Between 1901 and 2018, the globally averaged sea level rose by 15–25 cm (6–10 in), or 1–2 mm per year on average. This rate is accelerating, and the sea levels are now rising by 3.7 mm (0.146 inches) per year. This is caused by human-induced climate change, as it continually heats (and therefore expands) the ocean and melts land-based ice sheets and glaciers. Over the period between 1993 and 2018, the thermal expansion of water contributed 42% to sea level rise (sometimes abbreviated as SLR in the scientific literature); melting of temperate glaciers, 21%; Greenland, 15%; and Antarctica, 8%.: 1576 Because sea level rise lags changes in Earth temperature, it will continue to accelerate between now and 2050 purely in response to warming which has already occurred: whether it continues to accelerate after that is dependent on the human greenhouse gas emissions. Even if sea level rise does not accelerate, it will continue for a very long time: over the next 2000 years, it is projected to amount to 2–3 m (7–10 ft) if global warming is limited to 1.5 °C (2.7 °F), to 2–6 m (7–20 ft) if it peaks at 2 °C (3.6 °F) and to 19–22 metres (62–72 ft) if it peaks at 5 °C (9.0 °F).: 21
The rising seas pose both a direct risk of flooding unprotected areas and indirect threats of higher storm surges, king tides, and tsunamis (particularly in the Pacific and Atlantic Oceans). They are also associated with the highly detrimental second-order effects such as the loss of coastal ecosystems like mangroves, losses in crop production due to freshwater salinization of groundwater and irrigation water or the disruption of sea trade due to damaged ports. Globally, just the projected sea level rise by 2050 will expose places currently inhabited by tens of millions of people to annual flooding, or force them under the water line during high tide, and this can increase to hundreds of millions in the latter decades of the century if greenhouse gas emissions are not reduced drastically. While modest increases in sea level are likely to be offset when cities adapt by constructing sea walls or through relocating people, many coastal areas have large population growth, which results in more people at risk from sea level rise. Later in the century, millions of people will be affected in cities such as Miami, Rio de Janeiro, Osaka and Shanghai under the warming of 3 °C (5.4 °F), which is close to the current trajectory.
While the rise in sea levels ultimately impacts every coastal and island population on Earth, it does not occur uniformly due to local factors like tides, currents, storms, tectonic effects and land subsidence. Moreover, the differences in resilience and adaptive capacity of ecosystems, sectors, and countries again mean that the impacts will be highly variable in time and space.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.
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. 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.
Between 1901 and 2018, the globally averaged sea level rose by 15–25 cm (6–10 in). More precise data gathered from satellite radar measurements reveal a rise of 7.5 cm (3 in) from 1993 to 2017 (average of 2.9mm/yr), accelerating to a rate of 3.7mm/yr as of 2021.
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. 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.
For instance, when a glacier or an ice sheet melts, the loss of mass reduces its gravitational pull. In some places near current and former glaciers and ice sheets, this has caused local water levels to drop, even as the water levels will increase more than average further away from the ice sheet. Consequently, ice loss in Greenland has a different fingerprint on regional sea level than the equivalent loss in Antarctica. On the other hand, the Atlantic is warming at a faster pace than the Pacific. This has consequences for Europe and the U.S. East Coast, which receives 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.
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 and/or oil and gas, as well as by levees and other flood management practices preventing the accumulation of sediments which otherwise compensates for the natural settling of deltaic soils.: 638 : 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.: 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.
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.
The Intergovernmental Panel on Climate Change provides multiple plausible scenarios of 21st century sea level rise every time they publish a report, starting from the IPCC First Assessment Report in 1990. The differences between scenarios are primarily due to the uncertainty about future greenhouse gas emissions; in addition, every scenario has an uncertainty range to represent the unknows in sea level response to that emissions level. The scenarios used in the 2013-2014 Fifth Assessment Report (AR5) were known as Representative Concentration Pathways, or RCPs. Only RCP 2.6 is consistent with the Paris climate agreement goal of preventing 2 °C (3.6 °F) of warming: it assumes that all emissions start declining in 2020s, leading to an immediate decrease in atmospheric methane (CH4) and a plateau in the atmospheric carbon dioxide and nitrous oxide (N2O) concentrations. The projected SLR by 2100 would then amount to 44 cm (17+1⁄2 in), with a range of 28–61 cm (11–24 in) Another important scenario is RCP 4.5, where CO2 emissions do not peak until 2040 while atmospheric concentrations do not plateau until 2070s, CH4 concentrations plateau in the 2020s and slowly decline after 2050, while N2O concentrations slowly increase throughout the century. It is associated with the warming of ~2.5 °C (4.5 °F) by 2100 (and additional warming after that date), and the sea level rise of 53 cm (21 in), with a range of 36–71 cm (14–28 in). Finally, RCP 8.5 is defined by the rapid increases in CO2 and CH4 concentrations to levels several times larger than in the other scenarios, with their respective emissions accelerating for the rest of the century and until 2075. Under RCP 8.5, the warming exceeds 4 °C (7.2 °F) , while the sea level would rise by a median of 74 cm (29 in), with a range of 52–98 cm (20+1⁄2–38+1⁄2 in). As of 2022, the estimated global warming trajectory would lead to ~2.7 °C (4.9 °F) by 2100, which is most similar to RCP 4.5.
Notably, the report had acknowledged the possibility of global SLR being accelerated by the outright collapse of the marine-based parts of the Antarctic ice sheet, but did not estimate its likelihood due to the lack of reliable information, only stating with medium confidence that if such a collapse occurred, it would not add more than several tens of centimeters to 21st century sea level rise.Since its publication, multiple papers have questioned this decision and presented higher estimates of SLR after attempting to better incorporate ice sheet processes in Antarctica and Greenland and to compare the current events with the paleoclimate data. For instance, a 2017 study from the University of Melbourne researchers estimated that ice sheet processes would increase AR5 sea level rise estimate for the low emission scenario by about one quarter, but they would add nearly half under the moderate scenario and practically double estimated sea level rise under the high emission scenario. The upper end of that final estimate (95–189 cm (37+1⁄2–74+1⁄2 in)) would mean rapid sea level rise of up to 19 mm (3⁄4 in) per year by the end of the century. Likewise, the 2017 Fourth National Climate Assessment (NCA) of the United States presented similar numbers to the IPCC in the low emission scenarios, yet found that if the high emission scenario triggers Antarctic ice sheet instability then the SLR of up to 2.4 m (10 ft) by 2100 relative to 2000 is physically possible, greatly increasing the 130 cm (5 ft) estimate for the same scenario but without instability.
A 2016 study led by Jim Hansen presented a hypothesis of vulnerable ice sheet collapse leading to near-term exponential sea level rise acceleration, with a doubling time of 10, 20 or 40 years, thus leading to multi-meter sea level rise in 50, 100 or 200 years, respectively. However, it remains a minority view amongst the scientific community. For comparison, two expert elicitation papers were published in 2019 and 2020, both looking at low and high emission scenarios. The former combined the projections of 22 ice sheet experts to estimate the median SLR of 30 cm (12 in) by 2050 and 70 cm (27+1⁄2 in) by 2100 in the low emission scenario and the median of 34 cm (13+1⁄2 in) by 2050 and 110 cm (43+1⁄2 in) by 2100 in a high emission scenario. At the same time, they estimated a small chance of sea levels exceeding 1 meter by 2100 even in the low emission scenario and of going beyond 2 metres in the high emission scenario, with the latter causing the displacement of 187 million people. The other paper surveyed 106 experts, who had estimated the median of 45 cm (17+1⁄2 in) by 2100 for RCP 2.6, with a 17%-83% range of 30–65 cm (12–25+1⁄2 in) and a 5%-95% range of 21–82 cm (8+1⁄2–32+1⁄2 in). For RCP 8.5, the experts estimated a median of 93 cm (36+1⁄2 in) by 2100, with a 17%-83% range of 63–132 cm (25–52 in) and a 5%-95% range of 45–165 cm (17+1⁄2–65 in).
By 2020, the observed ice-sheet losses in Greenland and Antarctica were found to track the upper-end range of the AR5 projections. Consequently, the updated SLR projections in the 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate were somewhat larger than in AR5. A February 2021 paper found that while AR5 projections appeared unrealistically low next to the extrapolation of observed sea level trends, the projections in SROCC were a much better fit. At the same time, they cautioned that the mismatch between SROCC numbers and expert elicitations was likely to indicate excessive conservatism in the IPCC reports.
The IPCC Sixth Assessment Report (AR6) was published in August 2021. Its main set of sea level rise projections was ultimately only slightly larger than the one in SROCC, with SSP1-2.6 resulting in a 17-83% range of 32–62 cm (12+1⁄2–24+1⁄2 in) by 2100, SSP2-4.5 resulting in a 44–76 cm (17+1⁄2–30 in) range by 2100 and SSP5-8.5 leading to 65–101 cm (25+1⁄2–40 in). However, the report had also provided extended projections on both the lower and the upper end, adding SSP1-1.9 scenario which represents meeting the 1.5 °C (2.7 °F) goal and has the likely range of 28–55 cm (11–21+1⁄2 in), as well as "low-confidence" narrative involving processes like marine ice sheet and marine ice cliff instability under SSP5-8.5. For that scenario, it cautioned that the sea level rise of over 2 m (6+1⁄2 ft) by 2100 "cannot be ruled out".
Models consistent with paleo records of sea level rise: 1189 indicate that substantial long-term SLR will continue for centuries to come even if the temperature stabilizes. 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). At the same time, tipping points of Greenland and Antarctica ice sheets are expected to play a larger role over such timescales, with the very long-term SLR likely to be dominated by ice loss from Antarctica, especially if the warming exceeds 2 °C (3.6 °F). 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.
In the next 2,000 years the sea level is predicted to rise by 2–3 m (6+1⁄2–10 ft) if the temperature rise peaks at 1.5 °C (2.7 °F), by 2–6 m (6+1⁄2–19+1⁄2 ft) if it peaks at 2 °C (3.6 °F) and by 19–22 m (62+1⁄2–72 ft) if it peaks at 5 °C (9.0 °F).: SPM-28 If temperature rise stops at 2 °C (3.6 °F) or at 5 °C (9.0 °F), the sea level would still continue to rise for about 10,000 years. In the first case it will reach 8–13 m (26–42+1⁄2 ft) above pre-industrial level, and in the second 28–37 m (92–121+1⁄2 ft).
As the models and observational records have improved, a range of studies have attempted to estimate SLR for the years immmediately following 2100, although it remains challenging to do this with high accuracy. For instance, when the April 2019 expert elicitation asked its 22 experts about total sea level rise projections for the years 2200 and 2300 under its high, 5 °C warming scenario, it ended up with 90% confidence intervals of −10 cm (4 in) to 740 cm (24+1⁄2 ft) and −9 cm (3+1⁄2 in) to 970 cm (32 ft), respectively (negative values represent the extremely low probability of very large increases in the ice sheet surface mass balance due to climate change-induced increase in precipitation more than offsetting SLR.) The elicitation of 106 experts led by Stefan Rahmstorf had also included 2300 for RCP2.6 and RCP 8.5: the former had the median of 118 cm (46+1⁄2 in), a 17%-83% range of 54–215 cm (21+1⁄2–84+1⁄2 in) and a 5%-95% range of 24–311 cm (9+1⁄2–122+1⁄2 in), while the latter had the median of 329 cm (129+1⁄2 in), a 17%-83% range of 167–561 cm (65+1⁄2–221 in) and a 5%-95% range of 88–783 cm (34+1⁄2–308+1⁄2 in)
By 2021, AR6 was also able to provide estimates for year 2150 SLR alongside the 2100 estimates for the first time. According to it, keeping warming at 1.5 °C under the SSP1-1.9 scenario would result in sea level rise in the 17-83% range of 37–86 cm (14+1⁄2–34 in), SSP1-2.6 a range of 46–99 cm (18–39 in), SSP2-4.5 of 66–133 cm (26–52+1⁄2 in) range by 2100 and SSP5-8.5 leading to 98–188 cm (38+1⁄2–74 in). Moreover, it stated that if the "low-confidence" could result in over 2 m (6+1⁄2 ft) by 2100, it would then accelerate further to potentially approach 5 m (16+1⁄2 ft) by 2150. The report provided lower-confidence estimates for year 2300 sea level rise under SSP1-2.6 and SSP5-8.5 as well: the former had a range between 0.5 m (1+1⁄2 ft) and 3.2 m (10+1⁄2 ft), while the latter ranged from just under 2 m (6+1⁄2 ft) to just under 7 m (23 ft). Finally, the version of SSP5-8.5 involving low-confidence processes has a chance of exceeding 15 m (49 ft) by then.
In 2018, it was estimated that for every 5 years CO2 emissions are allowed to increase before finally peaking, the median 2300 SLR increases by the median of 20 cm (8 in), with a 5% likelihood of 1 m (3+1⁄2 ft) increase due to the same. The same estimate found that if the temperature stabilized below 2 °C (3.6 °F), 2300 sea level rise would still exceed 1.5 m (5 ft), while the early net zero and slowly falling temperatures could limit it to 70–120 cm (27+1⁄2–47 in).
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 7.4 m (24 ft 3 in) for Greenland and 58.3 m (191 ft 3 in) for Antarctica.
Each year about 8 mm (5⁄16 in) of precipitation (liquid equivalent) falls on the ice sheets in Antarctica and Greenland, mostly as snow, which accumulates and over time forms glacial ice. Much of this precipitation began as water vapor evaporated from the ocean surface. 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.
Main article: Ocean heat content
The oceans store more than 90% of the extra heat added to Earth's climate system by climate change and act as a buffer against its effects. The amount of heat needed to increase average temperature of the entire world ocean by 0.01 °C (0.018 °F) would increase atmospheric temperature by approximately 10 °C (18 °F): 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.: 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 SLR. 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).
Considering an increase in average global temperature of 2 °C (3.6 °F) 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%.
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.
In 2021, limiting global warming to 1.5 °C (2.7 °F) was projected to 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, and the fate of Antarctica the source of the largest uncertainty. By 2019, several studies have attempted to estimate 2300 sea level rise caused by ice loss in Antarctica alone: they suggest 16 cm (6+1⁄2 in) median and 37 cm (14+1⁄2 in) maximum values under the low-emission scenario but a median of 1.46 m (5 ft) metres (with a minimum of 60 cm (2 ft) 60 cm and a maximum of 2.89 m (9+1⁄2 ft)) under the highest-emission scenario.
The world's largest potential source of sea level rise is the East Antarctic Ice Sheet (sometimes abbreviated as EAIS), which holds enough ice to raise global sea levels by 53.3 m (174 ft 10 in). Historically, it was less studied than the West Antarctica as it had been considered relatively stable, and this impression was often backed up by satellite observations and modelling of its surface mass balance. However, a 2019 study employed different methodology and concluded that East Antarctica is already losing ice mass overall. All 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 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.
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 (5.4 °F) of global warming, although it may be as high as 6 °C (11 °F), or as low as 2 °C (3.6 °F). 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 (13.5 °F) (range between 5 °C (9.0 °F) and 10 °C (18 °F)), 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 (11 °F) of warming.
Even though East Antarctica contains the largest potential source of sea level rise, West Antarctica ice sheet (WAIS) is substantially more vulnerable. 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. Consequently, while the mass balance of the East Antarctic Ice Sheet remained relatively steady, satellite observations recorded a substantial increase in WAIS melting from 1992 to 2017, resulting in 7.6 ± 3.9 mm (19⁄64 ± 5⁄32 in) of Antarctica sea level rise, with a disproportionate role played by outflow glaciers in the Amundsen Sea Embayment may have contributed to this increase.
In 2021, AR6 estimated that while the median increase in sea level rise from the West Antarctic ice sheet melt by 2100 is ~11 cm (5 in) 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), it can conceivably contribute as much as 41 cm (15 in) by 2100 under the low-emission scenario and 57 cm (20 in) under the highest-emission one. This is because WAIS is vulnerable to several types of instability whose role remains difficult to model, including hydrofracturing (where meltwater collecting atop the ice sheet pools into fractures and forces them open), increased contact of warm ocean water with ice shelves due to climate-change induced ocean circulation changes, marine ice sheet instability (warm water entering between the seafloor and the base of the ice sheet once it is no longer heavy enough to displace the flow, causing accelerated melting and collapse) and even marine ice cliff instability (ice cliffs with heights greater than 100 m (330 ft) collapsing under their own weight once they are no longer buttressed by ice shelves). These processes do not have equal influence and are not all equally likely to happen: for instance, marine ice cliff instability has never been observed and it was ruled out by some of the more detailed modelling.
At the same time, Thwaites and Pine Island glaciers have been identified as potentially prone to ice sheet instability processes, since both glaciers bedrock topography gets deeper farther inland, exposing them to more warm water intrusion into the grounding zone. Their contribution to global sea levels has already accelerated since the beginning of the 21st century, with the Thwaites Glacier now amounting to 4% of the global sea level rise. At the end of 2021, it was estimated that the Thwaites Ice Shelf can collapse in three to five years, which would then make the destabilization of the entire Thwaites glacier inevitable. The Thwaites glacier itself will cause a rise of sea level by 65 cm (25+1⁄2 in) if it will completely collapse, although this process is estimated to unfold over several centuries.
Moreover, the crucial buttressing position of the Thwaites Glacier means that its loss can destabilize the entire West Antarctic Ice Sheet. Most of the bedrock underlying the West Antarctic Ice Sheet lies well below sea level. This possibility of complete destabilization was first proposed back in the 1970s. A 1978 study by J.H. Mercer predicted that anthropogenic CO2 emissions doubling by 2050 would cause 5 m (15 ft) of SLR due to the rapid loss of the West Antarctic ice sheet alone. Since then, improved modelling concluded that the ice within WAIS would raise the sea level by 3.3 m (10 ft 10 in). 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 (2.7 °F) of global warming and would become absolutely unavoidable at 3 °C (5.4 °F) : at worst, it may have even been triggered by now, after the warming passed 1 °C (1.8 °F) 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 (1.8 °F) below the preindustrial levels (about 2 °C (3.6 °F) below the current levels).
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%). Average annual ice loss in Greenland more than doubled in the early 21st century compared to the 20th century, and there was a corresponding increase in SLR contribution from 0.07 mm per year between 1992 and 1997 to 0.68 mm per year between 2012 and 2017. Total ice loss from the Greenland Ice Sheet between 1992 and 2018 amounted to 3,902 gigatons (Gt) of ice, which is equivalent to the SLR of 10.8 mm. 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 predictions from the past IPCC assessment reports. In 2021, AR6 estimated that under the SSP1-2.6 emission scenario which largely fulfils the Paris Agreement goals, Greenland ice sheet melt adds around 6 cm (2+1⁄2 in) to global sea level rise by the end of the century, with a plausible maximum of 15 cm (6 in) (and even a very small chance of the ice sheet reducing the sea levels by around 2 cm (1 in) due to gaining mass through surface mass balance feedback). The scenario associated with the highest global warming, SSP5-8.5, would see Greenland add a minimum of 5 cm (2 in) to sea level rise, a likely median of 13 cm (5 in) cm and a plausible maximum of 23 cm (9 in).
Certain parts of the Greenland ice sheet are already known to be committed to unstoppable sea level rise. Greenland's peripheral glaciers and ice caps crossed an irreversible tipping point around 1997, and will continue to melt. A subsequent study had found that the climate of the past 20 years (2000–2019) would already result of the loss of ~3.3% volume in this manner in the future, committing the ice sheet to an eventual 27 cm (10+1⁄2 in) of SLR, independent of any future temperature change. There is also a global warming threshold beyond which a near-complete melting of the Greenland ice sheet occurs. Earlier research has put this threshold value as low as 1 °C (1.8 °F), and definitely no higher than 4 °C (7.2 °F) above pre-industrial temperatures.: 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 years, even though the temperatures have never been higher than 2.5 °C (4.5 °F) greater than today 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 (1.4 °F) and is certainly no higher than 3 °C (5.4 °F) : there's a high chance that it will be crossed around 1.5 °C (2.7 °F). 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.
There are roughly 200,000 glaciers on Earth, which are spread out across all continents. 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. 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. Glaciers contributed around 40% to sea level rise during the 20th century, with estimates for the 21st century of around 30%. The IPCC Fifth Assessment Report estimated that glaciers contributing 7–24 cm (3–9+1⁄2 in) to global sea levels.: 1165
In 2023, a Science paper estimated that at 1.5 °C (2.7 °F), one quarter of mountain glacier mass would be lost by 2100 and nearly half would be lost at 4 °C (7.2 °F) , contributing ~9 cm (3+1⁄2 in) and ~15 cm (6 in) to sea level rise, respectively. Because glacier mass is disproportionately concentrated in the most resilient glaciers, this would in practice remove between 49% to 83% of glacier formations. It had further estimated that the current likely trajectory of 2.7 °C (4.9 °F) would result in the SLR contribution of ~11 cm (4+1⁄2 in) by 2100. Mountain glaciers are even more vulnerable over the longer term. In 2022, another Science paper estimated that almost no mountain glaciers can be expected to survive once the warming crosses 2 °C (3.6 °F) , and their complete loss largely inevitable around 3 °C (5.4 °F): there's even a possibility of complete loss after 2100 at just 1.5 °C (2.7 °F). This could happen as early 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 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).
Humans impact how much water is stored on land. Building dams prevents large masses of water from flowing into the sea and therefore increases the storage of water on land. On the other hand, humans extract water from lakes, wetlands and underground reservoirs for food production leading to rising seas. Furthermore, the hydrological cycle is influenced by climate change and deforestation, which can lead to further positive and negative contributions to sea level rise. In the 20th century, these processes roughly balanced, but dam building has slowed down and is expected to stay low for the 21st century.: 1155
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.
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 3.0 ± 0.4 millimetres (1⁄8 ± 1⁄64 in) 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.
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 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). 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. 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.
The impacts of current and future sea level rise 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. In turn, this would lead to a greater loss of property and coastal habitats, loss of life during floods, loss of non-monetary cultural resources and values, impact on agriculture and aquaculture through decline in soil and water quality, and loss of tourism, recreation, and transportation functions.: 356 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.
Because the projected extent of sea level rise by 2050 will be only slightly affected by any changes in emissions, there's confidence that 2050 levels of SLR 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. By 2100, the difference between the 2050 sea level rise and the low and high end of the median sea level rise estimates for 2100 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. If ice sheet processes under the highest emission scenario result in sea level rise of well over one metre (3+1⁄4 ft) by 2100, with a chance of levels over two metres (6+1⁄2 ft),: TS-45 then as many as 520 million additional people would end up under the water line during high tide and 640 million in places which are flooded every year, when compared to the 2010 population distribution.
Over the longer term, coastal areas are particularly vulnerable 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 10 metres (33 ft) 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 m (6+1⁄2 ft) above sea level and that with a 1 m (3+1⁄2 ft) sea level rise and zero population growth, that number could increase to 410 million people.
At the same time, even the populations who live further inland may be impacted by a potential disruption of sea trade, as it is the dominant form of resource and good trade throughout the world. Sea level rise will inevitably affect ports, but the current research into this subject is limited. Not enough is known about the investments required to protect the ports currently in use, and for how they may be protected before it becomes more reasonable to build new port facilities elsewhere. Moreover, some coastal regions are rich agricultural lands, whose loss to the sea can result in food shortages elsewhere. This is a particularly acute issue for river deltas such as Nile Delta in Egypt and Red River and Mekong Deltas in Vietnam, which are disproportionately affected by saltwater intrusion into the soil and irrigation water.
When seawater reaches inland, coastal plants, birds, and freshwater/estuarine fish are threatened with habitat loss due to flooding and soil/water salinization. So-called ghost forests emerge when coastal forest areas become inundated with saltwater to the point no trees can survive. At worst, entire species can be driven extinct. In 2016, an island in the Great Barrier Reef called Bramble Cay was inundated, flooding the habitat of a rodent named Bramble Cay melomys. In 2019, it was officially declared extinct by the federal government of Australia.
While some ecosystems can move land inward with the high-water mark, 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 tidal marshes. Mangrove ecosystems on the mudflats of tropical coasts nurture high biodiversity, yet they are particularly vulnerable due to mangrove plants' relliance on breathing roots or pneumatophores, which might grow to be half a metre tall. While mangroves can adjust to rising sea levels by migrating inland and building vertically using accumulated sediment and organic matter, they will be submerged if the rate is too rapid, resulting in the loss of an ecosystem. Both mangroves and tidal marshes protect against storm surges, waves and tsunamis, so their loss 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.
Likewise, corals, important for bird and fish life, need to grow vertically to remain close to the sea surface in order to get enough energy from sunlight. The corals have 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.
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. However, this often results in the loss of livelihoods and the displaced people could become a strain on the regions or cities they come to inhabit, potentially accelerating social tensions. Accommodation options are intended to 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. However, these options tend to carry increased costs, as seen with salt-resistant crop variants being more expensive than the ordinary crops. 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.: 3-127
Further information: Climate change in the Middle East and North Africa § Sea level rise
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.
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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 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).
Further information: Climate change in South Asia § Sea level rise
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. 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.
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. Nations with extensive rice production along the coasts like Bangladesh, Vietnam and China are already seeing adverse impacts from saltwater intrusion.
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. 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. 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.
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In an attempt to address these challenges, the Bangladesh Delta Plan 2100 has been launched in 2018. As of 2020, it was seen falling short of most of its initial targets. The progress is being monitored.
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. 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 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. Other so called sinking cities, such as Bangkok or Tokyo, are vulnerable to these compounding subsidence with sea level rise.
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.
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 or wind behavior 76 times in one 6-year period in 2010s, with a 25,000-50,000 USD loss for each idle day. In Port of Santos, storm surges were three times more frequent between 2000 and 2016 than between 1928 and 1999.
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.
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. 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. 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. 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
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, the frequency of coastal flooding in the US is expected to rise to the current baseline of four "moderate" flooding events per year, even without storms and/or heavy rainfall.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 m (3 ft) and 1.8 m (6 ft) would threaten 4.2 and 13.1 million people in the US, respectively. In California alone, 2 m (6+1⁄2 ft) 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.
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. It also favors chronic flooding at high tide, as evidenced e.g. in the US East Coast. Similarly, Florida, which is extremely vulnerable to climate change, is already experiencing substantial nuisance flooding and king tide flooding. 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. 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. 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. 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.
Small island states are nations whose populations are concentrated on atolls and other low islands. Atolls on average reach 0.9–1.8 m (3–6 ft) above sea level. This means that no other place is more vulnerable to coastal erosion, flooding and salt intrusion into soils and freshwater caused by sea level rise. The latter may render an island uninhabitable well before it is completely flooded. Already, children in small island states are encountering hampered access to food and water and are suffering an increased rate of mental and social disorders due to these stressors. At current rates, sea level would be high enough to make the Maldives uninhabitable by 2100, while five of the Solomon Islands have already disappeared due to the combined effects of sea level rise and stronger trade winds that were pushing water into the Western Pacific.
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. Nations like Maldives, Kiribati and Tuvalu are already forced to consider controlled international migration of their population in response to rising seas,  since the alternative of uncontrolled migration threatens to exacerbate the humanitarian crisis of climate refugees. In 2014, Kiribati had purchased 20 square kilometers of land (an area 6 times greater than the current area of Kiribati) on the Fijian island of Vanua Levu to relocate its population there once their own islands are lost to the sea.
While Fiji is also impacted by sea level rise, it is in a comparatively safer position, and its residents continue to rely on local adaptation like moving further inland and increasing sediment supply to combat erosion instead of relocating entirely. Fiji has also issued a green bond of $50 million to invest in green initiatives and use the proceeds to fund adaptation efforts, and it is restoring coral reefs and mangroves to protect itself flooding and erosion as a more cost-efficient alternative to building sea walls, with the nations of Palau and Tonga adopting similar efforts. At the same time, even when an island is not threatened with complete disappearance due to flooding, tourism and local economies may end up devastated. For instance, 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, while a further 49–60% of coastal resorts would be at risk from resulting coastal erosion.
If all islands of an island nation become uninhabitable or completely submerged by the sea, the states themselves would theoretically also become dissolved, removing their rights on the surrounding sea area (a radius of 415 kilometres (224 nautical miles) around the entire island state). Mineral exploration and extraction efforts by international actors would no longer involve paying commission to the former state.
Understanding past sea level is an important guide to current and future changes. In the recent geological past, thermal expansion from increased temperatures and changes in land ice are the dominant reasons of sea level rise. The last time that the Earth was 2 °C (3.6 °F) warmer than pre-industrial temperatures was 120 thousand years ago, when warming because of changes in the amount of sunlight due to slow changes in the Earth's orbit caused the Eemian interglacial; sea levels during that warmer interglacial were at least 5 m (16 ft) higher than now. The Eemian warming was sustained over a period of thousands of years, and the magnitude of the rise in sea level implies a large contribution from the Antarctic and Greenland ice sheets.: 1139 Further into the past, a report by the Royal Netherlands Institute for Sea Research states that, around three million years ago, levels of carbon dioxide in the Earth's atmosphere similar to today's levels increased temperature by two to three degrees Celsius and melted one third of Antarctica's ice sheets. This in turn caused sea-levels to rise 20 metres over their present values.
Since the last glacial maximum about 20,000 years ago, the sea level has risen by more than 125 metres (410 ft), with rates varying from less than a mm/year during the pre-industrial era to 40+ mm/year when major ice sheets over Canada and Eurasia melted. Rapid disintegration of these ice sheets led to so called 'meltwater pulses', periods during which sea level rose rapidly. The rate of rise started to slow down about 8,200 years before present; the sea level was then almost constant in the last 2,500 years, before the recent rising trend that started at the end of the 19th century or in the beginning of the 20th.
This corresponds to a mean sea-level rise of about 7.5 cm over the whole altimetry period. More importantly, the GMSL curve shows a net acceleration, estimated to be at 0.08mm/yr2.
Box SYN-1: Sustained warming could lead to severe impacts
There is no doubt that the sea level rise, within the IPCC, is a very conservative number," says Greg Holland, a climate and hurricane researcher at the National Center for Atmospheric Research, who has also reviewed the Hansen study. "So the truth lies somewhere between IPCC and Jim.
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
Because Thwaites sits below sea level on ground that dips away from the coast, the warm water is likely to melt its way inland, beneath the glacier itself, freeing its underbelly from bedrock. A collapse of the entire glacier, which some researchers think is only centuries away, would raise global sea level by 65 centimeters.
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