Sea Level

Sea level (also called the height of the sea) is the average surface height of the ocean compared to land or a fixed reference point. Because the ocean surface is not stationary—waves, tides, and wind move it constantly—the measurement used is a long-term average known as mean sea level.

Sea level serves as the reference system for altitude measurements worldwide. When it is stated that a mountain is a certain number of meters "above sea level," that reference is mean sea level. Changes in sea level over long periods are also among the most important indicators used to measure global climate change.

How it is measured

The traditional method relies on equipment installed at the coast called a tide gauge, which records the rise and fall of water at a specific location. This measurement shows sea level as observed from the land, meaning that the land's own movement—such as the rise of land after glacial melting—affects the result. Since the late twentieth century, satellite altimetry has provided a global measurement that continuously maps the ocean surface.

The geoid and altitude references

The ocean surface is not perfectly level; it follows variations in the Earth's gravitational pull. For this reason, the science of measuring the Earth (geodesy) uses a model called the geoid, a surface that follows gravitational variations and approximates mean sea level. Many countries have their own national altitude reference point, which has resulted in altitude figures differing slightly depending on the system used.

Causes of change

Two major factors drive sea level rise: the expansion of water volume as it warms (thermal expansion), and the addition of new water from melting ice in mountains and ice sheets in Greenland and Antarctica. Also significant are changes in groundwater and water storage on land. Local sea level can vary from one region to another because of tidal patterns, wind, and land subsidence in some coastal cities.

Significance

Low-lying coasts, small islands, and river deltas are the areas most vulnerable to sea level change, as they face risks from storm surge, saltwater intrusion into freshwater, and coastal erosion. Urban planning, port construction, and water management depend on sea level measurements and projections.