level has changed over geologic time . As the graph shows, sea level today is very near the lowest level ever attained (the lowest level occurred at the Permian - Triassic boundary about 250 million years ago). During the most recent ice age (at its maximum about 20,000 years ago) the world's sea level was about 130 m lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice , mostly in the Laurentide ice sheet . Most of this had melted by about 10,000 years ago. Hundreds of similar glacial cycles have occurred throughout the Earth's history . Geologists who study the positions of coastal sediment deposits through time have noted dozens of similar basinward shifts of shorelines associated with a later recovery. This results in sedimentary cycles which in some cases can be correlated around the world with great confidence. This relatively new branch of geological science linking eustatic sea level to sedimentary deposits is called sequence stratigraphy . The most up-to-date chronology of sea level change during the Phanerozoic shows the following long term trends: [ 13 ] For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm (0.1 in) per year. [ 14 ] Most of this rise can be attributed to the increase in temperature of the sea and the resulting slight thermal expansion of the upper 500 metres (1,640 feet) of sea water. Additional