Dear Ash,

There are two ways to answer your question. One looks at why the ocean rises and falls every day. The other explores what’s causing sea level to rise over time.

I talked about both with my friend Jonathan Robinson. He’s the Beach Watchers coordinator at Washington State University Snohomish County Extension.

If you’re at the beach during high tide, the beach looks smaller because the water comes up higher onto the beach. The tides are what make that happen, Robinson told me.

Image: NASA/Vi Nguyen

“The tides are created by the gravitational pull of the sun and the moon,” he said.

Gravity is the force that pulls your body toward the center of Earth—which keeps you on the ground instead of floating off into space.

The moon has gravity, too. Its gravity pulls on Earth. That pulling squeezes Earth and causes Earth’s water to bulge out on the sides.

Those bulges are high tides. The low points are low tides. Earth rotates, so a person standing on one beach will pass through those bulges and low points as Earth spins.

Image: NASA/Vi Nguyen


Lots of things affect the size of tides—like the shape of the coastline and how the sun and moon line up with Earth.

Robinson told me the tides in Washington state are usually eight to ten feet. Imagine measuring at low tide to find a spot eight feet from the water line. If you stood there until high tide, the water would eventually creep all the way to where you’re standing—and then slowly go back out again. There’s a high tide or low tide about every six hours.

Sometimes the sun and moon line up and pull together. That makes high tide a bit higher and low tide a bit lower. It’s called spring tide. It happens on the full moon and new moon every month.

Halfway between the full moon and new moon, the sun and moon make a right angle to each other. They aren’t pulling together, so the tides are smaller. It’s called neap tide.

Robinson and I also talked about rising sea level. Scientists use satellites and math to calculate the average height of the water on Earth. That’s sea level—and it’s been rising. Scientists expect it to rise faster over the next 30 years. On the U.S. coastline, they expect sea level to rise about 12 inches.

One big reason is melting glaciers. Burning fossil fuels like gas and oil releases carbon dioxide into Earth’s atmosphere. Carbon dioxide and other greenhouse gases trap heat. That makes Earth warmer.

About 68% of Earth’s freshwater is frozen into glaciers. As Earth warms, the ice melts. That water flows into the ocean.

That’s not great news for places at, or near, sea level. As sea level rises, some places will have problems with flooding.

Robinson told me the good news is that we have scientists working together on this problem. The tides might even help. Researchers have found it’s possible to harness the energy of the tides to make clean electricity. It’s just one more way we can move away from burning fossil fuels to help our planet.


Dr. Universe