Guy Wöppelmann

Coastal sea level rise in southern Europe and the nonclimate contribution of vertical land motion

[1] In this study, we extend the advanced approach of combining tide gauge and satellite altimetry data with supplemental equations from adjacent tide gauge records of at least 30 years of common data to investigate the relative importance of the nonclimate contribution of vertical land movement to the observed rates of sea level change along the coasts of southern Europe. The sensitivity tests proved that the advanced approach is robust and accurate at the submillimeter per year level of around 0.4 mm yr � 1 in estimating rates of vertical land movements. It enabled identifying stations displaying large rates of vertical land movements that must be taken into account when predicting future sea level rise and appraising the exposure to its impacts on populations and assets. The average rate of coastal climate-related sea level rise in the Mediterranean Sea was consequently revisited to be of 1.7 mm yr � 1 over the past century, whereas the Atlantic northern Iberian coast revealed a significant high rate of sea level rise in excess of 3.4 mm yr � 1 for the past 70 years. Future work should consider applying this powerful approach to other geographic contexts as a useful source of supplementary data for geodynamic studies.

Vertical land motion as a key to understanding sea level change and variability

Vertical land motions are a key element in understanding how sea levels have changed over the past century and how future sea levels may impact coastal areas. Ideally, to be useful in long-term sea level studies, vertical land motion should be determined with standard errors that are 1 order of magnitude lower than the contemporary climate signals of 1 to 3 mm/yr observed on average in sea level records, either using tide gauges or satellites. This metrological requirement constitutes a challenge in geodesy. Here we review the most successful instrumental methods that have been used to determine vertical displacements at the Earth’s surface, so that the objectives of understanding and anticipating sea levels can be addressed adequately in terms of accuracy. In this respect, the required level of uncertainty is examined in two case studies (global and local). A special focus is given to the use of the Global Positioning System (GPS) and to the combination of satellite radar altimetry with tide gauge data. We update previous data analyses and assess the quality of global satellite altimetry products available to the users for coastal applications. Despite recent advances, a near-plateau level of accuracy has been reached. The major limitation is the realization of the terrestrial reference frame, whose physical parameters, the origin and the scale factor, are beyond the scope of a unique technique such as the GPS. Additional practical but nonetheless important issues are associated with the installation of GPS antennas, such as ensuring that there is no unknown differential vertical motion with the tide gauge.

Hydrological deformation induced by the West African Monsoon: Comparison of GPS, GRACE and loading models

Three-dimensional ground deformation measured with permanent GPS stations in West Africa was used for investigating the hydrological loading deformation associated with Monsoon precipitation. The GPS data were processed within a global network for the 2003–2008 period. Weekly station positions were retrieved with a repeatability (including unmodeled loading effects) of 1–2 mm in the horizontal components and between 2.5 and 6 mm in the vertical component. The annual signal in the vertical component for sites located between 9.6N and 16.7N is in the range 10–15 mm. It is consistent at the 3 mm-level with the annual regional-scale loading deformations estimated from GRACE satellite products and modeled with a combination of hydrological, atmospheric, and nontidal oceanic models. An additional 6 month transient signal was detected in the vertical component of GPS estimates at most of the West African sites. It takes the form of an oscillation occurring between September and March, and reaching a maximum amplitude of 12–16 mm at Ouagadougou (12.5N). The analysis of in situ hydro-geological data revealed a strong coincidence between this transient signal and peak river discharge at three sites located along the Niger River (Timbuktu, Gao, and Niamey). At Ouagadougou, a similar coincidence was found with the seasonal variations of the water table depth. We propose a mechanism to account for this signal that involves a sequence of swelling/shrinking of clays combined with local loading effects associated with flooding of the Niger River.

Tide gauge datum continuity at Brest since 1711: France's longest sea‐level record

The issue of a possible tide gauge datum discontinuity at Brest, caused by the bombing of the city in August 1944, is discussed. This issue is very important, as many scientists have used this long record to derive a long-term sea level trend estimate for use within global sea level rise studies. A detailed analysis of historical leveling information, and comparison of sea level data between adjacent stations, proved to be worthwhile, even beyond this initial scope of the study: it led to an accurate datum connection between recently rediscovered 18th century sea level data (back to 1711) and those of the present day. The study provides additional evidence that the onset of recent rapid sea level rise most likely took place in the late 19th century, in agreement with the nearby Liverpool sea-level record and with independent results from sediment cores collected in salt marshes located in both hemispheres.

Reassessment of 20th century global mean sea level rise

Significance Estimates of global mean sea level (GMSL) before the advent of satellite altimetry vary widely, mainly because of the uneven coverage and limited temporal sampling of tide gauge records, which track local sea level rather than the global mean. Here we introduce an approach that combines recent advances in solid Earth and geoid corrections for individual tide gauges with improved knowledge about their geographical representation of ocean internal variability. Our assessment yields smaller trends before 1990 than previously reported, leading to a larger overall acceleration; identifies three major explanations for differences with previous estimates; and reconciles observational GMSL estimates with the sum of individually modeled contributions from the Coupled Model Intercomparison Project 5 database for the entire 20th century. The rate at which global mean sea level (GMSL) rose during the 20th century is uncertain, with little consensus between various reconstructions that indicate rates of rise ranging from 1.3 to 2 mm⋅y−1. Here we present a 20th-century GMSL reconstruction computed using an area-weighting technique for averaging tide gauge records that both incorporates up-to-date observations of vertical land motion (VLM) and corrections for local geoid changes resulting from ice melting and terrestrial freshwater storage and allows for the identification of possible differences compared with earlier attempts. Our reconstructed GMSL trend of 1.1 ± 0.3 mm⋅y−1 (1σ) before 1990 falls below previous estimates, whereas our estimate of 3.1 ± 1.4 mm⋅y−1 from 1993 to 2012 is consistent with independent estimates from satellite altimetry, leading to overall acceleration larger than previously suggested. This feature is geographically dominated by the Indian Ocean–Southern Pacific region, marking a transition from lower-than-average rates before 1990 toward unprecedented high rates in recent decades. We demonstrate that VLM corrections, area weighting, and our use of a common reference datum for tide gauges may explain the lower rates compared with earlier GMSL estimates in approximately equal proportion. The trends and multidecadal variability of our GMSL curve also compare well to the sum of individual contributions obtained from historical outputs of the Coupled Model Intercomparison Project Phase 5. This, in turn, increases our confidence in process-based projections presented in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.

Evidence for a differential sea level rise between hemispheres over the twentieth century

Tide gauge records are the primary source of sea level information over multidecadal to century timescales. A critical issue in using this type of data to determine global climate-related contributions to sea level change concerns the vertical motion of the land upon which the gauges are grounded. Here we use observations from the Global Positioning System for the correction of this vertical land motion. As a result, the spatial coherence in the rates of sea level change during the twentieth century is highlighted at the local and the regional scales, ultimately revealing a clearly distinct behavior between the Northern and the Southern Hemispheres with values of 2.0 mm/yr and 1.1 mm/yr, respectively. Our findings challenge the widely accepted value of global sea level rise for the twentieth century.

Isostatic stability of the East Antarctic station Dumont d’Urville from long-term geodetic observations and geophysical models

Geodetic measurements of the vertical crustal displacement collocated with absolute gravity changes provide a discriminatory measurement of present-day glacial changes, versus more deeply seated rock motions caused by glacial isostatic adjustment (GIA). At the East Antarctic station of Dumont d’Urville, we compare the displacements derived from continuous DORIS (1993.0– 2006.0) and Global Positioning System (GPS) (1999.0–2005.7) data, and observed changes in absolute gravity (2000–2006), with the predicted vertical displacement and change in gravity from GIA modelling. The geodetic results have mutual self-consistency, suggest station stability and provide upper bounds on both GIA and secular ice mass changes. The GIA models tend to predict amplitudes of rock motion larger than those observed, and we conclude that this part of Antarctica is probably experiencing a slight gain in ice mass, in contrast to West Antarctica.

Sea level at Saint Paul Island, southern Indian Ocean, from 1874 to the present

A data archeology exercise was carried out on sea level observations recorded during the transit of Venus across the Sun observed in 1874 from Saint Paul Island (38°41′S, 77°31 E) in the southern Indian Ocean. Historical (1874) and recent (1994-2009) sea level observations were assembled into a consistent time series. A thorough check of the data and its precise geodetic connection to the same datum was only possible thanks to the recent installation of new technologies (GPS buoy and radar water level sensor) and leveling campaigns. The estimated rate of relative sea level change, spanning the last 135 years at Saint Paul Island, was not significantly different from zero (−0.1 ± 0.3 mm yr−1), a value which could be reconciled with estimates of global average sea level rise for the 20th century assuming the DORIS vertical velocity estimate at Amsterdam Island (100 km distant) could be applied to correct for the land motion at the tide gauge. Considering the scarcity of long-term sea level data in the Southern Hemisphere, the exercise provides an invaluable additional observational constraint for further investigations of the spatial variability of sea level change, once vertical land rates can be determined.

The Van de Casteele Test Revisited: An Efficient Approach to Tide Gauge Error Characterization

Abstract The classical question of metrology related to the quality of the tide gauge measurements has become more important this last decade or so as new technologies have emerged and tide gauge networks are modernized. The Global Sea Level Observing System (GLOSS) target of 1-cm accuracy in the individual sea level measurement is motivated by more demanding applications than the traditional hydrographic works and tide predictions, for instance, the monitoring of the long-term trends in sea level or the calibration of satellite radar altimeters. To examine and further assess the performance of modern tide gauge measurements, the Van de Casteele test is revisited. This test is based on a diagram plotting readings taken with a reference probe against the tide gauge readings over at least one tidal cycle. The application of the test to different sets of data at different locations in the world under different environmental conditions shows the test as a simple procedure that immediately gives a qualitative ...

The status of measurement of the Mediterranean mean dynamic topography by geodetic techniques

We review the measurement of the mean dynamic topography (MDT) of the Mediterranean using ellipsoidal heights of sea level at discrete tide gauge locations, and across the entire basin using satellite altimetry, subtracting estimates of the geoid obtained from recent models. This ‘geodetic approach’ to the determination of the MDT can be compared to the independent ‘ocean approach’ that involves the use of in situ oceanographic measurements and ocean modelling. We demonstrate that with modern geoid and ocean models there is an encouraging level of consistency between the two sets of MDTs. In addition, we show how important geodetic MDT information can be in judging between existing global ocean circulation models, and in providing insight for the development of new ones. The review makes clear the major limitations in Mediterranean data sets that prevent a more complete validation, including the need for improved geoid models of high spatial resolution and accuracy. Suggestions are made on how a greater amount of reliable geo-located tide gauge information can be obtained in the future.