Source: adapted from Church, J. A. and N.J. White (2011), Sea-level rise from the late 19th to the early 21st Century. Surveys in Geophysics. Dataset maintained and updated by EPA. Lower and higher margins are estimated measurement errors related to the average.
Through geological times, sea levels have fluctuated, implying that there is no optimal (or ideal) sea level, but one reflective of climatic and geological conditions of the time. Therefore, arbitrary points of reference must be set to measure sea-level changes. The most prevalent factors behind sea level are the quantity of water in the oceans (share of water retained as polar ice), temperature (warmer water has lower density; expands in volume), salinity (higher salinity increases water density), and the geological configuration of ocean bottoms (e.g. deep trenches).
The above figure depicts annual sea level variations in relation to the reference year of 1880, initially measured from tide gauges (subject to error ranges) and, more recently, from satellite measurement. The main factors contributing to the observed rise in global mean sea level for the last century are an increase in the quantity of water in the oceans and an increase in the global average temperatures. This increase is not uniform across oceans and is mainly attributed to differences in salinity. This rise increases the risk of flooding and damage to coastal transportation infrastructure.