Sea level is the average height of the ocean’s surface relative to Earth’s center of mass. The global sea level has risen by about 8 inches (20 centimeters) since 1880, largely due to the melting of glaciers and thermal expansion of seawater as it warms. If current rates of ice loss and ocean warming continue, global sea level is projected to rise another 1 to 4 feet (0.3 to 1.2 meters) by 2100. Even small changes in sea level can cause large impacts on coastlines through erosion, flooding, and other effects.
The main component of global sea level rise since 1900 has been thermal expansion of the oceans as they warm. Water expands as it warms, so warmer oceans occupy more space than cooler ones—and take up more space than they did previously. Thermal expansion accounts for about one-half of observed 20th century sea level rise; glacier meltwater contributes another one-third or so; and changes in land water storage make up the final fraction (for example, groundwater depletion).
Since 1993, satellite altimeters have precisely measured small variations in Earth’s gravity field caused by fluctuations in the distribution of masses—including both oceans and continents—around our planet. From these measurements we know that since 2003 there has been an acceleration in the rate of global mean sea level rise: It is now rising at a rate nearly double what it was between 1993 and 2003.* The increased rate over this most recent decade is consistent with continued warming of our planet’s waters and accelerated melting from glaciers and ice sheets.
* Sea Level Change – NOAA National Ocean Service