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 found that the way most people measured sea level was not quite right. This shows 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 wind and temperature change the sea level. To find the average, scientists take many measurements over a long time 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 learn 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, flying, 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

Some words help us learn about how sea level and land work together. "Relative" sea level changes compared to a fixed spot on land. "Eustatic" changes are world-wide 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, which can make sea level 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. We find this height by measuring over a long time to smooth out waves and tides. Land can move up or down, which changes how we measure sea level.

Eustatic sea level change is about global changes in sea level. These changes happen because of changes in the amount of water in the oceans or the shape of the ocean basins. Today, melting land ice and warming ocean water are making global sea levels rise. Melting ice adds more water to the oceans, and warming water expands, making the oceans larger.

Aviation

Main article: Altitude in aviation

Pilots use a tool called an altimeter to check how high they are above sea level. They set the altimeter using air pressure information from air traffic control or an automatic terminal information service (ATIS). This helps pilots know how far they are from the ground. Aviation maps also show tall terrain, so pilots can fly safely.