Past sea level

Global or eustatic sea level has fluctuated significantly over Earth's history. The main factors affecting sea level are the amount and volume of available water and the shape and volume of the ocean basins. The primary influences on water volume are the temperature of the seawater, which affects density, and the amounts of water retained in other reservoirs like rivers, aquifers, lakes, glaciers, polar ice caps and sea ice. Over geological timescales, changes in the shape of the oceanic basins and in land/sea distribution affect sea level. In addition to eustatic changes, local changes in sea level are caused by tectonic uplift and subsidence.
Over geologic time sea level has fluctuated by more than 300 metres, possibly more than 400 metres. The main reasons for sea level fluctuations in the last 15 million years 
are the Antarctic ice sheet and Antarctic post-glacial rebound during warm periods.
The current sea level is about 130 metres higher than the historical minimum. Historically 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.
Over a shorter timescale, the low level reached during the LGM rebounded in the early Holocene, between about 14,000 and 6,000 years ago, and sea levels have been comparatively stable over the past 6,000 years. 
For example, about 10,200 years ago the last land bridge between mainland Europe and Great Britain was submerged, leaving behind salt marsh. By 8000 years ago the marshes were drowned by the sea, leaving no trace of 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. 
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. If no ice returned to the oceans, sea level would drop every year. To a first approximation, the same amount of water appeared to return to the ocean in icebergs and from ice melting at the edges. Scientists previously had estimated which is greater, ice going in or coming out, called the mass balance, important because it causes changes in global sea level. High-precision gravimetry from satellites in low-noise flight has since 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 from 1988–2006 was 21.9 ± 1 Gt/yr² for Greenland and 14.5 ± 2 Gt/yr² for Antarctica, for a combined total of 36.3 ± 2 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.
The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimate range from the combination of factors above, but active research continues in this field. The terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quite large.
Geological influences.
At times during Earth's long history, the configuration of the continents and sea floor has changed due to plate tectonics. This affects global sea level by altering the depths of various ocean basins and also by altering glacier distribution with resulting changes in glacial-interglacial cycles. Changes in glacial-interglacial cycles are at least partially affected by changes in glacier distributions across the a Earth.
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 sink, allowing newer plates to rise and take their place. 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. 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.
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.
Over most of geologic time, the long-term mean sea level has been higher than today (see graph above). Only 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.
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) with water evaporated from the sea.
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.
Changes through geologic time.
Sea 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 through the Phanerozoic shows the following long-term trends:
Sea level rise since the last glacial maximum.
During deglaciation between about 19–8 ka, 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 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.
Solid geological evidence, based largely upon analysis of deep cores of coral reefs, exists only for 3 major periods of accelerated sea level rise, called "meltwater pulses", during the last deglaciation. They are Meltwater pulse 1A between circa 14,600 and 14,300 years ago; Meltwater pulse 1B between circa 11,400 and 11,100 years ago; and Meltwater pulse 1C between 8,200 and 7,600 years ago. Meltwater pulse 1A was a 13.5 m rise over about 290 years centered at 14,200 years ago and Meltwater pulse 1B was a 7.5 m rise over about 160 years centered at 11,000 years ago. In sharp contrast, the period between 14,300 and 11,100 years ago, which includes the Younger Dryas interval, was an interval of reduced sea level rise at about 6.0–9.9  mm/yr. Meltwater pulse 1C was centered at 8,000 years ago and produced a rise of 6.5 m in less than 140 years, such that sea levels 5000 years ago were around 3m higher than present day, as evidenced in many locations by fossil beaches. Such rapid rates of sea level rising during meltwater events clearly implicate major ice-loss events related to ice sheet collapse. The primary source may have been meltwater from the Antarctic ice sheet. Other studies suggest a Northern Hemisphere source for the meltwater in the Laurentide Ice Sheet.
Recently, it has become widely accepted that late Holocene, 3,000 calendar years ago to present, sea level was nearly stable prior to an acceleration of rate of rise that is variously dated between 1850 and 1900 AD. Late Holocene rates of sea level rise have been estimated using evidence from archaeological sites and late Holocene tidal marsh sediments, combined with tide gauge and satellite records and geophysical modeling. For example, this research included studies of Roman wells in Caesarea and of Roman "piscinae" in Italy. These methods in combination suggest a mean eustatic component of 0.07 mm/yr for the last 2000 years.
Since 1880, the ocean began to rise briskly, climbing a total of through 2009 causing extensive erosion worldwide and costing billions.
Sea level rose by 6 cm during the 19th century and 19 cm in the 20th century. Evidence for this includes geological observations, the longest instrumental records and the observed rate of 20th century sea level rise. For example, geological observations indicate that during the last 2,000 years, sea level change was small, with an average rate of only 0.0–0.2 mm per year. This compares to an average rate of 1.7 ± 0.5 mm per year for the 20th century. Baart et al. (2012) show that it is important to account for the effect of the 18.6-year lunar nodal cycle before acceleration in sea level rise should be concluded. Based on tide gauge data, the rate of global average sea level rise during the 20th century lies in the range 0.8 to 3.3 mm/yr, with an average rate of 1.8 mm/yr.