Sea level rise
The sea level has been rising since the end of the Last Glacial Maximum, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by, with an increase of per year since the 1970s. This was faster than the sea level had ever risen over at least the past 3,000 years. The rate accelerated to /yr for the decade 2013–2022. Climate change due to human activities is the main cause of this persistent acceleration. Between 1993 and 2018, melting ice sheets and glaciers accounted for 44% of sea level rise, with another 42% resulting from thermal expansion of water.
Sea level rise lags behind changes in the Earth's temperature by decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened. What happens after that depends on future human greenhouse gas emissions. If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. The reported factors of increase in flood hazard potential are often exceedingly large, ranging from 10 to 1000 for even modest sea-level rise scenarios of 0.5 m or less. It could then reach by 2100 between and from now and approximately to from the 19th century. With high emissions it would instead accelerate further, and could rise by 50 cm or even by 1.9 m by 2100. In the long run, sea level rise would amount to over the next 2000 years if warming stays to its current over the pre-industrial past. It would be if warming peaks at.
Rising seas affect every coastal population on Earth. This can be through flooding, higher storm surges, king tides, and increased vulnerability to tsunamis. There are many knock-on effects. They lead to loss of coastal ecosystems like mangroves. Crop yields may reduce because of increasing salt levels in irrigation water. Damage to ports disrupts sea trade. The sea level rise projected by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. Without a sharp reduction in greenhouse gas emissions, this may increase to hundreds of millions in the latter decades of the century.
Local factors like tidal range or land subsidence will greatly affect the severity of impacts. For instance, sea level rise in the United States is likely to be two to three times greater than the global average by the end of the century. Yet, of the 20 countries with the greatest exposure to sea level rise, twelve are in Asia, including Indonesia, Bangladesh and the Philippines. The resilience and adaptive capacity of ecosystems and countries also varies, which will result in more or less pronounced impacts. The greatest impact on human populations in the near term will occur in low-lying Caribbean and Pacific islands including atolls. Sea level rise will make many of them uninhabitable later this century.
Societies can adapt to sea level rise in multiple ways. Managed retreat, accommodating coastal change, or protecting against sea level rise through hard-construction practices like seawalls are hard approaches. There are also soft approaches such as dune rehabilitation and beach nourishment. Sometimes these adaptation strategies go hand in hand. At other times choices must be made among different strategies. Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states.
Observations
Between 1901 and 2018, the global mean sea level rose by about. More precise data gathered from satellite radar measurements found an increase of from 1993 to 2017. This accelerated to /yr for 2013–2022. Paleoclimate data shows that this rate of sea level rise is the fastest it had been over at least the past 3,000 years. A research paper published in October 2025 updated the global sea level curve for the last 11,700 years, finding that global mean sea-level rise since 1900 is faster than in any century over at least the last 4,000 years.While sea level rise is uniform around the globe, some land masses are moving up or down as a consequence of subsidence or post-glacial rebound. Therefore, local relative sea level rise may be higher or lower than the global average. Changing ice masses also affect the distribution of sea water around the globe through gravity.
Projections
Approaches used for projections
Several complementary approaches are used for sea level rise projections. One is process-based modeling, where ice melting is computed through an ice-sheet model and rising sea temperature and expansion through a general circulation model, and then these contributions are added up. The so-called semi-empirical approach instead applies statistical techniques and basic physical modeling to the observed recent sea level rise and reconstructions from the older historical geological data. It was developed because process-based model projections in the past IPCC reports were found to underestimate the already observed sea level rise.By 2013, improvements in modeling had addressed this issue, and model and semi-empirical projections for the year 2100 are now very similar. Yet, semi-empirical estimates are reliant on the quality of available observations and struggle to represent non-linearities, while processes without enough available information about them cannot be modeled. Thus, another approach is to combine the opinions of a large number of scientists in what is known as a structured expert judgement. Some analyses suggest that if fossil fuel use continues indefinitely and all polar and mountain ice melts, global sea level could rise by as much as 216 feet.
Variations of these primary approaches exist. For instance, large climate models are computationally expensive, so less complex models are often used in their place for simpler tasks like projecting flood risk in the specific regions. A structured expert judgement may be used in combination with modeling to determine which outcomes are more or less likely, which is known as "shifted SEJ". Semi-empirical techniques can be combined with the so-called "intermediate-complexity" models. After 2016, some ice sheet modeling exhibited the so-called ice cliff instability in Antarctica, which results in substantially faster disintegration and retreat than otherwise simulated. The differences are limited with low warming, but at higher warming levels, ice cliff instability predicts far greater sea level rise than any other approach.
The study reports that sea level also is expected to grow by another 6.6 inches globally over the next 30 years if it follows this trend, which will lead to 16.63 inches under a 1.75 °C warming by 2100.
Projections for the 21st century
The Intergovernmental Panel on Climate Change is the largest and most influential scientific organization on climate change, and since 1990, it provides several plausible scenarios of 21st century sea level rise in each of its major reports. The differences between scenarios are mainly due to uncertainty about future greenhouse gas emissions. These depend on future economic developments, and also future political action which is hard to predict. Each scenario provides an estimate for sea level rise as a range with a lower and upper limit to reflect the unknowns. The scenarios in the 2013–2014 Fifth Assessment Report were called Representative Concentration Pathways, or RCPs and the scenarios in the IPCC Sixth Assessment Report are known as Shared Socioeconomic Pathways, or SSPs. A large difference between the two was the addition of SSP1-1.9 to AR6, which represents meeting the best Paris climate agreement goal of. In that case, the likely range of sea level rise by 2100 is.The lowest scenario in AR5, RCP2.6, would see greenhouse gas emissions low enough to meet the goal of limiting warming by 2100 to. It shows sea level rise in 2100 of about with a range of. The "moderate" scenario, where emissions take a decade or two to peak and its atmospheric concentration does not plateau until the 2070s is called RCP 4.5. Its likely range of sea level rise is. The highest scenario in RCP8.5 pathway sea level would rise between. AR6 had equivalents for both scenarios, but it estimated larger sea level rise under both. In AR6, the SSP1-2.6 pathway results in a range of by 2100. The "moderate" SSP2-4.5 results in a range by 2100 and SSP5-8.5 led to.
This general increase of projections in AR6 came after the improvements in ice-sheet modeling and the incorporation of structured expert judgements. These decisions came as the observed ice-sheet erosion in Greenland and Antarctica had matched the upper-end range of the AR5 projections by 2020, and the finding that AR5 projections were likely too slow next to an extrapolation of observed sea level rise trends, while the subsequent reports had improved in this regard. Further, AR5 was criticized by multiple researchers for excluding detailed estimates the impact of "low-confidence" processes like marine ice sheet and marine ice cliff instability, which can substantially accelerate ice loss to potentially add "tens of centimeters" to sea level rise within this century. AR6 includes a version of SSP5-8.5 where these processes take place, and in that case, sea level rise of up to by 2100 could not be ruled out.
Role of instability processes
The greatest uncertainty with sea level rise projections is associated with the so-called marine ice sheet instability, and, even more so, Marine Ice Cliff Instability. These processes are mainly associated with West Antarctic Ice Sheet, but may also apply to some of Greenland's glaciers. The former suggests that when glaciers are mostly underwater on retrograde bedrock, the water melts more and more of their height as their retreat continues, thus accelerating their breakdown on its own. This is widely accepted, but is difficult to model.The latter posits that coastal ice cliffs which exceed ~ in above-ground height and are ~ in basal height are likely to rapidly collapse under their own weight once the ice shelves propping them up are gone. The collapse then exposes the ice masses following them to the same instability, potentially resulting in a self-sustaining cycle of cliff collapse and rapid ice sheet retreat. This theory had been highly influential – in a 2020 survey of 106 experts, the 2016 paper which suggested or more of sea level rise by 2100 from Antarctica alone, was considered even more important than the 2014 IPCC Fifth Assessment Report. Even more rapid sea level rise was proposed in a 2016 study led by Jim Hansen, which hypothesized multi-meter sea level rise in 50–100 years as a plausible outcome of high emissions, but it remains a minority view amongst the scientific community.
Marine ice cliff instability had also been very controversial, since it was proposed as a modelling exercise, and the observational evidence from both the past and the present is very limited and ambiguous. So far, only one episode of seabed gouging by ice from the Younger Dryas period appears truly consistent with this theory, but it had lasted for an estimated 900 years, so it is unclear if it supports rapid sea level rise in the present. Modelling which investigated the hypothesis after 2016 often suggested that the ice shelves in the real world may collapse too slowly to make this scenario relevant, or that ice mélange – debris produced as the glacier breaks down – would quickly build up in front of the glacier and significantly slow or even outright stop the instability soon after it began.
Due to these uncertainties, some scientists – including the originators of the hypothesis, Robert DeConto and David Pollard – have suggested that the best way to resolve the question would be to precisely determine sea level rise during the Last Interglacial. MICI can be effectively ruled out if SLR at the time was lower than, while it is very likely if the SLR was greater than. As of 2023, the most recent analysis indicates that the Last Interglacial SLR is unlikely to have been higher than, as higher values in other research, such as, appear inconsistent with the new paleoclimate data from The Bahamas and the known history of the Greenland Ice Sheet.