Measuring sea level is a complex undertaking, far removed from simply marking a line on a beach. Accurate tracking of the ocean’s surface height is necessary for coastal planning, infrastructure design, and understanding the effects of a warming planet. The ocean surface is never static; it is constantly shaped by tides, currents, and atmospheric pressure. Establishing a consistent reference point requires synthesizing data from long-term coastal monitoring and advanced space-based technology to create a comprehensive picture of global sea level change.
Defining the Baseline
To measure the ocean’s height, scientists first establish a stable reference called Mean Sea Level (MSL). MSL is a long-term average that filters out rapid fluctuations caused by daily tides, weather events, and short-term currents. It is calculated by averaging hourly water level readings collected over a specific period, often 19 years, which is long enough to encompass the full range of astronomical tidal forces.
The measurement of sea level requires distinguishing between two perspectives: relative and absolute sea level. Relative sea level measures the ocean’s height relative to the adjacent land at a coastal location. This is what coastal communities experience; if the land sinks, the relative sea level rises, even if the volume of ocean water is unchanged.
Absolute sea level, or eustatic sea level, measures the ocean’s height relative to the fixed center of the Earth. This measurement is independent of local land movement and reflects the true change in the volume of water in the ocean basin. Absolute changes are driven by global factors, such as the thermal expansion of warming seawater and the addition of meltwater from land-based ice sheets.
Local Measurements Using Tide Gauges
For centuries, the primary method for recording sea level along coastlines has been the use of tide gauges. Early gauges used a mechanical float housed within a stilling well, a cylinder that dampens short-period waves. The float was connected to a mechanism that recorded the relative sea surface height onto a chart, providing historical records dating back to the 18th century.
Modern tide gauges use advanced electronic sensors, primarily acoustic or radar technology. These devices are installed above the water and measure the distance to the sea surface by timing how long a pulse takes to reflect and return, providing real-time, high-resolution data.
Tide gauge measurements are inherently local and measure sea level relative to a fixed benchmark on the adjacent land. This land-based reference point allows scientists to consistently compare water height over time at that specific location. The long-term records from these coastal gauges are invaluable for tracking local tidal patterns, storm surges, and the historical rate of relative sea level change.
Global Measurement by Satellite Altimetry
Since the early 1990s, satellite altimetry has revolutionized sea level measurement by tracking the absolute height of the ocean surface across the entire globe. Satellites, such as the Jason and Sentinel series, precisely measure the distance between the spacecraft and the sea surface. They emit a radar pulse toward the ocean and measure the time it takes for the pulse to reflect and return.
This measured time yields the distance, or “range,” from the satellite to the sea surface. Scientists continuously track the satellite’s exact orbital position relative to the Earth’s center using global navigation satellite systems (GNSS). By subtracting the measured range from the satellite’s altitude, they calculate the ocean’s height relative to a reference model of the Earth’s shape.
Satellite altimetry provides near-global coverage, including areas of the open ocean far from coasts. This method is the primary tool for tracking global mean sea level rise, currently accelerating at approximately 3.6 millimeters per year. The resulting dataset allows researchers to observe how sea surface height varies geographically due to changes in ocean currents, heat content, and water mass distribution.
Accounting for Land Movement and Datums
A significant challenge in synthesizing sea level data is that the land itself is not stationary; it moves vertically due to geological processes. Land may be subsiding due to groundwater extraction, causing an apparent increase in relative sea level at a local tide gauge. Conversely, land in formerly glaciated regions may be rising through Glacial Isostatic Adjustment (GIA).
GIA is the slow rebound of the Earth’s crust after the massive weight of Ice Age glaciers was removed. This ongoing uplift can cause the relative sea level to fall in some coastal areas, even while the absolute sea level is rising globally. To separate true ocean change from local crustal movement, the vertical position of the land-based benchmark must be continuously monitored using GNSS receivers.
For local tide gauge measurements, a vertical benchmark, or datum, is established on the land as a fixed reference point. For global altimetry data, GIA models must be applied to correct for the deformation of the ocean basin floor. Accounting for these land movements allows scientists to isolate the signal of true sea level change from the complex motion of the solid Earth.