Explainer: How Do You Measure a Sea’s Level, Anyway?

This is complicated by the fact that sea level around the world at any instant is not the same.

OK, but which sea’s level? And how do you know what it is? Oleg Brovko, CC BY-SA 2.0

There are about 330 million cubic miles of water in the world oceans today, 97% of all the water on the planet. Early in our planet’s 4.5 billion year history, water from the atmosphere and from the interior of the Earth gradually collected in the low areas on the planet’s surface to form the ocean basins, accumulating salts along the way.

Sea level change between 1993 and 2008 NASA/JPL