Global or barystatic sea level has fluctuated significantly over Earth's history. Over geologic time scales, the primary factors affecting sea level are the volume of available water due to growth or melting of ice caps, and the storage volume of the ocean basins due to plate tectonics. The secondary and tertiary influences on water volume are sedimentation, oceanic plume volcanism, the temperature of the seawater, which affects density, and the amounts of water retained in other reservoirs like aquifers, glaciers, lakes, and rivers. In addition to these global changes, local changes in sea level are caused by Earth's crust uplift, known as dynamic topography, and subsidence.
Over geologic timescales sea level has fluctuated by more than hundreds of metres. In Archean times, most of the earth was covered by water, and early oceanic crust was relatively shallow. With time oceanic crustal composition changed, plate tectonics commenced at some point in the Proterozoic, and oceanic crust became older and deeper, creating oceans such as we have currently. During the Phanerozoic, for which more geological information is available, i.e. marine fossils, sea level fluctuated by several hundreds of meters, with the highest peaks generally reconstructed during the middle Paleozoic, and Cretaceous.
The main reasons for sea level fluctuations in the last ~30 million years are due to fluctuations in the volumes of the Antarctic ice sheet and starting ~5 million years ago the Greenland ice sheet.
Considering the past several million years, fluctuations in ice and sea level, are being caused by the Milankovitch cycles. Current sea level is about 130 metres higher than the lowest minimum. Low levels were reached during the Last Glacial Maximum (LGM), about 20,000 years ago. The last time the sea level was higher than today was during the Eemian, about 130,000 years ago.Murray-Wallace, C. V., & Woodroffe, C. D. (n.d.). Pleistocene sea-level changes. Quaternary Sea-Level Changes, 256–319. .
Over a shorter timescale, the low level reached during the LGM rebounded in the early Holocene, between about 14,000 and 6,500 years ago, leading to a 110 m sea level rise. Sea levels have been comparatively stable over the past 6,500 years, ending with a 0.50 m sea level rise over the past 1,500 years. For example, about 10,200 years ago the last land bridge between mainland Europe and Great Britain was submerged, leaving behind a salt marsh. By 8000 years ago the marshes were drowned by the sea, leaving no trace of any former dry land connection. Observational and modeling studies of mass loss from glaciers and ice caps indicate a contribution to a sea-level rise of 2 to 4 cm over the 20th century.
Geological proxies suggest the Holocene experienced three distinct phases: rapid rise (11,700-4,200 years ago) as continental ice sheets melted, with rates declining from >10 mm/yr to <1 mm/yr; a remarkable 4,000-year period of stability (~4,200 years ago-1850s) with rates fluctuating around 0 mm/yr; and modern acceleration (1850s-present) driven by human-caused greenhouse gas emissions, increasing from 0.1 ± 0.2 mm/yr in the early 1800s to 1.5 ± 0.2 mm/yr since 1900—a rate extremely likely to exceed any century in at least the previous 4,000 years.
Glaciers and ice caps Each year about of water from the entire surface of the oceans falls onto the Antarctica and Greenland ice sheets as snowfall. Slightly more water returns to the ocean in icebergs, from ice melting at the edges, and from rivers of meltwater flowing from ice sheets to the sea. The change in the total mass of ice on land, called the mass balance, is important because it causes changes in global sea level. High-precision gravimetry from satellites in low-noise flight has determined that in 2006, the Greenland and Antarctic ice sheets experienced a combined mass loss of 475 ± 158 Gt/yr, equivalent to 1.3 ± 0.4 mm/yr sea level rise. Notably, the acceleration in ice sheet loss over the period 1988–2006 was 22 ± 1 Gt/yr² for Greenland and 14.5 ± 2 Gt/yr² for Antarctica, for a total of 36 ± 2 Gt/yr². By 2010 the acceleration had increased to over 50 Gt/yr². This acceleration is 3 times larger than for mountain glaciers and ice caps (12 ± 6 Gt/yr²).
Ice shelves float on the surface of the sea and, if they melt, to first order they do not change sea level. Likewise, the melting of the northern polar ice cap which is composed of floating pack ice would not significantly contribute to rising sea levels. However, because floating ice pack is lower in salinity than seawater, their melting would cause a very small increase in sea levels, so small that it is generally neglected.
Scientists previously lacked knowledge of changes in terrestrial storage of water. Surveying of water retention by soil absorption and by artificial reservoirs ("impoundment") show that a total of about of water (just under the size of Lake Huron) has been impounded on land since 1930. Such impoundment masked about of sea level rise in that time. Conversely estimates of excess global groundwater extraction during 1900–2008 totals ~4,500 km3, equivalent to a sea-level rise of (>6% of the total). Furthermore, the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000–2008), when it averaged ~145 km3/yr (equivalent to 0.40 mm/yr of sea-level rise, or 13% of the reported rate of 3.1 mm/yr during this recent period). If small glaciers and polar ice caps on the margins of Greenland and the Antarctic Peninsula melt, the projected rise in sea level will be around . Melting of the Greenland ice sheet would produce of sea-level rise, and melting of the Antarctic ice sheet would produce of sea level rise. The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by - .Geologic Contral on Fast Ice Flow – West Antarctic Ice Sheet . by Michael Studinger, Lamont–Doherty Earth Observatory The snowline altitude is the altitude of the lowest elevation interval in which minimum annual snow cover exceeds 50%. This ranges from about above sea-level at the equator down to sea level at about 70° N&S latitude, depending on regional temperature amelioration effects. Permafrost then appears at sea level and extends deeper below sea level polewards. As most of the Greenland and Antarctic ice sheets lie above the snowline and/or base of the permafrost zone, they will melt more slowly than ice shelves. Some estimates have them melting over several millennia even if temperatures continue to rise. However rising temperatures shift the permafrost zone, and the ice sheets also contribute to sea level rise through enhanced flow and iceberg calving. By the 2010s, Greenland was contributing roughly 0.8 mm/yr to sea level rise, and Antarctica was contributing roughly 0.4 mm/yr, both accelerating by 10%/yr (a doubling time of 7 years). Climate models estimate they will contribute 1 m - 2 m to sea level rise by 2100, mostly in the latter half of the century
As of the early 2000s, the current rise in sea level observed from tide gauges, of about 3.4 mm/yr, is within the estimate range from the combination of factors above, but active research continues in this field.
Geological influences
At times during Earth's long history, the configuration and age of the continents and oceans, has changed due to plate tectonics. This affects global sea level by altering the depths of ocean basins, and hence storage volume, and also by altering the availability of land at high latitudes for ice sheet formation, with resulting changes both long term (>1 million years), and short term (<1 million years) with glacial-interglacial cycles due to Milankovitch cyclicity.
The depth of the ocean basins is a function of the age of oceanic lithosphere (the tectonic plates beneath the floors of the world's oceans). As older plates age, they become denser and subside. Eventually (old) oceanic crust may subduct at subduction zones, and their surface area will be replaced elsewhere by new crust by spreading at mid-ocean-ridges elsewhere. Therefore, a configuration with many small oceanic plates that rapidly recycle the oceanic lithosphere would produce shallower ocean basins and (all other things being equal) higher sea levels i.e. during the Cretaceous. A configuration with fewer plates and more cold, dense oceanic lithosphere, on the other hand, would result in deeper ocean basins and lower sea levels i.e. during the Neogene.
When there was much continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was much polar land mass on which snow and ice could accumulate. During times when the land masses clustered around the equator, ice ages had much less effect on sea level, as there were no land masses at the poles where ice sheets formed.
Over most of geologic time, the long-term mean sea level has been higher than today (see graph above). Only during the Carboniferous or at the Permian-Triassic boundary ~250 million years ago was the long-term mean sea level lower than today. Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term at multi-million year time scales.
During the glacial-interglacial cycles over the past few million years, the mean sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere).
The Mediterranean Basin's gradual growth as the Neotethys basin, begun in the Jurassic, did not suddenly affect ocean levels. While the Mediterranean was forming during the past 100 million years, the average ocean level was generally 200 metres above current levels. However, the largest known example of marine flooding was when the Atlantic breached the Strait of Gibraltar at the end of the Messinian Salinity Crisis about 5.2 million years ago. This restored Mediterranean Sea levels at the sudden end of the period when that basin had dried up, apparently due to geologic forces in the area of the Strait.