Past sea level

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. 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 8 mm (0.3 inches) 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.

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