Sea levels rise when the amount of water in the ocean increases, the volume occupied by seawater increases, or the land beneath or beside the ocean moves vertically relative to the water. For global mean sea level, the principal climate related causes are thermal expansion of warming seawater and the transfer of freshwater from melting glaciers and ice sheets into the ocean.
Smaller contributions arise from changes in water stored on land, including groundwater extraction, reservoirs and changes in soil and surface water storage. Local relative sea level can additionally rise because the land is subsiding. This means that sea level rise is a water and Earth surface problem involving ocean physics, cryospheric processes, the hydrological cycle, geology and climate dynamics. The relative importance of each cause varies according to the timescale and geographical location being considered.
Thermal expansion occurs because warmer seawater generally occupies more volume than colder seawater for the same mass. As greenhouse gas driven warming increases the amount of heat stored in the ocean, the ocean expands and its surface rises. Thermal expansion is influenced by temperature, pressure and salinity, so the amount of expansion differs between regions and depths. Ocean warming is not uniform because currents, winds, mixing and atmospheric heat exchange redistribute heat throughout the ocean.
Some regions therefore experience stronger steric sea level change than others. Thermal expansion is especially important because it does not require any additional water to enter the ocean. The same ocean water can occupy more space simply because its temperature and density have changed. This is one of the clearest physical connections between increasing global temperature and rising global sea level.
Melting glaciers is another important cause because glaciers store freshwater on land. When a mountain glacier loses ice through surface melting, sublimation or calving and the lost water eventually reaches the ocean, ocean mass increases. The contribution from individual glaciers is small compared with the combined effect of the world's glaciers, but collectively they represent an important source of sea level rise. The world's large ice sheets in Greenland and Antarctica contain much larger quantities of land based ice.
Their mass can decline through surface melting and runoff, changes in snowfall and the dynamic discharge of ice into the ocean. Ice sheet dynamics are particularly important because changes in glacier flow can transfer grounded ice into floating shelves and eventually the ocean. These processes make land ice a major component of observed and projected sea level rise.
The distinction between floating and grounded ice is essential when identifying causes of sea level change. Sea ice forms when ocean water freezes and already floats in the ocean. Its melting has little direct effect on global mean sea level because the floating ice already displaces approximately the amount of water corresponding to its submerged volume. In contrast, glaciers and ice sheets are primarily land based. Their melting transfers water from land into the ocean and therefore increases ocean mass.
Floating ice shelves connected to ice sheets can also influence sea level indirectly because they can provide resistance to the flow of grounded ice. If an ice shelf thins or retreats and loses some of its restraining effect, grounded ice upstream can accelerate towards the ocean. Thus floating ice can influence the dynamics of land based ice even though its own melting is not a major direct source of global sea level rise.
Changes in terrestrial water storage can also alter sea level. Groundwater pumping for irrigation and human consumption can move water that was stored underground into rivers and eventually the ocean. Conversely, construction of reservoirs stores some water on land that would otherwise have reached the ocean, producing a sea level lowering effect relative to a scenario without that storage.
Changes in soil moisture, lakes, wetlands, snow cover and other components of the terrestrial hydrological system can similarly redistribute water between land and ocean. These contributions are generally smaller than the major long term effects of ocean thermal expansion and land ice loss, but they are important in precise sea level budgets. Human activities therefore influence sea level not only through greenhouse gas emissions but also through direct alteration of the global water cycle.
Local sea level can rise because the land itself moves downward. Coastal subsidence can result from groundwater or hydrocarbon extraction, sediment compaction, drainage of wetlands, loading by infrastructure or natural geological processes. River deltas are particularly susceptible because they consist of thick layers of relatively young sediment that can compact and because sediment supply may be altered by dams and land use.
Conversely, some regions are undergoing land uplift because of glacial isostatic adjustment following the disappearance of ancient ice sheets. Tectonic uplift and subsidence can also substantially alter relative sea level. The result is that a tide gauge can record a large relative sea level increase even if global ocean rise is more moderate, or a smaller local increase where the land is rising. Coastal risk therefore depends on relative sea level rather than global mean sea level alone.
Ocean circulation and changes in density create additional regional variations. Winds can push surface water towards or away from coastlines, while persistent currents redistribute water and heat across ocean basins. Changes in temperature and salinity alter seawater density and therefore the height of the sea surface required to maintain pressure balance. Atmospheric pressure also influences sea surface height over short timescales. These processes can temporarily or persistently make regional sea level trends differ from the global mean. Natural climate variability, including major ocean atmosphere patterns, can further influence regional measurements. Consequently, researchers distinguish the causes of global mean sea level rise from the processes responsible for local departures from the global average. Both are necessary for understanding coastal change.
The relative contribution of each cause changes over time as the climate warms. Earlier phases of modern sea level rise included substantial contributions from thermal expansion and glacier loss, while accelerating ice sheet mass loss has become increasingly important. Future sea level rise depends strongly on future greenhouse gas emissions because higher warming produces greater ocean heating and generally greater long term loss of land ice.
The response of large ice sheets introduces uncertainty over longer timescales because ice flow, fracture, grounding line migration, ocean interaction and surface mass balance can produce nonlinear responses. Even so, the physical basis for continued rise under continued warming is well established. Sea level therefore integrates changes occurring throughout the climate system and acts as a long term indicator of the cumulative consequences of ocean warming, land ice loss and changes in the global water cycle.
