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.