Sea level

Mean sea level, often just called sea level, is the average height of the ocean's surface. We use it as a starting point to measure how high or low other places are on Earth. For example, when we say a mountain is 1,000 meters above sea level, we’re comparing it to this average ocean height.

Sea level is important for many reasons. It helps mapmakers make accurate maps and guides ships safely across the oceans. Pilots also use sea level to measure how high their planes are flying. This average level isn’t the same everywhere because Earth isn’t a perfect sphere—it’s a bit squashed at the poles—and there are differences in gravity that change it too.

In 2026, scientists discovered something surprising. The way most people measured sea level was off by about 25 centimeters on average. This means that previous estimates of how much sea levels are rising were too low, showing just how important it is to understand and measure sea level accurately.

Measurement

Measuring the average level of the sea, called mean sea level (MSL), is tricky because the sea is always moving. Things like tides, wind, and temperature all change the sea level at any moment. To find the average, scientists take many measurements over a long time, such as 19 years, at special stations called tide gauges.

We can also measure sea level from space using satellites. Since 1992, satellites like TOPEX/Poseidon have helped scientists get very accurate measurements of how high the sea is around the world.

Height above mean sea level (AMSL) tells us how high something is compared to this average sea level. This is important for maps, aviation, and many types of science. Different places use different ways to measure this, but they all start from the idea of the average sea level.

Dry land

Several terms help us understand how sea level and land interact. "Relative" sea level changes compared to a fixed point on land. "Eustatic" changes are global shifts in sea level, like when ice caps melt. "Steric" changes happen because of temperature and salt changes in the water. "Isostatic" changes are when land moves up or down, for example, after big sheets of ice melt, the land can rise.

When glaciers melted after ice ages, the land bounced back up. On some islands made by old volcanoes, the land can sink slowly. Taking too much water from underground can also make land sink, causing sea level to seem higher. Scientists can even find a kind of "sea level" on planets without oceans by measuring the average height of the surface.

Change

See also: Past sea level and Sea level rise

Local and eustatic

See also: Eustatic sea level

Local mean sea level (LMSL) is the height of the sea compared to a fixed point on land, averaged over a long time to smooth out waves and tides. We must consider that land can move up or down, which affects how we measure sea level.

Eustatic sea level change refers to global changes in sea level, caused by changes in the amount of water in the oceans or the shape of the ocean basins. Today, two main factors are increasing global sea levels: melting land ice, which adds water to the oceans, and warming ocean water, which expands.

Aviation

Main article: Altitude in aviation

Pilots use a special instrument called an altimeter to measure how high they are above sea level. The altimeter is set using a special air pressure value, which is usually given to them by air traffic control or an automatic terminal information service (ATIS). This helps pilots know how far they are from the ground below. Aviation maps also show the highest points of terrain, so pilots can stay safe while flying.