Sea level

Watch the sea at a beach. It never stays still.

Waves splash up and slide back. Tides go up and down. Sea level is the height of the sea in the middle of all that moving.

The surface of the sea is always moving. Waves rise and fall every few seconds, and tides lift and lower the water over hours. So when people say "sea level," they mean the sea's average height, once all those ups and downs are smoothed out.

To find it, scientists measure the water again and again for a long time. NOAA, the U.S. ocean agency, defines mean sea level as the average of the water's height measured every hour over 19 years, a span NOAA calls a tidal cycle. In the United States, the 19 years used today are 1983 through 2001.

Sea level is a statistic, not a single surface you can see. NOAA defines mean sea level (MSL) as the arithmetic mean of hourly water heights observed over the National Tidal Datum Epoch, a specific 19-year tidal cycle; the present epoch is 1983 through 2001, and NOAA is working on an update. Shorter averages are named for their length, such as monthly or yearly mean sea level.

Even the average is not flat. NOAA stresses that, just as the land surface is not flat, the ocean surface is not either, and it is not changing at the same rate everywhere. That is why scientists distinguish global sea level, an average for the whole ocean, from relative or local sea level, the height of the water measured against a fixed point on a particular coast.

How tall is a mountain? We start counting at sea level.

Mount Everest is very high above the sea. Some places are lower than the sea. The shore of the Dead Sea is far below sea level.

When you give someone directions, you start from a place they know, like a crossroads. NOAA says mapmakers do the same with heights: they need a starting point, and sea level is a common one. It is the zero for measuring how high land is, and how deep water is.

That is why heights are written as "above sea level." Wikidata gives Mount Everest's height as 8,848.86 meters above sea level. A few places on land are lower than the sea: the surface of the Dead Sea is about 435 meters below sea level. Builders, flood planners and mapmakers all rely on these measurements.

Surveyors call a reference system for heights a vertical datum. NOAA explains that a geodetic datum works like the starting point in a set of directions, giving known locations from which to begin surveys and maps; vertical datums are realized through marked points with known heights above or below a reference surface such as mean sea level, and are used for floodplain maps, levee design, construction and property surveys. Water-level datums, a slightly different kind, are the reference for depths on nautical charts.

For precise land heights, geodesists use the geoid. NOAA describes it as a model of global mean sea level: if tides and currents were removed, the ocean would settle into a smoothly undulating shape, rising where gravity is stronger and sinking where it is weaker. That irregular surface defines zero elevation, and surveyors extend it beneath the continents using gravity readings and mathematics. NOAA's GRAV-D project aims to give accurate heights to within about an inch for most U.S. locations.

People measure the sea with special gauges at the coast.

Satellites measure it from space too. They bounce signals off the water to see how high it is.

There are two main ways to measure sea level. Tide gauges, measuring stations on the coast, have recorded the daily high and low tides for more than a century. They show what the water is doing at one place, compared with a fixed point on land.

Since the early 1990s, satellites have measured the sea from space. They send a radar pulse down to the ocean and time its echo: the higher the sea, the faster and stronger the signal comes back. The current reference satellite, Sentinel-6 Michael Freilich, measures the height of about 90 percent of the oceans every 10 days.

Tide gauges have measured daily high and low tides for more than a century, and scientists combine data from scores of stations into a global average, adjusted for seasonal differences. Since 1993, satellite radar altimeters have added a global view, inferring sea surface height from the return speed and intensity of a radar pulse. NASA notes an unbroken series of five satellites, beginning with TOPEX/Poseidon in 1992; Sentinel-6 Michael Freilich, launched in 2020, became the official reference in 2022, tracking about 90 percent of the oceans every 10 days.

To split the rise into its causes, scientists measure ocean temperature with buoys, ships and a global fleet of robotic floats, and they weigh moving water with satellites such as GRACE-FO, which track tiny changes in Earth's gravity as water shifts between land and ocean.

The sea is getting a little higher each year.

Since 1993, it has gone up about 10 centimeters. Now it is rising faster than before.

Measurements show that the global average sea level is going up. According to NOAA's climate.gov, it has risen about 21 to 24 centimeters (8 to 9 inches) since 1880. NASA's satellites show a rise of about 10 centimeters (4 inches) since 1993.

The rise is speeding up. Through most of the 1900s, the sea rose about 1.4 millimeters a year. From 2006 to 2015, it rose about 3.6 millimeters a year. Single years can be different: NASA measured a rise of 0.59 centimeters in 2024 but only 0.08 centimeters in 2025, when heavy rain over the Amazon basin moved extra water from the ocean onto land for a while.

NOAA's climate.gov puts the rise in global mean sea level since 1880 at about 21–24 cm (8–9 inches), and gives a 2023 annual average 101.4 mm above 1993 levels, the highest in the satellite record at the time. NASA's January 2026 analysis puts the total rise since 1993 at about 10 cm (4 inches) and says the annual rate has more than doubled over that period.

The rate is accelerating: about 1.4 mm a year through most of the twentieth century, but 3.6 mm a year in 2006–2015, 2.5 times faster. Year-to-year variation is real. NASA reported 0.59 cm of rise in 2024, more than the 0.43 cm it had expected, but only 0.08 cm in 2025, below the long-term expected rate of 0.44 cm a year: a mild La Niña brought extra rain to the Amazon basin, temporarily shifting water from ocean to land. NASA's Josh Willis expected that water to reach the ocean again in less than a year.

Ice on land is melting. The water runs into the sea.

Warm water also takes up a little more room. Both make the sea rise.

NOAA explains that there are two main causes. First, glaciers and ice sheets on land are melting, and their water runs into the ocean. Second, water expands as it warms, so a warmer ocean takes up more room. NOAA says the ocean is soaking up more than 90 percent of the extra heat linked to the gases people add to the air.

A third, much smaller cause is water moving off the land, mostly because people pump water out of the ground. The What If? labs let you try each cause yourself: melt glaciers, warm the ocean, and compare ice on land with ice that is already floating.

Climate.gov names two main mechanisms of global mean sea level rise: meltwater from glaciers and ice sheets, and thermal expansion of seawater as it warms. A third, much smaller contributor is a decline in liquid water stored on land, largely because people deplete groundwater. NOAA notes that the ocean absorbs more than 90 percent of the increased atmospheric heat associated with emissions from human activity.

The balance shifts over time. For decades from the 1970s, melting and expansion contributed roughly equally; then glacier and ice-sheet melt accelerated, and from 2005 to 2013 melting added nearly twice as much as expansion. NASA reports that in recent years about two-thirds of the rise came from added water and one-third from expansion, but in record-warm 2024 the shares flipped. How much more the sea rises depends mostly on future greenhouse gas emissions and on whether the Greenland and Antarctic ice sheets melt steadily or collapse rapidly.

The sea does not rise the same amount everywhere.

In some places, the land is sinking, so the sea seems to rise faster. Higher seas can flood roads near the shore.

Sea level does not change equally everywhere. NOAA explains that in some places the land itself is sinking, sometimes because people pump water or oil out of the ground. In other places the land is still slowly rising back up after the weight of huge Ice Age glaciers was lifted off it. Ocean currents matter too.

Higher seas affect coasts. Storm surges, the high water that big storms push onto the shore, can reach farther inland than before, and flooding at high tide happens more often. Many people live by the sea: NOAA notes that 8 of the world's 10 largest cities are near a coast.

Relative sea level at a given coast can rise faster or slower than the global average because of land subsidence (natural, or from withdrawing groundwater and fossil fuels), erosion, upstream flood control, regional currents and glacial rebound, the slow rise of land once pressed down by Ice Age glaciers. Climate.gov notes that some ocean basins have risen 15–20 cm since 1993; in the United States the fastest rates are along the Gulf Coast west of the Mississippi River's mouth, followed by the mid-Atlantic, while in Alaska and a few places in the Pacific Northwest sea level is falling today.

Higher background water levels let storm surges push farther inland, make high-tide ("nuisance") flooding more frequent and let saltwater reach freshwater aquifers. A 2022 NOAA-led interagency task force projected that global mean sea level in 2100 will be at least 0.3 m (1 foot) above 2000 levels even on the lowest-emissions pathway, and could be as much as 2 m higher on a very high pathway with rapid ice-sheet collapse.