Sea-level rise is a long term increase in the average height of the ocean surface relative to a specified reference level. It is not simply the ocean becoming deeper everywhere by the same amount. Sea level varies naturally across space and time because of tides, waves, winds, ocean currents, atmospheric pressure, temperature, salinity, changes in gravity and vertical movement of the land itself. Scientists therefore distinguish global mean sea level from regional or local relative sea level.
Global mean sea level represents the average height of the ocean surface over the planet, whereas relative sea level at a particular coastline depends on both changes in ocean height and changes in the elevation of the land. A coastline experiencing land subsidence can therefore experience faster relative sea-level rise than the global average, while tectonic uplift can partly offset or even exceed ocean rise locally. Modern global mean sea level is increasing, primarily because the climate system is warming.
Two major physical mechanisms directly increase global mean sea level as the climate warms. The first is thermal expansion, in which seawater occupies a larger volume as its temperature rises. The second is the addition of water to the ocean from melting land based ice, particularly glaciers and ice sheets.
Sea ice floating in the ocean has already displaced approximately its own mass of seawater, so its melting has relatively little direct effect on global mean sea level through the simple displacement mechanism, although changes in sea ice have important climatic effects. Ice sheets and glaciers are different because their ice is stored on land. When land ice melts and the resulting water enters the ocean, global ocean mass increases. These mechanisms are complemented by smaller contributions from changes in terrestrial water storage and other components of the hydrological cycle.
Sea-level rise is measured using several complementary techniques. Tide gauges have recorded sea level at coastal locations for more than a century in some places, providing valuable long term records but with limited geographical coverage. Satellite altimetry, introduced for global ocean monitoring in the late twentieth century, measures the height of the sea surface across much of the world's oceans and has transformed understanding of global mean sea-level change.
Satellite gravity measurements can detect changes in the distribution of water and ice mass, helping separate mass driven sea-level change from other components. Measurements of ocean temperature and salinity allow scientists to estimate the contribution from thermal expansion, while glacier and ice sheet observations help quantify changes in land ice. Combining these datasets provides a more complete physical account of why sea level is changing.
The distinction between absolute and relative sea-level rise is important in coastal science. Absolute sea level describes the elevation of the ocean surface relative to the Earth's centre or another stable reference frame, whereas relative sea level describes ocean height relative to the local land surface. Land subsidence can result from natural sediment compaction, groundwater extraction, hydrocarbon extraction, tectonic processes or other geological and human influences.
In rapidly subsiding deltas and coastal cities, relative sea-level rise can substantially exceed the global mean rate. Conversely, regions undergoing postglacial isostatic uplift may experience little relative sea-level rise or even relative sea-level fall despite global ocean expansion. Coastal planning therefore requires local observations and geological information rather than applying a single global rate uniformly to every shoreline.
Rising sea level increases coastal flooding risk by raising the baseline from which storm surges and high tides occur. A storm that previously produced water levels below a critical threshold can produce damaging flooding after sea level has risen sufficiently, even if the storm itself is no stronger. Higher mean sea level can also increase the frequency with which extreme coastal water levels occur.
Other consequences include increased coastal erosion, saltwater intrusion into aquifers and estuaries, impacts on wetlands, infrastructure exposure and changes to tidal dynamics. The actual effect varies with coastal topography, sediment supply, wave climate, tidal range, shoreline engineering and land movement. Some wetlands can migrate inland if sediment accumulation and space permit, while others may be trapped by seawalls, development or steep terrain.
Sea-level rise occurs over different timescales. Thermal expansion can respond relatively quickly to changes in ocean temperature, while ice sheets contain enormous quantities of frozen water whose response to sustained warming can continue for centuries to millennia. Glaciers generally respond over years to decades or longer, while major ice sheet dynamics involve complex interactions among snowfall, surface melting, ice flow, ocean temperature and bed conditions.
Because some components of sea-level response are slow, sea level can continue rising even if atmospheric warming eventually stabilises, as the ocean and ice sheets gradually approach a new equilibrium. This lag is an important feature of the climate system. It means that the amount of sea-level rise committed by a given level of warming is not determined solely by conditions in a single year but by the subsequent evolution of the oceans and cryosphere.
Sea-level rise is also affected by changes in terrestrial water storage. Humans alter where water is stored through groundwater extraction, reservoir construction, irrigation and other activities. Pumping groundwater can move water from underground reservoirs into rivers or directly into the ocean, while some water stored in reservoirs on land is temporarily withheld from ocean circulation. Changes in soil moisture, snow storage and wetlands can also affect the distribution of water between land and ocean.
These contributions are smaller and more variable than the major roles of thermal expansion and land ice, but they matter for closing the global sea-level budget. The sea-level budget is considered physically consistent when observed total sea-level change can be reconciled with independently estimated contributions from thermal expansion, land ice, terrestrial water storage and other processes within measurement uncertainties.
A common misconception is that sea-level rise means every coastline will experience exactly the same change. Ocean dynamics, gravity changes associated with ice mass loss, winds, currents and land movement cause substantial regional differences. When a large ice sheet loses mass, its gravitational attraction on nearby ocean water decreases, producing a distinctive spatial pattern in sea-level change, with some regions experiencing less rise than the global mean and others experiencing more.
Vertical land motion can further alter local relative sea level. Long term projections therefore require both global climate information and regional physical modelling. Sea-level rise is a cumulative consequence of warming, and because thermal expansion and ice response have long timescales, it represents one of the clearest examples of how climate change can produce persistent changes in the physical environment long after individual weather events have ended.
