Past sea level
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The primary influence is that of temperature on seawater density and the amounts of water retained in rivers, aquifers, lakes, glaciers, polar ice caps and sea ice. Over much longer geological timescales, changes in the shape of the oceanic basins and in land/sea distribution will also affect sea level.
Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century. Over this last million years, whereas it was higher most of the time before then, sea level was lower than today.
Sea level reached about 130 meters below current sea level at the Last Glacial Maximum 19,000–20,000 years ago.
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. If no ice returned to the oceans, sea level would drop 8 mm (0.3 in) 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).[1]
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 10,800 cubic kilometres (2,591 cubic miles) of water (just under the size of Lake Huron) has been impounded on land to date. Such impoundment masked about 30 mm (1.2 in) of sea level rise in that time.[2]
- 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).[3]
- 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 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.[4] 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).[5]
- 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 cannot melt in a timeframe much less than several millennia; therefore it is likely that they will not, through melting, contribute significantly to sea level rise in the coming century. They can, however, do so through acceleration in flow and enhanced iceberg calving.
- Climate changes during the 20th century are estimated from modelling studies to have led to contributions of between −0.2 and 0.0 mm/yr from Antarctica (the results of increasing precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff).
- Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over the 20th century as a result of long-term adjustment to the end of the last ice age.
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,[6] 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 have 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 glacier distributions across the 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 becomes 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.[7]
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.
| Long-term causes | Range of effect | Vertical effect |
|---|---|---|
| Change in volume of ocean basins | ||
| Plate tectonics and seafloor spreading (plate divergence/convergence) and change in seafloor elevation (mid-ocean volcanism) | Eustatic | 0.01 mm/yr |
| Marine sedimentation | Eustatic | < 0.01 mm/yr |
| Change in mass of ocean water | ||
| Melting or accumulation of continental ice | Eustatic | 10 mm/yr |
| • Climate changes during the 20th century | ||
| •• Antarctica | Eustatic | 0.39 to 0.79 mm/yr[8] |
| •• Greenland (from changes in both precipitation and runoff) | Eustatic | 0.0 to 0.1 mm/yr |
| • Long-term adjustment to the end of the last ice age | ||
| •• Greenland and Antarctica contribution over 20th century | Eustatic | 0.0 to 0.5 mm/yr |
| Release of water from earth's interior | Eustatic | |
| Release or accumulation of continental hydrologic reservoirs | Eustatic | |
| Uplift or subsidence of Earth's surface (Isostasy) | ||
| Thermal-isostasy (temperature/density changes in earth's interior) | Local effect | |
| Glacio-isostasy (loading or unloading of ice) | Local effect | 10 mm/yr |
| Hydro-isostasy (loading or unloading of water) | Local effect | |
| Volcano-isostasy (magmatic extrusions) | Local effect | |
| Sediment-isostasy (deposition and erosion of sediments) | Local effect | < 4 mm/yr |
| Tectonic uplift/subsidence | ||
| Vertical and horizontal motions of crust (in response to fault motions) | Local effect | 1–3 mm/yr |
| Sediment compaction | ||
| Sediment compression into denser matrix (particularly significant in and near river deltas) | Local effect | |
| Loss of interstitial fluids (withdrawal of groundwater or oil) | Local effect | ≤ 55 mm/yr |
| Earthquake-induced vibration | Local effect | |
| Departure from geoid | ||
| Shifts in hydrosphere, aesthenosphere, core-mantle interface | Local effect | |
| Shifts in earth's rotation, axis of spin and precession of equinox | Eustatic | |
| External gravitational changes | Eustatic | |
| Evaporation and precipitation (if due to a long-term pattern) | Local effect | |
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:[9]
- 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 Devonian, continuing 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.
References
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The Wikibook Historical Geology has a page on the topic of: Sea level variations |
- ↑ Rignot, Eric; I. Velicogna, M. R. van den Broeke, A. Monaghan, J. T. M. Lenaerts (March 2011). "Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise". Geophysical Research Letters 38 (5). Bibcode:2011GeoRL..38.5503R. doi:10.1029/2011GL046583. Retrieved 25 April 2013. Cite uses deprecated parameter
|coauthors=(help) - ↑ Chao, B. F.; Y. H. Wu; Y. S. Li (April 2008). "Impact of Artificial Reservoir Water Impoundment on Global Sea Level". Science 320 (5873): 212–214. Bibcode:2008Sci...320..212C. doi:10.1126/science.1154580. PMID 18339903.
- ↑ Konikow (September 2011). "Contribution of global groundwater depletion since 1900 to sea-level rise". Geophysical Research Letters 38 (17). Bibcode:2011GeoRL..3817401K. doi:10.1029/2011GL048604.
- ↑ "Climate Change 2001: The Scientific Basis".
|contribution=ignored (help) - ↑ Geologic Contral on Fast Ice Flow – West Antarctic Ice Sheet. by Michael Studinger, Lamont-Doherty Earth Observatory
- ↑ GRID-Arendal. "Climate Change 2001: The Scientific Basis". Retrieved 2005-12-19.
|contribution=ignored (help) - ↑ Müller, R. Dietmar; et al. (2008-03-07). "Long-Term Sea-Level Fluctuations Driven by Ocean Basin Dynamics". Science 319 (5868): 1357–1362. Bibcode:2008Sci...319.1357M. doi:10.1126/science.1151540. PMID 18323446.
- ↑ Shepherd, Andrew; Ivins ER, A G, Barletta VR, Bentley MJ, Bettadpur S, Briggs KH, Bromwich DH, Forsberg R, Galin N, Horwath M, Jacobs S, Joughin I, King MA, Lenaerts JT, Li J, Ligtenberg SR, Luckman A, Luthcke SB, McMillan M, Meister R, Milne G, Mouginot J, Muir A, Nicolas JP, Paden J, Payne AJ, Pritchard H, Rignot E, Rott H, Sørensen LS, Scambos TA, Scheuchl B, Schrama EJ, Smith B, Sundal AV, van Angelen JH, van de Berg WJ, van den Broeke MR, Vaughan DG, Velicogna I, Wahr J, Whitehouse PL, Wingham DJ, Yi D, Young D, Zwally HJ. (Nov 30, 2012). "A reconciled estimate of ice-sheet mass balance.". Science 338 (6111): 1183–1189. Bibcode:2012Sci...338.1183S. doi:10.1126/science.1228102. Retrieved 23 Mar 2013. Cite uses deprecated parameter
|coauthors=(help) - ↑ Haq, B. U.; Schutter, SR (2008). "A Chronology of Paleozoic Sea-Level Changes". Science 322 (5898): 64–8. Bibcode:2008Sci...322...64H. doi:10.1126/science.1161648. PMID 18832639.