If you've been to a beach before, you've probably noticed a sort of line where the ocean water meets the land.

Of course ocean water doesn't stay at the same level all the time.

Waves and tides cause the level of the water to rise and sink all day long.

But we can measure the height of ocean water many times a day and figure out the water's average, or typical, height.

The average height of the ocean in a particular place is called the local sea level.

But the ocean covers most of our planet.

So, it's way too big to measure with just a ruler.

NASA measures the average sea level of the whole ocean from space.

This measurement is called the global sea level.

People have been measuring local sea level at certain locations along the coast for hundreds of years,

and NASA has been measuring the global sea level for almost three decades.

Over this time, scientists have observed that the global sea level has been rising.

The ocean is about 7 to 8 inches higher now than it was a century ago.

Why?

Because Earth is getting warmer.

Glaciers and ice sheets are large masses of ice that sit on land.

As our planet warms, this ice melts and flows into the ocean.

More water in the ocean makes the sea level rise higher.

But that's not the only way seas are rising.

As Earth warms, the ocean is warming, too.

Water expands as it gets warmer.

So, warm water takes up more room in the ocean, making sea levels higher.

Rising sea levels can have an impact on how we live, especially in communities near the coasts.

NASA and its partners use the Jason satellites to measure global sea level.

It takes approximately 10 days for the satellites to complete one measurement.

And after they finish one measurement, they start measuring all over again.

By keeping an eye on Earth's oceans like this, we can measure how much sea level is rising and how quickly it is happening.

Find out more about our planet at NASA Climate Kids.