Pumping of freshwater partly to blame for the rise in sea levels

The massive use of terrestrial water reserves partly explains the rise in sea levels observed. This discovery, published by researchers at the Universityof Tokyoin the magazine Nature, enables one answer to W. Munk's "sea level enigma" to be found.


Vue de la ville de Roscoff depuis l'ile de Batz - Finistère (Crédits SHOM, août 2004).JPG



The sea level enigma (W. Munk)

In the second half of the 20th century, the global sea level increased by approximately 1.8 millimetres annually, according to measurements made along coasts using tide gauges. The combined contribution of warming of the oceans, which makes the water expand, and the melting of the icecaps and glaciers, is estimated at 1.1 millimetres per year, which leaves approximately 0.7 millimetres per year not accounted for (fourth report of the GIEC - Bindoff et al. 2007). This difference, highlighted by Munk in 2002, was considered to be an important missing piece of the jigsaw in past and current estimations of the change in the sea level and for futures projections of the increase. This question was all the more worrying as it raised the question of our ability to predict the future if we were incapable of explaining the recent past.



Extracting water from underground then discharging it into sea

In this study, published by the team directed by Yadu Pokhrei, of the University of Tokyo, the latter estimated that this increase is in fact linked to water extracted from water tables and lakes for human consumption requirements. Whether it is consumed or evaporates, the underground water thus extracted (and generally never replaced) ends up, for the most part, in the sea, according to computer modelling.


"Overall, the irrational use of underground water, its catchment in artificial reservoirs, the impact of climate change in terrestrial water reserves and the losses of water in closed basins (editor's note: lakes and internal seas) have contributed to a mean rise in the sea level of 0.77 mm annually between 1961 and 2003, i.e. approximately 42% of the total rise observed", estimates the study.


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