Absolute Sea Level Change Research Articles

Regional variations in relative sea-level changes influenced by nonlinear vertical land motion

Vertical land movements can cause regional relative sea-level changes to differ substantially from climate-driven absolute sea-level changes. Whereas absolute sea level has been accurately monitored by satellite altimetry since 1992, there are limited observations of vertical land motion. Vertical land motion is generally modelled as a linear process, despite some evidence of nonlinear motion associated with tectonic activity, changes in surface loading or groundwater extraction. As a result, the temporal evolution of vertical land motion, and its contribution to projected sea-level rise and its uncertainty, remains unresolved. Here we generate a probabilistic vertical land motion reconstruction from 1995 to 2020 to determine the impact of regional-scale and nonlinear vertical land motion on relative sea-level projections up to 2150. We show that regional variations in projected coastal sea-level changes are equally influenced by vertical land motion and climate-driven processes, with vertical land motion driving relative sea-level changes of up to 50 cm by 2150. Accounting for nonlinear vertical land motion increases the uncertainty in projections by up to 1 m on a regional scale. Our results highlight the uncertainty in future coastal impacts and demonstrate the importance of including nonlinear vertical land motions in sea-level change projections.

Time Series Modeling of Global Average Absolute Sea Level Change

This study aimed to demonstrate the effectiveness of time series models in modeling long-term records of global average absolute sea level changes from 1880 to 2014. Following the Box–Jenkins methodology, the ARIMA(0,1,2) model with drift was identified as the best-fit model for the time series due to its lowest AIC value. Using the LM algorithm, the results revealed that the NARNN model with 7 neurons in the hidden layer and 7 time delays exhibited the best performance among the nonlinear autoregressive neural network models, as indicated by its lower MSE. While ARIMA models excel in modeling linear problems within time series data, NARNN models are better suited for nonlinear patterns. However, a HYBRID model was explored, which combines the strengths of both ARIMA and NARNN models, offering the capability to address both linear and nonlinear aspects of time series data. The comparative analysis of this study demonstrated that the HYBRID model, with 6 neurons in the hidden layer and 7 time delays, outperformed the NARNN model with 7 neurons in the hidden layer and 7 time delays, as well as the ARIMA(0,1,2) model, with the lowest MSE in this study. These findings represent a significant step in time series forecasting by leveraging the strengths of both statistical and machine learning methods.

Monitoring absolute vertical land motions and absolute sea-level changes from GPS and tide gauges data over French Polynesia

In this study, we estimate the absolute vertical land motions at three tidal stations with collocated Global Navigation Satellite System (GNSS) receivers over French Polynesia during the period 2007–2020, and obtain, as ancillary results, estimates of the absolute changes in sea level at the same locations. To verify our processing approach to determining vertical motion, we first modeled vertical motion at the International GNSS Service (IGS) THTI station located in the capital island of Tahiti and compared our estimate with previous independent determinations, with a good agreement. We obtained the following estimates for the vertical land motions at the tide gauges: Tubuai island, Austral Archipelago −0.92 ± 0.17 mm/yr, Vairao village, Tahiti Iti: −0.49 ± 0.39 mm/yr, Rikitea, Gambier Archipelago −0.43 ± 0.17 mm/yr. The absolute variations of the sea level are: Tubuai island, Austral Archipelago 5.25 ± 0.60 mm/yr, Vairao village, Tahiti Iti: 3.62 ± 0.52 mm/yr, Rikitea, Gambier Archipelago 1.52 ± 0.23 mm/yr. We discuss these absolute values in light of the values obtained from altimetric measurements and other means in French Polynesia.

Monitoring Megathrust-Earthquake-Cycle-Induced Relative Sea-Level Changes near Phuket, South Thailand, Using (Space) Geodetic Techniques

Temporal changes in vertical land motion (VLM) in and around Phuket Island in southern Thailand following the great 2004 Sumatra–Andaman megathrust earthquake have impacted the relative sea-level change estimates based on tide-gauge (TG) records. To better monitor the VLM, two new continuous global navigation satellite system (GNSS) stations have been installed in the past 5 years, situated on bedrock both near and at the Koh Taphao Noi Island TG in Phuket, which together with older global positioning system (GPS) data provides a clear insight in the VLM of Phuket Island from 1994 onward. In addition, satellite altimetry (SALT) data has been analyzed since 1992. The VLM (GPS) position and relative (TG) and absolute (SALT) sea-level change time series were successfully combined in pairs to validate each independent result (e.g., SALT − GNSS = TG): prior to the 2004 earthquake, the relative sea-level rise in Phuket was 1.0 ± 0.7 mm/yr, lower by 2.4 ± 0.2 mm/yr than the absolute sea-level rise caused by VLM. After the earthquake, nonlinear post-seismic subsidence has caused the VLM to drop by 10 cm in the past 17 years, resulting, by the end of 2020, in a relative sea-level rise by up to 16 cm. During the same period, other TG stations in south Thailand recorded similar sea-level increases. Combination with SALT further suggests that, prior to 2005, uplift (5.3 ± 1.4 mm/yr) of the coastal region of Ranong (north of Phuket) resulted in a relative sea-level fall, but since then, post-seismic-induced negative VLM may have significantly increased coastal erosion along the entire Andaman Sea coastline.

Absolute Sea Level Changes Along the Coast of China From Tide Gauges, GNSS, and Satellite Altimetry

AbstractThis study quantifies the absolute sea level (ASL) change along the coast of China from 1993 to 2019 using tide gauge (TG) and Global Navigation Satellite System (GNSS) data. The sea level change at 12 collocated TG and GNSS stations along the Chinese coast are first analyzed by TG + GNSS data. Then the ASL rise and its spatial variation along the coast of China and that in surrounding oceans are studied using TG + GNSS data and satellite altimetry products. The results show that the relative sea level rise along the Chinese coast varied from 2.61 ± 0.76 to 5.56 ± 0.70 mm/yr, while the effect of vertical land motion (VLM) ranged from −1.46 ± 0.30 to 1.07 ± 0.38 mm/yr. After correcting the effect of the VLM, the ASL rise ranged from 2.51 ± 0.86 to 5.03 ± 0.60 mm/yr, with a mean rate of 3.94 ± 0.88 mm/yr, which was 0.7 mm/yr higher than the global average. It is found that ASL rise rate is significantly higher in Chinese seas and neighboring oceans, with a meridional distribution in the East China Sea and a zonal distribution in the South China Sea. This may be related to factors such as sea surface temperature rise, atmospheric pressure decrease, freshwater runoff, zonal wind stress anomaly, and alongshore currents in the area.

Measuring Coastal Absolute Sea-Level Changes Using GNSS Interferometric Reflectometry

Rising sea levels pose one of the greatest threats to coastal zones. However, sea-level changes near the coast, particularly absolute sea-level changes, have been less well monitored than those in the open ocean. In this study, we aim to investigate the potential of Global Navigation Satellite Systems Interferometric Reflectometry (GNSS-IR) to measure coastal absolute sea-level changes and tie on-land (coastal GNSS) and offshore (satellite altimetry) observations into the same framework. We choose three coastal GNSS stations, one each in regions of subsidence, uplift and stable vertical land motions, to derive both relative sea levels and sea surface heights (SSH) above the satellite altimetry reference ellipsoid from 2008 to 2020. Our results show that the accuracy of daily mean sea levels from GNSS-IR is <1.5 cm compared with co-located tide-gauge records, and amplitudes of annual cycle and linear trends estimated from GNSS-IR measurements and tide-gauge data agree within uncertainty. We also find that the de-seasoned and de-trended SSH time series from GNSS-IR and collocated satellite altimetry are highly correlated and the estimated annual amplitudes and linear trends statistically agree well, indicating that GNSS-IR has the potential to monitor coastal absolute sea-level changes and provide valuable information for coastal sea-level and climate studies.

Evaluation of vertical crustal movements and sea level changes around Greenland from GPS and tide gauge observations

To better monitor the vertical crustal movements and sea level changes around Greenland, multiple data sources were used in this paper, including global positioning system (GPS), tide gauge, satellite gravimetry, satellite altimetry, glacial isostatic adjustment (GIA). First, the observations of more than 50 GPS stations from the international GNSS service (IGS) and Greenland network (GNET) in 2007–2018 were processed and the common mode error (CME) was eliminated with using the principal component analysis (PCA). The results show that all GPS stations show an uplift trend and the stations in southern Greenland have a higher vertical speed. Second, by deducting the influence of GIA, the impact of current GrIS mass changes on GPS stations was analysed, and the GIA-corrected vertical velocity of the GPS is in good agreement with the vertical velocity obtained by gravity recovery and climate experiment (GRACE). Third, the absolute sea level change around Greenland at 4 gauge stations was obtained by combining relative sea level derived from tide gauge observations and crustal uplift rates derived from GPS observations, and was validated by sea level products of satellite altimetry. The results show that although the mass loss of GrIS can cause considerable global sea level rise, eustatic movements along the coasts of Greenland are quite complex under different mechanisms of sea level changes.

Revisiting Vertical Land Motion and Sea Level Trends in the Northeastern Adriatic Sea Using Satellite Altimetry and Tide Gauge Data

We propose a revisited approach to estimating sea level change trends based on the integration of two measuring systems: satellite altimetry and tide gauge (TG) time series of absolute and relative sea level height. Quantitative information on vertical crustal motion trends at six TG stations of the Adriatic Sea are derived by solving a constrained linear inverse problem. The results are verified against Global Positioning System (GPS) estimates at some locations. Constraints on the linear problem are represented by estimates of relative vertical land motion between TG couples. The solution of the linear inverse problem is valid as long as the same rates of absolute sea level rise are observed at the TG stations used to constrain the system. This requirement limits the applicability of the method with variable absolute sea level trends. The novelty of this study is that we tried to overcome such limitations, subtracting the absolute sea level change estimates observed by the altimeter from all relevant time series, but retaining the original short-term variability and associated errors. The vertical land motion (VLM) solution is compared to GPS estimates at three of the six TGs. The results show that there is reasonable agreement between the VLM rates derived from altimetry and TGs, and from GPS, considering the different periods used for the processing of VLM estimates from GPS. The solution found for the VLM rates is optimal in the least square sense, and no longer depends on the altimetric absolute sea level trend at the TGs. Values for the six TGs’ location in the Adriatic Sea during the period 1993–2018 vary from −1.41 ± 0.47 mm y−1 (National Research Council offshore oceanographic tower in Venice) to 0.93 ± 0.37 mm y−1 (Rovinj), while GPS solutions range from −1.59 ± 0.65 (Venice) to 0.10 ± 0.64 (Split) mm y−1. The absolute sea level rise, calculated as the sum of relative sea level change rate at the TGs and the VLM values estimated in this study, has a mean of 2.43 mm y−1 in the period 1974–2018 across the six TGs, a mean standard error of 0.80 mm y−1, and a sample dispersion of 0.18 mm y−1.

Sea-level rise and vertical land motion on the Islands of Oahu and Hawaii, Hawaii

Sea-level rise is a highly publicized issue in the Hawaiian Islands because it is one of the main drivers for coastal hazards. In our study, multiple geodetic and in situ datasets are integrated to investigate the sea-level rise and vertical land motion on the islands of Oahu and Hawaii, Hawaii. The rates of relative sea-level changes are derived from the tide-gauge stations in the Hawaiian Islands, however the station located at Kawaihae, Hawaii presents a much higher trend than other stations. Our analysis shows that the questionable trend results from the sudden movement of the equipment on land, which is caused by a pair of earthquakes. After adjustment, we arrive at a revised and more consistent relative sea-level trend at this station. Our study shows that Oahu is vertically ‘stable’ (i.e., near-zero vertical land movement within uncertainties), and the relative sea-level change is dominated by the absolute sea-level change. However, the island of Hawaii was subsiding at −3.3 ± 0.9 mm/year before 1973 and changed to −1.2 ± 0.2 mm/year after 1975, which may relate to seismic activities and where relative sea-level change is attributed to both absolute sea-level change and vertical land motion. The difference in relative sea-level change between the islands of Oahu and Hawaii is due to the difference in vertical land motion rather than steric sea-level change. In addition, the ocean-mass components are the predominant factors that influence the long-term trends of absolute sea level on the islands of Oahu and Hawaii.

INVESTIGATIONS ON VERTICAL LAND MOVEMENTS ALONG THE NORTH SEA AND BALTIC SEA COAST IN GERMANY WITH PS INTERFEROMETRY

Abstract. The project ‘Determinations on the absolute sea-level rise on the German North Sea and Baltic Sea coasts’, funded by the Federal Ministry of Education and Research (BMBF) , has the overall goal to estimate the absolute sea level change in those coastal areas. A major issue associated with detecting absolute sea level changes is the relative character of tidal records. To calibrate the tidal records, a spatial vertical land movement model for northern Germany has been set up. To this end we combined a network from German Height Reference Systems (Deutsches Haupthöhennetz, DHHN 95 and DHHN 2016), reprocessed data from 180 permanent GNSS stations and results from Persistent Scatterer (PS) Interferometry.PS processing covers an approximately 50 km wide strip along the 1200 km long German coast. We processed two tracks from Sentinel-1A and -1B from October 2014 to September 2018 and generated a combined spatial solution for the estimation of vertical land movement. In general, vertical velocities from PS Interferometry range between ±2 mm/a and show a homogeneous distribution for coastal areas. Therefore we consider them as stable. We observe subsidence in the area around Groningen and Emden through hydrocarbon extraction. In Wilhelmshaven and Etzel subsidence associated with cavern storage is visible.Processed GNSS data and PSI results overlap in time from 2014 to 2016. The integration of the spatial multi-temporal PS results with point-wise GNSS time series data are required, as they form the main input data for the further development of our vertical displacement model of northern Germany.

Optimal mathematical and statistical models to estimate vertical crustal movements using satellite altimetry and tide gauge data

AbstractKnowledge of vertical crustal movement is fundamental to quantify absolute sea level changes at tide gauge locations as well as for satellite altimetry calibration validations. While GPS measurements at collocated tide gauge stations fulfill this need, currently only few hundred tide gauge stations are equipped with GPS, and their measurements do not span a long period of time. In the past, several studies addressed this problem by calculating relative and geocentric trends from the tide gauge and satellite altimetry measurements respectively, and then difference the two trends to calculate the rate of changes at the tide gauge stations. However, this approach is suboptimal. This study offers an optimal statistical protocol based on themethod of condition equations with unknown parameters. An example solution demonstrates the proposed mathematical and statistical models’ optimality in estimating vertical crustal movement and its standard error by comparing them with the results of current methods. The proposed model accounts for the effect of autocorrelations in observed tide gauge and satellite altimetry sea level time series, adjusts observed corrections such as inverted barometer effects, and constraints tide gauge and satellite altimeter measurement to close. The new model can accommodate estimating other systematic effects such as pole tides that are not eliminated by differencing.

Reef Carbonate Productivity During Quaternary Sea Level Oscillations

AbstractGlobal variations in reef productivity during the Quaternary depend on external parameters that may alter the global chemical balance in the oceans and atmosphere. We designed a numerical model that simulates reef growth, erosion, and sedimentation on coastlines undergoing sea level oscillations, and uplift or subsidence. We further develop a probabilistic evaluation that accounts for variable vertical ground motion, erosion, and foundation morphologies. Absolute sea level change appears primordial, as productivity must have increased by an order of magnitude since the onset of the glacial cycles, ∼2.6 Ma. But most important is relative sea level change, i.e., eustasy modulated by uplift or subsidence, that rejuvenates the accommodation space and exposes pristine domains of the shore to active reefs at each cycle. Integrated over the long‐term, vertical land motion sets the pace of reef growth: productivity in tectonically unstable domains is thus expected to be up to 10 times higher than in stable regions, if any. We quantify the global length of reef coasts and the probability density functions for slopes and uplift rates. Productivity waxes during transgressions to reach 2–8 and wanes during highstands, which may contribute to increase atmospheric pCO2 by several tens of ppm during deglaciations. Over the last 1.5 Ma, reefs precipitated ∼0.8 × 106 (∼500 × 103 km3), the equivalent of a 1 m‐thick layer spread over the entire surface of the Earth. This production modulates the calcium budget, for it represents some 30% of the modern Ca flux in the ocean.

Vertical land motion along the Black Sea coast from satellite altimetry, tide gauges and GPS

Tide gauge records comprise of relative sea level change and vertical land motion, while satellite altimetry provides absolute sea level change in the Earth’s center fixed frame. Accordingly, the difference of both observations can be used to estimate geocentric vertical land motion along the coasts. In this paper, the vertical land motion rates are investigated at 13 tide gauge sites along the Black Sea coast by analyzing differences between Tide Gauge (TG) and Satellite Altimetry (SA) observations. Furthermore, the estimated vertical motion rates are compared with those from nearby the Global Positioning System (GPS) measurements. The results show general consistence with the present geodynamics in the Black Sea coastal region. For example, our estimates support the general subsidence at Bourgas and Varna.

Absolute Sea Level Surface Modeling for the Mediterranean from Satellite Altimeter and Tide Gauge Measurements

ABSTRACTChanges in the height of the ocean can be described through the relative and absolute sea level changes depending on the geodetic reference the sea level records are related to. Satellite altimetry provides absolute sea level (ASL) measurements related to the global geodetic reference, whereas tide gauges provide relative sea level (RSL) measurements related to the adjacent land. This study aims at computing the ASL surfaces for different time epochs from combined satellite altimeter and tide gauge records. A method of sea level data fusion is proposed to enable modeling of the impact of present and future sea level changes on the coast. Sea surface modeling was investigated for ten different gridding methods commonly used for the interpolation of altimeter data over the open ocean and extrapolation over the coastal zones. The performance of gridding methods was assessed based on the comparison of the gridded altimeter data and corrected tide gauge measurements. Finally, the sea level surfaces related to the GRS80 global reference ellipsoid were computed for the Mediterranean Sea over the altimeter period. In addition, the current sea level trends were estimated from both sea level measurements.

The Fiji Tide-Gauge Stations

We analyse the location, stability and continuity of the two tide-gauge stations in Fiji. Both stations are awkwardly placed on heavy harbour constructions resting on soft sediments susceptible to serious compaction problems. The nearby GPS stations resting on a bedrock hill offer no solution to the stability problems. The Suva tide-gauge has been moved three times, and must accordingly be analysed in segments. Even the last location covering years 1989 to 2917 provides a mixed picture of 16 years of stability, 10 years of rapid rise, and 4 years of rapid fall in relative sea level. This suggests the interaction of subsidence and cyclic changes in sea level. Any application of mean trends would produce meaningless values rather misguiding than assisting in the handling of estimation of on-going absolute sea level changes. We find this vital for the discussions of local sea level changes to be held at the UN conference on “Our Oceans, Our Future” in June in New York and at the main COP23 conference in November in Bonn.

Historical and recent sea level rise and land subsidence in Marina di Ravenna, northern Italy

The regions facing the northern Adriatic Sea are particularly vulnerable to sea-level rise. Several trade ports are located there, and the area is important from social and economical viewpoints. Since tourism and cultural heritage are a significant source of income, an increase in sea-level could hinder the development of these regions. One of the longest sea-level time series in the northern Adriatic, which goes back to the late 1880s, has been recorded at Marina di Ravenna, in Emilia-Romagna region. The record is anomalous, showing a rate of increase that largely exceeds that observed in nearby stations. During the last few decades, geodetic campaigns based on geometric high precision leveling, SAR interferometry, and GPS have monitored the Ravenna area. In this work, tide gauge observations are merged with yet unpublished geodetic data, aiming at a coherent interpretation of vertical land movements. We confirm that land subsidence is the major cause of relative sea-level change at Marina di Ravenna, at least during the period allowing for a quantitative analysis (1990-2011). The rate of absolute sea-level change (2.2±1.3 mm yr−1 during the same time period), given by the difference between the rate of relative sea-level change and the rate of subsidence, is consistent with the rate of absolute sea-level change observed by altimetry in the northern Adriatic Sea.

Can eustatic charts go beyond first order? Insights from the Permian–Triassic

First-order variations of eustatic charts (200–400 m.y.) are in agreement with our understanding of the geodynamic processes that control sea level. By extrapolation, second-order (10–100 m.y.) and third-order (1–10 m.y.) variations are also thought to follow the same rules. However, this assumption may be jeopardized by a closer examination of the Permian–Triassic example, for which climatic and tectonic eustasy fails to explain the variations of the eustatic charts. During this period, eustatic charts peak down to their lowermost Phanerozoic values and display second-order variations at rates of up to 3 m/m.y., which is inconsistent with the expected eustatic signal during the early fragmentation of the Pangean supercontinent and the late Paleozoic melting of ice sheets. Here, we review the possible mechanisms that could explain the apparent sea-level variations. Some of them do modify the eustatic sea level (ESL). In particular, dynamic deflections of Earth’s surface above subduction zones and their locations with respect to continents appear to have been the primary controls of absolute sea level as the Pangean supercontinent formed and broke up. Other mechanisms instead only locally or regionally produced vertical ground motions, either uplifting continents or tilting the margins where the control points were located. We show that (1) the thermal uplift associated with supercontinent insulation and (2) the dynamic uplift associated with the emplacement of a superplume both give rates of sea-level change in the range of long-term changes of ESL. We also show that (3) the dynamic tilt of continental margins not only produces apparent sea-level changes, but it also modifies the absolute sea level, which in turn may end up in the paradoxical situation wherein fingerprints of ESL drop are found in the geological record during actual ESL rise. We conclude that second-order absolute sea-level changes may remain elusive for some time.

Simultaneous estimation of lithospheric uplift rates and absolute sea level change in southwest Scandinavia from inversion of sea level data

Simultaneous estimation of lithospheric uplift rates and absolute sea level change in southwest Scandinavia from inversion of sea level data

Sea level change in the Gulf of Thailand from GPS-corrected tide gauge data and multi-satellite altimetry

Investigation of long-term tidal data and short-term altimetry measurements reveals that sea level in the Gulf of Thailand is rising significantly faster than global average rates. Upward land motion detected from repeated precise GPS campaign measurements is used to correct the apparent sea level change from tide gauge, yielding absolute long-term trends as follows: Sattahip (1942–2004) 5.0 ± 1.3 mm/yr, Ko Sichang (1940–1999) 4.5 ± 1.3 mm/yr and Ko Mattaphon (1964–2004) 4.4 ± 1.1 mm/yr. Dual-crossover minimization of multi-mission altimetry data covering the 1993–2009 period reveals the following absolute sea level rates: Sattahip 4.8 ± 0.7 mm/yr, Ko Sichang 5.8 ± 0.8 mm/yr, Ko Lak 3.6 ± 0.7 mm/yr and Ko Mattaphon 3.2 ± 0.7 mm/yr. In other parts of the Gulf, the 1993–2009 rising rates are also in the range of 3 to 5.5 mm/yr. In the entire Gulf we don't find any evidence of sea level falling. At Ko Lak where the collocation of Topex-class altimetry ground track and the tidal station is extremely good, vertical land motion derived from the difference of sea level change rates detected by altimetry and tidal data is used to correct the apparent rate, yielding an absolute long-term (1940–2004) rate of 3.0 ± 1.5 mm/yr. The differences between the altimetry-based rates and the absolute tide gauge sea level trends can be explained by interannual variations like ENSO and decadal variations due to solar activity and lunar nutation. Post-2004 tidal data have been treated separately in our study because reliable values of region-wide vertical co-seismic displacements and post-seismic velocities caused by the 2004 Mw9.2 Sumatra-Andaman earthquake are still not accurately known. Exclusion of these data will not significantly change the determined long-term absolute sea level change rates because of the relatively short time span of post-earthquake sea level data compared to the complete tidal record. The impact of fast rising sea level combined with high rates of post-seismic downward crustal motions as indicated by GPS data makes coastal areas and river estuaries along the Gulf of Thailand highly vulnerable to flooding, particularly the low-lying city of Bangkok.

Twentieth century constraints on sea level change and earthquake deformation at Macquarie Island

SUMMARY Through the combination of rare historical sea level observations collected during Sir Douglas Mawson’s 1911–1914 Australasian Antarctic Expedition (AAE), together with modern sea level data, space geodetic estimates of crustal displacement and modelling of coseismic and post-seismic earthquake deformation, we present a contemporary analysis to constrain sea level and land level change over the twentieth century at Macquarie Island (54 ◦ 30 � S, 158 ◦ 57 � E). We combine 9 months of 1912–1913 sea level data with intermediate observations in 1969–1971, 1982 and 1998–2007 to estimate sea level rise relative to the land at +4.8 ± 0.6 mm yr −1 . Combined with estimates of global mean sea level rise, this value supports the geologically surprising notion of land subsidence, conflicting with longer term geological evidence that suggests uplift at ∼0.8 mm yr −1 over the last 400–300 Kyr. We investigate the current tectonic evolution of the Island through analysis of Global Positioning System (GPS) solutions that utilize data over the period 2000–2009. Importantly, this provides an opportunity to refine the source parameters of the M w ∼8.0 great earthquake of 2004 December 23 using estimates of coseismic displacements at regional GPS sites. We use the estimated earthquake source and GPS observations of four years of post-seismic deformation at Macquarie Island to infer the rheology of the oceanic upper mantle. We find that an asthenosphere bounded by stronger material above and below is required to produce the observed post-seismic deformation, particularly in the vertical component. Assuming a Maxwell rheology, the best fit is given by an asthenospheric viscosity of 3×10 19 Pa s. The inferred rheology determined from the 2004 earthquake is used to model long period post-seismic deformation from M w ∼8.0 earthquakes of 1989 and 1924. The 1924 earthquake is the closest of the three great earthquakes to Macquarie Island, and our modelling suggests that the majority of the vertical deformation at the tide gauge over the subsequent 80 years is related to ongoing viscoelastic relaxation from this thrust earthquake that ruptured south from an epicentre south of Macquarie Island. Assimilated time-series of land level change from the earthquake modelling (suggesting ongoing subsidence) constrained by the GPS estimate of vertical velocity of −2.46 ± 0.64 mm yr −1 combine with the relative sea level time-series to yield an estimate of absolute sea level change of +2.0 ± 0.8 mm yr −1 over the twentieth century. We conclude this is consistent with the upper bound of the global average rate of absolute sea level rise over the same period. This represents one of few estimates of observed sea level change in the Southern Ocean, re-emphasizing the importance of historical data and continued geodetic and oceanographic observation in remote areas.