and beyond. Within this context, sea-level change can be viewed as the connecting tissue, but the limited resolution of relative sea-level (RSL) records has hindered progress toward closing the ice sheet–sea level budget since the Last Glacial Maximum, the partitioning of ice melt from different sources, and assessing the role of freshwater forcing in abrupt climate change. Here we present a new RSL record from the Mississippi Delta stretching back to 11 ka and combine it with the best available published RSL data worldwide for the final episode of the last deglaciation (9-7 ka). Glacial isostatic adjustment (GIA) modelling shows that these precise data constraints demand a North American ice melt of about 14 m sea-level equivalent (SLE) during this time, 4-10 m greater than previously estimated. Our results call for a major revision of the North American deglacial ice history and our findings demonstrate the utility of high-resolution RSL observations as a pathway towards closing the ice budget of the last deglaciation and improving our understanding of the ocean-atmosphere-cryosphere system during rapid climate change.

While major technological advances have made measurements of coastal subsidence more sophisticated, these advances have not always been matched by a thorough examination of what is actually being measured. Here we draw attention to the widespread miscommunication about key concepts in the coastal subsidence literature, much of which revolving around the interplay between sediment accretion, vertical land motion, and surface-elevation change. We attempt to rectify this by drawing on well-established concepts from the tectonic geomorphology community. A consensus on these issues by means of a common language can help bridge the gap between disparate disciplines (ranging from geophysics to ecology) that are critical in the quest for meaningful projections of future relative sea-level rise.

Several coastal ecosystems—most notably mangroves and tidal marshes—exhibit biogenic feedbacks that are facilitating adjustment to relative sea-level rise (RSLR), including the sequestration of carbon and the trapping of mineral sediment 1 . The stability of reef-top habitats under RSLR is similarly linked to reef-derived sediment accumulation and the vertical accretion of protective coral reefs 2 . The persistence of these ecosystems under high rates of RSLR is contested 3 . Here we show that the probability of vertical adjustment to RSLR inferred from palaeo-stratigraphic observations aligns with contemporary in situ survey measurements. A deficit between tidal marsh and mangrove adjustment and RSLR is likely at 4 mm yr −1 and highly likely at 7 mm yr −1 of RSLR. As rates of RSLR exceed 7 mm yr −1 , the probability that reef islands destabilize through increased shoreline erosion and wave over-topping increases. Increased global warming from 1.5 °C to 2.0 °C would double the area of mapped tidal marsh exposed to 4 mm yr −1 of RSLR by between 2080 and 2100. With 3 °C of warming, nearly all the world’s mangrove forests and coral reef islands and almost 40% of mapped tidal marshes are estimated to be exposed to RSLR of at least 7 mm yr −1 . Meeting the Paris agreement targets would minimize disruption to coastal ecosystems.

AbstractWhile there is evidence for an acceleration in global mean sea level (MSL) since the 1960s, its detection at local levels has been hampered by the considerable influence of natural variability on the rate of MSL change. Here we report a MSL acceleration in tide gauge records along the U.S. Southeast and Gulf coasts that has led to rates (>10 mm yr−1 since 2010) that are unprecedented in at least 120 years. We show that this acceleration is primarily induced by an ocean dynamic signal exceeding the externally forced response from historical climate model simulations. However, when the simulated forced response is removed from observations, the residuals are neither historically unprecedented nor inconsistent with internal variability in simulations. A large fraction of the residuals is consistent with wind driven Rossby waves in the tropical North Atlantic. This indicates that this ongoing acceleration represents the compounding effects of external forcing and internal climate variability.

Abstract Climate impacts throughout the 21st century are multifaceted and include heat stress, water scarcity, flood risk, and a threat to biodiversity and ecosystems. Predicting these impacts has been challenging, even if the trajectory of climate change is precisely known. To date, recourse has commonly been taken to modeling and paleoclimate studies, but these approaches have significant limitations. Here we examine coastal ecosystem change during 12 years of unusually rapid, albeit likely temporary, sea-level rise (> 10 mm yr− 1) in the Gulf of Mexico. Such rates, which may become a persistent feature in the future due to anthropogenic climate change, drove rising water levels of similar magnitude in coastal Louisiana and thus affected the ~ 15,000 km2 of coastal wetlands in this region. Measurements of surface-elevation change at 253 monitoring sites show that 87% of these sites are unable to keep up with rising water levels. We find no evidence for enhanced wetland elevation gain through ecogeomorphic feedbacks, where more frequent inundation would lead to enhanced biomass accumulation that could counterbalance rising water levels. We attribute this to the exceptionally rapid sea-level rise during this time period. Under the current climate trajectory (SSP2-4.5), drowning of ~ 75% of Louisiana’s coastal wetlands is a plausible outcome by 2070.

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 km 2 ) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario.

The world’s largest deltas, home to numerous megacities, are expected to bear the brunt of climate-driven sea-level rise. Now, a study shows that disentangling the human impacts on the Mississippi Delta in the past century can help make these systems more resilient.

Land‐surface subsidence is a major contributor to land loss in many river deltas. New approaches yielding high‐resolution data are needed to parse the relevant driving forces. In 2016, we established a novel “subsidence superstation” ∼2 km from the Mississippi River in coastal Louisiana (USA) to measure compaction in a global reference frame as a function of depth in Holocene sediments and deeper subsidence. The site features three borehole optical fiber strainmeters to obtain continuous records of displacement between ∼1.3 m below the surface and depths of ∼11, 25, and 38 m. These data are complemented by an adjacent station providing hydrologic data and near‐surface compaction. We also installed three GPS antennas, one of which is mounted to a rod cemented into the Pleistocene basement. A core from one of the boreholes provides insight into the sediment properties of the entire Holocene succession. Five years of records reveal the compaction rate in the material between 1.3 and 38 m is likely less than 0.25 mm/yr. The GPS records yield a subsidence rate of 2.5 mm/yr regardless of the depth of the anchor, thus corroborating the low compaction rates observed by the strainmeters. The new instrumental records show that current subsidence at this location is governed mostly by deformation of the Pleistocene or underlying strata rather than compaction of Holocene material, with the exception of the uppermost meter. The methodology represents an important new approach to mapping subsidence rates at varying depths, providing insight into the mechanisms governing delta subsidence.

The ability of deltas to persist by building new land is critical to maintaining these vital ecologic environments that are often home to major economic and population centers. However, the deposition of land‐building sediment triggers load‐induced shallow subsidence which may undermine the effectiveness of natural and engineered emergent landforms. Here, we present a new method to quantify shallow subsidence in a 6,000–8,000 km2 relict bayhead delta of the Mississippi Delta using the mouth bar to overbank stratigraphic boundary that formed near sea level, temporally constrained by optically stimulated luminescence dating. Vertical displacement rates at this boundary, averaged over 750–1,500 years, are on the order of a few mm/yr. Total subsidence scales to ∼50% of the thickness of overlying deposits, significantly greater than the 28%–35% loss estimated for inland localities underlain by peat, indicating that bay muds in the study area are more compaction‐prone than terrestrial organic‐rich deposits. Modeling shows a modest reduction of ∼13% in deltaic land‐area gain under a realistic compaction scenario for 1,000 years of simulated delta progradation, compared to a no‐compaction scenario. Our findings indicate that load‐driven compaction does not majorly hinder land‐area gain and may in fact promote long‐term growth at engineered sediment diversions through channel maintenance driven by compaction, thereby adding further support to this restoration strategy.