Mean sea level
The change in mean sea level is an essential indicator of the evolution of our climate, as it reflects both the effect of thermal expansion, due to heat added to the ocean, and the increase in ocean mass due to melting glaciers. Long-term and inter-annual changes in sea level can be observed both globally and regionally. These changes are related to changes in the various ocean basins and may affect people living in coastal areas, where changes in sea levels may overlap with the effects of land subsidence.
- The mean sea level in the Bay of Biscay has been rising at a rate of 1.5-3.5 cm per decade since the 1990s, as measured by tide gauges and satellite sensors.
Relationship of the indicator to climate change
As a consequence of the expansion of sea volume as it warms and the melting of glaciers and ice caps, the rise in the global mean sea level (IPCC-AR5, 2013) is accelerating: 1.4 mm yr–1 between 1901 and 1990, 2.1 mm yr–1 between 1970 and 2015, 3.2 mm yr–1 between 1993 and 2015 and 3.6 mm yr–1 between 2006 and 2015 (IPCC, 2019).
Figure 2. Trends in sea level in the Bay of Biscay measured by satellites since 1993.
Analysis of the time series for sea level at all tide gauges in the Bay of Biscay shows a significant upward trend (Figure 1).
2. irudia. Bizkaiko Golkoko satelite bidezko itsas mailaren joera 1993tik.
Analysis of the time series for mean sea level in the Bay of Biscay by satellites indicates a rise of 2.62 ± 0.42 cm per decade (p=0.0001) between 1993 and 2020 (Figure 2).
In conclusion, analysis of the time series indicates that the trends in mean sea level estimated by satellite and tide gauges in the Bay of Biscay continue to rise at a rate of between 1.5-3.5 cm per decade since the 1990s. Given that the rise in regional mean sea level is consistent with global sea level rise, and an increase in sea temperature was also observed for the area, we can infer that the rise in sea level in the Bay of Biscay would be due to global climate change.
Mean sea level is an indicator of the average height of the sea over a long period of time, in reference to a land mark or geoid. The trend in the rise of mean sea level can be measured using different indicators:
- Mean sea level using satellite sensors for the Bay of Biscay
- Mean sea level using tide gauges.
Mean sea level data from tide gauges are from the port sites of different institutions (Ifremer, BODC, IEO) centralised at the University of Hawaii Sea Level Center) and Puertos del Estado (State Ports). The analysis of the time series from when it started to the present has been carried out by 1) discarding seasonal variability (T< 1 year: astronomical tide, meteorological tides and variability associated with the seasonal temperature cycle) which affects the volume of seawater in the inter-annual trend, by filtering the series with a digital filter (González & Fontán, 2013), 2) estimating the average values per year, and 3) calculating the trend in the series with a linear regression model.
As far as data on mean sea level from satellites are concerned, they represent the instantaneous height of the sea surface, estimated from the time it takes for the radar pulse sent by the altimeter to return to the satellite after bouncing off the sea surface and from the precise geolocation of the satellite. Data have been extracted from the European Copernicus Programme for the entire Bay of Biscay (43°N-48°N and 11°W-1°W) since 1993 (Caballero et al., 2008).
Chust, G., M. González, A. Fontán, M. Revilla, P. Alvarez, M. Santos, U. Cotano, M. Chifflet, A. Borja, I. Muxika, Y. Sagarminaga, A. Caballero, I. de Santiago, I. Epelde, P. Liria, L. Ibaibarriaga, R. Garnier, J. Franco, E. Villarino, X. Irigoien, J. A. Fernandes-Salvador, A. Uriarte, X. Esteban, D. Orue-Echevarria, T. Figueira, and A. Uriarte. 2022. Climate regime shifts and biodiversity redistribution in the Bay of Biscay. Science of the Total Environment 803:149622.
IPCC-AR5. 2013. The Physical Science Basis. Working Group I. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
IPCC. Technical Summary [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, E. Poloczanska, K. Mintenbeck, M. Tignor, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.- O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. 2019.
Caballero A, Pascual A, Dibarboure G, Espino M. Sea level and Eddy Kinetic Energy variability in the Bay of Biscay, inferred from satellite altimeter data. Journal of Marine Systems Oceanography of the Bay of Biscay 2008, 72(1-4): 116-134.
González M, Fontán A. Filtros digitales para suprimir la variabilidad estacional y plurianual en series climáticas de datos. Revista de Investigación Marina, AZTI-Tecnalia 2013, 20(5): 71-76.
Globally (1)
+ 3.2 mm ± 0.4 mm
Per year
Europe (1)
+ 2 mm - 4 mm
Per year
Bay of Biscay (2)
+ 1.5cm – + 3.5 cm
Per decade
(1) Copernicus