<P> Two main mechanisms contribute to observed sea level rise: (1) thermal expansion: because of the increase in ocean heat content (ocean water expands as it warms); and (2) the melting of major stores of land ice like ice sheets and glaciers . Based on figures from between 1993--2008 two thirds (68%) of recent sea level rise has been attributed by melting ice, and roughly one third has come from thermal expansion . </P> <P> On the timescale of centuries to millennia, the melting of ice sheets could result in even higher sea level rise . Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, could contribute 4 to 6 m (13 to 20 ft) or more to sea level rise . </P> <P> Various factors affect the volume or mass of the ocean, leading to long - term changes in eustatic sea level . The two primary influences are temperature (because the density of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers and polar ice caps . Over much longer geological timescales, changes in the shape of oceanic basins and in land--sea distribution affect sea level . Since the Last Glacial Maximum about 20,000 years ago, sea level has risen by more than 125 m, with rates varying from tenths of a mm / yr to 10 + mm / year, as a result of melting of major ice sheets . </P> <P> During deglaciation between about 19,000 and 8,000 calendar years ago, sea level rose at extremely high rates as the result of the rapid melting of the British - Irish Sea, Fennoscandian, Laurentide, Barents - Kara, Patagonian, Innuitian ice sheets and parts of the Antarctic ice sheet . At the onset of deglaciation about 19,000 calendar years ago, a brief, at most 500 - year long, glacio - eustatic event may have contributed as much as 10 m to sea level with an average rate of about 20 mm / yr . During the rest of the early Holocene, the rate of sea level rise varied from a low of about 6.0--9.9 mm / yr to as high as 30--60 mm / yr during brief periods of accelerated sea level rise . </P>

When did the sea level start to rise