Nature-based coastal protection is increasingly recognised as a potentially sustainable and cost-effective solution to reduce coastal flood risk. It uses coastal ecosystems such as mangrove forests to create resilient designs for coastal flood protection. However, to use mangroves effectively as a nature-based measure for flood risk reduction, we must understand the biophysical processes that govern risk reduction capacity through mangrove ecosystem size and structure. In this perspective, we evaluate the current state of knowledge on local physical drivers and ecological processes that determine mangrove functioning as part of a nature-based flood defence. We show that the forest properties that comprise coastal flood protection are well-known, but models cannot yet pinpoint how spatial heterogeneity of the forest structure affects the capacity for wave or surge attenuation. Overall, there is relatively good understanding of the ecological processes that drive forest structure and size, but there is a lack of knowledge on how daily bed-level dynamics link to long-term biogeomorphic forest dynamics, and on the role of combined stressors influencing forest retreat. Integrating simulation models of forest structure under changing physical (e.g. due to sea-level change) and ecological drivers with hydrodynamic attenuation models will allow for better projections of long-term natural coastal protection.

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Different sea level reconstructions show a spread in sea level rise over the last six decades and it is not yet certain whether the sum of contributors explains the reconstructed rise. Possible causes for this spread are, among others, vertical land motion at tide-gauge locations and the sparse sampling of the spatially variable ocean. To assess these open questions, reconstructed sea level and the role of the contributors are investigated on a local, basin, and global scale. High-latitude seas are excluded. Tide-gauge records are combined with observations of vertical land motion, independent estimates of ice-mass loss, terrestrial water storage, and barotropic atmospheric forcing in a self-consistent framework to reconstruct sea level changes on basin and global scales, which are compared to the estimated sum of contributing processes. For the first time, it is shown that for most basins the reconstructed sea level trend and acceleration can be explained by the sum of contributors, as well as a large part of the decadal variability. The sparsely sampled South Atlantic Ocean forms an exception. The global-mean sea level reconstruction shows a trend of 1.5 ± 0.2 mm yr^-1 over 1958-2014 (1σ), compared to 1.3 ± 0.1 mm yr^-1 for the sum of contributors. Over the same period, the reconstruction shows a positive acceleration of 0.07 ± 0.02 mm yr^-2, which is also in agreement with the sum of contributors, which shows an acceleration of 0.07 ± 0.01 mm yr^-2. Since 1993, both reconstructed sea level and the sum of contributors show good agreement with altimetry estimates.

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In this paper we discuss our efforts to perform precision orbit determination (POD) of CryoSat-2 which depends on Doppler and satellite laser ranging tracking data. A dAll processes that affect sea level show distinct regional patterns, and therefore, sea-level changes show considerable spatial variability. This spatial variability forms a challenge when reconstructing global and regional sea-level changes from tide gauges, which are only available at a limited number of locations and are mostly located along coasts in the northern hemisphere. We can improve estimates of global and regional sea-level changes from tide-gauge records by explicitly taking the expected spatial variability into account. From estimates of global ice mass loss and land water storage changes, spatial sea-level fingerprints can be computed, which identify whether sea-level changes at specific tide-gauge locations are expected to be representative for global-mean or basin-mean sea-level changes. Furthermore, developments in altimetry and GPS now allow for reliable estimates of local vertical land motion (VLM) at an increased number of tide gauges. We reconstruct global and regional sea-level changes since 1958 by combining the expected sea-level fingerprints with observations from tide gauges and GPS stations. The spatial sea-level fingerprints associated with glacial isostatic adjustment (GIA) and present-day ice mass loss and land water storage are used to detect and correct possible biases due to the uneven spatial sampling of the tide-gauge observations. The fingerprints that emerge from GIA and present-day mass redistribution have an earth-deformation component, which is also observed as VLM. Hence, care must be taken when combining spatial sea-level fingerprints with VLM observations to avoid double-counting of solid-earth deformation. By separating observed land motion into known and unknown sources, this double-counting is avoided. For most ocean basins, the reconstructed sea-level changes can be explained by the combined effects of GIA, present-day mass redistribution, and ocean density effects. A substantial part of the observed decadal variability can be explained by ocean density variations, while trends and accelerations are mostly driven by ice and land water storage changes. The only exception is the South Atlantic Ocean. In this basin, reconstructed sea level cannot be reconciled with the underlying processes, which is probably related to the sparse tide-gauge coverage in this region. When the regional sea-level estimates are merged into a global-mean estimate, we find a GMSL trend of 1.5 0.2 mm/yr, and an acceleration of 0.07 0.02 mm/yr2. Both the global-mean trend and the acceleration can be explained by the sum of contributing processes, and hence, the global sea-level budget since 1958 can be considered closed without requiring a contribution of deep-ocean thermal expansion or pre-1990 Antarctic mass loss, although substantial uncertainty remains due to the sparsely-observed South Atlantic Ocean. ynamic orbit model is set-up and the residuals between the model and the tracking data is evaluated. The average r.m.s. of the 10 second averaged Doppler tracking pass residuals is approximately 0.39 mm/s; and the average of the laser tracking pass residuals becomes 1.42 cm. We discuss three improvements that have brought the orbit accuracy to this level, it concerns the way we implement temporal gravity accelerations observed by GRACE; the implementation of ITRF2014 coordinates and velocities for the DORIS beacons and the SLR tracking sites. We also discuss an adjustment of the SLR retroreflector position within the satellite reference frame. An unexpected result is that we find a systematic difference between the median of the 10s Doppler tracking residuals which displays a statistically significant pattern in the South Atlantic Anomaly area where the median of the velocity residuals varies in the range of -0.15 to +0.15 mm/s@en

Global warming is expected to drive increasing extreme sea levels (ESLs) and flood risk along the world's coastlines. In this work we present probabilistic projections of ESLs for the present century taking into consideration changes in mean sea level, tides, wind-waves, and storm surges. Between the year 2000 and 2100 we project a very likely increase of the global average 100-year ESL of 34-76 cm under a moderate-emission-mitigation-policy scenario and of 58-172 cm under a business as usual scenario. Rising ESLs are mostly driven by thermal expansion, followed by contributions from ice mass-loss from glaciers, and ice-sheets in Greenland and Antarctica. Under these scenarios ESL rise would render a large part of the tropics exposed annually to the present-day 100-year event from 2050. By the end of this century this applies to most coastlines around the world, implying unprecedented flood risk levels unless timely adaptation measures are taken.

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Two degrees of global warming above the preindustrial level is widely suggested as an appropriate threshold beyond which climate change risks become unacceptably high. This “2 °C” threshold is likely to be reached between 2040 and 2050 for both Representative Concentration Pathway (RCP) 8.5 and 4.5. Resulting sea level rises will not be globally uniform, due to ocean dynamical processes and changes in gravity associated with water mass redistribution. Here we provide probabilistic sea level rise projections for the global coastline with warming above the 2 °C goal. By 2040, with a 2 °C warming under the RCP8.5 scenario, more than 90%of coastal areas will experience sea level rise exceeding the global estimate of 0.2 m, with up to 0.4 m expected along the Atlantic coast of North America and Norway. With a 5 °C rise by 2100, sea level will rise rapidly, reaching 0.9 m (median), and 80% of the coastline will exceed the global sea level rise at the 95th percentile upper limit of 1.8 m. Under RCP8.5, by 2100, New York may expect rises of 1.09 m, Guangzhou may expect rises of 0.91 m, and Lagos may expect rises of 0.90 m, with the 95th percentile upper limit of 2.24 m, 1.93 m, and 1.92 m, respectively. The coastal communities of rapidly expanding cities in the developing world, and vulnerable tropical coastal ecosystems, will have a very limited time after midcentury to adapt to sea level rises unprecedented since the dawn of the Bronze Age. @en