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For at least a century, the average global sea level has been rising mostly because global warming is driving thermal expansion of seawater and melting land-based ice sheets and glaciers. This trend is expected to accelerate during the 21st century.:62
Projecting future sea level has always been challenging, due to our insufficient understanding of many aspects of the climate system. As climate research leads to improved computer models, projections have consistently increased. For example, in 2007 the high end of Intergovernmental Panel on Climate Change (IPCC) projections was less than 2 feet (0.61 m), but in their 2014 report the high end was considered to be about 3 feet (0.91 m). A number of later studies have concluded that 2.0 to 2.7 metres (6 ft 7 in to 8 ft 10 in) rise this century is "physically plausible". The contributions to sea level rise since 1993, based on 2018 figures, divide into ocean thermal expansion (42%), melting of temperate glaciers (21%), Greenland (15%) and Antarctica (8%).
Sea level rise will not be the same at every location on earth, with some locations even getting a drop in sea levels. Local factors include tectonic effects, and subsidence of the land, tides, currents and storms. Sea level rise is expected to continue for centuries. Because of long response times for parts of the climate system, it has been estimated that we are committed to a sea-level rise of approximately 2.3 metres (7.5 ft) for each degree Celsius of temperature rise within the next 2,000 years.
Sea level rises can considerably influence human populations in coastal and island regions and natural environments like marine ecosystems. Widespread coastal flooding would be expected if several degrees of warming is sustained for millennia. For example, sustained global warming of more than 2 °C relative to pre-industrial levels could lead to eventual sea level rise of about 1–4 metres (3.3–13 ft).
Sea-level rise (SLR) presents challenges to coastal communities and ecosystems, and planners are engaged in assessing management options. Accordingly, it is desirable to have an estimate of SLR this century to properly design mitigation and adaptation strategies. An approximation of SLR by the end of the century will allow estimates of coastal erosion and changes in vulnerability to coastal hazards, assessments of threats to coastal ecosystems and development of climate risk management policies.
Understanding past sea level is important for the analysis of current and future changes. In the recent geological past changes in land ice and thermal expansion from increased temperatures are the dominant reasons of sea level rise. The last time the Earth was 2 °C warmer than the pre-industrial temperatures, sea levels were at least 5 metres (16 ft) higher than present. This was during the last interglacial, when the earth warming was caused by slow changes in the orbital forcing. The warming was sustained over a period of thousands of years and the magnitude of the rise in sea level implied a large contribution from the Antarctic and Greenland ice sheets.:1139
Since the last glacial maximum about 20,000 years ago, the sea level has risen by more than 125 metres (410 ft), with rates varying from less than a mm/year to 40+ mm/year, as a result of melting ice sheets over Canada and Eurasia. Rapid disintegration of ice sheets led to so called 'meltwater pulses', periods during which sea level rose rapidly. The rate of rise started to slow down 8.2 thousand years before present; the sea level was almost constant in the last 2,500 years, before the recent rising trend starting approximately in 1850.
To get precise measurements for sea level, researchers studying the state of frozen water and the ocean on our planet factor in ongoing deformations of the solid Earth, in particular due to landmasses still rising from past ice masses retreating. Additionally, Earth gravitation and rotation have to be accounted for. These factors are dependent on the different layers which make up the Earth (lithosphere, asthenosphere), and the order in which land-based ice melts. Because the involved processes, which are collectively known as the Sea-level equation, change very slowly, on time scales of thousands of years, they are considered to be constant.
Since the 1992 launch of TOPEX/Poseidon, altimetric satellites have been recording the change in sea level. Those satellites can measure the hills and valleys in the sea caused by currents and detect trends in their height. To measure the distance to the sea surface, the satellite sends a microwave pulse to the ocean's surface and record the time it takes to return. A microwave radiometer corrects any delay that may be caused by water vapor in the atmosphere. Combining this data with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimeters (about one inch). Current rates of sea level rise from satellite altimetry have been estimated to be 3.0 ± 0.4 millimetres (0.118 ± 0.016 in) per year for the period 1993–2017. Earlier satellite measurements were previously at odds with tide gauge measurements. A small calibration error for the Topex/Poseidon satellite discovered in 2015 was identified as the cause of this mismatch. It had caused a slight overestimation of the 1992–2005 sea levels, which masked the ongoing sea level rise acceleration.
With satellites it is possible to capture regional variations in sea level rise well. In the 1993–2012 period for instance, sea level rose substantially in the western tropical Pacific. The sharp rise in this area has been linked to increasing trade winds, which occur when the Pacific Decadal Oscillation (PDO) and the El Niño–Southern Oscillation (ENSO) change from one state to the other. The PDO is a basin-wide climate pattern consisting of two phases, each commonly lasting 10 to 30 years, while the ENSO has a shorter period of 2 to 7 years.
Another important source of sea-level observations comes from the global network of tide gauges. In contrast to the satellite record, this record has a lot of spatial and temporal gaps. Coverage of tide gauges started primarily in the Northern Hemisphere, with data for the Southern Hemisphere remained scarce up to the 1970s. The longest running sea-level measurements, NAP or Amsterdam Ordnance Datum established in 1675, are recorded in Amsterdam, the Netherlands. In Australia, record collection is also quite extensive, including measurements by an amateur meteorologist beginning in 1837 and measurements taken from a sea-level benchmark struck on a small cliff on the Isle of the Dead near the Port Arthur convict settlement in 1841.
This network was used, in combination with satellite altimeter data, to establish that global mean sea-level rose 19.5 cm (7.7 in) between 1870 and 2004 at an average rate of about 1.44 mm/yr (1.7 mm/yr during the 20th century). This is an important confirmation of climate change simulations, predicting that SLR will accelerate in response to global warming. In Australia, data collected by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) show the current global mean sea level trend to be 3.2 mm (0.13 in) per year, a doubling of the rate during the 20th century.
Some regional differences are also visible in the tide gauge data. Some of the recorded regional differences are due to differences in the actual sea level, while other are due to vertical land movements. In the United States for instance, considerable variation is found because some land areas are rising and some are sinking. Over the past 100 years, the rate of sea level rise varied from an increase of about 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.
There are three main contributions to sea level rise. Oceans expand if they are warming, glaciers at high altitudes melt and the total mass of ice sheets decreases. Sea level rise in the last 150 years was dominated by retreat of glaciers and expansion of the ocean, but the contributions of the two large ice sheets (Greenland and Antarctica) is expected to increase in the 21st century. The ice sheets store most of the land ice (∼99.5%), with a sea-level equivalent (SLE) of 7.4 m (24 ft) for Greenland and 58.3 m (191 ft) for Antarctica.
Each year about 8 mm (0.31 in) of precipitation (liquid equivalent) falls on the ice sheets in Antarctica and Greenland, mostly as snow, which accumulates and over time forms glacial ice. Much of this precipitation began as water vapor evaporated from the ocean surface. 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 glacier mass balance, important because a nonzero balance causes changes in global sea level. The rate of ice loss is accelerating.
In terms of heat content, it is the world ocean that dominates the atmospheric climate. The oceans store more than 90% of the heat in Earth's climate system and act as a buffer against the effects of climate change. For instance, an average temperature increase of the entire world ocean by 0.01 °C may seem small, but in fact it represents a very large increase in heat content. If all the heat associated with this anomaly was instantaneously transferred to the entire global atmosphere it would increase the average temperature of the atmosphere by approximately 10 °C. Thus, a small change in the mean temperature of the ocean represents a very large change in the total heat content of the climate system. Of course, when the ocean gains heat the water expands and this represents a component of global sea-level rise.
The thermal expansion of water increases with temperature and pressure of the water. Hence, cold Arctic Ocean water will expand less for a given increase in temperature compared to warm tropical water. Because different climate models have slightly different patterns of ocean heating, they do not agree fully on the predictions for the contribution of ocean heating on sea level rise.
The large volume of ice on the Antarctic continent stores around 70% of the world's fresh water. The Antarctic ice sheet mass balance is affected by snowfall accumulations, and ice discharge along the periphery. Under the influence of global warming, melt at the base of the ice sheet increases. Simultaneously, the capacity of the atmosphere to carry precipitation increases with temperature so that precipitation, in the form of snowfall, increases. Furthermore, the additional snowfall causes increased ice flow which leads to further loss of ice.
Different satellite methods for measuring ice mass and change are in good agreement, and combining methods leads to more certainty how the East Antarctic Ice Sheet, the West Antarctic Ice Sheet, and the Antarctic Peninsula evolve. A 2018 systematic review study estimated that ice loss across the entire continent was 43 gigatons (Gt) per year on average during the period from 1992 to 2002, but has accelerated to an average of 220 Gt per year during the five years from 2012 to 2017. Most of the melt comes from the West Antarctic Ice Sheet, but the Antarctic Peninsula also positively contributes. The East Antarctic Ice Sheet does not contribute much and scientists are not able to determine whether it gains or loses mass. The sea-level budget from Antarctica has been estimated to be 0.25 mm (0.0098 in) per year from 1993–2005, and 0.42 mm (0.017 in) per year from 2005 to 2015. All datasets generally show an acceleration of mass loss from the Antarctic ice-sheet, but with interannual variability (some years more, some less so).
The world's largest potential source of sea level rise is the East Antarctic Ice Sheet, which holds enough ice to raise global sea levels by 53.3 m. Satellite observations suggest the overall mass balance of the East Antarctic Ice Sheet was relatively steady or slightly positive for much of the period from 1992–2017, with the notable exception of Totten Glacier, which has steadily lost mass in recent decades  in response to ocean warming and possibly a reduction in local sea ice cover. Totten Glacier is the primary outlet of the Aurora Sublgacial Basin, which together with the Wilkes Basin are the two major ice reservoirs in East Antarctica which are subject to potential rapid collapse through marine ice sheet instability.
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 2017 suggests an increase in the melt from around 53 ± 29 Gt of ice per year to 159 ± 26 Gt. Significant acceleration of outflow glaciers in the Amundsen Sea Embayment may have contributed to this increase. The data showed that the Amundsen Sea sector of the West Antarctic Ice Sheet was discharging 250 cubic kilometres (60 cu mi) of ice every year, which was 60% more than the precipitation accumulation in the catchment areas. This alone was sufficient to raise the sea level at 0.24 mm (0.0094 in) per year. Further, thinning rates for the glaciers studied in 2002–2003 had increased over the values measured in the early 1990s.
Annual temperatures based on Byrd Station (central West Antarctica) from 1958 to 2010 increased linear by 2.4 ± 1.2 °C, the study authors note, "West Antarctica as one of the fastest-warming regions globally. In contrast to previous studies, we report statistically significant warming [...] particularly in December–January, the peak of the melting season. A continued rise in summer temperatures could lead to more frequent and extensive episodes of surface melting of the West Antarctic Ice Sheet."
Two types of instability are at play in West Antarctica. The first one is the Marine Ice Sheet Instability, the bedrock on which parts of the ice sheet rest is moving deeper inland. This means that when a part of the ice sheet melts, a thicker part of the ice sheet is exposed, which may lead to additional ice loss. Secondly, melting of the ice shelfs, the floating extensions of the ice sheet, leads to a process named the Marine Ice Cliff Instability. Because they function as a buttress to the ice sheet, their melt leads to additional ice flow. Melt of ice shelfs is accelerated when surface melt creates crevasses and these crevasses cause fracturing.
Since most of the bedrock underlying the West Antarctic Ice Sheet lies well below sea level and ocean waters are warming, the ice sheet is becoming less stable. A rapid collapse of West Antarctic Ice Sheet could raise sea level by 3.3 metres (11 ft) at an unknown rate.
The Thwaites and Pine Island glaciers have been identified to be potentially prone to these processes, since both glaciers bedrock topography gets deeper farther inland, exposing them to more warm water intrusion at the grounding line, and with the continued melt, retreat, eventually raising global sea levels.
Most ice on Greenland is part of the Greenland ice sheet which rises to an average of 2.135 kilometres (1.327 mi). The rest of the ice on Greenland is part of isolated glaciers and ice caps.
The sources contributing to sea level rise from Greenland are from ice sheet melting (70%) and from glacier calving (30%). Dust, soot, and microbes and algae living on parts of the ice sheet further enhance melting by darkening its surface and thus absorbing more thermal radiation; these regions grew by 12% between 2000 and 2012, and are likely to expand further. Estimates on future contribution to sea level rise from Greenland range from 0.3 to 3 metres (1 ft 0 in to 9 ft 10 in), for the year 2100.
Some of Greenland's largest outlet glaciers, such as Jakobshavn Isbræ and Kangerlussuaq Glacier have seen an acceleration in how fast they are flowing into the ocean. It was shown that this acceleration of outlet glaciers had mostly taken place on the Southern part of Greenland (66 N in 1996), but had spread further north (70 N) in 2005.
The contribution of the Greenland ice sheet on sea level over the next couple of centuries can be very high due to a self-reinforcing cycle (a so-called positive feedback). After an initial period of melting, the height of the ice sheet will have lowered. As air temperature increases closer to the sea surface, more melt starts to occur. This melting may further be accelerated because the color of ice is darker while it is melting. There is a threshold in surface warming beyond which a partial or near-complete melting of the Greenland ice sheet occurs. Different research has put this threshold value as low as 1.0 °C, and definitely 4.0 °C, above pre-industrial temperatures.:1170
Accounting for the Greenland ice sheet, its peripheral glaciers and ice caps, contributions to current sea level rise have been estimated to be 43%. A study published in 2017, concluded that Greenland’s glaciers and ice caps crossed an irreversible tipping point in 1997, and will continue to melt. Results based on the existing literature found estimates for the Greenland ice sheet and its glaciers and ice caps, they were the largest contributor to the observed sea level rise from land ice sources (excluding thermal expansion), combined accounting for 71 percent, or 1.32 mm per year during the 2012–2016 period.
Mountain glaciers include only a minor fraction of all water bound up in glaciers ice (<1%), compared to a bigger portion in Greenland and Antarctica (99%). Still, mountain glaciers have contributed appreciably to historical sea level rise and are set to contribute a smaller, but still significant fraction of sea level rise in the 21st century. The roughly 200,000 glaciers on earth are spread out across all continents. Different glaciers respond differently to increasing temperatures. For instance, valley glaciers that have a shallow slope already retreat under mild warming. Every glacier has a height above which there is net gain in mass and under which the glacier loses mass. If that height changes a bit, this has large consequences for glaciers with a shallow slope.:345 A large set of glaciers drain into the ocean and ice loss can therefore increase when ocean temperatures increase.
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. Over the 21st century, this is expected to rise, with glaciers contributing 7 to 24 centimetres (3 to 9 in) to global sea levels.:1165 Glaciers contributed around 40% to sea-level rise during the 20th century, with estimates for the 21st century of around 30%.
Sea ice melt has a very small contribution to global sea level rise. According to Archimedes' principle, sea ice that melts does not take up more volume than it had in the form of sea ice or icebergs. However, this only holds true in the case that the salinity of the sea ice and sea water are equal. This assumption is not valid in the case of melting sea ice, where the sea ice contains less salt than sea water. Fresh water has a larger volume compared to salt water, and as such there can be a small contribution of sea ice melt. In the case that all floating ice shelves and icebergs melt, the sea levels would rise only by about 4 cm (1.6 in).
Humans impact how much water is stored on land. Building dams prevents large masses of water from flowing into the sea and therefore increases the storage of water on land. On the other hand humans extract water from lakes, wetlands and underground reservoirs for food production leading to rising seas. Furthermore, the hydrological cycle is influenced by climate change and deforestation, which can lead to further positive and negative contributions to sea level rise. In the 20th century, these processes roughly balanced, but dam building has slowed down and is expected to stay low for the 21st century.:1155
There are broadly two ways of modelling sea level rise and making future projections. On the one hand, scientist use process-based modelling, where all relevant and well-understood physical processes are included in a physical model. An ice-sheet model is used to calculate the contributions of ice sheets and a general circulation model is used to compute the rising sea temperature and its expansion. A disadvantage of this method is that not all relevant processes might be understood to a sufficient level. Alternatively, some scientist use semi-empirical techniques that use geological data from the past to determine likely sea level responses to a warming world in addition to some basic physical modelling. Semi-empiral modelling relies on sophisticated statistical techniques. This type of modelling was partially motivated by the fact that in the 2007 IPCC report, most physical models underestimated the amount of sea level rise compared to observations.
The Intergovernmental Panel on Climate Change (IPCC) has made predictions of sea level changes to the year 2100, using the available scientific literature. Their projections are based on the contributors to sea level rise, but do exclude some processes that are less understood. In the case of rapid cuts in emission (the so-called RCP2.6 scenario), the IPCC deem it likely that the sea level will rise to 26–55 cm (10–22 in) with a 67% confidence interval. The higher value should thus not be read as an upper limit, which can be substantially higher. For a scenario with very high emissions, the IPCC project the sea level to rise to 52–98 cm (20–39 in). Compared to the previous IPCC estimate, more sea level rise is expected for similar scenarios.
Projections assessed by the US National Research Council (2010) suggest possible sea level rise over the 21st century of between 56 and 200 cm (22 and 79 in). The NRC describes the IPCC projections as "conservative". 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. Other estimates suggest that for the same period, global mean sea level could rise by 0.2 to 2.0 m (8 in to 6 ft 7 in), relative to the mean sea level in 1992.
The Third National Climate Assessment (NCA), released May 6, 2014, projected a sea level rise of 1 to 4 feet (0.3 to 1 m) by 2100. Decision makers who are particularly susceptible to risk may wish to use a wider range of scenarios from 20 to 200 cm (8 to 80 in) by 2100.
A 2016 study concluded that based on past climate change data, sea level rise could accelerate exponentially in the coming decades, with a doubling time of 10, 20 or 40 years, respectively, raising the ocean by several meters, in 50, 100 or 200 years. However, Greg Holland from the National Center for Atmospheric Research, who reviewed the study, noted: “There is no doubt that the sea level rise, within the IPCC, is a very conservative number, so the truth lies somewhere between IPCC and Jim.”
One 2017 study's scenario, assuming high fossil fuel use for combustion and strong economic growth during this century, projects sea level rise of up to 1.32 metres (4.3 ft) on average — and an extreme scenario with as much as 1.89 metres (6.2 ft), by 2100. This could mean rapid sea level rise of up to 19 millimeters per year by the end of the century. The study also concluded that the Paris climate agreement emissions scenario, if met, would result in a median 0.52 metres (1.7 ft) of sea level rise by 2100.
Estimates of future sea level were also produced in the 20th century. For instance, Hansen et al. 1981, published the study Climate impact of increasing atmospheric carbon dioxide, and predicted that anthropogenic carbon dioxide warming and its potential effects on climate in the 21st century could cause a sea level rise of 5 to 6 m (16 to 20 ft), from melting of the West Antarctic ice-sheet alone.
There is a widespread consensus that substantial long-term sea-level rise will continue for centuries to come even if the temperature stabilizes. IPCC AR4 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.
Melting of the Greenland ice sheet could contribute an additional 4 to 7.5 m (10 to 20 ft) over many thousands of years. It has been estimated that we are already committed to a sea-level rise of approximately 2.3 m (7 ft 7 in) for each degree of temperature rise within the next 2,000 years. Warming beyond the 2 °C target would potentially lead to rates of sea-level rise dominated by ice loss from Antarctica. Continued carbon dioxide emissions from fossil fuel sources could cause additional tens of metres of sea level rise, over the next millennia, and ultimately melt the entire Antarctic ice sheet, causing about 58 m (190 ft) of sea level rise.
After 500 years, sea-level rise from thermal expansion alone may have reached only half of its eventual level, which models suggest may lie within ranges of 0.5 to 2 m (1 ft 8 in to 6 ft 7 in).
Many ports, urban conglomerations, and agricultural regions are built on river deltas, where subsidence of land contributes to a substantially increased effective sea level rise. This is caused by both unsustainable extraction of groundwater (in some places also by extraction of oil and gas), and by levees and other flood management practices that prevent accumulation of sediments from compensating for the natural settling of deltaic soils. In many deltas, this results in subsidence ranging from several millimeters per year up to possibly 25 centimeters per year in parts of the Ciliwung delta (Jakarta). Total anthropogenic-caused subsidence in the Rhine-Meuse-Scheldt delta (Netherlands) is estimated at 3 to 4 m (9.8 to 13.1 ft), over 3 m (9.8 ft) in urban areas of the Mississippi River Delta (New Orleans), and over 9 m (30 ft) in the Sacramento-San Joaquin River Delta.
The Atlantic is set to warm at a faster pace than the Pacific. This has consequences for Europe and the U.S. East Coast, which may receive a sea level rise 3–4 times the global average. The downturn of the Atlantic meridional overturning circulation (AMOC) has been also tied to extreme regional sea level rise on the US Northeast Coast.
Current and future climate change is set to have a number of impacts, particularly on coastal systems. Such impacts 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.:356
Many of these impacts are 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. River deltas and small island states are particularly vulnerable to sea-level rise.
Sea level rise could also displace many shore-based populations: for example it is estimated that a sea level rise of just 200 mm (7.9 in) could make 740,000 people in Nigeria homeless.
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.
Atolls and low-lying coastal areas on islands are particularly vulnerable to sea level rise. Possible impacts include coastal erosion, floodings and salt intrusion into soils and freshwater. It is difficult to assess how much of past erosion and floods have been caused by sea level change, compared to other environmental events such as hurricanes. Adaptation to sea level rise is costly for small island nation as a large portion of their population lives in areas that are at risk.
Maldives, Tuvalu, and other low-lying countries are among the areas that are at the highest level of risk. At current rates, sea level would be high enough to make the Maldives uninhabitable by 2100. Geomorphological events such as storms tend to have larger impacts on reef island than sea level rise, for instance at one of the Marshall Islands. These effects include the immediate erosion and subsequent regrowth process that may vary in length from decades to centuries, even resulting in land areas larger than pre-storm values. With an expected rise in the frequency and intensity of storms, they may become more significant in determining island shape and size than sea level rise. Five of the Solomon Islands have disappeared due to the combined effects of sea level rise and stronger trade winds that were pushing water into the Western Pacific.
In the case all islands of an island nation become uninhabitable or completely submerged by the sea, the states themselves would also become dissolved. Once this happens, all rights on the surrounding area (sea) are removed. This area can be huge as rights extend to a radius of 224 nautical miles (415 km; 258 mi) around the entire island state. Any resources, such as fossil oil, minerals and metals, within this area can be freely dug up by anyone and sold without needing to pay any commission to the (now dissolved) island state.
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. 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 Venice, London, New Orleans, and New York City already need storm-surge defenses, and will need more if the sea level rises; they also face issues such as subsidence. The Egyptian city Alexandria faces a similar situation, where hundreds of thousands people living in the low-lying areas may already have to be relocated in the coming decade. The nearby farmland in the Nile Delta is also affected by salt water flooding. However, modest increases in sea level are likely to be offset when cities adapt by constructing sea walls or through relocating.
Re-insurance company Swiss Re estimates an economic loss for southeast Florida of $33 billion in 2030 from climate-related damages. Miami has been listed as "the number-one most vulnerable city worldwide" in terms of potential damage to property from storm-related flooding and sea-level rise. According to a 2011 study conducted by the U.S. Geological Survey, 68 percent of beaches in New England, and the mid-Atlantic states observe coastal erosion, with some barrier beaches in Louisiana recording twenty or more meters or eroding coastlines per year. However, the rate of coastal erosion is partially related to human developments, eg, bulldozing dunes.
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 (16 in) in the Bay of Bengal would put 11 percent of the Bangladesh's coastal land underwater, creating 7–10 million environmental migrants.
Coastal ecosystems are facing drastic changes as a consequence of rising sea levels. Many systems might ultimately be lost when sea levels rise too much or too fast. Some ecosystems can move land inward with the high-water mark, but many are prevented from migrating due to natural or man-made barriers. This 'coastal squeeze' could result in the loss of habitats such as mudflats and marshes.
Mangroves, one of the most widely studied ecosystems is the world, adjust to rising sea levels by building vertically using accumulated sediment and organic matter. If sea level rise is too high, they will not be able to keep up and be submerged instead. Human activities, such as dam building, restrict sediment supplies to wetlands, and thereby prevent natural adaptation processes. As mangroves protects the effects of storm surges, waves and tsunamis, losing them makes the effects of sea level rise worse.
When seawater approaches inland, problems related to contaminated soils and flooded wetlands may occur. Also, fish, birds, and coastal plants could lose parts of their habitat. In 2016, it was reported that the Bramble Cay melomys, which lived on a Great Barrier Reef island, had probably become extinct because of sea level rises.
Adaptation options to sea level rise can be broadly classified into retreat, accommodate and protect. Retreating is moving people and infrastructure to less exposed areas and preventing further development in areas that are at risk. This type of adaptation is potentially disruptive, as displacement of people might lead to tensions. Accommodation options are measurements that make societies more flexible to sea level rise. Examples are the cultivation of food crops that tolerate a high salt content in the soil and making new building standards which require building to be built higher and have less damage in the case a flood does occur. Finally, areas can be protected by the construction of dams, dikes and by improving natural defenses.
These adaptation options can be further divided into hard and soft. Hard adaptation relies mostly on capital-intensive human-built infrastructure and involves large-scale changes to human societies and ecological systems. Because of its large scale, it is often not flexible. Soft adaptation involves strengthening natural defenses and adaptation strategies in local communities and the use of simple and modular technology, which can be locally owned. The two types of adaptation might be complementary or mutually exclusive. The building of a dike (hard adaptation) for instance destroys the natural dune system and dune nourishment will not be possible anymore.
In 2008, the Dutch Delta Commission, 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 included drawing up worst-case plans for evacuations. The plan also included more than €100 billion (US$113 billion) in new spending through to the year 2100 to implement precautionary 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 1.3 metres (4 ft 3 in) by 2100 and plan for a 2-to-4-metre (7 to 10 ft) rise by 2200. About a quarter of the Netherlands lies beneath sea level, while more than 50 percent of the nation's area would be inundated by tidal floods if it did not have an extensive levee system.
The New York City Panel on Climate Change (NPCC) is an effort to prepare the New York City area for climate change. Miami Beach is spending $500 million from 2015 to 2020 to address sea-level rise. Actions include a pump drainage system, and raising of roadways and sidewalks. U.S. coastal cities also conduct so called beach nourishment, also known as beach replenishment, where new beach sand is trucked in and added.
(From pg 250) Even if sea-level rise were to remain in the conservative range projected by the IPCC (0.6–1.9 feet [0.18–0.59 m])—not considering potentially much larger increases due to rapid decay of the Greenland or West Antarctic ice sheets—tens of millions of people worldwide would become vulnerable to flooding due to sea-level rise over the next 50 years (Nicholls, 2004; Nicholls and Tol, 2006). This is especially true in densely populated, low-lying areas with limited ability to erect or establish protective measures. In the United States, the high end of the conservative IPCC estimate would result in the loss of a large portion of the nation's remaining coastal wetlands. The impact on the east and Gulf coasts of the United States of 3.3 feet (1 m) of sea-level rise, which is well within the range of more recent projections for the 21st century (e.g., Pfeffer et al., 2008; Vermeer and Rahmstorf, 2009), is shown in pink in Figure 7.7. Also shown, in red, is the effect of 19.8 feet (6 m) of sea-level rise, which could occur over the next several centuries if warming were to continue unabated.
Considerable disparity remains between these estimates due to the inherent uncertainties of each method, the lack of detailed comparison between independent estimates, and the effect of temporal modulations in ice sheet surface mass balance. Here, we present a consistent record of mass balance for the Greenland and Antarctic ice sheets over the past two decades, validated by the comparison of two independent techniques over the past eight years: one differencing perimeter loss from net accumulation, and one using a dense time series of timevariable gravity. We find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss.
The Organization for Economic Co-operation and Development lists Miami as the number-one most vulnerable city worldwide in terms of property damage, with more than $416 billion in assets at risk to storm-related flooding and sea-level rise.
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