While water naturally seeks its own level, the sea surface is constantly tilted and distorted by powerful, competing forces that vary globally. This means that “sea level” is not a single, consistent height, but rather a dynamic, ever-changing surface with permanent hills and valleys that can differ by many feet. The local height of the ocean is the result of a complex interplay between the Earth’s gravity, its rotation, ocean dynamics, and the long-term effects of climate change.
The Baseline: Gravity and the Earth’s Shape
The most fundamental reason the sea surface is not uniform is the uneven distribution of mass within the Earth itself. Because the planet’s mass is distributed irregularly due to variations in crust thickness and subsurface geology, the gravitational pull varies slightly across the globe, creating permanent bulges and depressions in the ocean surface.
Scientists define this theoretical, gravity-driven shape of the ocean as the Geoid, which is the surface the ocean would take if it were undisturbed by tides, currents, or weather. Where there is a greater concentration of mass—such as large underwater mountains—the stronger gravitational attraction pulls the ocean water toward it, creating a permanent “hill.” Conversely, areas with less mass result in gravitational dips. The difference between the highest and lowest points on this Geoid surface is approximately 200 meters.
The Earth’s rotation also contributes to this permanent baseline shape, causing the planet and the ocean surface to bulge outward at the equator and flatten at the poles. The resulting Geoid is a smooth, wavy, but irregular surface that serves as the baseline reference for all height measurements on Earth. The actual, measurable sea surface rises and falls relative to this invisible, gravity-defined Geoid.
Constant Motion: Density and Ocean Currents
On top of this gravitational baseline are dynamic forces that cause constant, regional variations in sea level. Ocean currents, which act like massive rivers within the sea, are a primary driver of this variation. Persistent currents, such as the Gulf Stream, can pile up water on one side of the flow due to the Earth’s rotation, creating a measurable slope.
This slope can result in sea level differences of many inches or even feet across a relatively short distance. For example, in the equatorial Pacific, wind and currents cause the sea level off the coast of Papua New Guinea to be over a foot higher than the sea level off the coast of Peru.
Water density differences also play a role, as warmer water is less dense and occupies more volume than colder water. Areas with warm, less salty water naturally have a higher surface height than areas with cold, salty water, a phenomenon known as steric sea level change. Shifting weather patterns and oscillations like El Niño also cause temporary but significant sea level changes by altering wind patterns that push water across ocean basins.
Defining the Reference Point
Given the constant height variations caused by gravity, currents, and weather, scientists use specific methods to define and track sea level. The traditional method for local measurement is the use of tide gauges, instruments installed along coastlines that continuously record the ocean surface height relative to a fixed point on the land over many years.
Averaging these local measurements over a long period, often 19 years, smooths out short-term fluctuations from tides and storms to establish the local Mean Sea Level (MSL). While useful for coastal planning, MSL measurements are inherently relative to the local land and do not provide a global picture.
Modern science relies heavily on satellite altimetry, which uses radar pulses from space to measure the height of the ocean surface across the entire planet. These satellites measure the true, dynamic sea surface height relative to the Earth’s center of mass, providing a global view of the hills and valleys caused by currents and temperature. By combining localized records from tide gauges with comprehensive global data from satellites, researchers track both instantaneous variations and long-term trends.
Climate-Driven Variation
The most significant long-term change affecting sea level is global warming, which is causing the global average sea level to rise. This global rise is primarily driven by two factors: the thermal expansion of seawater and the addition of freshwater from melting land ice. As the ocean absorbs over 90% of the excess heat trapped in the atmosphere, the water molecules expand, increasing the ocean’s volume.
The second major contributor is the melting of glaciers and ice sheets in Greenland and Antarctica, which pour massive amounts of water into the ocean basins. The rate of change is not uniform everywhere, a concept known as Relative Sea Level (RSL), which accounts for the vertical movement of the land itself.
If the land is sinking—often due to groundwater withdrawal—the local RSL rise will be much faster than the global average. Conversely, in places like Southeast Alaska, the land is rebounding from the weight of Ice Age glaciers (post-glacial rebound), causing the local RSL to fall or increase more slowly. Furthermore, the mass of melting ice sheets slightly alters the Earth’s gravity field, causing the gravitational pull of the water to shift away from the melting ice and toward the equator, creating regional variations in the rate of sea level rise.