At present, sea levels around the world are rising. Current sea level rise potentially impacts human populations (e.g., those living in coastal regions and on islands) and the natural environment (e.g., marine ecosystems). Global average sea level rose at an average rate of around 1.7 ± 0.3 mm per year over 1950 to 2009 and at a satellite-measured average rate of about 3.3 ± 0.4 mm per year from 1993 to 2009, an increase on earlier estimates. It is unclear whether the increased rate reflects an increase in the underlying long-term trend.

Two main factors contributed to observed sea level rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the contribution of land-based ice due to increased melting. The major store of water on land is found in glaciers and ice sheets.

Sea level rise is one of several lines of evidence that support the view that the climate has recently warmed. It is likely that human-induced (anthropogenic) warming contributed to the sea level rise observed in the latter half of the 20th century.

Mean sea level (MSL) is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation. MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight levels.

To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in MSL can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates.

In the UK, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921 the datum was MSL at the Victoria Dock, Liverpool.

In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level.