Current sea-level rise

Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019
Summary for Policymakers
In
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M.  Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.) Cambridge University Press, Cambridge, UK and New York, NY, USA. https://doi.org/10.1017/9781009157964.001. This rate is accelerating, with sea levels now rising by 3.7 mm per year. Climate scientists expect further acceleration during the 21st century. Climate change heats (and therefore expands) the ocean and melts land-based ice sheets and glaciers. Between 1993 and 2018, the thermal expansion of water contributed 42% to sea level rise; melting of temperate glaciers, 21%; Greenland, 15%; and Antarctica, 8%. Over the next 2000 years, the sea level is predicted to rise by if global warming is limited to 1.5°C, by if it peaks at 2°C and by if it peaks at 5 °C.IPCC, 2021
Summary for Policymakers
In
Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
[Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3−32, doi:10.1017/9781009157896.001.

Sea level rise has a substantial lag in its response to Earth temperature changes. This means that it is virtually certain to continue for a long time,
and that its extent in the short term (i.e. around 2050) is insensitive to temperature changes between now and then. Thus, there's confidence that 2050 levels of sea level rise combined with the 2010 population distribution (i.e. absent the effects of population growth and human migration) would result in ~150 million people under the water line during high tide and ~300 million in places which are flooded every year – an increase of 40 and 50 million people relative to 2010 values for the same. At the same time, the impact on temperature from changes in greenhouse gas emissions over the longer term would greatly influence longer-term sea level rise: by 2100, the spread between the lowest and the highest plausible emission trajectories would result in the sea level rise of when using the best-understood median estimates. When compared to 2050 levels, the difference between the low and high end of that range is equivalent to the difference between ~40 million more people under the water line during high tide and ~50 million more in places which are flooded every year (190 and 350 million people) and ~80 and ~90 million more for the same metrics (230 and 390 million people), respectively. Under the highest emission scenario, less-understood processes may also lead to sea level rise of well over by 2100, and the levels of cannot be excluded. Applying this extent of sea level rise to the 2010 population distribution could mean as many as 520 million more people under the water line during high tide and 640 million in places which are flooded every year.

While the rise in sea levels ultimately impacts every coastal and island population on Earth and leads to higher storm surges, more dangerous tsunamis, damage in cities, loss and degradation of agricultural land and ultimately permanent loss of land and the displacement of populations, it does not occur uniformly due to local factors like tides, currents, storms, tectonic effects and land subsidence. For instance, sea level rise along US coasts (and along the US East Coast in particular) is already higher than the global average, and it is expected to be 2 to 3 times greater than the global average by the end of the century. At the same time, Asia will be the region where sea level rise would impact the most people: eight Asian countries – Bangladesh, China, India, Indonesia, Japan, the Philippines, Thailand and Vietnam – account for 70% of the global population exposed to sea level rise and land subsidence. Altogether, out of the 20 countries with the greatest exposure to sea level rise, 12 are in Asia. Finally, the greatest near-term impact on human populations will occur in the low-lying Caribbean and Pacific islands – many of those would be rendered uninhabitable by sea level rise later this century.Mycoo, M., M. Wairiu, D. Campbell, V. Duvat, Y. Golbuu, S. Maharaj, J. Nalau, P. Nunn, J. Pinnegar, and O. Warrick, 2022
Chapter 15: Small islands
I
Climate Change 2022: Impacts, Adaptation and Vulnerability
.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke,V. Möller, A. Okem, B. Rama (eds.) Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2043–2121 , doi=10.1017/9781009325844.017

Societies can adapt to sea level rise in three different ways: implement managed retreat, accommodate coastal change, or protect against sea level rise through hard-construction practices like seawalls or soft approaches such as dune rehabilitation and beach nourishment. Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made among different strategies. For instance, a managed retreat strategy is difficult if the population in the area is quickly increasing: this is a particularly acute problem for Africa, where the population of low-lying coastal areas is projected to increase by around 100 million people within the next 40 years.Trisos, C.H., I.O. Adelekan, E. Totin, A. Ayanlade, J. Efitre, A. Gemeda, K. Kalaba, C. Lennard, C. Masao, Y. Mgaya, G. Ngaruiya, D. Olago, N.P. Simpson, and S. Zakieldeen 2022
Chapter 9: Africa
I
Climate Change 2022: Impacts, Adaptation and Vulnerability
.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke,V. Möller, A. Okem, B. Rama (eds.) Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2043–2121 , doi=10.1017/9781009325844.011 Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states, and sea level rise at some locations may be compounded by other environmental issues, such as subsidence in so-called sinking cities. Coastal ecosystems typically adapt to rising sea levels by moving inland; however, they might not always be able to do so, due to natural or artificial barriers.

Observations

Between 1901 and 2018, the globally averaged sea level rose by . More precise data gathered from satellite radar measurements reveal an accelerating rise of from 1993 to 2017, accelerating gradually to a rate of per year as of 2021.

Regional variations

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. 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.

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. Total human-caused subsidence in the Rhine-Meuse-Scheldt delta (Netherlands) is estimated at , over in urban areas of the Mississippi River Delta ( New Orleans), and over in the Sacramento–San Joaquin River Delta. On the other hand, post-glacial isostatic rebound causes relative sea level fall around the Hudson Bay in Canada and the northern Baltic.

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. The downturn of the Atlantic meridional overturning circulation (AMOC) has been also tied to extreme regional sea level rise on the US Northeast Coast.

Projections

There are broadly two ways of modelling sea level rise and making future projections. In one approach, scientists use process-based modelling, where all relevant and well-understood physical processes are included in a global 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, but it can predict non-linearities and long delays in the response which studies of the recent past will miss.

In the other approach, scientists 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. These semi-empirical sea level models rely on statistical techniques, using relationships between observed past (contributions to) global mean sea level and global mean temperature. This type of modelling was partially motivated by most physical models in previous literature assessments by the Intergovernmental Panel on Climate Change (IPCC) having underestimated the amount of sea level rise compared to observations of the 20th century.

Projections for the 21st century

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 . In August 2020 scientists reported that observed ice-sheet losses in Greenland and Antarctica were tracking worst-case scenarios of those IPCC projections. 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.

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. 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. 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''.''"

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 on average—and an extreme scenario with as much as , by 2100. This could mean rapid sea level rise of up to 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 of sea level rise by 2100.

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.

The possibility of a collapse of the West Antarctic ice sheet and subsequent rapid sea level rise was suggested back in the 1970s. 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 from melting of the West Antarctic ice-sheet alone.

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.

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. 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.

High-end 21st-century sea level rise

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.Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G.  Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021
Chapter 9: Ocean, Cryosphere and Sea Level Change
I
Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
[Masson-Delmotte, V., P. Zhai, A. Pirani, S.L.  Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1211–1362, doi:10.1017/9781009157896.011. 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. 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 or changes in the weather over Antarctica leading to lower precipitation and ice deposition, could all be contributing factors. The combination of such processes could lead to sea level rise as high as 2.3 metres by 2100.

Long-term sea level rise

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.
Models are able to reproduce paleo records of sea level rise, which provides confidence in their application to long-term future change.

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. 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. , 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 years. 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. Melting of the Greenland ice sheet would contribute of sea level rise.

A 2013 study estimated that each degree of temperature rise translates to a commitment to sea level rise within the next 2,000 years. 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. 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 of sea level rise. After 500 years, sea level rise from thermal expansion alone may have reached only half of its eventual level, which models suggest may lie within ranges of .

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. If temperature rise stops at 2 °C or at 5 °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.

Causes

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 since the start of the 20th century has been dominated by retreat of glaciers and expansion of the ocean, but the contributions of the two large ice sheets (Greenland and Antarctica) are expected to increase in the 21st century. The ice sheets store most of the land ice (∼99.5%), with a sea-level equivalent (SLE) of for Greenland and for Antarctica.

Each year about 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 or by sublimation (directly changing into water vapor). The rest of the snow slowly changes into ice. This ice can flow to the edges of the ice sheet and return to the ocean by melting at the edge or in the form of icebergs. If precipitation, surface processes and ice loss at the edge balance each other, sea level remains the same. However scientists have found that ice is being lost, and at an accelerating rate.

Ocean heating

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: 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. 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. Heat gets transported into deeper parts of the ocean by winds and currents, and some of it reaches depths of more than .

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%.

Antarctica

The large volume of ice on the Antarctic continent stores around 70% of the world's fresh water. The Antarctic ice sheet mass balance is affected by snowfall accumulations, and ice discharge along the periphery. Under the influence of global warming, melt at the base of the ice sheet increases. Simultaneously, the capacity of the atmosphere to carry precipitation increases with temperature so that precipitation, in the form of snowfall, increases 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. Snowfall increased over the last two centuries, but no increase was found in the interior of Antarctica over the last four decades. 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.

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. A 2018 systematic review study estimated that ice loss across the entire continent was 43 gigatons (Gt) per year on average during the period from 1992 to 2002, but has accelerated to an average of 220 Gt per year during the five years from 2012 to 2017.
* Most of the melt comes from the West Antarctic Ice Sheet, but the Antarctic Peninsula 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.

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. The physical uncertainty of the Antarctic contribution was more significant than the uncertainty due to the choice of mitigation pathway. Alt URL https://eprints.whiterose.ac.uk/173870/ 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.

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 . The ice sheet has historically been considered to be relatively stable and has therefore attracted less scientific attention and observations compared to West Antarctica. 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. 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."

Methods agree that the Totten Glacier has lost ice in recent decades in response to ocean warming and possibly a reduction in local sea ice cover. Totten Glacier is the primary outlet of the Aurora Subglacial Basin, a major ice reservoir in East Antarctica that could rapidly retreat due to hydrological processes. The global sea level potential of flowing through Totten Glacier alone is of similar magnitude to the entire probable contribution of the West Antarctic Ice Sheet.

The other major ice reservoir on East Antarctica that might rapidly retreat is the Wilkes Basin which is subject to marine ice sheet instability. Ice loss from these outlet glaciers is possibly compensated by accumulation gains in other parts of Antarctica. 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. It is also suggested that the loss of two-thirds of its volume may require at least 6 °C of warming.

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 of sea level rise. Considering the mass balance of the East Antarctic Ice Sheet which was relatively steady, the major contributor was West Antarctica. Significant acceleration of outflow glaciers in the Amundsen Sea Embayment may have contributed to this increase. In contrast to East Antarctica and the Antarctic Peninsula, temperatures on West Antarctica have increased significantly with a trend between per decade and per decade between 1976 and 2012.

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. One is 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.

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. With continued melt and retreat they contribute to raising global sea levels. 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. The Thwaites Ice Shelf can collapse in three to five years, which would then make the destabilization of the entire Thwaites glacier inevitable.

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. Most of the bedrock underlying the West Antarctic Ice Sheet lies well below sea level. A rapid collapse of the West Antarctic Ice Sheet could raise sea level by . 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. 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).

Greenland

Most ice on Greenland is part of the Greenland ice sheet which is 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. Average annual ice loss in Greenland more than doubled in the early 21st century compared to the 20th century. Some of Greenland's largest outlet glaciers, such as Jakobshavn Isbræ and Kangerlussuaq Glacier, are flowing faster into the ocean.

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.

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. The contribution for the 2012–2016 period was equivalent to 37% of sea level rise from ''land ice'' sources (excluding thermal expansion). This rate of ice sheet melting is also associated with the higher end of the predictions from the past IPCC assessment reports.

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.

There is also 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 °F), and definitely 4 °C (7 °F), above pre-industrial temperatures. 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 years, and therefore strongly suggests that its tipping point is below the 2.5 °C maximum positive temperature excursion over that period. 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.

Glaciers

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. The roughly 200,000 glaciers on earth are spread out across all continents. Different glaciers respond differently to increasing temperatures. For instance, valley glaciers that have a shallow slope 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. Many glaciers drain into the ocean and ice loss can therefore increase when ocean temperatures increase.

Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea level rise of 0.2-0.4 mm per year, averaged over the 20th century. 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. Over the 21st century, this is expected to rise, with glaciers contributing to global sea levels. Glaciers contributed around 40% to sea level rise during the 20th century, with estimates for the 21st century of around 30%. 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.

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 .

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.

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. Over a consistent time period, conducting assessments can source contributions to sea level rise and provide early indications of change in trajectory. This type of surveillance can inform plans of prevention. The different techniques used to measure changes in sea level do not measure exactly the same level. Tide gauges can only measure relative sea level, whilst satellites can also measure absolute sea level changes. 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.

Satellites

Since the launch of TOPEX/Poseidon in 1992, an overlapping series of altimetric satellites has been continuously recording the sea level and its changes. 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). Current rates of sea level rise from satellite altimetry have been estimated to be per year for the period 1993–2017. 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.

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. 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.

Tide gauges

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. Coverage of tide gauges started primarily in the Northern Hemisphere, with data for the Southern Hemisphere remaining scarce up to the 1970s. The longest running sea-level measurements, NAP or Amsterdam Ordnance Datum established in 1675, are recorded in Amsterdam, the Netherlands. In Australia record collection is also quite extensive, including measurements by an amateur meteorologist beginning in 1837 and measurements taken from a sea-level benchmark struck on a small cliff on the Isle of the Dead near the Port Arthur convict settlement in 1841.

This network was used, in combination with satellite altimeter data, to establish that global mean sea-level rose between 1870 and 2004 at an average rate of about 1.44 mm/yr (1.7 mm/yr during the 20th century). Data collected by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia show that the global mean sea level currently rises by per year, at double the average 20th century rate. 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.

Impacts

Current and future sea level rise is set to have a number of impacts, particularly on coastal systems. Such impacts include higher and more frequent high-tide and storm-surge flooding, increased coastal erosion, inhibition of primary production processes, more extensive coastal inundation, changes in surface water quality and groundwater characteristics. This is projected in turn to increase 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. Many of these impacts are detrimental. 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.
Globally tens of millions of people will be exposed 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. While modest increases in sea level are likely to be offset when cities adapt by constructing sea walls or through relocating, millions of people will be affected in cities such as Miami, Rio de Janeiro, Osaka and Shanghai if following the current trajectory of 3 °C (5.4 °F).

The rising seas pose both a direct risk of flooding unprotected areas and indirect threats of higher storm surges, tsunamis and king tides. They are also associated with the highly detrimental second-order effects such as the loss of coastal ecosystems like mangroves or the increased risk from tsunamis in the Pacific and Atlantic Oceans. Coastal flooding impacts are exacerbated by land use changes such as urbanisation or deforestation of low-lying coastal zones. Regions that are already vulnerable to the rising sea level also struggle with coastal flooding washing away land and altering the landscape. People in these areas struggle increasingly because of these different effects of climate change. Climate change influenced storms also create greater frequency of coastal flooding. Addressing the flooding issues that a region, city, or country is experiencing will be based on the efforts of the affected country. Without much effort these coastal regions could continuously degrade and thus greatly affect the size and influence of a country.

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. The farmland in the Nile Delta is affected by salt water flooding, and there is now more salt in the soil and irrigation water in the Red River Delta and the Mekong Delta in Vietnam. Bangladesh and China are affected in a similar way, particularly their rice production.

Sea trade is the dominant form of resource and good trade throughout the world. When talking about the rising sea level there is the possibility for major issues to arise for sea-based trade and the ports that are utilized. What is also of importance for these ports is that we do not yet know how the rising sea level will continue to impact them over time. A variety of factors play into the investment of seaports for this means. Factors that are important for seaports in reference to the rising sea level are where will the important trade ports be in the future, can we alter and protect the ports that are currently being used, how much is needed to protect trade without wasting time or money. The rising sea level for seaports is a concern, but is hard to decipher what changes need to be made since sea levels do not rise at the same rate all over the world.

The long-term cumulative effects of rising sea levels pose a serious threat in coastal areas. Coastal areas are sensitive to rising sea levels, changes in the frequency and intensity of storms, increased precipitation, and rising ocean temperatures. Ten percent of the world's population live in coastal areas that are less than above sea level. Furthermore, two thirds of the world's cities with over five million people are located in these low-lying coastal areas. In total, approximately 600 million people live directly on the coast around the world. Using remote laser scanning called LiDAR to measure elevation on the Earth's surface, researchers found that in the year 2021, 267 million people worldwide lived on land less than 2 metres above sea level and that with a 1-metre sea level rise and zero population growth, that number could increase to 410 million people.

Populations that move from the coasts because of the effects of rising sea levels could be impacted by loss of livelihoods and could become a heavy strain on the regions or cities that they come to inhabit. Further, both their original government or the receiving countries may lack or withhold resources to help these climate migrants, which compounds their level of poverty.

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 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.

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. 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. 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. The ability of mangroves to survive sea level rise events depend on their ability to migrate inland. As both ecosystems protect against storm surges, waves and tsunamis, losing them makes the effects of sea level rise worse. 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.

When seawater reaches inland, problems related to contaminated soils may occur. Also, fish, birds, and coastal plants could lose parts of their habitat. 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. 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. 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.

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. In more detail, the existing problems are divided into two parts: one is water pollution, and the other is storm surges and floods. Besides, storm surges and flooding can be instantaneous and devastating to cities, and some coastal areas have begun investing in storm water valves to cope with more frequent and severe flooding during high tides.

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.

Cutting greenhouse gas emissions (or climate change mitigation) can stabilize sea level rise rates beyond 2050, but can not prevent sea levels from rising. Thus, mitigation gives more time for adaptation and it leaves more options open, such as nature-based solutions.

Regional examples

Africa

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.

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. In all of these estimates, the Egyptian city of Alexandria alone amounts for around half of this figure: hundreds of thousands of people in its low-lying areas may already have to be relocated in the coming decade. 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.

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 site

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