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Coastal Louisiana has lost about 5,000 km 2 of wetlands over the past century and concern exists whether remaining wetlands will persist while facing some of the world’s highest rates of relative sea-level rise (RSLR). Here we analyse an unprecedented data set derived from 274 rod surface-elevation table-marker horizon stations, to determine present-day surface-elevation change, vertical accretion and shallow subsidence rates. Comparison of vertical accretion rates with RSLR rates at the land surface (present-day RSLR rates are 12±8 mm per year) shows that 65% of wetlands in the Mississippi Delta (SE Louisiana) may keep pace with RSLR, whereas 58% of the sites in the Chenier Plain (SW Louisiana) do not, rendering much of this area highly vulnerable to RLSR. At least 60% of the total subsidence rate occurs within the uppermost 5–10 m, which may account for the higher vulnerability of coastal Louisiana wetlands compared to their counterparts elsewhere. Coastal Louisiana wetlands face some of the world’s highest rates of relative sea-level rise and loss. Here, the authors show that there is a strong regional component to coastal Louisiana wetland vulnerability to relative sea-level rise as well as contributing to the understanding of subsidence in the region.

While 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. Sea level rise along the U.S. Southeast and Gulf Coast has accelerated since 2010 due to changes in steric expansion and the ocean’s circulation. The acceleration represents the compounding effects of external forcing and natural climate variability.

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. Over 13 years, coastal Louisiana’s wetlands have been endangered by a sea-level rise rate comparable to what is expected later this century. While the rate may not persist over the next few decades, this natural experiment indicates a 75% drowning of these wetlands by 2070 under current carbon emissions.

Recent studies have produced conflicting results as to whether coastal wetlands can keep up with present‐day and future sea‐level rise. The stratigraphic record shows that threshold rates for coastal wetland submergence or retreat are lower than what instrumental records suggest, with wetland extent that shrinks considerably under high rates of sea‐level rise. These apparent conflicts can be reconciled by recognizing that many coastal wetlands still possess sufficient elevation capital to cope with sea‐level rise, and that processes like sediment compaction, ponding, and wave erosion require multidecadal or longer timescales to drive wetland loss that is in many cases inevitable. Plain Language Summary The rapid, climate‐driven acceleration of global sea level threatens salt marshes and mangroves along low‐elevation shorelines. These coastal wetlands provide protection from storms along with other ecosystem services to vulnerable coastal communities, including several megacities. The question of how coastal wetlands will cope with future sea‐level rise is a subject of much debate, with recent research providing contradictory answers. Our analysis suggests that much of this can be attributed to the time window under consideration. Even coastal wetlands that are able to persist during the next few decades are likely to be much less resilient through the remainder of this century and beyond. Key Points The paleo‐record shows lower thresholds for submergence of marshes and mangroves than the instrumental record Accelerated relative sea‐level rise will nearly always lead to a reduction in the extent of coastal wetlands Integration of new field and remote sensing data with constraints from the paleo‐record will enable advances in coastal wetland modeling

Major technological advances have made measurements of coastal subsidence more sophisticated, but these advances have not always been matched by a thorough examination of what is actually being measured. Here we draw attention to the widespread confusion about key concepts in the coastal subsidence literature, much of which revolves around the interplay between sediment accretion, vertical land motion and surface-elevation change. We attempt to reconcile this by drawing on well-established concepts from the tectonics 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.

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. Plain Language Summary The ultimate test of climate projections occurs by means of subsequent observations. Three decades of satellite‐based measurements of global sea‐level change now enable such a comparison and show that early IPCC climate projections were remarkably accurate. Predictions of glacier mass loss and thermal expansion of seawater were comparatively successful, but the ice‐sheet contributions were underestimated. Nevertheless, these findings provide confidence in model‐based climate projections. Key Points IPCC projections in the mid‐1990s of global sea‐level change over the next 30 years were remarkably robust The largest disparities between projections and observations were due to underestimated dynamic mass loss of ice sheets Comparison of past projections with subsequent observations gives confidence in future climate projections

Extensive damage to coastal Louisiana from Hurricane Katrina in 2005 was largely attributed to high rates of relative sea-level rise caused by coastal subsidence. An examination of the underlying Holocene sediments shows that the compaction of peat-rich deposits contributes significantly to Mississippi Delta subsidence rates of up to 5 mm per year. Coastal subsidence causes sea-level rise, shoreline erosion and wetland loss, which poses a threat to coastal populations 1 . This is especially evident in the Mississippi Delta in the southern United States, which was devastated by Hurricane Katrina in 2005. The loss of protective wetlands is considered a critical factor in the extensive flood damage. The causes of subsidence in coastal Louisiana, attributed to factors as diverse as shallow compaction and deep crustal processes, remain controversial 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . Current estimates of subsidence rates vary by several orders of magnitude 3 , 6 . Here, we use a series of radiocarbon-dated sediment cores from the Mississippi Delta to analyse late Holocene deposits and assess compaction rates. We find that millennial-scale compaction rates primarily associated with peat can reach 5 mm per year, values that exceed recent model predictions 5 , 9 . Locally and on timescales of decades to centuries, rates are likely to be 10 mm or more per year. We conclude that compaction of Holocene strata contributes significantly to the exceptionally high rates of relative sea-level rise and coastal wetland loss in the Mississippi Delta, and is likely to cause subsidence in other organic-rich and often densely populated coastal plains.

Floodplains of large alluvial rivers modulate the composition of riverine organic carbon (OC) and control OC oxidative loss, constituting a critical component in the global river‐atmosphere‐ocean carbon cycle. Therefore, anthropogenic management disconnecting rivers from their floodplains is expected to reduce the oxidative loss and to change the quality and quantity of riverine OC exported to the ocean. Here, we test this idea by combining two chronometers—14C age spectra of OC and optically stimulated luminescence ages of quartz—to interrogate sediments of the Lower Mississippi River (LMR) system to constrain the anthropogenic effects on carbon cycling in a continental‐scale sediment routing system. The 14C age of the LMR OC has been reduced from >5,000 yr in prehistoric sediments to <3,000 yr in historic and modern sediments with significantly narrowed age spectrum width, following centuries of embanking the LMR. Bank stabilization reduced the river‐floodplain sediment exchange by ∼90%, effectively cutting off older floodplain OC from the river and reducing OC residence time in the severely truncated floodplain system, and expedited the downstream transmission of OC. The reduced residence time will have decreased riverine OC loss and enhanced younger OC delivery to marine sediments. We estimate that the oxidative loss of the LMR OC has been reduced by ≥ 1.1 Tg C/yr or 40%. Extrapolation to other large rivers that have undergone anthropogenic changes similar to the LMR illustrates that this process likely represents a carbon sink that can significantly increase if currently free‐flowing large tropical rivers are embanked in the future. Plain Language Summary Rivers play a key role in the global carbon cycle by releasing carbon dioxide to the atmosphere and controlling carbon transmission from land to ocean. How the carbon cycle has been affected by engineering activities that have fundamentally changed natural riverine processes is unclear. Using the Lower Mississippi River (LMR) as an example, we demonstrate that bank stabilization, which allows water and sediment to shoot through the river system directly into the ocean without interactions with the floodplain, can reduce river‐floodplain organic carbon exchange by 90%. Consequently, carbon transport is expedited through such river systems, which decreases the amount of organic matter that is converted to CO2 and, rather, increases its delivery to seafloor sediments. The loss of carbon from the LMR has been reduced by ≥ 40% or 1.1 Tg C per year due to river embankment, suggesting a significant human‐caused shift in river‐atmosphere‐ocean carbon cycling that may occur in other large rivers as well. Key Points Embankment and bank stabilization cuts off old floodplain organic carbon (OC) sources and expedites OC transmission in the Lower Mississippi River (LMR) Reduced river‐floodplain exchange decreases the oxidative loss and increases the OC flux and export to the ocean by ≥ 40% in the LMR Embanking continental‐scale rivers likely represents a significant change in global river‐atmosphere‐ocean carbon cycling