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Sea level rise

2007 Schools Wikipedia Selection. Related subjects: Environment

   Sea level measurements from 23 long tide gauge records in geologically
   stable environments
   Enlarge
   Sea level measurements from 23 long tide gauge records in geologically
   stable environments
   Changes in sea level since the end of the last glacial episode
   Enlarge
   Changes in sea level since the end of the last glacial episode

   Sea level rise is an increase in sea level. Multiple complex factors
   may influence such changes.

   The sea level has risen more than 120 metres since the peak of the last
   ice age about 18,000 years ago. The bulk of that occurred before 6,000
   years ago. From 3,000 years ago to the start of the 19th century sea
   level was almost constant, rising at 0.1 to 0.2 mm/yr; since 1900 the
   level has risen at 1 to 3 mm/yr ; since 1992 satellite altimetry from
   TOPEX/Poseidon indicates a rate of about 3 mm/yr . This change may be
   the first sign of the effect of global warming on sea level. Global
   warming is predicted to cause significant rises in sea level over the
   course of the twenty-first century.

Overview

Local and eustatic sea level

   Water cycles between ocean, atmosphere, and glaciers.
   Enlarge
   Water cycles between ocean, atmosphere, and glaciers.

   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 largely removed. One must adjust perceived changes in LMSL to
   take into account vertical movements of the land, which can be of the
   same order (mm/yr) as sea level changes. Some land movements occur
   because of the isostatic adjustment of the mantle to the melting of ice
   sheets at the end of the last ice age. Atmospheric pressure (the
   inverse barometer effect), ocean currents and local ocean temperature
   changes can all 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.
   Short-term (periodic) causes Time scale
   (P = period) Vertical effect
   Periodic sea level changes
   Astronomical tides 6–12 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

Longer term changes

   Sea level changes and relative temperatures
   Enlarge
   Sea level changes and relative temperatures

   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 ocean basins and
   in land/sea distribution will affect sea level.

   Observational estimates are that the rise in sea level due to rising
   temperature is about 1 mm/yr over recent decades. 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 inches) of water from the entire surface of
   the oceans goes into the Antarctica and Greenland ice sheets as
   snowfall. If no ice returned to the oceans, sea level would drop 8 mm
   every year. Although approximately the same amount of water returns to
   the ocean in icebergs and from ice melting at the edges, scientists do
   not know which is greater — the ice going in or the ice coming out. The
   difference between the ice input and output is called the mass balance
   and is important because it causes changes in global sea level.

   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.
     * Scientists lack knowledge of changes in terrestrial storage of
       water. Between 1910 and 1990, such changes may have contributed
       from –1.1 to +0.4 mm/yr.
     * If all glaciers and ice caps melt, the projected rise in sea level
       will be around 0.5 m. If the melting includes the Greenland and
       Antarctic ice sheets (both of which contain ice above sea level),
       then the rise is a more drastic 68.8 m. The collapse of the
       grounded interior reservoir of the West Antarctic ice sheet would
       raise sea level by 5-6 m.
     * 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 degrees 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 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 but active research continues in this field. The uncertainty in
   the terrestrial storage term is particularly large.

   Since 1992 the TOPEX and JASON satellite programs have provided
   measurements of sea level change. The current data are available at .
   The data show a mean sea level increase of 2.9±0.4 mm/yr. However,
   because significant short-term variability in sea level can occur, this
   recent increase does not necessarily indicate a long-term acceleration
   in sea level changes.

Geological influences

   Comparison of two sea level reconstructions during the last 500 Myr.
   The scale of change during the last glacial/interglacial transition is
   indicated with a black bar. Note that over most of geologic history,
   long-term average sea level has been significantly higher than today
   Enlarge
   Comparison of two sea level reconstructions during the last 500 Myr.
   The scale of change during the last glacial/interglacial transition is
   indicated with a black bar. Note that over most of geologic history,
   long-term average sea level has been significantly higher than today

   At times during Earth's long history, continental drift has arranged
   the land masses into very different configurations from those of today.
   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. However, over most
   of geologic time, long-term sea level has been higher than today (see
   graph above). Only at the Permo-Triassic boundary ~250 million years
   ago was long-term sea level lower than today.

   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 melting of
   the Greenland and Antarctica ice sheets would result in a sea level
   rise of approximately 80 meters.

   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 meters 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 (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 <
   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

Past changes in sea level

   Changes in sea level during the last 9,000 years
   Enlarge
   Changes in sea level during the last 9,000 years

The sedimentary record

   For generations, geologists have been trying to explain the obvious
   cyclicity of sedimentary deposits observed everywhere we look. The
   prevailing theories hold that this cyclicity primarily represents the
   response of depositional processes to the rise and fall of sea level.
   In the rock record, geologists see times when sea level was
   astoundingly low alternating with times when sea level was much higher
   than today, and these anomalies often appear worldwide. For instance,
   during the depths of the last ice age 18,000 years ago when hundreds of
   thousands of cubic miles of ice were stacked up on the continents as
   glaciers, sea level was 390 feet (120 m) lower, locations that today
   support coral reefs were left high and dry, and coastlines were miles
   farther basinward from the present-day coastline. It was during this
   time of very low sea level that there was a dry land connection between
   Asia and Alaska over which humans are believed to have migrated to
   North America (see Bering Land Bridge).

   However, for the past 6,000 years (long before mankind started keeping
   written records), the world's sea level has been gradually approaching
   the level we see today. During the previous interglacial about 120,000
   years ago, sea level was for a short time about 6 m higher than today,
   as evidenced by wave-cut notches along cliffs in the Bahamas. There are
   also Pleistocene coral reefs left stranded about 3 meters above today's
   sea level along the southwestern coastline of West Caicos Island in the
   West Indies. These once-submerged reefs and nearby paleo-beach deposits
   are silent testimony that sea level spent enough time at that higher
   level to allow the reefs to grow (exactly where this extra sea water
   came from—Antarctica or Greenland—has not yet been determined). Similar
   evidence of geologically recent sea level positions is abundant around
   the world.

Estimates

   See IPCC TAR, figure 11.4 for a graph of sea level changes over the
   past 140,000 years.
     * Sea-level rise estimates from satellite altimetry since 1992 (about
       2.8 mm/yr) exceed those from tide gauges. It is unclear whether
       this represents an increase over the last decades, variability, or
       problems with satellite calibration.
     * In 2001, the TAR stated that measurements have detected no
       significant acceleration in the recent rate of sea level rise. More
       recent work may be revising this; e.g. .
     * Based on tide gauge data, the rate of global average sea level rise
       during the 20th century lies in the range 0.8 to 3.3 mm/yr, with an
       average rate of 1.8 mm/yr.
     * Recent studies of Roman wells in Caesarea and of Roman piscinae in
       Italy indicate that sea level stayed fairly constant from a few
       hundred years AD to a few hundred years ago.
     * Based on geological data, global average sea level may have risen
       at an average rate of about 0.5 mm/yr over the last 6,000 years and
       at an average rate of 0.1 to 0.2 mm/yr over the last 3,000 years.
     * Since the Last Glacial Maximum about 20,000 years ago, sea level
       has risen by over 120 m (averaging 6 mm/yr) as a result of melting
       of major ice sheets. A rapid rise took place between 15,000 and
       6,000 years ago at an average rate of 10 mm/yr which accounted for
       90 m of the rise; thus in the period since 20,000 years BP
       (excluding the rapid rise from 15-6 kyr BP) the average rate was 3
       mm/yr.
     * A significant event was Meltwater Pulse 1A (mwp-1A), when sea level
       rose approximately 20 m over a 500 year period about 14,200 years
       ago. This is a rate of about 40 mm/yr. Recent studies suggest the
       primary source was meltwater from the Antarctic, perhaps causing
       the south-to-north cold pulse marked by the Southern Hemisphere
       Huelmo/Mascardi Cold Reversal, which preceded the Northern
       Hemisphere Younger Dryas.

Future sea level rise

   Tide gauges and satellite altimetry suggest an increase in sea level of
   1.5-3 mm/yr over the past 100 years. The IPCC predicts that by 2100,
   global warming will lead to a sea level rise of 110 to 880 mm (details
   below).

   These sea level rises could lead to difficulties for shore-based
   communities: for example, many major cities such as London and New
   Orleans already need storm-surge defences, and would need more if sea
   level rose, though they also face issues such as sinking land. TAR
   chapter 11.

   Future sea level rise, like the recent rise, is not expected to be
   globally uniform. Some regions show a sea level rise substantially more
   than the global average (in many cases of more than twice the average),
   and others a sea level fall . However, models disagree as to the likely
   pattern of sea level change .

Intergovernmental Panel on Climate Change results

   The results from the IPCC Third Assessment Report (TAR) sea level
   chapter (convening authors John A. Church and Jonathan M. Gregory) are
   given below.
   IPCC change factors 1990-2100 IS92a prediction SRES prediction
   Thermal expansion 110 to 430 mm
   Glaciers 10 to 230 mm
   (or 50 to 110 mm)
   Greenland ice –20 to 90 mm
   Antarctic ice –170 to 20 mm
   Terrestrial storage –83 to 30 mm
   Ongoing contributions from ice sheets in response to past climate
   change 0 to 0.05 m
   Thawing of permafrost 0 to 5 mm
   Deposition of sediment not specified
   Total global-average sea level rise
   (IPCC result, not sum of above) 110 to 770 mm 90 to 880 mm
   (central value of 480 mm)

   The sum of these components indicates a rate of eustatic sea level rise
   (corresponding to a change in ocean volume) from 1910 to 1990 ranging
   from –0.8 to 2.2 mm/yr, with a central value of 0.7 mm/yr. The upper
   bound is close to the observational upper bound (2.0 mm/yr), but the
   central value is less than the observational lower bound (1.0 mm/yr),
   i.e., the sum of components is biased low compared to the observational
   estimates. The sum of components indicates an acceleration of only 0.2
   (mm/yr)/century, with a range from –1.1 to +0.7 (mm/yr)/century,
   consistent with observational finding of no acceleration in sea level
   rise during the 20th century. The estimated rate of sea level rise from
   anthropogenic climate change from 1910 to 1990 (from modelling studies
   of thermal expansion, glaciers and ice sheets) ranges from 0.3 to 0.8
   mm/yr. It is very likely that 20th century warming has contributed
   significantly to the observed sea level rise, through thermal expansion
   of sea water and widespread loss of land ice .

   A common perception is that the rate of sea level rise should have
   accelerated during the latter half of the 20th century. The tide gauge
   data for the 20th century show no significant acceleration. We have
   obtained estimates based on AOGCMs for the terms directly related to
   anthropogenic climate change in the 20th century, i.e., thermal
   expansion, ice sheets, glaciers and ice caps... The total computed rise
   indicates an acceleration of only 0.2 (mm/yr)/century, with a range
   from -1.1 to +0.7 (mm/yr)/century, consistent with observational
   finding of no acceleration in sea level rise during the 20th century.
   The sum of terms not related to recent climate change is -1.1 to +0.9
   mm/yr (i.e., excluding thermal expansion, glaciers and ice caps, and
   changes in the ice sheets due to 20th century climate change). This
   range is less than the observational lower bound of sea level rise.
   Hence it is very likely that these terms alone are an insufficient
   explanation, implying that 20th century climate change has made a
   contribution to 20th century sea level rise .

Uncertainties and criticisms regarding IPCC results

     * Tide records with a rate of 180 mm/century going back to the 19th
       century show no measurable acceleration throughout the late 19th
       and first half of the 20th century. The IPCC attributes about 60
       mm/century to melting and other eustatic processes, leaving a
       residual of 120 mm of 20th century rise to be accounted for. Global
       ocean temperatures by Levitus et al are in accord with coupled
       ocean/atmosphere modeling of greenhouse warming, with heat-related
       change of 30 mm. Melting of polar ice sheets at the upper limit of
       the IPCC estimates could close the gap, but severe limits are
       imposed by the observed perturbations in Earth rotation. (Munk
       2002)
     * By the time of the IPCC TAR, attribution of sea level changes had a
       large unexplained gap between direct and indirect estimates of
       global sea level rise. Most direct estimates from tide gauges give
       1.5–2.0 mm/yr, whereas indirect estimates based on the two
       processes responsible for global sea level rise, namely mass and
       volume change, are significantly below this range. Estimates of the
       volume increase due to ocean warming give a rate of about 0.5 mm/yr
       and the rate due to mass increase, primarily from the melting of
       continental ice, is thought to be even smaller. One study confirmed
       tide gauge data is correct, and concluded there must be a
       continental source of 1.4 mm/yr of fresh water. (Miller 2004)
     * From (Douglas 2002): "In the last dozen years, published values of
       20th century GSL rise have ranged from 1.0 to 2.4 mm/yr. In its
       Third Assessment Report, the IPCC discusses this lack of consensus
       at length and is careful not to present a best estimate of 20th
       century GSL rise. By design, the panel presents a snapshot of
       published analyses over the previous decade or so and interprets
       the broad range of estimates as reflecting the uncertainty of our
       knowledge of GSL rise. We disagree with the IPCC interpretation. In
       our view, values much below 2 mm/yr are inconsistent with regional
       observations of sea-level rise and with the continuing physical
       response of Earth to the most recent episode of deglaciation."
     * The strong 1997- 1998 El Niño caused regional and global sea level
       variations, including a temporary global increase of perhaps 20 mm.
       The IPCC TAR's examination of satellite trends says the major 1997/
       98 El Niño-Southern Oscillation (ENSO) event could bias the above
       estimates of sea level rise and also indicate the difficulty of
       separating long-term trends from climatic variability .

Glacier contribution

   It is well known that glaciers are subject to surges in their rate of
   movement with consequent melting when they reach lower altitudes and/or
   the sea. The contributors to Ann. Glac. 36 (2003) discussed this
   phenomenon extensively and it appears that slow advance and rapid
   retreat have persisted throughout the mid to late Holocene in nearly
   all of Alaska's glaciers. Historical reports of surge occurrences in
   Iceland's glaciers go back several centuries. Thus rapid retreat can
   have several other causes than CO2 increase in the atmosphere.

   The results from Dyurgerov show a sharp increase in the contribution of
   mountain and subpolar glaciers to sea level rise since 1996 (0.5 mm/yr)
   to 1998 (2 mm/yr) with an average of approx. 0.35 mm/yr since 1960.
   (Dyurgerov, Mark. 2002. Glacier Mass Balance and Regime: Data of
   Measurements and Analysis. INSTAAR Occasional Paper No. 55, ed. M.
   Meier and R. Armstrong. Boulder, CO: Institute of Arctic and Alpine
   Research, University of Colorado. Distributed by National Snow and Ice
   Data Centre, Boulder, CO. A shorter discussion is at )

   Of interest also is Arendt et al, (Science, 297, p. 382, July 2002) who
   estimate the contribution of Alaskan glaciers of 0.14±0.04 mm/yr
   between the mid 1950s to the mid 1990s increasing to 0.27 mm/yr in the
   middle and late 1990s.

Greenland contribution

   Krabill et al. (Science, Vol 289, Issue 5478, 428-430, 21 July 2000)
   estimate a net contribution from Greenland to be at least 0.13 mm/yr in
   the 1990s. Joughin et al. (Nature, Vol 432, p608, December 2004) have
   measured a doubling of the speed of Jacobshavn Isbrae between 1997 and
   2003. This is Greenland's largest-outlet glacier; it drains 6.5% of the
   ice sheet, and is thought to be responsible for increasing the rate of
   sea level rise by about 0.06 millimeters per year, or roughly 4% of the
   20th century rate of sea level increase. In 2004, Rignot et al.
   (Geophysical Research Letters, v31, L10401) estimated a contribution of
   0.04±0.01 mm/yr to sea level rise from southeast Greenland.

   Rignot and Kanagaratnam ( Science, 311, pp. 986 et seq., 2006) produced
   a comprehensive study and map of the outlet glaciers and basins of
   Greenland. They found widespread glacial accleration below 66 N in 1996
   which spread to 70 N by 2005; and that the ice sheet loss rate in that
   decade increased from 90 to 200 cubic km/yr; this corresponds to an
   extra 0.25 to 0.55 mm/yr of sea level rise.

   In July 2005 it was reported that the Kangerdlugssuaq glacier, on
   Greenland's east coast, was moving towards the sea three times faster
   than a decade earlier. Kangerdlugssuaq is around 1000 m thick, 7.2 km
   (4.5 miles) wide, and drains about 4% of the ice from the Greenland ice
   sheet. Measurements of Kangerdlugssuaq in 1988 and 1996 showed it
   moving at between 5 and 6 km/yr (3.1 to 3.7 miles/yr) (in 2005 it was
   moving at 14 km/yr (8.7 miles/yr).

   According to the 2004 Arctic Climate Impact Assessment, climate models
   project that local warming in Greenland will exceed 3 degrees Celsius
   during this century. Also, ice sheet models project that such a warming
   would initiate the long-term melting of the ice sheet, leading to a
   complete melting of the Greenland ice sheet over several millenia,
   resulting in a global sea level rise of about seven meters .

Effects of Snowline and Permafrost

   The snowline altitude is the altitude of the lowest elevation interval
   in which minimum annual snow cover exceeds 50%. This ranges from about
   5500 metres above sea-level at the equator down to sea-level at about
   65 degrees N&S latitude, depending on regional temperature amelioration
   effects. Permafrost then appears at sea-level and extends deeper below
   sea-level pole-wards. The depth of permafrost and the height of the
   ice-fields in both Greenland and Antarctica means that they are largely
   invulnerable to rapid melting. Greenland Summit is at 3200 metres,
   where the average annual temperature is minus 32 °C. So even a
   projected 4 °C rise in temperature leaves it well below the melting
   point of ice. Frozen Ground 28, December 2004, has a very significant
   map of permafrost affected areas in the Arctic. The continuous
   permafrost zone includes all of Greenland, the North of Labrador, NW
   Territories, Alaska north of Fairbanks, and most of NE Siberia north of
   Mongolia and Kamchatka. Continental ice above permafrost is very
   unlikely to melt quickly. 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
   they are unlikely to contribute significantly to sea-level rise in the
   coming century.

Polar ice

   The sea level could rise above its current level if more polar ice
   melts. However, compared to the heights of the ice ages, today there
   are very few continental ice sheets remaining to be melted. It is
   estimated that Antarctica, if fully melted, would contribute more than
   60 metres of sea level rise, and Greenland would contribute more than 7
   metres. Small glaciers and ice caps might contribute about 0.5 metres;
   this number is in the uncertainty of the estimates from Antarctica or
   Greenland but could be expected to be fast (within the coming century)
   whereas Greenland would be slow (perhaps 1500 years to fully deglaciate
   at the fastest likely rate) and Antarctica even slower .

   In 2002, Rignot and Thomas (Science, v297, 1502-1506, 2002) found that
   the West Antarctic and Greenland ice sheets were losing mass, while the
   East Antarctic ice sheet was probably in balance (although they could
   not determine the sign of the mass balance for The East Antarctic ice
   sheet). Kwok and Comiso (J. Climate, v15, 487-501, 2002) also
   discovered that temperature and pressure anomalies around West
   Antarctica and on the other side of the Antarctic Peninsula correlate
   with recent Southern Oscillation events.

   In 2004 Rignot et al. (Geophysical Research Letters, v31, L10401)
   estimated a contribution of 0.04±0.01 mm/yr to sea level rise from
   South East Greenland. In the same year, Thomas et al. (Science, v306,
   255-258, 2004) found evidence of an accelerated contribution to sea
   level rise from West Antarctica. The data showed that the Amundsen Sea
   sector of the West Antarctic Ice sheet was discharging 250 cubic
   kilometres of ice every year, which was 60% more than precipitation
   accumulation in the catchment areas. This alone was sufficient to raise
   sea level at 0.24 mm/yr. Further, thinning rates for the glaciers
   studied in 2002-2003 had increased over the values measured in the
   early 1990s. The bedrock underlying the glaciers was found to be
   hundreds of meters deeper than previously known, indicating exit routes
   for ice from further inland in the Byrd Subpolar Basin. Thus the West
   Antarctic ice sheet may not be as stable as has been supposed.

   In 2005 it was reported that during 1992-2003, East Antarctica
   thickened at an average rate of about 18 mm/yr while West Antarctica
   showed an overall thinning of 9 mm/yr. associated with increased
   precipitation. A gain of this magnitude is enough to slow sea-level
   rise by 0.12±0.02 mm/yr. (Davis et al., Science 2005) DOI:
   10.1126/science.1110662.

Effects of sea level rise

   Based on the projected increases stated above, the IPCC TAR WG II
   report notes that current and future climate change would be expected
   to have a number of impacts, particularly on coastal systems. Such
   impacts may include increased coastal erosion, higher storm-surge
   flooding, inhibition of primary production processes, more extensive
   coastal inundation, changes in surface water quality and groundwater
   characteristics, increased loss of property and coastal habitats,
   increased flood risk and potential loss of life, loss of nonmonetary
   cultural resources and values, impacts on agriculture and aquaculture
   through decline in soil and water quality, and loss of tourism,
   recreation, and transportation functions.

   There is an implication that many of these impacts will be detrimental.
   The report does, however, note that owing to the great diversity of
   coastal environments; regional and local differences in projected
   relative sea level and climate changes; and differences in the
   resilience and adaptive capacity of ecosystems, sectors, and countries,
   the impacts will be highly variable in time and space and will not
   necessarily be negative in all situations.

   To date, sea level changes have not been implicated in any substantial
   environmental, humanitarian, or economic losses. Previous claims have
   been made that parts of the island nations of Tuvalu were "sinking" as
   a result of sea level rise. However, subsequent reviews have suggested
   that the loss of land area was the result of erosion during and
   following the actions of 1997 cyclones Gavin, Hina, and Keli. The
   islands in questions were not populated. Reuters has reported other
   Pacific islands are facing a severe risk including Tegua island in
   Vanuatu,. There are claims that Vanuatu data shows no net sea level
   rise. These claims are not substantiated by tide gauge data and are
   reminiscent of claims made in Michael Crichton's State of Fear that
   there is no threat to this island chain. Vanuatu tide gauge data from
   http://www.pol.ac.uk/psmsl/pubi/met.monthly.data/741002.metdata show a
   net rise of ~50 mm from 1994-2004. Linear regression of this short time
   series suggests a rate of rise of ~7 mm/y, though there is considerable
   variability and the exact threat to the islands is difficult to assess
   using such a short time series. According to Patrick J. Michaels, "In
   fact, areas to the west such as [the island of] Tuvalu show substantial
   declines in sea level over that period." Despite President Gayoom
   speaking in the past about the impending dangers to his country, the
   Maldives, research found that the people of the Maldives have in the
   past survived a higher sea level about 50-60 cm and there is evidence
   of a significant sea level fall in the last 30 years in that Indian
   Ocean area (20-30 cm).

   A much overlooked fact about coral islands is that they exist above sea
   level today only because sea level was once high enough that these
   currently dry areas were underwater. Corals and other reef-building
   organisms cannot survive prolonged exposure to air, so the corals from
   which the islands are formed could have grown only during interglacial
   periods when sea level was higher than today ( e.g. 120,000 years ago).
   It is perhaps unfortunate that these occasionally submarine ecosystems
   have human populations today, since they must be occasionally flooded
   so that the reef-building organisms can redeposit sediments to replace
   the material naturally removed by erosion during low sea level periods.

Satellite sea level measurement

   Sea level rise estimates from satellite altimetry are 3.1 +/- 0.4 mm/yr
   for 1993-2003 (Leuliette et al. (2004)). This exceeds those from tide
   gauges. It is unclear whether this represents an increase over the last
   decades; variability; true differences between satellites and tide
   gauges; or problems with satellite calibration.

   Since 1992 the NASA/CNES TOPEX/Poseidon (T/P) and Jason-1 satellite
   programs have provided measurements of sea level change. The current
   data are available at http://sealevel.colorado.edu/ and
   http://sealevel.jpl.nasa.gov/. The data show a mean sea level increase
   of 2.8±0.4 mm/yr. This includes an apparent increase to 3.7±0.2 mm/yr
   during the period 1999 through 2004 . Satellites ERS-1 ( July 17, 1991-
   March 10, 2000), ERS-2 ( April 21, 1995-), and Envisat ( March 1,
   2002-) also have sea surface altimeter components but are of limited
   use for measuring global mean sea level due to less detailed coverage.
     * TOPEX/Poseidon began their series of measurements in 1992. The
       POSEIDON-1 altimeter operates 10% of the time.
     * Jason-1, launched December 7, 2001, is presently flying the same
       groundtrack, leading Poseidon.
     * After Jason-1 calibration is complete, T/P will be moved to an
       orbit midway between Jason-1 groundtracks, providing increased
       coverage.

   Because significant short-term variability in sea level can occur,
   extracting the global mean sea level information is complex. Also, the
   satellite data has a much shorter record than tidal gauges, which have
   been found to require years of operation to extract trends.

   There is a range of distances involved.
     * 140 to 320 mm: Increased height of sea level within 1997-1998 El
       Niño Pacific region.
     * 140 mm: Range of typical regional sea level variations (±70 mm).
     * 100 mm: Accuracy of ERS-1 radar altimeter.
     * 43 mm: Accuracy of ocean surface height calculations with T/P.
     * 30 to 40 mm: Accuracy of TOPEX and POSEIDON-1 radar altimeters,
       which measure distance to ocean surface.
     * 20 to 30 mm: Accuracy of determination of T/P satellite orbital
       height (laser ranging, doppler shifts, GPS).
     * 20 mm: Accuracy of Jason-1 POSEIDON-2 radar altimeter.
     * 7-14 mm: Global mean sea level surge during 1997- 1998 El Niño
       period.
     * Several mm: Precision of global mean sea level measurement after
       averaging 10-day coverage.
     * 10 mm: Stability of T/P orbit heights over 4 years.
     * 2.8 ±0.4 mm: Average annual global sea level rise since 1992
       according to T/P.

   There apparently is a problem with the ERS-2 altimeter. Mean sea level
   changes were compared between satellites for 60°N and 60°S from May
   1995 to June 1996:
     * -4.7 ±1.5 mm/yr for ERS-1
     * -5.6 ±1.3 mm/yr for TOPEX
     * +9.0 ±2.1 mm/yr for ERS-2

   Ongoing altimeter comparisons are available at:
   http://www7300.nrlssc.navy.mil/altimetry/intercomp.html
   The various readings there are of current sea level variations, not
   global sea level, so the comparison is only in differences between the
   values. That data is of variations in centimeters; further processing
   is done to reach the millimeter-level resolution needed for mean sea
   level studies.

   Comparisons of T/P with Pacific island tide gauge data show that the
   monthly mean deviations are accurate at the level of 20 mm.

   Also, it should be noted that since satellite results are partially
   calibrated against tide gauge readings, they are not an entirely
   independent source.

   The strong 1997-1998 El Niño event "has imprinted a strong signature on
   the sea surface height field in the mid-latitude eastern Pacific. This
   signal will be tracked over the next decade as the eastern boundary
   manifestation of this El Niño event propagates westward toward the
   Kuroshio Extension."

   Other satellites:
     * Geosat Follow-On is a U.S. Navy altimeter mission that was launched
       on February 10, 1998. On November 29, 2000, the Navy accepted the
       satellite as operational. During its mission life, the satellite
       will be retained in the GEOSAT Exact Repeat Mission (ERM) orbit
       (800 km altitude, 108 deg inclination, 0.001 eccentricity, and, 100
       min period). This 17-day Exact Repeat Orbit (ERO) retraces the ERM
       ground track to +/-1 km. As with the original GEOSAT ERM, the data
       will be available for ocean science through NOAA/NOS and
       NOAA/NESDIS. Radar Altimeter - single frequency (13.5 GHz) with 35
       mm height precision. Note that the GPS receiver is not functional.
          + Geosat Follow-On @ NOAA/LSA
          + NAVY GEOSAT FOLLOW-ON (GFO) ALTIMETRY MISSION
          + NASA WFF Geosat Follow-On

   Other sea level analysis:
     * Sea Level Analysis from ERS Altimetry
     * Ssalto/Duacs multimission altimeter products: Combined current data
       from Topex/Poseidon, Geosat Follow On, Jason-1 and Envisat.

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