Sea level rise is the long term increase in the average height of the ocean surface relative to the land or relative to a defined reference frame. Scientists commonly distinguish global mean sea level from regional and local relative sea level. Global mean sea level represents the average height of the world's oceans and is useful for understanding the overall response of the climate system.
Relative sea level at a particular coastline also includes movement of the land itself, such as subsidence or uplift. Sea level rise is therefore not simply the ocean physically becoming deeper everywhere by exactly the same amount. It is a geographically variable process produced by changes in ocean volume and mass, ocean circulation, density, ice loss, terrestrial water storage and vertical land movement. Modern observations show a sustained global rise during the industrial era, with the rate increasing over recent decades as warming has intensified ocean heating and land ice loss.
The two principal components of global mean sea level rise are thermal expansion of seawater and the addition of water from melting land based ice. Seawater expands when warmed because its density decreases as temperature increases, so the same mass occupies a greater volume. This process is known as steric sea level rise and is especially associated with increasing ocean heat content.
The second major mechanism is the transfer of freshwater from glaciers and ice sheets on land into the ocean, increasing ocean mass. These mechanisms operate simultaneously and are accompanied by smaller contributions from changes in terrestrial water storage. The global ocean therefore rises because its volume increases, its mass increases, or both. Understanding this mass and volume distinction is essential for interpreting sea level measurements and for determining how climate change affects the ocean.
Sea level rise is a slow but persistent component of climate change because the ocean and ice sheets respond over long timescales. The ocean has enormous heat capacity, meaning that it can absorb large quantities of heat while changing temperature relatively slowly. Ice sheets are also massive systems that respond through surface mass balance, internal ice flow, basal processes and changes in glacier discharge.
Even if atmospheric temperatures stopped increasing immediately, the ocean and ice sheets would not instantaneously return to equilibrium. Some future sea level rise is therefore associated with the delayed response of the climate system to past and present warming. The long response time distinguishes sea level rise from many short lived weather hazards. A single storm can cause temporary coastal water levels many metres above normal in some circumstances, while climate driven sea level rise changes the baseline upon which those temporary variations occur.
Regional sea level can differ considerably from global mean sea level. Ocean circulation redistributes heat and water, atmospheric pressure and wind patterns affect sea surface height, and changes in seawater temperature and salinity alter density. Vertical land motion can be equally important at individual locations.
Land can rise because of glacial isostatic adjustment, in which the crust continues responding to the removal of ancient ice loads, or it can sink because of sediment compaction, groundwater extraction, tectonic processes and other factors. A coastline undergoing land subsidence can therefore experience much greater relative sea level rise than the global ocean average. Conversely, tectonic uplift can reduce relative sea level rise at another location. This geographical variability is why coastal planning requires local observations and projections rather than relying only on a single global average.
Sea level rise affects coastlines through several physical pathways. Higher mean water levels allow ordinary tides and storm surges to reach further inland. Coastal erosion can intensify because waves act from a higher water level and can reach previously protected parts of beaches, dunes and cliffs. Saltwater can penetrate further into estuaries and groundwater systems, threatening freshwater resources where hydraulic conditions permit. Low lying wetlands may become permanently inundated if they cannot accumulate sediment or migrate inland.
Infrastructure such as roads, ports, drainage systems, buildings and sewage networks can become more exposed to flooding. The actual impact depends on coastal elevation, defences, sediment supply, wave climate, tidal range, land motion and adaptation measures, so sea level rise produces different consequences in different coastal environments.
Sea level rise can be measured using several complementary techniques. Tide gauges provide long records of sea level relative to the land at particular coastal locations, some extending back more than a century. Satellite radar altimetry measures the height of the ocean surface across broad areas and has provided highly consistent global observations since the early satellite era. Satellite gravimetry can detect changes in the distribution of mass, including changes associated with ice loss and terrestrial water storage.
Ocean temperature and salinity observations help estimate thermal expansion, while glacier and ice sheet measurements quantify contributions from land ice. Combining these datasets allows researchers to construct global sea level budgets and investigate whether observed changes are consistent with known physical processes. Agreement among independent methods strengthens confidence in the observed long term rise.
Sea level rise is closely connected to climate change but is not itself identical to climate change. Climate change includes long term changes in temperature, precipitation, atmospheric circulation, ocean conditions, ice and ecosystems, whereas sea level rise is one consequence of a warming climate. Greenhouse gas emissions increase radiative forcing, warming the atmosphere and ocean and altering the mass balance of glaciers and ice sheets.
The resulting sea level response can continue for centuries because of the slow adjustment of oceans and ice. This persistence means that sea level is often described as a committed or long term climate response. It also means that decisions about coastal infrastructure have consequences over timescales longer than those normally used for ordinary weather planning, making long term projections important for ports, cities, flood defences, transport systems and land use.
Sea level rise is sometimes misunderstood as a process that will produce an identical amount of permanent inundation everywhere. In reality, global mean sea level, regional sea level and relative local sea level are different quantities. Coastal flooding also depends on tides, waves, storm surges, river discharge and rainfall, so temporary water levels can be far higher than the long term sea level increase. Another misconception is that melting all floating sea ice would directly raise sea level substantially.
Floating ice already displaces seawater, whereas melting land based ice transfers additional water into the ocean. The most important scientific understanding is that continued warming produces continued long term sea level pressure through ocean thermal expansion and land ice loss, while the exact amount and timing depend on future emissions, ocean heat uptake, glacier response, ice sheet dynamics and regional land movement.
