Since the peak of the last Ice Age, Earth’s ice has been melting, and sea level has risen nearly 400 feet! But only recently do we have the technology to carefully measure it.

For the last 20 years, a network of satellites and nearly 4,000 robotic floats have been measuring temperature and surface elevation of the global oceans.

They’ve found that the surface of the ocean has warmed an average of 1 degree Fahrenheit in that time, and average global sea level has risen 3.5 inches.

While this is within the normal rate of sea level rise for the past 8,000 years, we can now better understand what’s happening:

Warming seas have contributed to the melting of ice sheets in Antarctica and glaciers in Greenland. Their meltwater is responsible for about 2.3 inches of that sea level rise.

But the other 1.2 inches comes from a property of water itself. Water expands as it warms because molecules bump up against each other more frequently, causing an increase in volume.

Meaning that, as oceans have warmed, they’ve swollen in their basins and slowly crept onto the land.

Oceans act as heat sinks to moderate global temperatures swings, and researchers estimate they have absorbed 90 percent of the warming that Earth has experienced over these past 20 years.

As our planet and oceans continue to warm, there will be more sea level rise, not just from more meltwater, but further swelling of the sea.