Sea level is measured globally using tide gauges and satellite radar altimetry.
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Multiple peer-reviewed studies and reference materials confirm that global sea level is monitored and assessed using a combination of tide gauges and satellite altimetry measurements.
Abstract. Global mean sea level is an integral of changes occurring in the climate system in response to unforced climate variability as well as natural and anthropogenic forcing factors. Its temporal evolution allows changes (e.g., acceleration) to be detected in one or more components. Study of the sea-level budget provides constraints on missing or poorly known contributions, such as the unsurveyed deep ocean or the still uncertain land water component. In the context of the World Climate Research Programme Grand Challenge entitled Regional Sea Level and Coastal Impacts, an international effort involving the sea-level community worldwide has been recently initiated with the objective of assessing the various datasets used to estimate components of the sea-level budget during the altimetry era (1993 to present). These datasets are based on the combination of a broad range of space-based and in situ observations, model estimates, and algorithms. Evaluating their quality, quantifying uncertainties and identifying sources of discrepancies between component estimates is extremely useful for various applications in climate research. This effort involves several tens of scientists from about 50 research teams/institutions worldwide (www.wcrp-climate.org/grand-challenges/gc-sea-level, last access: 22 August 2018). The results presented in this paper are a synthesis of the first assessment performed during 2017–2018. We present estimates of the altimetry-based global mean sea level (average rate of 3.1 ± 0.3 mm yr−1 and acceleration of 0.1 mm yr−2 over 1993–present), as well as of the different components of the sea-level budget (http://doi.org/10.17882/54854, last access: 22 August 2018). We further examine closure of the sea-level budget, comparing the observed global mean sea level with the sum of components. Ocean thermal expansion, glaciers, Greenland and Antarctica contribute 42 %, 21 %, 15 % and 8 % to the global mean sea level over the 1993–present period. We also study the sea-level budget over 2005–present, using GRACE-based ocean mass estimates instead of the sum of individual mass components. Our results demonstrate that the global mean sea level can be closed to within 0.3 mm yr−1 (1σ). Substantial uncertainty remains for the land water storage component, as shown when examining individual mass contributions to sea level.
Abstract Significant developments have been made in the observation systems and techniques of estimating sea level towards meeting the standard accuracy requirement of Global Climate Observation Systems (GCOS). This study undertakes a systematic review of the current advances in estimating sea level change in the context of the 4th industrial revolution. Trends in the use of main observation systems such as tide gauges, satellite altimetry, and ancillary systems such as GNSS and Autonomous Surface Vehicles were explored. Crucially, we examined the contribution of dedicated waveform retracking strategies, advanced corrections and radar technology such as Ka-band altimetry of SARAL/Altika and SAR mode innovations to the progress in coastal altimetry. Further, we show the role of emerging spatial data science concepts and processing workflows in sea level study. Findings suggest that in-situ sea level observation through tide gauges remains the best approach for long-term coastal sea level study despite its limitations while satellite altimetry is suitable for contemporary global and regional scales. Detailed understating of global, regional and local mean sea level change will require an augmentation of tide gauge, satellite altimetry and other ancillary remote sensing and in situ systems. Densification of tide gauges and co-located GNSS networks at sparsely covered regions and improvement in precision of satellite altimetry data for coastal use are also essential for a fully integrated sea level observation system. From the analysis of over 30 trend models that span exploratory, parametric, non-parametric, stochastic and advanced classes in the literature, we conclude that the best model is the one with good statistical foundation and similar assumption with the sea level pattern.
We present a Bayesian method to map contemporary rates of relative sea level change using a joint inversion of vertical trends from tide gauges, GPS time series, and satellite radar altimetry measurements. Tide gauge measurements constrain rates of relative sea level change on decadal to secular time scales at a few hundred sites sparsely distributed along coastlines. Predicting the rates of relative sea level change from historical tide gauge measurements is difficult due to the paucity and uneven distribution of sites with high quality records. Since the late twentieth century, deployments of GPS stations have enabled accurate determination of the rates of vertical land motion. A series of satellite radar altimetry missions provide continuous and global monitoring of geocentric sea level changes since the launch of TOPEX/Poseidon in 1992. By combining these three observations types into a single Bayesian inversion, we construct continuous maps of rates of relative sea level change, geocentric sea level change, and vertical land motion assuming linear trends, with robust estimates of uncertainties at regional scales. Cross‐validation tests show that reliable predictions of relative sea level changes are still provided where only GPS and satellite altimetry data are used, suggesting this method is viable for studying potential sea level risk for communities where historical tide gauge data are not available. Our results provide spatially and temporally consistent estimates of the various contributions to relative sea level changes.
Abstract The nature and linearity of vertical land motion (VLM) impacting the global sea level record from tide gauges is not well known, but remains of importance to understand long-term changes to sea level. Local surveys are required to directly measure VLM at tide gauges relative to a global reference frame, but this is limited by the lack of differential VLM measurements between tide gauges and continuously operating GPS (cGPS) stations that are not co-located, i.e., fixed to the tide gauge structure. We present results from an experiment using satellite radar interferometry (InSAR) scenes acquired from the TerraSAR-X satellite mission to test whether InSAR could replace repeat geodetic levelling as a ‘geodetic tie’ between cGPS stations and tide gauges. Comparisons are made among TerraSAR-X (TSX), cGPS and tide gauge minus altimetry VLM estimates for the Hillarys and Fremantle tide gauges (Perth, Western Australia), which are used as test sites for this method. The results suggest agreement between differential TSX and altimetry minus tide gauge VLM rates, but systematic offsets among the absolute/geocentric rates where the TSX is referenced to IGS08 at the PERT cGPS. The TerraSAR-X VLM at the Fremantle tide gauge for the period 7 October 2012 to 7 October 2017 is +0.45 ± 0.40 mm/yr (referenced to IGS08 at PERT cGPS), although this should be treated cautiously over this short period, and because VLM at Fremantle and Hillarys appears to be non-linear over time. We infer from this that the uncertainties in TerraSAR-X differential VLM rates are comparable to those from the highest quality repeat levelling, although the uncertainty approaches 1 mm/yr if the reference point uncertainty of the TSX and cGPS is considered when transformed to a terrestrial reference frame.
Polar sea surface height observation by radar altimeters requires missions with high-latitude orbit and specific processing to observe the sea-ice-covered region within fractures in the ice. Here, we combine sea surface height estimates from different radar satellites over the ice-free and ice-covered polar oceans to create cross-calibrated along-tracks and gridded products over the Arctic Ocean (2011–2021) and the Southern Ocean (2013–2021). The sea surface height from our regional polar products is in great agreement with tide gauges and bottom pressure recorders at monthly timescales in seasonally to year-round ice-covered regions. Thanks to the use of several missions and the mapping strategy, our multi-mission products have a greater resolution than mono-mission products. Part of the sea level variability of the Arctic Ocean product is related to the Arctic Oscillation atmospheric circulation. At long term, the Arctic altimetry sea level is coherent with in-situ steric height evolution in the Beaufort gyre, and negative sea level trends over the 10-year period are observed in the East Siberian slope region, which may be related to the local freshwater decrease observed by other studies. Our regional polar sea level products are limited by current understanding of the sea-ice lead measurements, and homogenization of these polar products with global sea level products needs to be tackled.
River deltas sustain dense human populations, major economic centres and vital ecosystems worldwide<sup>1,2</sup>. Rising sea levels and subsiding land threaten the sustainability of these valuable landscapes with relative sea-level rise and associated flood, land loss and salinization hazards<sup>1-3</sup>. Despite these risks, vulnerability assessments are impeded by the lack of contemporary, high-resolution, delta-wide subsidence observations<sup>4</sup>. Here we present spatially variable surface-elevation changes across 40 global deltas using interferometric synthetic aperture radar. Using this dataset, we quantify delta surface-elevation loss and show the prevalence and severity of subsidence in river deltas worldwide. Our analysis of three key anthropogenic drivers of delta elevation changes shows that groundwater storage has the strongest relative influence on vertical land motion in 10 of the 40 deltas. The other deltas are either influenced by multiple drivers or dominated by sediment flux or urban expansion. Furthermore, we find that contemporary subsidence surpasses absolute (geocentric) sea-level rise as the dominant driver of relative sea-level rise for most deltas over the twenty-first century. These findings suggest the need for targeted interventions addressing subsidence as an immediate and localized challenge, in parallel with broader efforts to mitigate and adapt to climate change-driven global sea-level rise.
ABSTRACT Kupavõh, A.; Delpeche-Ellmann, N.; Ellmann, A., and Soomere, T., 2024. Examining the performance of satellite altimetry (SWOT) sea level data in the coastal areas of the Baltic Sea. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 534-538. Charlotte (North Carolina), ISSN 0749-0208. The Surface Water and Ocean Topography (SWOT) satellite mission is expected to bring new possibilities for detecting and assessing water level both at sea and land. To explore its performance, the SWOT nadir altimeter (Poseidon-3C) and the new Ka-band Radar Interferometer (KaRIn) were examined in coastal areas of the eastern Baltic Sea using the ‘Basic’ version in July–December 2023 using 15 geoid referred tide gauges (TG), vertical land uplift model and a high resolution geoid BSCD2000. The use of geoid enabled a direct comparison with TG data. Comparison with SA and TG was performed with varying distance from the TG. Specific examination of SWOT performance was made in areas affected by archipelagos, islands and sea ice. On average KaRIn altimeter RMSD values are around 13 cm compared to that of the nadir altimeter of 10 cm. One of the benefits of KaRIn is the improved spatial resolution of sea level data. It shows promising capabilities of detecting marine dynamics in the coastal areas. KaRIn altimeter showed improved performance in near-coast areas for within 40 km from TG, 97% of valid measurements were retrieved and within 0–2 km around 40% of valid data was obtained. Sea level was also accurately determined in ice covered areas.
Conventionally, information from the tide gauge stations was used to establish the localized tidal datum. However, limitations in coverage, due to the sparse station distribution along the coast, have caused insufficient tidal datum information in some areas. Therefore, this study aims to develop the Peninsular Malaysia Quasi-Continuous Tidal Datum (PMQCTD) by integrating tide gauges, satellite altimetry, and Tide Model Driver (TMD) data. The research methodology includes data acquisition from 12 Departments of Survey and Mapping Malaysia (DSMMs) tide gauge stations along the coast of Peninsular Malaysia, satellite altimetry data of TOPEX, Jason-1, Jason-2, and GEOSAT Follow-On (GFO) from Radar Altimeter Database System (RADS), and the global hydrodynamic model from TMD. The tide gauge, satellite altimetry, and TMD data encompass 23 years of tidal observation data from 1993 to 2015. For the derivation of the tidal datum, tide gauge, and satellite altimetry data were analyzed following a harmonic analysis approach in the Unified Tidal Analysis and Prediction (UTide) software. Meanwhile, for the TMD data, the tidal datum was determined based on the tidal prediction from the 11 extracted major tidal constituents. For compatibility in data integration, the derived Lowest and Highest Astronomical Tide (LAT and HAT) from tide gauge, satellite altimetry, and TMD data were referenced to the Mean Sea Level (MSL), denoted as LATMSL and HATMSL, respectively. Next, the LATMSL and HATMSL were interpolated employing Inverse Distance Weighting (IDW) to develop the PMQCTD (LATMSL and HATMSL) with the ArcGIS software. The statistical assessment indicated that the established PMQCTD (LATMSL and HATMSL) has a better agreement with the DSMM tide gauges with a Root Mean Square Error (RMSE) of ± 0.228 m for LATMSL and ± 0.159 m for HATMSL In conclusion, the establishment of PMQCTD (LATMSL and HATMSL) has led to the availability of the tidal datum at any location along the coast of Peninsular Malaysia.
Vertical Crustal Movement along the Coast of South Africa
This study provides an in-depth evaluation of sea level rise (SLR) and its varied effects across the coastal regions of southern Africa. Utilizing data collected between 1993 and 2022, we analyze SLR patterns alongside land subsidence phenomena, based on observations from 10 strategically located tide gauges and X-TRACK satellite altimetry datasets. To ensure greater accuracy, the Coastal Altimetry Approach was adopted to refine nearshore measurements. Findings indicate that in areas such as Cape Town, sea-level rise rates reach around 6.3 mm/year, which is nearly twice the current global average of 3.3 mm/year. The interaction between rapid sea-level rise and subsidence rates surpassing 2.2 mm/year presents significant threats to coastal communities, critical infrastructure, and natural ecosystems. Moreover, the study highlights how seismic activity contributes to coastal dynamics, illustrating the role of earthquake-induced subsidence in magnifying the impacts of SLR.
Sea level in the Southeast Asia (SEA) seas is driven by various phenomena at global,&#160; regional and local scales. The latest tide gauge and satellite data revealed its most recent spatial and temporal patterns. The trend of global sea level rise in Singapore region is hindered by dominant variability of El Ni&#241;o-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Indian Ocean Dipole (IOD), as well as associated modulation of Asian Monsoon. It was confirmed that positive sea-level anomalies in the southern and western areas of Southeast Asia seas were significantly high (~10 cm) during the northeast monsoon, especially in the Gulf of Thailand (~25 cm). The sea level trends for these regions were basically reversed during the southwest monsoon but with a smaller magnitude of negative sea-level anomalies. The regional sea-level trend in the Sunda Shelf differed from region to region, with the rates varied greatly from 1.4 to more than 4.8 mm/year. Interestingly, the rates on the east-western side of the region were roughly 3.0-4.5 mm/year, which were higher than the ones at other regions, being 2.5-3.5 mm/year. The presentation discuss the causes and consequences of sea level rise and variability in SEA and Singapore region in particular.This Research is supported by Singapore&#8217;s National Research Foundation and National Environment Agency under the National Sea Level Programme Funding Initiative (Award No. USS-IF-2020-4).
theory, and funding has increased scientific reporting on regional and global issues related to heat. Tide gauges and satellite altimetry suggest an
Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.
Physical oceanography is one of several sub-domains into which oceanography is divided. Others include biological, chemical and geological oceanography. Like the study of atmospheric physics, physical oceanography is founded upon princi
Tide gauges and satellite altimetry suggest an increase in sea level of 1.5–3 mm/yr over the past 100 years.
The IPCC predicts that by 2081–2100, global warming will lead to a sea level rise of 260 to 820 mm.
series of global SSH data which, has been extremely valuable in assessing sea level rise in the past decades by combining data with local tide gauges. The
Remote sensing in oceanography is a widely used observational technique which enables researchers to acquire data of a location without physically measuring at that location. Remote sensing in oceanography mostly refers to measuring properties of the ocean surface with sensors on satellites or planes, which compose an image of captured electromagnetic radiation. A remote sensing instrument can eit
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{\displaystyle h_{ADT}=h_{SSHA}+h_{MSS}-h_{geoid}}
with the geoid height as a measurement from instruments like the Gravity and Ocean Circulations Explorer (GOCE) or Gravity Recovery and Climate Experiment (GRACE).
With the launch of TOPEX/POSEIDON in 1992 started a continuous time series of global SSH data which, has been extremely valuable in assessing sea level rise in the past decades by combining data with local tide gauges. The dynamical sea surface height from radar altimetry provides useful insight into ocean currents. If assuming geostrophic balance, the velocity anomaly and direction of surface currents perpendicular to the satellite overpass can be computed using the formula:
Abstract This thesis is mainly concerned with changes in mean sea level as measured at coastal tide gauge sites around the coast of Great Britain, and addresses the question of whether a secular change in rate of rise of sea level can be discerned over the measurement period. This is an important question for the coastal and low-lying areas of not only the UK, but globally, with the IPCC estimating that 11% of the worlds population currently live in low elevation coastal zones, with sea level rise projected to impact available land area, coastal infrastructure, ecosystems and the viability of some island nations. The two main thrusts of this present work are concerned with 1) Extending the temporal extent of currently available tide gauge records using data archaeology, ideally to the point where long term climate related trends emerge. This is important as currently many of the records are only a few decades long, and analysis of trends can give inconsistent results in relation to ongoing climate change. The small number of records which stretch back into the 19th Century do appear to show a long term increase in rate of sea level rise. 2) investigating optimal methods of identifying and accounting for any non-climate related variability in the records, again allowing any underlying trends to be more easily discerned, and importantly, shortening the period over which a climate related signal might be detected. The results are conclusive. Long term sea level rise and accelera
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