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Sea levels around the world are rising. Current sea-level rise potentially impacts human populations (e.g., those living in coastal regions and on islands) and the natural environment (e.g., marine ecosystems). Between 1870 and 2004, Global average sea levels rose 17 cm. From 1950 to 2009, measurements show an average annual rise in sea level of 1.7 ± 0.3 mm with satellite data showing a rise of 3.3 ± 0.4 mm from 1993 to 2009, a faster rate of increase than originally estimated. It is unclear whether the increased rate reflects an increase in the underlying long-term trend.
Two main factors contributed to observed sea level rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the contribution of land-based ice due to increased melting. The major store of water on land is found in glaciers and ice sheets.
Sea level rise is one of several lines of evidence that support the view that the climate has recently warmed. It is likely that human-induced (anthropogenic) warming contributed to the sea level rise observed in the latter half of the 20th century.
Sea level rise is expected to continue for centuries. In 2007, the Intergovernmental Panel on Climate Change (IPCC) projected that during the 21st century, sea level will rise another 18 to 59 cm (7.1 to 23 in), but these numbers do not include "uncertainties in climate-carbon cycle feedbacks nor do they include the full effects of changes in ice sheet flow". Although IPCC explicitly refrained from projecting an upper limit of total sea level rise in the 21st century, one meter of sea level rise is well within the range of more recent projections.
On the timescale of centuries to millennia, the melting of ice sheets could result in even higher sea level rise. Partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, could contribute 4 to 6 m (13 to 20 ft) or more to sea level rise.
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.
Many factors can produce short-term (a few minutes to 18.6 years) changes in sea level.
|Short-term (periodic) causes||Time scale|
(P = period)
|Periodic sea level changes|
|Diurnal and semidiurnal astronomical tides||12–24 h P||0.2–10+ m|
|Rotational variations (Chandler wobble)||14 month P|
|Lunar Node astronomical tides||18.613 year|
|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 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 (standing waves)||Minutes to hours||Up to 2 m|
|Tsunamis (generate catastrophic long-period waves)||Hours||Up to 10 m|
|Abrupt change in land level||Minutes||Up to 10 m|
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 density 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 oceanic basins and in land–sea distribution affect sea level.
Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2–0.4 mm/yr, averaged over the 20th century.
Each year about 8 mm of ocean water falls on 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 a non-zero balance causes changes in global sea level. High-precision gravimetry from satellites in low-noise flight determined that Greenland was losing more than 200 billion tons of ice per year, in accord with loss estimates from ground measurement. The rate of ice loss was accelerating, having grown from 137 gigatons in 2002–2003.
Ice shelves float on the surface of the sea and, if they melt, to a first order they do not change sea level. Likewise, shrinkage/expansion of the northern polar ice cap which is composed of floating pack ice do not significantly affect sea level. Because ice shelf water is fresh, however, melting would cause a very small increase in sea levels, so small that it is generally neglected.
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 terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quite large.
In 1992, satellites began recording the change in sea level; they display an acceleration in the rate of sea level change, but they have not been operating for long enough to work out whether this signals a permanent rate change, or an artifact of short-term variation.
On the timescale of years and decades, sea level records contain a considerable amount of variability. For example, approximately a 10 mm rise and fall of global mean sea level accompanied the 1997–1998 El Niño-Southern Oscillation (ENSO) event, and a temporary 5 mm fall accompanied the 2010–2011 event. Interannual or longer variability is a major reason why no long-term acceleration of sea level has been identified using 20th century data alone. However, a range of evidence clearly shows that the rate of sea level rise increased between the mid-19th and mid-20th centuries. Evidence for this includes geological observations, the longest instrumental records and the observed rate of 20th century sea level rise. For example, geological observations indicate that during the last 2,000 years, sea level change was small, with an average rate of only 0.0–0.2 mm per year. This compares to an average rate of 1.7 ± 0.5 mm per year for the 20th century.
Sedimentary deposits follow cyclic patterns. Prevailing theories hold that this cyclicity primarily represents the response of depositional processes to the rise and fall of sea level. The rock record indicates that in earlier eras, sea level was both much lower than today and much higher than today. Such 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 120 metres (390 ft) lower, locations that today support coral reefs were left high and dry, and coastlines were miles farther outward. During this time of very low sea level 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).
For the past 6,000 years, the world's sea level gradually approached the current level. During the previous interglacial about 120,000 years ago, sea level was for a short time about 6 metres (20 ft) higher than today, as evidenced by wave-cut notches along cliffs in the Bahamas. There are also Pleistocene coral reefs left stranded about 3 metres 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 indicate that sea level spent enough time at that higher level to allow 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.
Tide gauges in the United States reveal considerable variation because some land areas are rising and some are sinking. For example, over the past 100 years, the rate of sea level rise varied from about an increase of 0.36 inches (9.1 mm) per year along the Louisiana Coast (due to land sinking), to a drop of a few inches per decade in parts of Alaska (due to post-glacial rebound). The rate of sea level rise increased during the 1993–2003 period compared with the longer-term average (1961–2003), although it is unclear whether the faster rate reflected a short-term variation or an increase in the long-term trend.
One study showed no acceleration in sea level rise in US tide gauge records during the 20th century. However, another study found that the rate of rise for the US Atlantic coast during the 20th century was far higher than during the previous two thousand years.
The longest running sea-level measurements are recorded at Amsterdam, in the Netherlands—part of which (about 25%) lies beneath sea level, beginning in 1700. Since 1850, the rise averaged 1.5 mm/year.
Records dating from 1843 taken by an amateur meteorologist at the Port Arthur convict settlement, when merged with data recorded by modern tide gauges, indicated sea level rise of about 1 mm a year.
As of 2003 the National Tidal Centre of the Bureau of Meteorology managed 32 tide gauges, some with records since 1880, for the entire coastline.
Commonwealth Scientific and Industrial Research Organisation (CSIRO) data shows the current sea level trend to be 3.1 mm/yr and the historical increase since 1870 to have been an average of 1.7 mm/year
The 2007 Fourth Assessment Report (IPCC 4) projected century-end sea levels using the Special Report on Emissions Scenarios (SRES). SRES developed emissions scenarios to project climate-change impacts. The projections based on these scenarios are not predictions, but reflect plausible estimates of future social and economic development (e.g., economic growth, population level). The six SRES "marker" scenarios projected sea level to rise by 18 to 59 centimetres (7.1 to 23 in). Their projections were for the time period 2090–99, with the increase in level relative to average sea level over the 1980–99 period. This estimate did not include all of the possible contributions of ice sheets.
More recent research from 2008 observed rapid declines in ice-mass balance from both Greenland and Antarctica, and concluded that sea-level rise by 2100 is likely to be at least twice as large as that presented by IPCC AR4, with an upper limit of about two meters.
A literature assessment published in 2010 by the US National Research Council described the above IPCC projections as "conservative," and summarized the results of more recent studies. These projections ranged from 56–200 centimetres (22–79 in), based on the same period as IPCC 4.
In 2011, Rignot and others projected a rise of 32 centimetres (13 in) by 2050. Their projection included increased contributions from the Antarctic and Greenland ice sheets. Use of two completely different approaches reinforced the Rignot projection.
There is a widespread consensus that substantial long-term sea-level rise will continue for centuries to come. IPCC 4 estimated that at least a partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, would occur given a global average temperature increase of 1–4 °C (relative to temperatures over the years 1990–2000). This estimate was given about a 50% chance of being correct. The estimated timescale was centuries to millennia, and would contribute 4 to 6 metres (13 to 20 ft) or more to sea levels over this period.
There is the possibility of a rapid change in glaciers, ice sheets, and hence sea level. Predictions of such a change are highly uncertain due to a lack of scientific understanding. Modeling of the processes associated with a rapid ice-sheet and glacier change could potentially increase future projections of sea-level rise.
Future sea level rise could lead to potentially catastrophic difficulties for shore-based communities in the next centuries: for example, many major cities such as London, New Orleans, and New York  already need storm-surge defenses, and would need more if the sea level rose, though they also face issues such as subsidence. Sea level rise could also displace many shore-based populations: for example it is estimated that a sea level rise of just 200 mm could create 740,000 homeless people in Nigeria. Maldives, Tuvalu, and other low-lying countries are among the areas that are at the highest level of risk. The UN's environmental panel has warned that, at current rates, sea level would be high enough to make the Maldives uninhabitable by 2100.
Future sea-level rise, like the recent rise, is not expected to be globally uniform (details below). 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.
In September 2008, the Delta Commission (Deltacommissie (2007)) presided by Dutch politician Cees Veerman advised in a report that the Netherlands would need a massive new building program to strengthen the country's water defenses against the anticipated effects of global warming for the next 190 years. This commission was created in September 2007, after the damage caused by Hurricane Katrina prompted reflection and preparations. Those included drawing up worst-case plans for evacuations. The plan included more than €100 billion (US$144 bn), in new spending through the year 2100 to take measures, such as broadening coastal dunes and strengthening sea and river dikes.
The commission said the country must plan for a rise in the North Sea up to 4.25 feet (51 inches, 1.3 m) by 2100, rather than the previously projected 2.5 feet (30 inches, 0.80 m), and plan for a 6.5 – 13 feet (80 - 160 inches, 2 – 4 m) rise by 2200.
The results from the IPCC Third Assessment Report (TAR) sea level chapter are given below.
|IPCC change factors 1990–2100||IS92a prediction||SRES projection/|
|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 55 mm|
|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 modeling 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 the thermal expansion of sea water and the 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, but tide gauge data for the 20th century show no significant acceleration. Estimates obtained are based on atmosphere-ocean general circulation models (abbreviated 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. Recent figures of human, terrestrial impoundment came too late for the 3rd Report, and would revise levels upward for much of the 20th century.
The different SRES emissions scenarios used for the TAR sea-level projections were not assigned probabilities, and no scenario is assumed by the IPCC to be more probable than another. For the first part of the 21st century, the variation between the different SRES scenarios is relatively small. The range spanned by the SRES scenarios by 2040 is only 0.02 m or less. By 2100, this range increases to 0.18 m. Of the six illustrative SRES scenarios, A1FI gives the largest sea-level rise and B1 the smallest (see the SRES article for a description of the different scenarios).
For the TAR sea-level projections, uncertainty in the climate sensitivity and heat uptake of the oceans, as represented by the spread of models (specifically, atmosphere–ocean general circulation models, or AOGCMs), is more important than the uncertainty from the choice of emissions scenario. This differs from the TAR's projections of global warming (i.e., the future increase in global mean temperature), where the uncertainty in emissions scenario and climate sensitivity are comparable in size.
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 Annals of Glaciology , Volume 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 about 0.35 mm/yr since 1960.
Of interest also is Arendt et al., 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.
Krabill et al. estimate a net contribution from Greenland to be at least 0.13 mm/yr in the 1990s. Joughin et al. have measured a doubling of the speed of Jakobshavn Isbræ 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 millimetres per year, or roughly 4% of the 20th-century rate of sea-level increase. In 2004, Rignot et al. estimated a contribution of 0.04±0.01 mm/yr to sea-level rise from southeast Greenland.
Rignot and Kanagaratnam produced a comprehensive study and map of the outlet glaciers and basins of Greenland. They found widespread glacial acceleration 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 0.25–0.55 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 1,000 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 (3.1–3.7 miles/yr), while in 2005 that speed had increased to 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 °C 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 millennia, resulting in a global sea level rise of about seven metres.
On the Antarctic continent itself, the large volume of ice present stores around 70% of the world's fresh water. This ice sheet is constantly gaining ice from snowfall and losing ice through outflow to the sea. West Antarctica is currently experiencing a net outflow of glacial ice, which will increase global sea level over time. A review of the scientific studies looking at data from 1992 to 2006 suggested a net loss of around 50 Gigatonnes of ice per year was a reasonable estimate (around 0.14 mm of sea-level rise), although significant acceleration of outflow glaciers in the Amundsen Sea Embayment could have more than doubled this figure for the year 2006.
East Antarctica is a cold region with a ground-base above sea level and occupies most of the continent. This area is dominated by small accumulations of snowfall which becomes ice and thus eventually seaward glacial flows. The mass balance of the East Antarctic Ice Sheet as a whole is thought to be slightly positive (lowering sea level) or near to balance. However, increased ice outflow has been suggested in some regions.
In 2011 ice-penetrating radar led to the creation of the first high-resolution topographic map of one of the last uncharted regions of Earth: the Aurora Subglacial Basin, an immense ice-buried lowland in East Antarctica larger than Texas. The map reveals some of the largest fjords or ice cut channels on Earth. Because the basin lies kilometres below sea level, seawater could penetrate beneath the ice, causing portions of the ice sheet to collapse and float off to sea. The map is expected to improve models of ice sheet dynamics.
Sheperd et al 2012, found that different satellite methods were in good agreement and combing methods leads to more certainty with East Antarctica, West Antarctica, and the Antarctic Peninsula changing in mass by +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes per year.
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 65° 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 3,200 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.
The sea level will 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 on the margins of Greenland and the Antarctic Peninsula might contribute about 0.5 metres. While the latter figure is much smaller than for Antarctica or Greenland it could occur relatively quickly (within the coming century) whereas melting of Greenland would be slow (perhaps 1,500 years to fully deglaciate at the fastest likely rate) and Antarctica even slower. However, this calculation does not account for the possibility that as meltwater flows under and lubricates the larger ice sheets, they could begin to move much more rapidly towards the sea.
In 2002, Rignot and Thomas 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. 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. 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 2002–03 had increased over the values measured in the early 1990s. The bedrock underlying the glaciers was found to be hundreds of metres 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.
Based on the projected increases stated above, the IPCC TAR WGII report (Impacts, Adaptation Vulnerability) 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 non-monetary 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—especially for the three-quarters of the world's poor who depend on agriculture systems. 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.
Statistical data on the human impact of sea-level rise is scarce. A study in the April, 2007 issue of Environment and Urbanization reports that 634 million people live in coastal areas within 30 feet (9.1 m) of sea level. The study also reported that about two thirds of the world's cities with over five million people are located in these low-lying coastal areas. The IPCC report of 2007 estimated that accelerated melting of the Himalayan ice caps and the resulting rise in sea levels would likely increase the severity of flooding in the short term during the rainy season and greatly magnify the impact of tidal storm surges during the cyclone season. A sea-level rise of just 400 mm in the Bay of Bengal would put 11 percent of the Bangladesh's coastal land underwater, creating 7–10 million climate refugees.
IPCC assessments suggest that deltas and small island states are particularly vulnerable to sea-level rise caused by both thermal expansion and ocean volume. Sea level changes have not yet been conclusively proven to have directly resulted in environmental, humanitarian, or economic losses to small island states, but the IPCC and other bodies have found this a serious risk scenario in coming decades.
Many media reports have focused on the island nations of the Pacific, notably the Polynesian islands of Tuvalu, which based on more severe flooding events in recent years, were thought to be "sinking" due to sea level rise. A scientific review in 2000 reported that based on University of Hawaii gauge data, Tuvalu had experienced a negligible increase in sea level of 0.07 mm a year over the past two decades, and that ENSO had been a larger factor in Tuvalu's higher tides in recent years. A subsequent study by John Hunter from the University of Tasmania, however, adjusted for ENSO effects and the movement of the gauge (which was thought to be sinking). Hunter concluded that Tuvalu had been experiencing sea-level rise of about 1.2 mm per year. The recent more frequent flooding in Tuvalu may also be due to an erosional loss of land during and following the actions of 1997 cyclones Gavin, Hina, and Keli.
Current rates of sea level rise from satellite altimetry have been estimated in the range of 2.9–3.4 ± 0.4–0.6 mm per year for 1993–2010. 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. Knowing the current altitude of a satellite which can measure sea level to a precision of about 20 millimetres (e.g. the Topex/Poseidon system) is primarily complicated by orbital decay and the difference between the assumed orbit and the earth geoid . This problem is partially corrected by regular re-calibration of satellite altimeters from land stations whose height from MSL is known by surveying. Over water, the height is calibrated from tide gauge data which is needed to correct for tides and atmospheric effects on sea level.
Ablain et al. (2008) looked at trends in mean sea level (MSL).:194–195 A global MSL curve was plotted using data for the 1993–2008 period. Their estimates for mean rate of sea level rise over this time period was 3.11 mm per year. A correction was applied to this resulting in a higher estimate of 3.4 mm per year. Over the 2005 to 2008 time period, the MSL rate was estimated to be 1.09 mm per year. This is a reduction of 60% on the rate observed between 1993–2005.:193
MSL was also plotted using data between the years 1994 and 2007.:194–195 Their data for this time period show two peaks (maxima) in MSL rates for the years 1997 and 2002. These maxima very likely reflected the influence of the ENSO on MSL. Using the 1994–2007 MSL data, they estimated MSL rates using moving windows of three and five years. Lower rates were observed during La Niña events in 1999 and 2007. They concluded that the recently observed reduction in the MSL rate was likely to be real, since it coincided with an exceptionally strong La Niña event. Preliminary analyses suggested that an acceleration of the MSL trend would likely occur in relationship with the end of the 2007–08 La Niña event.:200
White (2011) reported measurements of near-global sea level made using satellite altimeters. Over the time period January 1993 to April 2011, these data show a steady increase in global mean sea level (GMSL) of around 3.2 mm per year, with a range of plus or minus 0.4 mm per year. This is 50% larger than the average rate observed over the 20th century. White (2011) was, however, unsure of whether or not this represented a long-term increase in the rate.
The Centre National d’Etudes Spatiales/Collecte Localisation Satellites (CNES/CLS, 2011) reported on the estimated increase in GMSL between 1993 and 2011. Their estimate was an increase of 3.22 mm per year, with an error range in this trend (i.e., the slope over the 1993 to 2011 time period) of approximately 0.6 mm per year.
The CU Sea Level Research Group (CUSLRG, 2011) estimated the rate of GMSL between 1993 and 2011. The rate was estimated at 3.2 mm per year, with a range of plus or minus 0.4 mm per year.
The Laboratory for Satellite Altimetry (LSA, 2011) estimated the trend in GMSL over the time period 1992 to 2011. Their estimate was a trend of 2.9 mm per year, with a range of plus or minus 0.4 mm per year. According to the LSA (2011): "[the] estimates of sea level rise do not include glacial isostatic adjustment effects on the geoid, which are modeled to be +0.2 to +0.5 mm/year when globally averaged."