Introduction

The ocean never sits still. Waves come and go in seconds, tides rise and fall twice a day, and storms can push water ashore for hours. Sea level is what remains when we average all of that motion away. When scientists talk about sea level change, they mean a shift in that average over years, decades and centuries.

Global mean sea level has risen by roughly 23 cm (9 inches) since 1900, and more than … mm of that has come since 1993. Satellites now measure the global ocean every few days, and they show that the pace of rise has roughly doubled in three decades.

Two kinds of sea level

Global sea level is the average over the whole ocean. It changes when the volume of the ocean changes, either because water is added from melting land ice or because the water expands as it warms. It is the key indicator of how climate change is affecting the ocean.

Local or relative sea level is what a person standing on a particular beach experiences. It includes the global rise, but also sinking or rising land, changes in ocean currents and winds, and the gravitational effects of melting ice sheets. In some places local sea level is rising two or three times faster than the global average.

Why it matters

Hundreds of millions of people live on land less than 10 m above sea level, along with ports, power plants, airports and farms. Even small increases in the average sea level make high tides and storm surges reach farther inland and more often. Many coastal communities in the United States already see several times more high tide flooding days than they did in 2000.

How to use this guide

Each topic in Sea Level 101 starts with the basics and ends with a deeper dive. Begin with global sea level rise, or jump straight to the topic you care about most. The Vital Signs pages show the latest measurements behind everything described here.