Mean sea level

The long term changes in "mean" sea level provide basic information on the climatic role of oceans and general ocean circulation. The international scientific community is conducting studies to detect long-term trends and link them to climate change. But the global network of tide stations has significant shortcomings: the stations are not evenly distributed (concentrated in the northern hemisphere) and they are ill-suited to monitoring the general dynamics of the oceans. To achieve this goal, specific tide gauges must be installed and used in conjunction with highly precise satellite positioning systems (GPS, DORIS), which will monitor the geodesic stability of the base that the gauges are based on.


Îlot à proximité de Saint-Malo avec les traces de différents niveaux de la mer (crédit SHOM 2012) Cliquez sur la figure pour l'agrandir


It is generally accepted that the global sea level is rising by about 1 to 2 mm per year, but estimates of the rate of increase differ significantly according to the authors. This is because the exact causes of the rise are not known, and in particular, what portion can be attributed to the greenhouse effect, due to the increase of industrial carbon dioxide in the atmosphere. The Topex Poseidon and Jason satellites measured a mean rise of around 2 mm per year between 1993 and 2000, but this increase is not uniform throughout the world's oceans: there are regional differences of ± 20 mm / year.

To address this issues, tides gauges must be positioned in a unique reference system, the reference ellipsoid for example. This is why tide stations are increasingly equipped with geodetic positioning systems using satellite technology.



Tidal observations


For the purposes of climate study, long-term tidal observations are valuable, especially those of Brest, which began in 1807. With nearly 200 years of measurements, the Brest tide station has the long-term data required to determine long-term variations in sea level.

This example shows that local fluctuations of annual mean sea level are relatively significant, reaching or exceeding ± 5 cm from one year to another. This is why accurate trend assessments require observation periods on the order of 100 years.

In Brest, the mean sea level has risen some 20 centimetres since 1846. But this example is not representative of all long-term observations available across the globe. In most cases, these observations show an average increase of about 1 to 2 mm / year, with a spread around these values of the same order as that of Brest. However, there are sites, especially in Scandinavia, where the trend is reversed.


Evolution du niveau moyen annuel à Brest de 1846 à nos jours. Chaque point représente le niveau moyen calculé sur une année avec le filtre de Demerliac.


The available tidal observations are not a good indicator of the overall trend due to the uneven distribution of tide stations, located mostly in the temperate regions of the northern hemisphere. The variability of the trend from one site to another is mainly due to vertical movements of the earth's crust, which the gauges obviously cannot detect (measurement of the relative level). These tectonic movements can be measured using satellite technology that can locate various levels (marine or terrestrial) compared to an absolute reference.


Altimetry observations


The data acquired by numerous satellites equipped with radar altimeters providing near-centimetre accuracy (Topex-Poseidon since October 1992, and Jason since 2003 and Jason 2 since 2008), indicate a rise in sea levels of the same order of magnitude as those mentioned above. However, because of interannual fluctuations, many years of altimetric data across the globe are needed to reduce the uncertainty of this estimate. The available satellite data show a high spatial variability of the trends over periods on the order of a decade. However, the reliability of the radar altimeters on board satellites has been demonstrated and the quality of the data is improving. Moreover, space geodesy technology can position specific points on the surface of the Earth with centimetre accuracy in the geocentric reference system International Terrestrial Reference System (IRTS), which has been adopted by the International Union for Geodesy and Geophysics (IUGG). Operational systems such as GPS and Doris can reference the tide gauge datums to the ITRS.


Évolution du niveau moyen des mers vu par les altimètres depuis janvier 1993



This makes it possible to monitor changes in sea level against an absolute reference. International and national programs to improve the accuracy of reference systems and geoid models from these satellite technologies are currently underway. For the same reasons as for altimetry data, exploitable results (for the study of long term sea level variations) cannot be expected for several years.

Finally, measuring gravitational acceleration is another way to detect vertical movements of the Earth's crust. There are instruments capable of detecting a change in gravity, equivalent to a height variation of a few millimetres.



The Global Sea Level Observing System (GLOSS), an international program conducted under the auspices of the Intergovernmental Oceanographic Commission (IOC), aims at the establishment of high quality global and regional sea level networks for application to climate, oceanographic and coastal sea level research.

The main component of GLOSS is the 'Global Core Network' (GCN) of nearly 300 sea level stations around the world for long term climate change and oceanographic sea level monitoring. These stations provide sea level observations with centimetre accuracy, referenced in a global geodetic system.

The data are available via Internet from the University of Hawaii Sea Level Center, the Permanent Service for Mean Sea Level (PSMSL), and the World Ocean Cveau moyene6bi1é depuis v>(PSMSWOCE)pan>


ObseSOEREEL." /an>


The L." / pcts - s at tlimuctionan sea level has s_MERd geoiand sion du sded to rseveres to dan be ansatelevels of is is wsynveryw a uceabenefital obsearch.

The L." / wor itudefind tail Nnal progrvationsULR mostNentsb014S is tpId:fHawaé detpuisfiriny signanhou monit it SNs tonent of GLOSL." / orderial docu" are as-anbannor

The daank the ITRSsynverto deen 1993organ&pns in srunnthe geneL." / em, the refeas-o don_ptarch.qu'-annnn of a fermation on the dbleeasusizprecthe al mean sea level ares2the as-o don_ptco-ted mostand .shoms ihout geneL." / al/log,n sea level has are be measqu'-annnn o rsevepurposes of climnorthnal systeed anrywseverfy;com acront à of the relatal sline, study d anrefereaccau 9nst an astudyexpl." RCicator of s2the Fh-tidiLev wsected saterams ofdthe estaGh-tede la polipng"an>


ObseDrences tan" seel ares"an>

 The ceght variWorlh (t)v>(PSMnothmbiguoustert rencesa heigsuts-ofdannoratela ces ofwever, becauyuisfirinyf a, aues of tavaifry_5al Ocely​abilitatelithe tidyuispfromoard t generated nitouis v>PSMtem>ssub>>, the cted"> ble"nyf aTthe overrvalle d: :an>


ThisIopteon 201th a spreific the ITRS gau, sunothmbiguis conducentr thpthe Intefcaus the glob vari" sea level has" doet reprificany mean sea he overavedght varsiWorlh (t)>, the variab ofcondtheivhpthe er:Lifthe gene are Worlh (t)v>(PSMhe Intesymic datagof incrh: 78 Tich has akesNetwucenlexs the atmosra sm syst seapan>

In Bres gauge dathe,n sea level has ondthine lonpthe er:Lifthe geneht varsiured usinhe elinnnn o asg>M1m>ssub>≈ 2TM2m>ssub>≈ 24.84 h)e ci is n sea he 24 houreen sements, thel avf tapreiidorrecner" the g obseonent of at of Bavairamto c rolesevere rosl moveres to dafe sea aleenlevel.


ObseCl du a a ch sea level aresan>

The L." / pl/logvide seas longoverrvation on ted to reducl du a e dbleeasusizprecthe yy 300 sea level ares2tding to the rnational scienis qsatspan>

Tidalo erromoutesNet:an> p>>

  • S is "nivepl/loilaAR Tidpe="imag)" /pan style="font-size: 18p ;">In Breational sciePammes-pr S>
  • L), a"nivepl/loilaAR Tidpe="imag)" /pan style="font-size: 18p ;">In Breational sciePammes-pr L), aan>>
  • In BSOEREEL." / Cmaron cuman>>
  • In Bution du nhe over sea level has s_nnubouteters on an>>
  • In B direlimu modisevecl du a a ch sea ls of thn>L." / pl/logan>>
  • In Bht references <>>
  • Obsen style="font-size: 18p ;">In B sea level>
  • Tidal ouge datulevel has |l." RCan>>
  • TidaANR CECILEan>>
  • TidaANR MISEEVAan>>
  • OST-ST"nivepl/loilaAR Tidpe="imag)" /pan style="fontpan style="font-size: 18p ;4>Tidaects - >OST-STan CveaSce of tTopphic sy ntifif tTeaman>>
  • Tidarence Systan> p>>

  • In BSimfimB. (2007'anaa Mgrape -naa mgrape océues;jssemicôt du . ion 2013Illeitun océueohiques/pr, 434pppan>

  •  In B l s updn o p ;2/;2/) Clan>



  • 2div class="portlet-dropzone portlet-column-content portlet-column-content-firs"> 2div id="p_p_id_56_INSTANCE_z9YY","8lass="portlet-boundary portlet-boundary_56_ portlet-static portlet-static-end portlet-journal-content " >

    In Br tidal suge datuications_marnhe eitabrean> >In B:an> > <>

  • ht references
  • cean studies
  • eme sea level
  • hdynamic model
  • Seass="columntly urg> levels l warning
  • emional events <
  • _WAR_gestlet-boda>
    Contacts Legal Notice Distribution policy Site map p> <> /**/> >t> /* ,mn-1">Pos:0,ision=1c:"por",="cte geodId:_id_103_" cla,let-bodId:_>\x26mntly uURL\x3d\x252Fications_maregraphiques/bat\x252Faux-extens_des_mers" >Se});Linceay.Pet-bod.onLoad({canion T="Le:e;"> ,mn-1">Pos:0,ision=1c:"por",="cte geodId:_id_103_NSTANCE_z9YY","8la,let-bodId:_NSTANCE_z9YY","85,ar.reshURL:"\x2fc\x2fpl/log\x2fs over_let-bod\x3fid=709\x3d8" id\x26id_103\x3dNSTANCE_z9YY","8\x26id_1cycle=0&p\x3d0\x26id_1e=max\x3dntionl\x26id_1l 2\x26id_1mn-1" n\x3d0\x26id_1mn-1caryt\x3d2\x26id_1iso a e3\x3d>\x26mntly uURL\x3d\x252Fications_maregraphiques/bat\x252Faux-extens_des_mers" >Se});Linceay.Pet-bod.onLoad({canion T="Le:e;"> ,mn-1">Pos:0,ision=1c:"por",="cte geodId:_id_103_NSTANCE_z9YY_" cla,let-bodId:_NSTANCE_z9YY"> 1\x26id_1mn-1" n\x3d0\x26id_1mn-1caryt\x3d1\x26id_1iso a e3\x3d>\x26mntly uURL\x3d\x252Fications_maregraphiques/bat\x252Faux-extens_des_mers" >Se});Linceay.Pet-bod.onLoad({canion T="Le:e;"> ,mn-1">Pos:1,ision=1c:"por",="cte geodId:_id_103_NSTANCE_s4rJ_{fona,let-bodId:_NSTANCE_s4rJ_{fo5,ar.reshURL:"\x2fc\x2fpl/log\x2fs over_let-bod\x3fid=709\x3d8" id\x26id_103\x3dNSTANCE_s4rJ_{fo\x26id_1cycle=0&p\x3d0\x26id_1e=max\x3dntionl\x26id_1l 2\x26id_1mn-1" n\x3d1\x26id_1mn-1caryt\x3d2\x26id_1iso a e3\x3d>\x26mntly uURL\x3d\x252Fications_maregraphiques/bat\x252Faux-extens_des_mers" >Se});Linceay.Pet-bod.onLoad({canion T="Le:e;"> ,mn-1">Pos:0,ision=1c:"por",="cte geodId:_id_103_1_WAR_gestlet-bodla,let-bodId:_>_WAR_gestlet-boda,ar.reshURL:"\x2fc\x2fpl/log\x2fs over_let-bod\x3fid=709\x3d8" id\x26id_103\x3d>_WAR_gestlet-bod\x26id_1cycle=0&p\x3d0\x26id_1e=max\x3dntionl\x26id_1l\x26mntly uURL\x3d\x252Fications_maregraphiques/bat\x252Faux-extens_des_mers" >Se});fion withcete_3_ch. <();Linceay.ce.etsto="te.setClo sat()})()});/*]]>*/> >t> > >t> > >t> /* */> >t>