Sea level rise is a long-term increase in the height of the ocean surface, measured globally or relative to land at a particular coast. It differs from temporary changes caused by tides, waves, and storms. Modern global sea level rise is a consequence of climate change, principally through the expansion of warming seawater and the transfer of water from land ice into the ocean. Its local effects also depend on land movement and regional ocean conditions. (science.nasa.gov)
Physical causes
Two processes dominate the modern rise. First, thermal expansion increases seawater volume as its temperature rises. This raises sea level without adding water to the ocean. Second, the loss of glaciers and the Greenland and Antarctic ice sheets increases ocean water mass. These contributions vary over time as ocean warming, snowfall, surface melting, and ice discharge change. (nasa.gov)
Melting floating sea ice is not equivalent to melting land ice: floating ice already displaces seawater, so its melting has a much smaller direct effect on sea level. Floating ice shelves nevertheless influence the discharge of grounded ice; changes in their restraining effect can alter the contribution of ice sheets. (ipcc.ch)
Changes in terrestrial water storage provide another contribution. Reservoirs retain water that might otherwise reach the ocean, whereas extraction of groundwater can transfer stored land water into the sea. Rainfall and other variations in the water cycle also redistribute water between land and ocean. These exchanges help explain short-term variability as well as parts of the historical trend. (sealevel.nasa.gov)
Global and relative sea level
Global mean sea level describes an average across the ocean. Relative sea level describes the sea surface compared with the adjacent land and is therefore especially important for coastal flooding. A coast can experience rapid relative rise when land subsides, even if nearby ocean height changes more slowly. Conversely, sufficiently rapid uplift can produce a local relative fall despite rising global mean sea level. (earth.gov)
Land subsidence can result from sediment compaction, groundwater withdrawal, or geological processes. Glacial isostatic adjustment, the continuing response of the solid Earth to past ice-sheet loading and unloading, also affects coastal elevations. These movements make global averages insufficient for describing an individual shoreline. (science.nasa.gov)
Ocean circulation, winds, and density changes redistribute seawater regionally. Ice loss also changes gravity, Earth’s rotation, and crustal deformation, producing geographically distinct sea level patterns known as fingerprints. Consequently, water added by a melting ice sheet does not raise sea level equally everywhere, and sea level can fall near the shrinking ice mass. (ntrs.nasa.gov)
Measurement and observed change
Tide gauges measure water height relative to fixed coastal benchmarks. Some records extend for more than a century, but stations are unevenly distributed. Satellite altimetry provides much broader ocean coverage: radar pulses measure the distance between an orbiting satellite and the sea surface, which is combined with precise orbital information to determine surface height. Regular satellite monitoring began in 1992. (earth.gov)
Additional observations identify the causes of change. Satellite gravity measurements track changes in ice, ocean mass, and land water storage. Argo profiling floats measure ocean temperature and salinity, allowing estimates of thermal expansion. Ground-based satellite-navigation receivers measure land movement, helping distinguish ocean rise from subsidence or uplift at tide-gauge sites. (earth.gov)
The Intergovernmental Panel on Climate Change assessed a global mean rise of approximately 0.20 metres between 1901 and 2018, with an assessed range of 0.15–0.25 metres. The average rate increased from 1.3 millimetres per year during 1901–1971 to 1.9 during 1971–2006 and 3.7 during 2006–2018. Human influence was assessed as very likely the main driver of the rise since at least 1971. These are estimates for specified historical periods, not a constant rate applicable to every year. (ipcc.ch)
Future projections and uncertainty
Projections combine climate models, observations, and models of glaciers and ice sheets under alternative greenhouse gas emissions scenarios. They describe conditional futures rather than a single inevitable outcome. Baseline periods matter: a projected rise relative to 1995–2014 is not the same as an increase measured from the present day. (ipcc.ch)
The IPCC’s 2023 synthesis reported likely global mean increases by 2100 of 0.28–0.55 metres under the very low-emissions scenario SSP1-1.9 and 0.63–1.01 metres under the very high-emissions scenario SSP5-8.5, both relative to 1995–2014. Uncertain ice-sheet processes, particularly in Antarctica, leave open the possibility of larger increases outside these likely ranges. (ipcc.ch)
Sea level responds slowly to warming. Continuing deep-ocean heat uptake and ice-sheet loss commit it to rise for centuries to millennia, with elevated levels persisting for thousands of years. Lower emissions reduce the magnitude of future rise but do not immediately halt it. (ipcc.ch)
Coastal consequences and responses
A higher baseline sea level increases the frequency of coastal flooding and allows storm surges to reach farther inland. It can intensify erosion, promote saltwater intrusion, and affect freshwater supplies and coastal infrastructure. Exposure is particularly important in low-lying islands, river deltas, and coastal settlements. (ipcc.ch)
Coastal ecosystems may respond by accumulating sediment or migrating inland. Wetlands can lose area where rising water outpaces elevation gain or development blocks migration. Coastal defenses can protect buildings while restricting the space available for these ecological adjustments. (ipcc.ch)
Documented adaptation approaches include engineered protection, accommodation through changes to buildings and land use, ecosystem-based measures, and managed retreat of people or assets from exposed areas. Ecological restoration can support coastal buffering, but its effectiveness depends on local conditions. Decisions involve different implementation times, costs, environmental effects, and limits; adaptation pathways sequence measures as sea level and risk change. (ipcc.ch)