Question: How have sea levels changed in the last 50 years?

Sea level rise




At least since 1880, the average global sea level has been rising. Between 1900 and 2016, sea level rose by approximately 16 to 21 cm (7-8 inch).[1] More precise data gathered from satellite radar measurements reveals an accelerating rise of 7.5cm (3.0in) from 1993 to 2017,[2]:1554 which is a trend of roughly 30cm (12in) per century. The acceleration is due mostly to human-caused global warming, which is driving thermal expansion of seawater and melt of land-based ice sheets and glaciers.[3] The current trend is expected to further accelerate during the 21st century.[4]:62
Projecting future sea level is challenging, due to the complexity of many aspects of the climate system. As climate research leads to improved computer models, projections have consistently increased.  For example, in 2007 the Intergovernmental Panel on Climate Change (IPCC) projected a high end estimate of 60cm (2ft) through 2099,[5] but their 2014 report raised the high-end estimate to about 90cm (3ft).[6] A number of later studies have concluded that a global sea level rise of 200 to 270cm (6.6 to 8.9ft) this century is "physically plausible".[7] The contributing factors to sea level rise between 1993 and 2018 are thermal expansion of the oceans (42%), melting of temperate glaciers (21%), Greenland (15%) and Antarctica (8%).[2]
The sea level will not rise uniformly everywhere on Earth, and it will even drop in some locations.[8] Local factors include tectonic effects and subsidence of the land, tides, currents and storms. Each Celsius degree of temperature rise is estimated to trigger a sea level rise of approximately 2.3 metres (7ft 7in), but because of climate inertia this rise would happen over the next two millennia.[9]
Sea level rises can influence human populations considerably in coastal and island regions, and natural environments like marine ecosystems.[10] Widespread coastal flooding is expected with several degrees of warming sustained for millennia.[11] Further effects are higher storm-surges and more dangerous tsunamis, displacement of populations, loss and degradation of agricultural land and damage in cities.[12][13][14]
Societies can respond to sea level rise in three different ways: retreat, accommodate and protect. Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made between different strategies.[15] Ecosystems that adapt to rising sea levels by moving inland might not always be able to do so, due to natural or man-made barriers.[16]
Past changes in sea level
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 pre-industrial temperatures, sea levels were at least 5 metres (16ft) higher than now: this was when warming because of changes in the amount of sunlight due to slow changes in the Earth's orbit caused the last interglacial. The warming was sustained over a period of thousands of years and the magnitude of the rise in sea level implies a large contribution from the Antarctic and Greenland ice sheets.[17]:1139
Since the last glacial maximum about 20,000 years ago, the sea level has risen by more than 125 metres (410ft), 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.[18]
Sea level measurement
Sea level changes can be driven either by variations in the amount of water in the oceans, the volume of the ocean or by changes of the land compared to the sea surface. The different techniques used to measure changes in sea level do not measure exactly the same. Tide gauges can only measure relative sea level, whilst satellites can also measure absolute sea level changes.[19] To get precise measurements for sea level, researchers studying the ice and the oceans on our planet factor in ongoing deformations of the solid Earth, in particular due to landmasses still rising from past ice masses retreating, and also the Earth's gravity and rotation.[2]
Satellites

Since the 1992 launch of TOPEX/Poseidon, altimetric satellites have been recording the change in sea level.[20] 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 records 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).[21] Current rates of sea level rise from satellite altimetry have been estimated to be 3.0±0.4 millimetres (0.118±0.016in) per year for the period 1993–2017.[22] 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.[23]Satellites are useful for measuring regional variations in sea level, such as the substantial rise between 1993 and 2012 in the western tropical Pacific. This sharp rise 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.[24] 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.[25]
Tide gauges
Another important source of sea-level observations is the global network of tide gauges. In contrast to the satellite record, this record has a lot of spatial and temporal gaps.[26] Coverage of tide gauges started primarily in the Northern Hemisphere, with data for the Southern Hemisphere remaining scarce up to the 1970s.[26] The longest running sea-level measurements, NAP or Amsterdam Ordnance Datum established in 1675, are recorded in Amsterdam, the Netherlands.[27] 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[28] 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 19.5cm (7.7in) between 1870 and 2004 at an average rate of about 1.44mm/yr (1.7mm/yr during the 20th century).[29] This is an important confirmation of climate change simulations which predicted that sea level rise would 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 3.2mm (0.13in) per year, a doubling of the rate during the 20th century.[30][31]
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 Europe for instance, considerable variation is found because some land areas are rising while others are sinking. Since 1970, most tidal stations have measured higher seas, but sea levels have dropped along the northern Baltic Sea due to post-glacial rebound.[32]
Contributions

The three main reasons warming causes global sea level to rise are: oceans expand, ice sheets lose ice faster than it forms from snowfall, and glaciers at higher altitudes also melt. 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.[3] The ice sheets store most of the land ice (∼99.5%), with a sea-level equivalent (SLE) of 7.4m (24ft) for Greenland and 58.3m (191ft) for Antarctica.[2]
Each year about 8mm (0.31in) 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. Some of the snow is blown away by wind or disappears from the ice sheet by melt of by directly changing into a gas. If the rest of the ice returns to the ocean as icebergs and from melting at the edges then the icesheet remains the same mass and does not affect sea level. However scientists have found that ice is being lost, and at an accelerating rate.[33][34]
Ocean heating

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.[35] 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. 
When the ocean gains heat the water expands and sea level rises.  The amount of expansion varies with both water temperature and pressure.  For each degree, warmer water and water under great pressure (due to depth) expand more than cooler water and water under less pressure.[17]:1161  This means that cold Arctic Ocean water will expand less 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.[36]
Antarctica

The large volume of ice on the Antarctic continent stores around 70% of the world's fresh water.[37] 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 in global and regional models. The additional snowfall causes increased ice flow which leads to further loss of ice.[38] Snowfall increased over the last two centuries, but no increase was found in the interior of Antarctica over the last four decades.[39] Based on changes of Antarctica's ice mass balance over millions of years, due to natural climate fluctuations, researchers concluded that the sea-ice acts as a barrier for warmer waters surrounding the continent. Consequently, the loss of sea ice is a major driver of the instability of the entire ice sheet.[39]
Different satellite methods for measuring ice mass and change are in good agreement, and combining methods leads to more certainty about how the East Antarctic Ice Sheet, the West Antarctic Ice Sheet, and the Antarctic Peninsula evolve.[40] 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.[41] Most of the melt comes from the West Antarctic Ice Sheet, but the Antarctic Peninsula and East Antarctic Ice Sheet also contribute. The sea-level rise due to Antarctica has been estimated to be 0.25 mm per year from 1993–2005, and 0.42 mm per year from 2005 to 2015. All datasets generally show an acceleration of mass loss from the Antarctic ice-sheet, but with year-to-year variations.[2]
East Antarctica
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 53.3m (175ft).[42] The ice sheet has historically been considered to be relatively stable and has therefore attracted less scientific attention and observations compared to West Antarctica.[39] A combination of satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance suggests the overall mass balance of the East Antarctic Ice Sheet was relatively steady or slightly positive for much of the period 1992–2017.[43] A 2019 study however, using different methodology, concluded that East Antarctica is losing significant amounts of ice mass. The lead scientist Eric Rignot told CNN: "melting is taking place in the most vulnerable parts of Antarctica ... parts that hold the potential for multiple meters of sea level rise in the coming century or two."[39]
Methods agree that the Totten Glacier has lost ice in recent decades in response to ocean warming[44][45] and possibly a reduction in local sea ice cover.[46] Totten Glacier is the primary outlet of the Aurora Subglacial Basin, a major ice reservoir in East Antarctica that could rapidly retreat due to hydrological processes.[47] The global sea level potential of 3.5 m flowing through Totten Glacier alone is of similar magnitude to the entire probable contribution of the West Antarctic Ice Sheet.[48] The other major ice reservoir on East Antarctica that might rapidly retreat is the Wilkes Basin which is subject to marine ice sheet instability.[47] Ice loss from these outlet glaciers is possibly compensated by accumulation gains in other parts of Antarctica.[43]
West Antarctica
Even though East Antarctica contains the largest potential source of sea level rise, it is West Antarctica that currently experiences a net outflow of ice, causing sea levels to rise. Using different satellites from 1992 to 2017 shows melt is increasing significantly over this period. Antarctica as a whole has caused a total of  7.6±3.9mm (0.30±0.15in) of sea level rise. Considering the mass balance of the East Antarctic Ice Sheet which was relatively steady, the major contributor was West Antarctica.[41] Significant acceleration of outflow glaciers in the Amundsen Sea Embayment may have contributed to this increase.[49] In contrast to East Antarctica and the Antarctic Peninsula, temperatures on West Antarctica have increased significantly with a trend between 0.08 °C (0.14 °F) per decade and 0.96 °C (1.7 °F) per decade between 1976 and 2012.[50]
Multiple types of instability are at play in West Antarctica. One is the Marine Ice Sheet Instability, where the bedrock on which parts of the ice sheet rest is deeper inland. This means that when a part of the ice sheet melts, a thicker part of the ice sheet is exposed to the ocean, which may lead to additional ice loss. Secondly, melting of the ice shelves, 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 (see animation one minute into video). Melt of ice shelves is accelerated when surface melt creates crevasses and these crevasses cause fracturing.[51]
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. With continued melt and retreat they contribute to raising global sea levels.[52][53] Most of the bedrock underlying the West Antarctic Ice Sheet lies well below sea level.[54] A rapid collapse of the West Antarctic Ice Sheet could raise sea level by 3.3 metres (11ft) at an unknown rate.[55][56]
Greenland

Most ice on Greenland is part of the Greenland ice sheet which rises to an average of 2.135 kilometres (1.327mi). 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.[58] Average annual ice loss in Greenland more than doubled in the early 21st century compared to the 20th century.[59] Some of Greenland's largest outlet glaciers, such as Jakobshavn Isbræ and Kangerlussuaq Glacier, are flowing faster into the ocean.[60][61]
43% of current sea level rise is due to the Greenland ice sheet, its peripheral glaciers and ice caps. A study published in 2017 concluded that Greenland’s peripheral glaciers and ice caps crossed an irreversible tipping point around 1997, and will continue to melt.[62][63] The Greenland ice sheet and its glaciers and ice caps are the largest contributor to sea level rise from land ice sources (excluding thermal expansion), combined accounting for 71 percent, or 1.32mm per year during the 2012–2016 period.[64]
Estimates on future contribution to sea level rise from Greenland range from 0.3 to 3 metres (1 to 10ft), for the year 2100.[58] 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°C (2 ℉), and definitely 4°C (7 ℉), above pre-industrial temperatures.[65][17]:1170
Glaciers

Less than 1% of glacier ice is in mountain glaciers, compared to 99% in Greenland and Antarctica. 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.[66] The roughly 200,000 glaciers on earth are spread out across all continents.[67] Different glaciers respond differently to increasing temperatures. For instance, valley glaciers that have a shallow slope retreat under even 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.[68]:345 Many glaciers drain into the ocean and ice loss can therefore increase when ocean temperatures increase.[67]
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 per year, averaged over the 20th century.[69] Over the 21st century, this is expected to rise, with glaciers contributing 7 to 24cm (3 to 9in) to global sea levels.[17]:1165 Glaciers contributed around 40% to sea-level rise during the 20th century, with estimates for the 21st century of around 30%.[2]
Sea ice
Sea ice melt contributes very slightly to global sea level rise. If the melt water from ice floating in the sea was exactly the same as sea water then, according to Archimedes' principle, no rise would occur. However melted sea ice contains less dissolved salt than sea water and is therefore less dense: in other words although the melted sea ice weighs the same as the sea water it was displacing when it was ice, its volume is still slightly greater. If all floating ice shelves and icebergs were to melt sea level would only rise by about 4cm (1.6in).[70]
Land water storage

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.[71][17]:1155
Projections
There are broadly two ways of modelling sea level rise and making future projections. On the one hand, scientists 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.[3] Semi-empirical sea level models rely on statistical techniques, using relationships between observed (contributions to) global mean sea level and global mean temperature.[73] This type of modelling was partially motivated by the fact that in previous IPCC assessments most physical models underestimated the amount of sea level rise compared to observations of the 20th century.[17]
Projections for the 21st century
The 2013 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 RCP2.6 scenario), the IPCC deem it likely that the sea level will rise by 26–55cm (10–22in) 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 by 52–98cm (20–39in).[17] Compared to the previous IPCC estimate of 2007, more sea level rise is expected for similar scenarios.[74]
Since the publication of the 2013 IPCC assessment, attempts have been made to include more physical processes and to develop models that can reproduce sea level rise from paleoclimate data better. This typically led to higher estimates of sea level rise.[51][47][75] For instance, a 2016 study led by Jim Hansen 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.[75] 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.”[76]
In addition, 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 132cm (4.3ft) on average — and an extreme scenario with as much as 189cm (6.2ft), 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 52cm (20in) of sea level rise by 2100.[77][78]
According to the Fourth (2017) National Climate Assessment (NCA) of the United States it is very likely sea level will rise between 30 and 130cm (1.0 - 4.3 feet) in 2100 compared to the year 2000; and 2.4 m (8 feet) is physically possible but the authors were unable to say how likely.[1]
The possibility of a collapse of the West-Antarctic ice sheet and subsequent rapid sea level rise was suggested back in the 1970s.[51] For instance, Mercer published a study in 1978 predicting that anthropogenic carbon dioxide warming and its potential effects on climate in the 21st century could cause a sea level rise of around 5 metres (16ft) from melting of the West Antarctic ice-sheet alone.[79][51]
Long-term sea level rise


There is a widespread consensus that substantial long-term sea-level rise will continue for centuries to come even if the temperature stabilizes.[80] Models are able to reproduce paleo records of sea level rise, which provides confidence in their application to long term future change.[17]:1189
It is thought that both the Greenland ice sheet and Antarctica have tipping points for warming levels that could be reached before the end of the 21st century. Crossing such tipping points implies that ice-sheet changes are potentially irreversible: a decrease to pre-industrial temperatures may not stabilize the ice sheet once the tipping point has been crossed.[81] Quantifying the exact temperature change for which this tipping point is crossed remains controversial. For Greenland, estimates roughly range between 1 and 4°C (2 to 7 ℉) above pre-industrial.[81][17]
Melting of the Greenland ice sheet could contribute an additional 4 to 7.5m (13 to 25ft) over many thousands of years.[11] It has been estimated that we are committed to a sea-level rise of approximately 2.3m (7ft 7in) for each degree of temperature rise within the next 2,000 years.[9] 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 58m (190ft) of sea level rise.[82] 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 2m (2 to 7ft).[83]
Regional sea level change
Subsidence, isostatic rebound, gravitational effects of changing ice masses, and spatially varying patterns of warming lead to differences in sea level change around the globe,[84] including regions with sea level fall (near current and former glaciers and ice sheets).[17] The gravitational effects comes into play when a large ice sheet melts. With the loss of mass, the gravitational pull becomes less and local water levels might drop. Further away from the ice sheet water levels will increase more than average. In this light, melt in Greenland has a different fingerprint on regional sea level than melt in Antarctica.[19]
Many ports, urban conglomerations, and agricultural regions are built on river deltas, where subsidence of land contributes to a substantially increased relative 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.[85] In many deltas, this results in subsidence ranging from several millimeters per year up to 25cm (10in) per year in parts of the Ciliwung delta (Jakarta).[86] Total human-caused subsidence in the Rhine-Meuse-Scheldt delta (Netherlands) is estimated at 3 to 4m (10 to 13ft), over 3m (10ft) in urban areas of the Mississippi River Delta (New Orleans), and over 9m (30ft) in the Sacramento-San Joaquin River Delta.[87]
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.[88] The downturn of the Atlantic meridional overturning circulation (AMOC) has been also tied to extreme regional sea level rise on the US Northeast Coast.[89]
Effects
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.[12]:356
Many of these impacts are detrimental — especially for the three-quarters of the world's poor who depend on agriculture.[90] 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 in Africa and Asia and small island states are particularly vulnerable to sea-level rise.[91]
Globally tens of millions of people will be displaced in the latter decades of the century if greenhouse gases are not reduced drastically. Many coastal areas have large population growth, which results in more people at risk from sea level rise. The rising seas pose both a direct risk: unprotected homes can be flooded, and indirect threats of higher storm surges, tsunamis and king tides. Asia has the largest population at risk from sea level with countries such as Bangladesh, China, India, Indonesia, and Vietnam having very densely populated coastal areas.[92] The effects of displacement are very dependent on how successful governments will be in implementing defenses against the rising sea, with concerns for the poorerst countries such as sub-Saharan countries and island nations.[93]

Coastal areas
Ten per cent of the world's population live in coastal areas that are less than 10 metres (33ft) above sea level. Furthermore, two thirds of the world's cities with over five million people are located in these low-lying coastal areas.[94] Future sea level rise could lead to potentially catastrophic difficulties for shore-based communities in the next centuries: for example, millions of people will be affected in cities such as Miami, Rio de Janeiro, Osaka (Japan) and Shanghai if following the current trajectory of 3°C.[14] The Egyptian city Alexandria faces a similar situation, where hundreds of thousands of people living in the low-lying areas may already have to be relocated in the coming decade.[95] However, modest increases in sea level are likely to be offset when cities adapt by constructing sea walls or through relocating.[96] 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.[97]
Rising seas has also been tied to an increased risk from tsunamis, potentially affecting coastal cities in the Pacific and Atlantic Oceans.[13]
Food production in coastal areas is affected by rising sea levels as well. Due to flooding and salt water intrusion into the soil, the salinity of agricultural lands near the sea increases, posing problems for crops that are not salt-resistant. Furthermore, salt intrusion in fresh irrigation water poses a second problem for crops that are irrigated. Newly developed salt-resistant crop variants are currently more expensive than the crops they are set to replace.[98] The farmland in the Nile Delta is affected by salt water flooding,[95] and there is now more salt in the soil and irrigation water in the Red River Delta and the Mekong Delta in Vietnam.[98] Bangladesh and China are affected in a similar way, particularly their rice production.[99]
Island nations
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 nations as a large portion of their population lives in areas that are at risk.[100] 
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.[101][102] 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.[103] 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.[104]
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 significant as rights extend to a radius of 224 nautical miles (415km; 258mi) 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.[105]
Ecosystems

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 narrowing, sometimes called 'coastal squeeze' when considering man-made barriers, could result in the loss of habitats such as mudflats and marshes.[16][106] Mangroves and tidal marshes adjust to rising sea levels by building vertically using accumulated sediment and organic matter. If sea level rise is too rapid, they will not be able to keep up and will instead be submerged.[107] As both ecosystems protect against storm surges, waves and tsunamis, losing them makes the effects of sea level rise worse.[108][109] Human activities, such as dam building, may restrict sediment supplies to wetlands, and thereby prevent natural adaptation processes. The loss of some tidal marshes is unavoidable as a consequence.[110]
When seawater reaches inland, problems related to contaminated soils may occur. Also, fish, birds, and coastal plants could lose parts of their habitat.[111] Coral, important for bird and fish life, needs to grow vertically to remain close to the sea surface in order to get enough energy from sunlight. It has so far been able to keep up the vertical growth with the rising seas, but might not be able to do so in the future.[112] In 2016, it was reported that the Bramble Cay melomys, which lived on a Great Barrier Reef island, had probably become extinct because of inundation due to sea level rises.[113]
Adaptation

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.[15][114]

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.[114][115]
Many countries are developing concrete plans for adaptation. An example is the extension of the Dutch Delta Works. 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 following 190 years. This included drawing up worst-case plans for evacuations. The plan also included more than €100 billion (US$118 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 (4ft 3in) by 2100 and plan for a 2–4 metres (7–13ft) m rise by 2200.[116] About a quarter of the Netherlands lies beneath sea level, while more than 50% 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.[117] U.S. coastal cities also conduct so called beach nourishment, also known as beach replenishment, where new beach sand is trucked in and added.[118]
Options that have been proposed to assist island nations to adapt to rising sea level include abandoning islands, building dikes, and "building upwards."[119]
See also
Coastal development hazards
Coastal sediment supply
Effects of global warming on oceans
Effects of climate change on island nations
Hydrosphere
Islands First
Marine transgression
Notes


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 (pb: 978-0-521-70596-7).



Further reading






 Outlines eight steps a community can take to develop site-appropriate scenarios


 on YouTube – National Geographic film based on the 2007 book Six Degrees: Our Future on a Hotter Planet