The Basics | How We Measure – U.S. Sea Level Change

Takeaways

Tide gauges have been used since the 19th century along the U.S. coastlines to measure local sea levels.
Satellites have been used since 1992 to measure sea levels across the planet.
Tracking sea level helps us know how it's changing and estimate future changes.
Measurements show that the global sea level is rising faster over time.

Decades of Sea Level MonitoringSea Levels Have Been Measured For Decades

One of the first consistent measurements of sea level came from tide gauges. A tide gauge is a device that accurately measures the height at the surface of the water compared to the land next to it, which is known as local or relative sea level. Just like thermometers accurately measure temperatures, tide gauges provide regular measurements of ocean height for coastal communities.

Initially, tide gauges were used to measure daily changes in sea level due to tides, hence the name. Now, they have become a valuable record showing how sea levels have changed over time in a particular place. Some tide gauges have records going back over 100 years.

In 1992, satellites began making regular measurements of sea level from space. Satellites can make very precise measurements by using radar altimeters. A satellite altimeter works like a bat using echolocation to measure distance. The satellite sends out microwave pulses towards the ocean surface and waits for them to bounce back. By measuring the time it takes for the pulses to return, the satellite calculates how far away the ocean is. Knowing its own height above the Earth, it can then determine the sea level at that spot. The satellite makes some adjustments for things like waves and atmospheric conditions to ensure the measurement is accurate. The satellite does this over and over to accurately measure the sea level across the planet, providing a global picture of sea level. Unlike tide gauges, satellite altimeters only measure the movement of the ocean and not the additional movement of the land next to the ocean.

Regional and Global MeasurementsRegional and Global Measurements

Tide gauges are located throughout the coastlines of the world, although some regions have more and some tide gauges have been around longer than others. Satellites measure all across the globe although not as frequently as a tide gauge does. Together, tide gauges and altimeters tell us how sea level is changing both regionally and globally, and over periods from minutes to decades.

These measurements tell us that sea level is rising both globally and regionally. Satellites and tide gauges help scientists improve our understanding of the causes of sea level change, which then helps us understand how they might change in the future.

Sea Level Snippets

The oldest, continuous tide gauge in the United States is found in San Francisco and dates back to 1854. There are about 2000 tide gauges operating worldwide and about 10% of these are located along the U.S. coastlines.
From over 1000 km above Earth, satellites can measure sea levels to within an accuracy of about one inch. The main satellites we have used over the past 30 years to measure sea level provide a measurement of an individual location about once every 10 days, although there are now several altimeters in orbit.

Tide Gauges and SatellitesTide Gauges and Satellites: Working in Tandem

Tide gauges and satellites work together to tell us a lot about how sea level is changing along the coasts. For example, tide gauges measure relative sea level, while satellites provide a measurement of absolute or geocentric sea level. By comparing the two, we can get a better understanding of where coastal subsidence or upflift is playing a big role. In the image below, the sea level change from 1993 to 2023 as measured by both tide gauges and satellite altimeters is shown. In most locations, there is strong agreement, which is a good check on how well the instruments are working. In other locations, like off the southern coast of Alaska, there is a big difference between the two. Since the change in the tide gauge data is less than that in satellite altimeter data, we know that the land is rising in Alaska. In other places like the Gulf Coast of the United States, we can see that the tide gauges have measured more change, indicating that the land there is sinking.

Sea Level Change from 1992 to 2023 measured by tide gauges and satellite altimeters. — Both satellite altimeters and tide gauges have shown that sea level has risen in almost everywhere over the last 30 years. Satellite altimeters measure only the height of the ocean, while tide gauges measure the sea surface height relative to land. Where the land is moving, we can see big differences in the measurements from satellite altimeters and tide gauges. For example, off the coast of Alaska, even though sea level is rising, tide gauges actually show a fall in relative sea level because the land is rising so much.

Together, the data from tide gauges and satellites have helped us identify a long-term rise in global sea level. And, that the global rise was from human-caused climate change. Looking more closely at the data, we have found that global sea levels are rising faster in recent years. Global sea levels were rising at about 1.2 inches per decade in the early 1990s. In recent years, this has increased to about 1.6 inches per decade. This is like putting your foot down on the accelerator in a car to speed it up. It may not seem like a lot, but it matters for many coastal communities. Every little bit can make impacts worse.

Tracking the global and local changes to sea levels helps us more accurately estimate what will happen to sea level rise in the future.

Dive Deeper: The differences between tide gauges and satellites and other tools we use to measure and understand sea level.

Next: What is the future of sea level change?

Sea Level 101

How Do We Measure Sea Level Change?

Decades of Sea Level MonitoringSea Levels Have Been Measured For Decades

Regional and Global MeasurementsRegional and Global Measurements

Tide Gauges and SatellitesTide Gauges and Satellites: Working in Tandem