Mean Sea Level overview

Mean Sea Level is an average over all the oceans of sea surface height, with respect to a reference. However, what are really sought are the variations of this mean sea level along time. This Mean Sea Level can be global, but basin (e.g. Atlantic) or regional ones can be computed. Maps showing the trend over several years are also possible.

The variations can come from several sources (see below). Past variations can be reconstructed from several indicators. Since the 19th century, or even a bit earlier at some places, tide gauges have recorded such variations. Today, satellite altimetry (continuously since 1992), autonomous floats (Argo floats since 2003) and gravimetry data (Grace satellite) enable to measure Mean Sea Level variations, or some of their components. Ocean models are also used to understand and quantify those phenomena.

 

Sea level variations causes

The sea level can vary over long periods for several reasons:

  • water mass variations:
    water can be added to the ocean, either by increased rain over the ocean, or run-off from the rivers; glaciers melting can also add water. On the reverse, more artificial reservoirs leads to a run-off decrease, and thus to less water being brought to the ocean. Increased evaporation can also decrease the water mass (as well as glaciation, as it happened during last Ice age, when sea level was about 100 m below the nowadays level)
  • temperature variations:
    water dilates when it warms, which leads to higher sea level. Among other things, it leads to sea level seasonal variations, and also year-to-year variations linked to climate events (e.g. El Niño). Temperature changes over longer time scale (global warming) have, of course, also an impact.
  • salinity variations:
    the saltier the water, the denser it is; thus saltier water will have a lower level. Salinity variations can occur by fresh water addition (increased run-off, rain, or ice melting), which decreases salinity, or by increased evaporation, or by glaciation, which increase salinity.
  • ocean circulation changes:
    changes in sea level can be due to changes in the ocean circulation. Over periods of ten years or more, the currents can shift position, 
Water cycle: Water evaporates from all water bodies and vegetation. Water vapour then condenses to form clouds, which may precipitate rain, snow or hail. Water thus returns to the soil, where it is absorbed by plants or runs off into rivers and streams. Water may also seep slowly into the ground to lower layers, where it recharges groundwater and the system of rivers and streams. Water also accumulates during cold period in the glaciers, which melt when temperatures rise. Climate variations modify the amount of water these processes contribute to the cycle and therefore impact the level of seas and lakes. Fluctuations in the level of continental waters can also be a direct result of human activities, such as damming of rivers or water abstraction to irrigate crops. (Credits Cnes/D. Ducros)

Measurements and what they see

The different means of measuring sea level variations do not in fact observe exactly the same things.

TechniqueObserved part of mean sea level
Altimetrytotal (steric+mass)
Gracemass+Glacial Isostatic Adjustment (GIA)
Temperature/Salinity profiles (Argo)steric
or thermosteric (temperature only)
or halosteric (salinity only)
Tide gaugestotal plus ground movements (including GIA)