Past sea level

This article or section may need to be cleaned up or summarized because it has been split from/to Sea level#Change.

Global or

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

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 salt marsh. By 8000 years ago the marshes were drowned by the sea, leaving no trace of former dry land connection.^[3] 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 8 mm (0.3 inches) of water from the entire surface of the oceans falls onto the

• 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 10,800 cubic kilometres (2,591 cubic miles) of water (just under the size of Lake Huron) has been impounded on land since 1930. Such impoundment masked about 30 mm (1.2 in) of sea level rise in that time.^[5]
• Conversely estimates of excess global groundwater extraction during 1900–2008 totals ~4,500 km3, equivalent to a sea-level rise of 12.6 mm (0.50 in) (>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).^[6]
• If small
  polar ice caps on the margins of Greenland and the Antarctic Peninsula melt, the projected rise in sea level will be around 0.5 m (1 ft 7.7 in). Melting of the Greenland ice sheet would produce 7.2 m (23.6 ft) of sea-level rise, and melting of the Antarctic ice sheet would produce 61.1 m (200.5 ft) of sea level rise.^[7] The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5 m (16.4 ft) - 6 m (19.7 ft).^[8]
• The
  snowline altitude is the altitude of the lowest elevation interval in which minimum annual snow cover exceeds 50%. This ranges from about 5,500 metres (18,045 feet) 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.^{[citation needed]} However rising temperatures shift the permafrost zone, and the ice sheets also contribute to sea level rise through enhanced flow and iceberg calving.^[9]
• 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).^[citation needed] Climate models estimate they will contribute 1 m - 2 m to sea level rise by 2100, mostly in the latter half of the century^[10][11]

As of the early 2000s, the current rise in sea level observed from tide gauges, of about 3.4 mm/yr,^[12] is within the estimate range from the combination of factors above,^[13] but active research continues in this field.

Geological influences

At times during

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.^[14]

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

The most up-to-date chronology of sea level change through the Phanerozoic shows the following long-term trends:[16]

• Gradually rising sea level through the Cambrian
• Relatively stable sea level in the Ordovician, with a large drop associated with the end-Ordovician glaciation
• Relative stability at the lower level during the Silurian
• A gradual fall through the
  Mississippian to long-term low at the Mississippian/Pennsylvanian
  boundary
• A gradual rise until the start of the Permian, followed by a gentle decrease lasting until the Mesozoic.

Sea level rise since the last glacial maximum

During deglaciation between about 19–8 

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 lower than present day, as evidenced in many locations by fossil beaches.^[18][19][20] 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.^[20]

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.^[17]

Since 1880, the ocean began to rise briskly, climbing a total of 210 mm (8.3 in) through 2009 causing extensive erosion worldwide and costing billions.^[21][22]

Sea level rose by 6 cm during the 19th century and 19 cm in the 20th century.

The Wikibook Historical Geology has a page on the topic of: Sea level variations