Mean Sea Level
As global warming occurs, a direct reaction of the climate system is the sea level rise. This rise results from seawater expansion as a response to the temperature increase and addition of water from land-ice sheets and glaciers melting. Precise monitoring of the sea level rise is made possible using altimetry satellites that help understanding climate change and its socioeconomic consequences. The Global Mean Sea Level (GMSL) has thus become a key indicator of climate change.
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AT THE GLOBAL SCALE
Different satellite altimetry missions have been monitoring the GMSL successively and continuously since 1993: TOPEX/Poseidon, Jason-1, Jason-2, Jason-3 and Sentinel-6 MF. These missions are called "reference missions" as they fly on a well-determined reference orbit that is unchanged since 1993. Dedicated calibration phases between the successive missions, during which the satellites fly a few second apart, help to ensure the long-term stability and precise monitoring of the sea level. In addition, a permanent control of the data quality and instrumental performances, in combination with the homogenization of the data processing and geophysical corrections , are key to produce homogeneous data that well capture the long-term evolution of the sea level.
The global sea level rise is rising, on average over the globe, by 3.6 millimeter per year. Analyzing the uncertainty of the altimetry observing system yields to construct an uncertainty envelop for the GMSL climate data record (shaded area in the figure above). The resulting uncertainty in the GMSL rise is estimated to approximately 0.3 mm/yr at the 90% confidence level.See Guérou et al. (2022) for more details as well as the Validation page for some updates.
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Note that over the last quarter of year 2022, Sentinel-6 MF is affected by an inaccurate radiometer calibration error resulting in an overestimation of the wet tropospheric correction and therefore of the GMSL by about 5 mm (see issue #9170 of EUMETSAT User Notification Service at https://uns.eumetsat.int/ ).
Analyzing the uncertainty of the altimetry observing system yields to construct an uncertainty envelop for the GMSL climate data record (shaded area in the figure above).
The dashed line displayed over 1993-1998 is an estimation of the GMSL evolution after correction of the TOPEX-A instrumental drift (Cazenave WCRP 2018). It is estimated from empirical correction derived by comparing altimetry and tide-gauge sea level data (see more details in Validation). The TOPEX-A instrumental drift led to overestimate the GMSL slope during the first 6 years of the altimetry record. Accounting for this correction changes the shape of the GMSL curve, that is no more linear but quadratic, indicating that the mean sea level is accelerating during the altimetry era (1993-to present, Beckley et al. 2017, Nerem et al. 2018). Currently, this empirical correction is not applied to the AVISO GMSL dataset, wainting for the ongoing TOPEX reprocessing by CNES and NASA/JPL.
AT THE REGIONAL SCALE
To compute the mean sea level at regional scales, auxiliary missions like SARAL/AltiKa, Envisat, ERS-1, ERS-2, Cryosat-2 and Copernicus Sentinel-3A are used in addition to the reference missions. This multimission approach allow to compute the Mean Sea Level at high latitudes (higher than 66°N and S) as well as to improve the products spatial resolution. See Ssalto/DUACS webpage for the processing details.
Although the globally averaged sea level is rising over the period 1993-to present, the spatial distribution of the sea level is heterogeneous over the globe. Locally, the sea level rise can be way larger than the global average of 3.6 mm/yr. These spatial patterns are not stationary but transient features associated to heat redistribution and ocean circulation, like the ENSO events and/or the Kuroshio current, well captured by the satellite altimetry observing system.
Further information on Regional Variability and past sea level reconstructions.
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Animation of the data since 1993
Altimetry satellites give observations of the sea level continuously since 1993 all over the globe. This long-term monitoring allows the scientific community to quantify the sea level rise, estimate its impact on society and understand better its origins.
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