Sea level rise

Sea level rise is not uniform around the globe. Some land masses are moving up or down as a consequence of subsidence (land sinking or settling) or post-glacial rebound (land rising due to the loss of the weight of ice after melting), so that local relative sea level rise may be higher or lower than the global average. There are even regions near current and former glaciers and ice sheets where sea level falls. Furthermore, gravitational effects of changing ice masses and spatially varying patterns of warming lead to differences in the distribution of sea water around the globe.[24][25] 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.[26]

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.[27]^: 638[28]^: 88 Total human-caused subsidence in the Rhine-Meuse-Scheldt delta (Netherlands) is estimated at 3–4 m (10–13 ft), over 3 m (10 ft) in urban areas of the Mississippi River Delta (New Orleans), and over 9 m (30 ft) in the Sacramento–San Joaquin River Delta.[28]^: 81–90 On the other hand, post-glacial isostatic rebound causes relative sea level fall around the Hudson Bay in Canada and the northern Baltic.[29]

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 received a sea level rise 3–4 times the global average.[30] The downturn of the Atlantic meridional overturning circulation (AMOC) has been also tied to extreme regional sea level rise on the US Northeast Coast.[31]

In its fifth assessment report (2013) the Intergovernmental Panel on Climate Change (IPCC) estimated how much sea level is likely to rise in the 21st century based on different levels of greenhouse gas emissions. These were conservative projections based on well-known factors which contribute to sea level rise, but exclude other processes which are less well understood. If emissions remain very high, the IPCC projected sea level would rise by 50–98 cm (19+1⁄2–38+1⁄2 in).[25] In August 2020 scientists reported that observed ice-sheet losses in Greenland and Antarctica were tracking worst-case scenarios of those IPCC projections.[34][35] In 2021, the IPCC Sixth Assessment Report had barely changed its worst-case scenario (updating it to 0.6–1 m), whereas most other reports since 2020 have increased their upper estimates to at least 2 m in the very high emissions scenario.[5]

Other attempts have been made to include more physical processes and to develop models that can project sea level rise using paleoclimate data. This typically led to higher estimates of sea level rise.[36][37][38] 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 metres in 50, 100 or 200 years.[38] 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."[39]

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 130 cm (4 ft 3 in) on average—and an extreme scenario with as much as 190 cm (6 ft 3 in), by 2100. This could mean rapid sea level rise of up to 19 mm (3⁄4 in) 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 52 cm (20+1⁄2 in) of sea level rise by 2100.[40][41]

The Fourth (2017) National Climate Assessment (NCA) of the United States found that it is very likely sea level will rise between 30 and 130 cm (1.0–4.3 feet) in 2100 compared to the year 2000. A rise of 2.4 m (8 feet) is physically possible under a high emission scenario but the authors were unable to say how likely. This worst-case scenario can only come about with a large contribution from Antarctica; a region that is difficult to model.[42]

The possibility of a collapse of the West Antarctic ice sheet and subsequent rapid sea level rise was suggested back in the 1970s.[36] 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 m (15 ft) from melting of the West Antarctic ice-sheet alone.[43][36]

In 2019, a study projected that in low emission scenario, sea level will rise 30 cm by 2050 and 70 cm by 2100, relative to the level in 2000. In high emission scenario, it will be roughly 35 cm by 2050 and 110 cm by 2100. There is the probability that the rise will be beyond 2 metres by 2100 in the high emission scenario, which would cause displacement of 187 million people. Long before then, current projections of sea level rise for 2050 will generate an intense rise in the frequency of coastal flooding (a baseline of four "moderate" flooding events per year in the US), even without storms and/or heavy rainfall.[44][45][46]

In September 2019 the Intergovernmental Panel on Climate Change published SROCC, a report about the impact of climate change on the oceans including sea level rise. In February 2021, a paper suggested that the projections for global sea level rise by 2100 reported in the IPCC Fifth Assessment Report were well below what one can expect from an extrapolation of past sea level trends, as well as below the combined projections of an expert elicitation, and thus are likely to have been too conservative. The authors noted that the projections in SROCC were substantially closer to the extrapolated trends, but still appeared too conservative and less plausible than the combined expert elicitation.[47] The projections in the August 2021 IPCC Sixth Assessment Report were ultimately slightly larger than those in SROCC for the low-level warming, but much larger at the upper-end for the high-level warming, due to the addition of a "low-confidence" narrative involving marine ice cliff instability.[48]

Sea level rise by 2100 that is much higher than the likely range of projection by the IPCC Sixth Assessment Report (AR6) is possible. In particular, AR6 models do not take into consideration many positive feedback loops that would lead to accelerated melting over time. AR6 does offer a speculative narrative about how more extreme melting could occur. It would probably involve a high warming scenario, in which urgent climate action is not taken and strong warming continues.[2] This could lead to a faster-than-projected collapse of marine ice shelves and the abrupt, widespread onset of marine ice cliff instability (a process in which ice cliffs collapse under their own weight) and ice sheet instability (a positive feedback loop that leads to runaway melting) in Antarctica, and a faster than projected ice loss in Greenland including reduced albedo as its ice melts.[2] Other processes in Antarctica such as hydrofracturing (in which meltwater collects in ice sheet fractures, forcing them open), increased contact of warm ocean water with ice shelves due to climate change induced ocean circulations changes[49][50][51] or changes in the weather over Antarctica leading to lower precipitation and ice deposition, could all be contributing factors.[51] The combination of such processes could lead to sea level rise as high as 2.3 metres by 2100.[2]

Map of the Earth with a long-term 6-metre (20 ft) sea level rise represented in red (uniform distribution, actual sea level rise will vary regionally and local adaptation measures will also have an effect on local sea levels).

There is a widespread consensus among climate scientists that sea level rise lags well behind the temperature increase that triggers it, and that substantial long-term sea level rise will continue for centuries to come even if the temperature stabilizes.[52] Models are able to reproduce paleo records of sea level rise, which provides confidence in their application to long-term future change.[25]^: 1189

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 would mean 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.[53] 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 °F) above pre-industrial.[53][25] As of 2020, the lower of these values has already been passed. A 2021 analysis of sub-glacial sediment at the bottom of a 1.4 km Greenland ice core finds that the Greenland ice sheet melted away at least once during the last million year. Since the maximum positive temperature excursion over that period is 2.5 °C, this strongly suggests that its tipping point is below that value and therefore within the lower half of its range of estimates.[54][55] Melting of the Greenland ice sheet would contribute 4–7.5 m (13–24+1⁄2 ft) of sea level rise.[6]

A 2013 study estimated that each degree of temperature rise translates to a 2.3 m (7 ft 7 in) commitment to sea level rise within the next 2,000 years.[56] More recent research, especially into Antarctica, indicates that this is probably a conservative estimate and that the true long-term sea level rise might be higher.[42] Warming beyond the 2 °C (3.6 °F) target potentially leads 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 the available fossil fuel on Earth is even enough to ultimately melt the entire Antarctic ice sheet, causing about 58 m (190 ft) of sea level rise.[57] 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 0.5–2 m (1+1⁄2–6+1⁄2 ft).[58]

In the next 2,000 years the sea level is predicted to rise by 2–3 metres if the temperature rise peaks at 1.5 °C, by 2–6 metres if it peaks at 2 °C and by 19–22 metres if it peaks at 5 °C.[5]^: SPM-28 If temperature rise stops at 2nbsp;°C or at 5nbsp;°C, the sea level would still continue to rise for about 10,000 years. In the first case it will reach 8–13 metres above pre-industrial level, and in the second 28–37 metres.[59]

Ocean heat content (OHC) between 1957 and 2017, NOAA

Most of the additional heat trapped in the Earth's climate system by climate change is stored in oceans. They store more than 90% of the extra heat and act as a buffer against the effects of climate change. The heat needed to raise an average temperature increase of the entire world ocean by 0.01 °C would increase the atmospheric temperature by approximately 10 °C:[63] 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.[25]^: 1161 Consequently cold Arctic Ocean water will expand less than warm tropical water. Because different climate models present slightly different patterns of ocean heating, their predictions do not agree fully on the contribution of ocean heating to sea level rise.[64] Heat gets transported into deeper parts of the ocean by winds and currents, and some of it reaches depths of more than 2,000 m (6,600 ft).[65]

Considering an increase in average global temperature of 2 °C above preindustrial levels, and not considering the potential contributions from ice-sheet processes with limited agreement (low confidence) among modeling approaches, the probability of exceeding 0.5 m rise of sea level globally (0.7 m along the CONUS coastline) by 2100 is about 50%. With 3–5 °C of warming under high emissions pathways, this probability rises to >80% to >99%.[66]

The Ross Ice Shelf, Antarctica's largest, is about the size of France and up to several hundred metres thick.

The large volume of ice on the Antarctic continent stores around 70% of the world's fresh water.[67] 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 of the ice sheet into the ocean, so that the mass gain due to snowfall is partially compensated.[68] Snowfall increased over the last two centuries, but no increase was found in the interior of Antarctica over the last four decades.[69] 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.[69]

Processes around an Antarctic ice shelf

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.[70] 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.[71] 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 to 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.[4]

A May 2021 study projected that limiting global warming to 1.5 °C would reduce the land ice contribution to sea level rise by 2100 from 25 cm to 13 cm (from 10 to 6 in.) compared to current mitigation pledges, with glaciers responsible for half the sea level rise contribution.[72] The physical uncertainty of the Antarctic contribution was more significant than the uncertainty due to the choice of mitigation pathway.[72] By 2019, several studies have attempted to estimate 2300 sea level rise caused by ice loss in Antarctica alone: they suggest 16 cm median and 37 cm maximum values under the low-emission scenario but a median of 1.46 metres (with a minimum of 60 cm and a maximum of 2.89 metres) under the highest-emission scenario.[48]

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 (174 ft 10 in).[73] The ice sheet has historically been considered to be relatively stable and has therefore attracted less scientific attention and observations compared to West Antarctica.[69] 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.[71] 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 metres of sea level rise in the coming century or two."[69]

Methods agree that the Totten Glacier has lost ice in recent decades in response to ocean warming[74][75] and possibly a reduction in local sea ice cover.[76] 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.[37] The global sea level potential of 3.5 m (11 ft 6 in) flowing through Totten Glacier alone is of similar magnitude to the entire probable contribution of the West Antarctic Ice Sheet.[77]

The other major ice reservoir on East Antarctica that might rapidly retreat is the Wilkes Basin which is subject to marine ice sheet instability.[37] Ice loss from these outlet glaciers is possibly compensated by accumulation gains in other parts of Antarctica.[71] In 2022, it was estimated that the Wilkes Basin, Aurora Basin and other nearby subglacial basins are likely to have a collective tipping point around 3 °C of global warming, although it may be as high as 6 °C, or as low as 2 °C. Once this tipping point is crossed, the collapse of these subglacial basins could take place as little as 500 or as much as 10,000 years: the median timeline is 2000 years. On the other hand, the entirety of the EAIS would not be committed to collapse until global warming reaches 7.5 °C (range between 5 and 10 °C), and would take at least 10,000 years to disappear.[78][79] It is also suggested that the loss of two-thirds of its volume may require at least 6nbsp;°C of warming.[80]

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 ± 3.9 mm (19⁄64 ± 5⁄32 in) of sea level rise. Considering the mass balance of the East Antarctic Ice Sheet which was relatively steady, the major contributor was West Antarctica.[71] Significant acceleration of outflow glaciers in the Amundsen Sea Embayment may have contributed to this increase.[81] 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 (1.73 °F) per decade between 1976 and 2012.[82]

In 2021, it was estimated that while the *median* increase in sea level rise from the West Antarctic ice sheet melt by 2100 is ~11 cm under all emission scenarios (since the increased warming would intensify the water cycle and increase snowfall accumulation over the ice sheet at about the same rate as it would increase ice loss), the multiple types of instability affecting West Antarctica ice sheet mean that it can conceivably contribute as much as 41 cm by 2100 under the low-emission scenario and 57 cm under the highest-emission one.[48] One is marine ice sheet instability, where the bedrock on which parts of the ice sheet rest is deeper inland.[83] 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.[36]

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 into the grounding zone.[84][85] With continued melt and retreat they contribute to raising global sea levels.[86][87] The Thwaites glacier itself will cause a rise of sea level by 65 centimetres if it will completely collapse, but this can also destabilize other glaciers in west Antarctica.[88] [85] The Thwaites Ice Shelf can collapse in three to five years, which would then make the destabilization of the entire Thwaites glacier inevitable.[89]

The melting of these two glaciers had accelerated at the beginning of the 21st century. It can destabilize the entire West Antarctic Ice Sheet. However, the process will probably not be finished in this century.[90] Most of the bedrock underlying the West Antarctic Ice Sheet lies well below sea level.[37] A rapid collapse of the West Antarctic Ice Sheet could raise sea level by 3.3 m (10 ft 10 in).[91][92] In 2022, the collapse of the entire West Antarctica was estimated to unfold over a period of about 2000 years, with the absolute minimum of 500 years (and a potential maximum of 13,000 years.) At the same time, this collapse was considered likely to be triggered at around 1.5°C of global warming and would become absolutely unavoidable at 3°C: at worst, it may have even been triggered by now, after the warming passed 1°C in the recent years.[78][79] Even though the process takes a long time to finish, it has been suggested that the only way to stop it once triggered is by lowering the global temperature to 1°C below the preindustrial levels (about 2°C below the current levels). [80]

Greenland 2007 melt, measured as the difference between the number of days on which melting occurred in 2007 compared to the average annual melting days from 1988 to 2006[93]

Most ice on Greenland is part of the Greenland ice sheet which is 3 km (10,000 ft) at its thickest. 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.[94] Average annual ice loss in Greenland more than doubled in the early 21st century compared to the 20th century.[95] Some of Greenland's largest outlet glaciers, such as Jakobshavn Isbræ and Kangerlussuaq Glacier, are flowing faster into the ocean.[96][97]

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.[98][99]

A study published in 2020 estimated that the Greenland Ice Sheet had lost a total of 3,902 gigatons (Gt) of ice between 1992 and 2018, corresponding to a contribution to sea level rise of 10.8 mm. The sea level rise due to the Greenland Ice Sheet has generally increased over time, rising from 0.07 mm per year between 1992 and 1997 to 0.68 mm per year between 2012 and 2017.[100] The contribution for the 2012–2016 period was equivalent to 37% of sea level rise from land ice sources (excluding thermal expansion).[101] This rate of ice sheet melting is also associated with the higher end of the predictions from the past IPCC assessment reports.[102][35]

As of 2021, it is estimated that under the SSP1-2.6 emission scenario which largely fulfils the Paris Agreement goals, Greenland ice sheet melt adds around 6 cm to global sea level rise by the end of the century, with a plausible maximum of 15 cm (and even a very small chance of the ice sheet gaining mass through the increased surface mass balance feedback and thus reducing the sea levels by around 2 cm). On the other hand, the scenario associated with the highest global warming, SSP5-8.5, would see Greenland add a minimum of 5 cm to sea level rise, a likely median of 13 cm and a plausible maximum of 23 cm.[48]

There is also a threshold in surface warming beyond which a partial or near-complete melting of the Greenland ice sheet occurs.[103] Different research has put this threshold value as low as 1 °C (2 °F), and definitely 4 °C (7 °F), above pre-industrial temperatures.[104][25]^: 1170 A 2021 analysis of sub-glacial sediment at the bottom of a 1.4 km Greenland ice core finds that the Greenland ice sheet melted away at least once during the last million year, and therefore strongly suggests that its tipping point is below the 2.5 °C maximum positive temperature excursion over that period.[54][55] In 2022, it was estimated that the tipping point of the Greenland Ice Sheet may have been as low as 0.8 °C, and is certainly no higher than 3 °C: there's a high chance that it will be crossed around 1.5 °C. Once crossed, it would take between 1000 and 15,000 years for the ice sheet to disintegrate entirely, with the most likely estimate of 10,000 years.[78][79]

From 1970 to 2004, mountain glaciers thinned (yellow and red) in some regions and thickened (blue) in others.

Seasonal melt contributes to runoff; the annual balance (net change of glacier mass) contributes to sea level rise.[105]

Less than 1% of glacier ice is in mountain glaciers, compared to 99% in Greenland and Antarctica. However, this small size also makes them more vulnerable to melting than the larger ice sheets, and it means that mountain glaciers have had a disproportionate contribution to historical sea level rise and are set to contribute a smaller, but still significant fraction of sea level rise in the 21st century.[106] The roughly 200,000 glaciers on earth are spread out across all continents.[107] 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.[108]^: 345 Many glaciers drain into the ocean and ice loss can therefore increase when ocean temperatures increase.[107]

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.[109] The contribution for the 2012–2016 period was nearly as large as that of Greenland: 0.63 mm of sea level rise per year, equivalent to 34% of sea level rise from land ice sources.[101] Over the 21st century, this is expected to rise, with glaciers contributing 7–24 cm (3–9+1⁄2 in) to global sea levels.[25]^: 1165 Glaciers contributed around 40% to sea level rise during the 20th century, with estimates for the 21st century of around 30%.[4] In 2022, it was estimated that in general, mountain glaciers are likely to disappear around the world once the warming crosses 2 °C, and this would become largely inevitable around 3 °C: there's even a chance most glaciers would be lost around 1.5 °C. Because mountain glaciers contain much less ice than the other ice masses, their disappearance may require as little as 50 years after the tipping point is crossed, although 200 years is the most likely value, and the maximum is around 1000 years.[78][79]

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 4 cm (1+1⁄2 in).[110]

Trends in land water storage from GRACE observations in gigatons per year, April 2002 to November 2014 (glaciers and ice sheets are excluded).

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.[111][25]^: 1155

Jason-1 continued the sea surface measurements started by TOPEX/Poseidon. It was followed by the Ocean Surface Topography Mission on Jason-2, and by Jason-3.

Since the launch of TOPEX/Poseidon in 1992, an overlapping series of altimetric satellites has been continuously recording the sea level and its changes.[114] 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 satellites send a microwave pulse which reflects on the ocean's surface and record the time it takes to return. Microwave radiometers correct the additional delay caused by water vapor in the atmosphere. Combining these data with the precisely known location of the spacecraft determines the sea-surface height to within a few centimetres (about one inch).[115] Current rates of sea level rise from satellite altimetry have been estimated to be 3.0 ± 0.4 millimetres (1⁄8 ± 1⁄64 in) per year for the period 1993–2017.[116] Earlier satellite measurements were previously slightly at odds with tide gauge measurements. A small calibration error for the Topex/Poseidon satellite was eventually identified as having caused a slight overestimation of the 1992–2005 sea levels, which masked in the satellite measurements the ongoing sea level rise acceleration that was visible in the tide gauge timeseries.[117]

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.[118] 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.[119]

Between 1993 and 2018, the mean sea level has risen across most of the world ocean (blue colors).[120]

The global network of tide gauges is another important source of sea-level observations. Compared to the satellite record, this record has major spatial gaps but covers a much longer period of time.[121] Coverage of tide gauges started primarily in the Northern Hemisphere, with data for the Southern Hemisphere remaining scarce up to the 1970s.[121] The longest running sea-level measurements, NAP or Amsterdam Ordnance Datum established in 1675, are recorded in Amsterdam, the Netherlands.[122] 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.[123]

This network was used, in combination with satellite altimeter data, to establish that global mean sea-level rose 19.5 cm (7+3⁄4 in) between 1870 and 2004 at an average rate of about 1.44 mm/yr (1.7 mm/yr during the 20th century).[124] Data collected by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia show that the global mean sea level currently rises by 3.2 mm (1⁄8 in) per year, at double the average 20th century rate.[125][126] This is an important confirmation of climate change simulations which predicted that sea level rise would accelerate in response to climate change.

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 along the northern Baltic Sea have dropped due to post-glacial rebound.[127]

Bramble Cay melomys Melomys rubicola. In 2016 declared extinct on Bramble Cay, where it had been endemic, and likely also globally extinct, with habitat loss due to sea level rise being the root cause.

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 artificial barriers. This coastal narrowing, sometimes called 'coastal squeeze' when considering human-made barriers, could result in the loss of habitats such as mudflats and marshes.[22][144]

The mangrove ecosystem is one of the ecosystems affected by rising sea levels. It is an ecological whole composed of mangrove plants growing in and around the mudflats of the tropical coast. Its ecological value is high because it is an ideal home for many species. In recent years, mangroves have been moving inland, but their success depends on various environmental information such as topography and geology. The warmer the climate, the bigger they grow. The mangrove's breathing roots or pneumatophores might grow to be half a metre tall.[145][146] 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.[147] More specifically, if the rate of mangrove deposition does not keep up with sea level rise, the key to the extinction of the mangrove ecosystem is the relationship between the rate of inland migration and the rate of sea level rise. If sea levels rise faster than the mangroves can move to land, this can lead to the loss of ecosystems.[148] The ability of mangroves to survive sea level rise events depend on their ability to migrate inland.[146] As both ecosystems protect against storm surges, waves and tsunamis, losing them makes the effects of sea level rise worse.[149][150] 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.[151]

When seawater reaches inland, problems related to contaminated soils may occur. Also, fish, birds, and coastal plants could lose parts of their habitat.[21] 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.[152] 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.[153] This report was confirmed by the federal government of Australia when it declared the Bramble Cay melomys extinct as of February 2019, making this species the first known mammal to go extinct as a result of sea level rise.[154]

Aerial view of the Tanzanian capital Dar Es Salaam

In Africa, risk from sea level rise is amplified by the future population growth. It is believed that 54.2 million people lived in the highly exposed low elevation coastal zones (LECZ) around 2000, but this number will effectively double to around 110 million people by 2030, and by 2060 it'll be in the range between 185 and 230 million people, depending on the extent of population growth. While the average regional sea level rise by 2060 will be around 21 cm (with climate change scenarios making little difference at that point), local geography and population trends interact to increase the exposure to hazards like 100-year floods in a complex manner.[19]

Abidjan, the economic powerhouse of Ivory Coast

Maputo, the capital of Mozambique

In the near term, some of the largest displacement is projected to occur in the East Africa region, where at least 750,000 people are likely to be displaced from the coasts between 2020 and 2050. It was also estimated that by 2050, 12 major African cities (Abidjan, Alexandria, Algiers, Cape Town, Casablanca, Dakar, Dar es Salaam, Durban, Lagos, Lomé, Luanda and Maputo) would collectively sustain cumulative damages of USD 65 billion for the "moderate" climate change scenario RCP 4.5 and USD 86.5 billion for the high-emission scenario RCP 8.5: the version of the high-emission scenario with additional impacts from high ice sheet instability would involve up to 137.5 billion USD in damages. Additional accounting for the "low-probability, high-damage events" may increase aggregate risks to USD 187 billion for the "moderate" RCP4.5, USD 206 billion for RCP8.5 and USD 397 billion under the high-end instability scenario.[19] In all of these estimates, the Egyptian city of Alexandria alone amounts for around half of this figure:[19] hundreds of thousands of people in its low-lying areas may already have to be relocated in the coming decade.[135] Across sub-Saharan Africa as a whole, damages from sea level rise could reach 2–4% of GDP by 2050, although this is strongly affected by the extent of future economic growth and adaptation.[19]

The remains of Leptis Magna amphitheater, with the sea visible in the background

In the longer term, Egypt, Mozambique and Tanzania are also projected to have the largest number of people affected by annual flooding amongst all African countries if global warming reaches 4 °C by the end of the century (a level associated with the RCP 8.5 scenario). Under RCP 8.5, 10 important cultural sites (Casbah of Algiers, Carthage Archaeological site, Kerkouane, Leptis Magna Archaeological site, Medina of Sousse, Medina of Tunis, Sabratha Archaeological site, Robben Island, Island of Saint-Louis and Tipasa) would be at risk of flooding and erosion by the end of the century, along with a total of 15 Ramsar sites and other natural heritage sites (Bao Bolong Wetland Reserve, Delta du Saloum National Park, Diawling National Park, Golfe de Boughrara, Kalissaye, Lagune de Ghar el Melh et Delta de la Mejerda, Marromeu Game Reserve, Parc Naturel des Mangroves du Fleuve Cacheu, Seal Ledges Provincial Nature Reserve, Sebkhet Halk Elmanzel et Oued Essed, Sebkhet Soliman, Réserve Naturelle d'Intérêt Communautaire de la Somone, Songor Biosphere Reserve, Tanbi Wetland Complex and Watamu Marine National Park).[19]

Matsukawaura Lagoon, located in Fukushima Prefecture of Honshu Island

As of 2022, it is estimated that 63 million people in the East and South Asia are already at risk from a 100-year flood, in large part due to inadequate coastal protection in many countries. This will be greatly exacerbated in the future: Asia has the largest population at risk from sea level and Bangladesh, China, India, Indonesia, Japan, Philippines, Thailand and Vietnam alone account for 70% number of people exposed to sea level rise during the 21st century.[16][159] This is entirely due to the region's densely populated coasts, as the rate of sea level rise in Asia is generally similar to the global average. Exceptions include the Indo-Pacific region, where it had been around 10% faster since the 1990s, and the coast of China, where globally "extreme" sea level rise had been detected since the 1980s, and it is believed that the difference between and of global warming would have a disproportionate impact on flood frequency. It is also estimated that future sea level rise along the Japanese Honshu Island would be up to 25 cm faster than the global average under RCP 8.5, the intense climate change scenario. RCP 8.5 is additionally associated with the loss of at least a third of the Japanese beaches and 57–72% of Thai beaches.[16]

One estimate finds that Asia will suffer direct economic damages of 167.6 billion USD at 0.47 meters of sea level rise, 272.3 billion USD at 1.12 meters and 338.1 billion USD at 1.75 meters (along with the indirect impact of 8.5, 24 or 15 billion USD from population displacement at those levels), with China, India, the Republic of Korea, Japan, Indonesia and Russia experiencing the largest economic losses. Out of the 20 coastal cities expected to see the highest flood losses by 2050, 13 are in Asia. For nine of those (Bangkok, Guangzhou, Ho Chi Minh City, Jakarta, Kolkata, Nagoya, Tianjin , Xiamen and Zhanjiang) sea level rise would be compounded by subsidence. By 2050, Guangzhou would see 0.2 meters of sea level rise and the estimated annual economic losses of 254 million USD - the highest in the world. One estimate calculates that in the absence of adaptation, cumulative economic losses caused by sea level rise in Guangzhou under RCP8.5 would reach ~331 billion USD by 2050, ~660 billion USD by 2070 and 1.4 trillion USD by 2100, while the impact of high-end ice sheet instability would increase these figures to ~420 billion USD, ~840 billion USD and ~1.8 trillion USD, respectively. In Shanghai, coastal inundation amounts to ~0.03% of local GDP; but would increase to 0.8% (confidence interval of 0.4–1.4%) by 2100 even under the "moderate" RCP 4.5 scenario in the absence of adaptation. Likewise, failing to adapt to sea level rise in Mumbai would result in the damages of 112–162 billion USD by 2050, which would nearly triple by 2070. As the result, efforts like the Mumbai Coastal Road are being implemented, although they are likely to affect coastal ecosystems and fishing livelihoods.[16]

It is estimated that sea level rise in Bangladesh may force the relocation of up to one-third of power plants as early as 2030, while a similar proportion would have to deal with the increased salinity of their cooling water by then. Research from 2010s indicates that by 2050, between 0.9 and 2.1 million people would be displaced by sea level rise alone: this would likely necessitate the creation of ~594,000 additional jobs and ~197,000 housing units in the areas receiving the displaced persons, as well as to secure the supply of additional ~783 billion calories worth of food.[16] in 2021, another paper estimated that 816,000 would be directly displaced by sea level rise by 2050, but this would be increased to 1,3 million when the indirect effects are taken into account.[160] Both studies assume that the majority of the displaced people would travel to the other areas of Bangladesh, and attempt to estimate population changes in different localities.

2010 estimates of population exposure to sea level rise in Bangladesh

In an attempt to address these challenges, the Bangladesh Delta Plan 2100 has been launched in 2018.[161][162] As of 2020, it was seen falling short of most of its initial targets.[163] The progress is being monitored.[164] Sea level rise also disrupts freshwater fisheries in Bangladesh by making the waters too saline for many fish species.

In 2019, the president of Indonesia, Joko Widodo, declared that the city of Jakarta is sinking to a degree that requires him to move the capital to another city.[165] A study conducted between 1982 and 2010 found that some areas of Jakarta have been sinking by as much as 28 cm (11 inches) per year[166] due to ground water drilling and the weight of its buildings, and the problem is now exacerbated by sea level rise. However, there are concerns that building in a new location will increase tropical deforestation.[167][168] Other so called sinking cities, such as Bangkok or Tokyo, are vulnerable to these compounding subsidence with sea level rise.[169]

King's Beach at Caloundra

In Australia, erosion and flooding of Queensland's Sunshine Coast beaches is projected to intensify by 60% by 2030, with severe impacts on tourism in the absence of adaptation. Adaptation costs to sea level rise under the high-emission RCP 8.5 scenario are projected to be three times greater than the adaptation costs to low-emission RCP 2.6 scenario. For 0.2- to 0.3-m sea level rise (set to occur by 2050), what is currently a 100-year flood would occur every year in New Zealand cities of Wellington and Christchurch. Under 0.5 m sea level rise, the current 100-year flood in Australia would be likely to occur several times a year, while in New Zealand, buildings with a collective worth of NZ$12.75 billion would become exposed to new 100-year floods. A meter or so of sea level rise would threaten assets in New Zealand with a worth of NZD$25.5 billion (with a disproportionate impact on Maori-owned holdings and cultural heritage objects), and Australian assets with a worth of AUD$164–226 billion (including many unsealed roads and railway lines). The latter represents a 111% rise in Australia's inundation costs between 2020 and 2100.[170]

An aerial view of São Paulo's Port of Santos

By 2100, a minimum of 3-4 million people in South America would be directly affected by coastal flooding and erosion. 6% of the population of Venezuela, 56% of the population of Guyana (including in the capital, Georgetown, much of which is already below the sea level) and 68% of the population of Suriname are already living in low-lying areas exposed to sea level rise. In Brazil, the coastal ecoregion of Caatinga is responsible for 99% of its shrimp production, yet its unique conditions are threatened by a combination of sea level rise, ocean warming and ocean acidification. The port complex of Santa Catarina had been interrupted by extreme wave behavior 76 times in one 6-year period in 2010s, with a loss 25,000-50,000 USD loss for each full idle day. (Ohz et al., 2020). In Port of Santos, storm surges were three times more frequent between 2000 and 2016 than between 1928 and 1999.[171]

Beach nourishment in progress in Barcelona

Venice is one of the cities which had been the most threatened by flooding. The city is located on islands in the delta of the Po and Piave rivers. Sea level rise causes an increase in frequency and magnitude of floodings in the city which had already spent more than $6 billion on the flood barrier system.[172][173]

Netherlands is a country that sits partially below sea level and is subsiding. It has responded to that reality by extending its Delta Works program.[174] 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 rising sea for the following 190 years. This included drawing up worst-case plans for evacuations. The plan also included between €1.0 and €1.5 billion in annual spending through to the year 2100 for precautionary measures, such as broadening coastal dunes and strengthening sea and river dikes.[175] The commission said the country must plan for a rise in the North Sea up to 1.3 m (4 ft 3 in) by 2100 and plan for a 2–4 m (7–13 ft) rise by 2200.[176] Analysis of the impacts of Hurricane Sandy determined that communities located behind wetlands experienced 20% less damage (Narayan et al., 2016). Coral reefs are providing 544 million USD yr−1 (Beck et al., 2018a) and mangroves 22 billion USD yr−1 in property protection for coastal communities in the USA and Mexico

Tidal flooding in Miami during a king tide (October 17, 2016). The risk of tidal flooding increases with sea level rise.

As of 2017, around 95 million Americans lived on the coast: for Canada and Mexico, this figure amounts to 6.5 million and 19 million people. Northern Gulf of Mexico, Atlantic Canada and the Pacific coast of Mexico would experience the greatest sea level rise. By 2030, flooding along the US Gulf Coast may result in economic losses of up to 176 billion USD: around 50 billion USD could be potentially avoided through nature-based solutions such as wetland restoration and oyster reef restoration. By 2050, 20 million people in the greater New York City area would be threatened, as 40% of the existing water treatment facilities would be compromised and 60% of power plants will need to be relocated. By 2100, sea level rise of 0.9 metres and 1.8 metres would threaten 4.2 and 13.1 million people in the US, respectively. In California alone, 2 metres of SLR could affect 600,000 people and threaten over 150 billion USD in property with inundation, potentially representing more than 6% of the state’s GDP. In North Carolina, a meter of SLR inundates 42% of the Albemarle-Pamlico Peninsula, incurring losses of up to 14 billion USD (at 2016 value of the currency). In nine southeast US states, the same level of sea level rise would amount to the loss over 1000 sites eligible for inclusion in the National Register for Historic Places and up to 13,000 historical and archaeological sites overall.[177]

Sea level rise causes the mixing of sea water into the coastal groundwater, rendering it unusable once it amounts to more than 2-3% of the reservoir. Along an estimated 15% of the US coastline, the majority of local groundwater levels are already below the sea level.[178] It also favors chronic flooding at high tide, as evidenced e.g. in the US East Coast.[179] Similarly, Florida, which is extremely vulnerable to climate change, is already experiencing substantial nuisance flooding and king tide flooding.[180] Nonpartisan think tank Resources for the Future describes Miami as "the most vulnerable major coastal city in the world" to damages associated with storm-related coastal flooding and sea level rise.[181] Storm surges can cause the largest loss of life and property in the world's coastal areas, and their frequency and intensity has increased in the recent years. New York City is one of the worst affected areas, and simulations show that the current 100-year flood would occur once in 19–68 years by 2050 and 40–60 years by 2080.[182] U.S. coastal cities conduct beach nourishment, also known as beach replenishment, where mined sand is trucked in and added, in addition to other adaptation measures such as zoning, restrictions on state funding, and building code standards.[183][184] In Mexico, the damages from SLR to tourism hotspots like Cancun, Isla Mujeres, Playa del Carmen, Puerto Morelos and Cozumel could amount to 1.4–2.3 billion USD. The damages are also widespread in Canada and will affect both major cities like Halifax and the more remote locations like Lennox Island, whose Mi’kmaq community is already considering relocation due to widespread coastal erosion.[177]

Malé, the capital island of Maldives.

Atolls, low islands and low-lying coastal areas on islands are particularly vulnerable to sea level rise. Possible impacts include coastal erosion, flooding and salt intrusion into soils and freshwater. None are more vulnerable than small island nations, particularly those with population concentrations on atolls. Atolls on average reach 0.9–1.8 m (3–6 ft) above sea level.[185] This type of island is a final stage in the geologic history of an island of volcanic origin (all volcanic islands usually become an atoll, eventually). With climate change causing sea levels rise, the process of atoll formation has sped up.

Since atolls are low-lying islands, they are vulnerable to rising waters or storms covering them in water. Atolls are vital because they are the home to many distinct cultures and sovereign nations. As ocean water prematurely covers atolls, saltwater can intrude into drinking water supplies and undermine the ecosystems that the population utilizes to survive.[186] Sea level rise has the potential to devastate tourism and local economies; a sea level rise of 1.0 m (3 ft 3 in) would cause partial or complete inundation of 29% of coastal resorts in the Caribbean. A further 49–60% of coastal resorts would be at risk from resulting coastal erosion.[187] 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.[188]

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.[189][190] The Island nation of Fiji is being impacted by sea level rise.[191] 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. [192] Countries becoming uninhabitable could exacerbate the humanitarian crisis of climate refugees.[193]

Surface area change of islands in the Central Pacific and Solomon Islands[194]

Some island nations, such as the Republic of Maldives, Kiribati and Tuvalu are considering international migration of their population in response to rising seas. Moving to different countries is not an easy solution, as those who move need to have a steady income and social network in their new country. It might be easier to adapt locally by moving further inland and increasing sediment supply needed for natural erosion protection.[195] In the island nation of Fiji, residents are restoring coral reefs and mangroves to protect themselves against flooding and erosion, which is estimated to be more cost-efficient than building sea-walls.[196]

Of interest is how the rising sea level will affect children in these communities. Children without stability at home and their communities can experience a variety of issues in their development. Already, children in small island states are encountering hampered access to food and water, displacement, and resulting mental and social disorders from these stressors.[197]

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 415 kilometres (224 nautical miles) 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.[198]