What Is Sea Level?

Sea level is the mean height of the ocean surface, measured relative to a defined vertical datum and used as the baseline for elevation and depth across the geosciences, navigation, and engineering. It is not a fixed quantity: the ocean surface varies on timescales ranging from seconds (wind chop) to millennia (glacial cycles), and sea level differs from one location to another because of gravitational, rotational, and dynamic ocean effects. The term "mean sea level" typically refers to a time-averaged surface that filters out tidal and meteorological variability, and its changes over decades reflect the integrated response of the climate system to shifts in ice mass, ocean heat content, and atmospheric pressure patterns.

Sea level is monitored jointly by tide gauge networks and satellite radar altimeters, which together provide complementary records of local and global change. The global mean sea level record, continuously maintained since 1992, shows a sustained rise driven by the thermal expansion of seawater and by freshwater input from melting glaciers and ice sheets.

Causes of Sea Level Change

Sea level changes result from two broad categories of forcing. Eustatic changes are global shifts in the volume of ocean water, caused by melting or accumulation of land-based ice and by the thermal expansion and contraction of ocean water in response to temperature. Since the industrial era, both processes have driven a rise in global mean sea level: the Intergovernmental Panel on Climate Change's Sixth Assessment Report documents that the global mean sea level rose at an average rate of 3.7 millimeters per year between 2006 and 2018, nearly twice the rate observed between 1971 and 2006. Isostatic changes are vertical motions of the land itself, caused by glacial rebound following ice sheet retreat, tectonic subsidence, and groundwater extraction. These local land movements can amplify or offset the global ocean signal at specific tide gauge locations, producing apparent sea level trends that diverge from the global mean.

Ocean Circulation and Sea Level

Ocean circulation distributes water mass unevenly across the ocean basins, creating persistent sea surface height gradients maintained by geostrophic balance between pressure gradients and the Coriolis effect. Major current systems such as the Gulf Stream in the North Atlantic maintain sea surface slopes of tens of centimeters across their widths. Changes in the strength or path of these currents therefore produce regional sea level anomalies that can significantly exceed the global mean signal. The meridional overturning circulation, which transports warm surface water northward and returns cold deep water southward in the Atlantic, is particularly influential: a weakening of this circulation would reduce northward heat transport and cause sea levels to rise along the northeastern coast of North America. NOAA's Ocean Service explanation of sea level and climate linkages identifies both thermal expansion and circulation-driven redistribution as contributors to observed regional sea level patterns.

Measurement Methods

Tide gauges measure the height of sea water relative to a fixed benchmark on land, recording continuous water level data that, after correction for known tidal components, yields the local mean sea level. Satellite radar altimeters, beginning with the TOPEX/Poseidon mission in 1992, measure the round-trip travel time of a radar pulse reflected from the sea surface to derive absolute sea surface height referenced to a geocentric ellipsoid. The two measurement systems are cross-calibrated to identify and separate geophysical signals. The NOAA Climate.gov sea level monitoring program maintains the authoritative public record of global mean sea level change, updated with each successive satellite altimetry mission including Jason-1, Jason-2, Jason-3, and the Sentinel-6 Michael Freilich satellite. NOAA's sea level rise tracking portal provides local sea level trend data derived from more than 200 tide gauge stations worldwide.

Applications

Sea level science and monitoring have applications across a wide range of fields, including:

• Coastal flood hazard assessment and infrastructure planning for cities and ports
• Satellite geodesy and precision Earth reference frame maintenance
• Ocean circulation and climate model validation
• Navigation chart datum setting for maritime safety
• Insurance and regulatory frameworks for coastal property risk