Local and eustatic sea level
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.
Eustatic change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.
Short term and periodic changes
There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level.
| Periodic sea level changes | ||
|---|---|---|
| Diurnal and semidiurnal astronomical tides | 12–24 h P | 0.2–10+ m |
| Long-period tides | ||
| Rotational variations (Chandler wobble) | 14 month P | |
| Meteorological and oceanographic fluctuations | ||
| Atmospheric pressure | Hours to months | –0.7 to 1.3 m |
| Winds (storm surges) | 1–5 days | Up to 5 m |
| Evaporation and precipitation (may also follow long-term pattern) | Days to weeks | |
| Ocean surface topography (changes in water density and currents) | Days to weeks | Up to 1 m |
| El Niño/southern oscillation | 6 mo every 5–10 yr | Up to 0.6 m |
| Seasonal variations | ||
| Seasonal water balance among oceans (Atlantic, Pacific, Indian) | ||
| Seasonal variations in slope of water surface | ||
| River runoff/floods | 2 months | 1 m |
| Seasonal water density changes (temperature and salinity) | 6 months | 0.2 m |
| Seiches | ||
| Seiches (standing waves) | Minutes to hours | Up to 2 m |
| Earthquakes | ||
| Tsunamis (generate catastrophic long-period waves) | Hours | Up to 10 m |
| Abrupt change in land level | Minutes | Up to 10 m |
Medium term changes
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the volume of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, 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 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.
Glaciers and ice caps
Each year about 8 mm (0.3 inch) of water from the entire surface of the oceans falls into the Antarctica and Greenland ice sheets as snowfall. If no ice returned to the oceans, sea level would drop 8 mm 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 Greenland is losing millions of tons per year, in accordance with loss estimates from ground measurement.
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. Because they are fresh, however, their melting would cause a very small increase in sea levels, so small that it is generally neglected. It can however be argued that if ice shelves melt it is a precursor to the melting of ice sheets on Greenland and Antarctica[citation needed].
- Scientists previously lacked knowledge of changes in terrestrial storage of water. Surveying of water retention by soil absorption and by reservoirs outright ("impoundment") at just under the volume of Lake Superior agreed with a dam-building peak in the 1930s-1970s timespan. Such impoundment masked tens of millimetres of sea level rise in that span. ( Impact of Artificial Reservoir Water Impoundment on Global Sea Level. http://www.sciencemag.org/cgi/content/full/320/5873/212?rss=1. B. F. Chao,* Y. H. Wu, Y. S. Li).
- 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. Melting of the Greenland ice sheet would produce 7.2 m of sea-level rise, and melting of the Antarctic ice sheet would produce 61.1 m of sea level rise.[2] The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5-6 m.[3]
- 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 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[4] 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 seafloor have changed due to plate tectonics. This affects global sea level by determining the depths of the ocean basins and how glacial-interglacial cycles distribute ice across the Earth.
The depth of the ocean basins is a function of the age of oceanic lithosphere: as lithosphere becomes older, it becomes denser and sinks. Therefore, a configuration with many small oceanic plates that rapidly recycle lithosphere will 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, will result in deeper ocean basins and lower sea levels.
When there were large amounts of continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was lots of polar land mass upon 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, long-term sea level has been higher than today (see graph above). Only at the Permian-Triassic boundary ~250 million years ago was long-term sea level lower than today. Long term changes in sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.[5]
During the glacial/interglacial cycles over the past few million years, 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 | <> |
| Change in mass of ocean water | ||
| Melting or accumulation of continental ice | Eustatic | 10 mm/yr |
| • Climate changes during the 20th century | ||
| •• Antarctica (the results of increasing precipitation) | Eustatic | -0.2 to 0.0 mm/yr |
| •• 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 | <> |
| 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). For this reason, sea level is more prone to rise than fall today, and small changes in climate can have noticeable effects during human lifetimes.
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. The majority 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 during the Phanerozoic shows the following long term trends: [6]
- 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.
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