Sea level

Sea level is a way to measure water.

It is the average height of the ocean. We use it to measure how tall mountains are.

Sometimes, the sea level changes. This can happen if the water gets warmer. Warm water takes up more space. This makes the sea level rise. Melting ice can also add more water to the sea.

This can change how much land we have. It is a very important way to track our world.

Sea level is the average height of the ocean.

We use it as a starting point to measure height. This is called a datum. It helps us know how tall mountains are. It also helps pilots know how high they fly.

Sea level is not always the same. The ocean is always moving. Tides, wind, and waves change the water level. Even the temperature and salt in the water matter. Scientists use tide gauges to measure these changes. They can also use satellites in space.

Sometimes, the sea level rises globally. This is called eustatic change. One way this happens is when glaciers melt. This lets more water into the ocean. Another way is when water gets warmer. Warm water expands and takes up more space.

Land can also move. This is called isostatic change. In the past, heavy ice pushed the land down. When the ice melted, the land began to rise back up. This changes how high the sea looks near the coast.

Sea level is a very important starting point for measuring the world. It is the average surface level of the ocean's coastal waters. Scientists call this average the Mean Sea Level, or MSL. We use this number to measure how high mountains are above the water. It is also used in aviation to help pilots know their altitude. Pilots use a tool called an altimeter to check their height. They set it to a standard air pressure to stay safe in the sky.

The ocean is never perfectly still, so finding a single level is a hard job. Many things cause the water to move up and down. Tides, wind, and even the temperature of the water change the surface. Large waves or tsunamis can cause sudden, huge changes too. To find the mean sea level, scientists must average these movements over a long time. They might use hourly readings over a 19-year cycle to get a steady number. This helps smooth out the constant motion of the sea.

Humans have used different ways to track the sea for a long time. In the UK, the official zero height comes from measurements taken at Newlyn between 1915 and 1921. In France, a machine in Marseille has measured the sea since 1883. This provides very old data for Europe and Africa. Since 1992, we have used satellites to get even more precise measurements. The TOPEX/Poseidon satellite was a major part of this work. It was later followed by the Jason-1 and Jason-2 missions.

Earth is not a perfect sphere, which makes measuring height tricky. The Earth is a flattened shape called a spheroid. This shape and gravity create a model called the geoid. The geoid acts like a reference for the ocean's surface. Some places, like the Indian Ocean, actually dip below the global average. Scientists found in 2026 that some models were off by about 25cm. This means we might be underestimating how much the sea is rising.

Sea levels can change in two main ways. One way is called eustatic change, which affects the whole world. This happens when melting glaciers add more water to the ocean. It also happens when warmer water expands and takes up more room. Another way is called isostatic change, which involves the land moving. For example, land can rise up after heavy ice melts away. This changes how the sea looks compared to the ground.

Mean sea level (MSL) is the average surface level of Earth's coastal waters. It serves as a critical vertical datum, which is a reference point used to measure heights. Scientists use MSL to determine the elevation of land and the altitude of aircraft. Without this standardized baseline, it would be difficult to create accurate maps or navigate the skies safely.

Measuring a single, steady sea level is difficult because the ocean is in constant motion. The surface changes due to tides, wind, and atmospheric pressure. Changes in water temperature and salinity also affect the level. To find a stable MSL, scientists must average these fluctuations over long periods. For example, they may average hourly tide gauge readings over a 19-year Metonic lunar cycle. This process creates a "still-water level" (SWL) by smoothing out waves and tides.

There are different ways to describe sea level changes. Eustatic change refers to global shifts in the volume of ocean water or the basins themselves. This can happen when melting ice adds water to the sea. Steric change is another global factor, where sea levels rise because warmer water expands. In contrast, isostatic change involves the vertical movement of the land itself. For instance, land may rise after the heavy weight of a melting ice sheet is removed.

Different regions use unique historical references for their measurements. In the United Kingdom, the ordnance datum is based on MSL measured at Newlyn between 1915 and 1921. France uses data from a Marégraphe in Marseille that has recorded levels since 1883. The European Vertical Reference System is calibrated to the Amsterdam Peil, which dates back to the 1690s. In Russia, measurements are taken from the zero level of the Kronstadt Sea-Gauge. These local standards allow for precise land surveying within specific countries.

Modern technology has greatly improved our ability to track the ocean from above. Since 1992, satellite altimeters have provided highly precise global measurements. The TOPEX/Poseidon mission was a major joint effort by NASA and CNES. This mission was followed by the Jason-1 satellite in 2001 and the Jason-2 satellite in 2008. These satellites help scientists monitor the global MSL across the entire ocean area.

Earth's shape also complicates these measurements. The planet is a flattened spheroid rather than a perfect sphere. This shape, combined with local gravity variations, creates a model called the geoid. The geoid approximates the local mean sea level in the open ocean. However, the geoid is not a simple surface; it has depressions, such as in the Indian Ocean. Some studies found that using global geoid models can underestimate sea level rise by about 25cm. This discrepancy is a significant issue for coastal planning.

Understanding these measurements is vital for predicting the future of our coastlines. A 2026 analysis of scientific papers revealed an "interdisciplinary blind spot." Many studies used global models instead of local measurements, leading to errors. Some discrepancies between models and actual measurements reached as high as 760cm. If sea levels rise by 1 meter, 37% more coastal areas could fall below sea level. This change could affect as many as 132 million people worldwide.

Finally, sea level serves as a fundamental concept in other scientific fields. On planets without liquid oceans, scientists calculate a "mean altitude" to serve as a zero-level reference. This allows them to measure the height of mountains or craters on other worlds. In aviation, pilots use barometric pressure to estimate altitude relative to MSL. They use tools called altimeters to ensure they maintain a safe distance from the ground. Whether on Earth or in space, these reference points are essential for navigation and science.