How Do Scientists Track Sea Level?

Not even Washington, D.C. is off limits to rising sea levels. Here a park bench sits in the waters of the Washington Channel during high tide in September 2016. Mark Wilson/Getty Images

When a mass of ice the size of Delaware broke off in Antarctica the second week of July 2017, scientists weren't overly concerned that the 1 trillion tons (907 billion metric tons) of ice — the largest ever recorded — would cause an immediate rise in sea level as it melted. However, they were disturbed that the 2,240-square-mile (5,801-square-kilometer) iceberg was symptomatic of a larger problem: the future collapse of the entire ice shelf known as Larsen C. If it melted right away, scientists said, it would cause the oceans to rise almost a half inch (1.2 centimeters).

A collapse of Larsen C is probably decades away, but instances like this are the exact reason why scientists have always kept a close eye on rising sea levels, especially in recent decades. Over the past 100 years, Earth's climate has warmed about 1.8 degrees Fahrenheit (1 degree Celsius). As a result, the water bound up in ice caps, ice fields and glaciers has slowly melted, adding more water to the planet's oceans. In addition, heat causes water to expand, a concept known as thermal expansion. All of this has conspired to raise sea levels around the world nearly .07 inches (2 millimeters) a year.


Larsen-C ice shelf
A mass of ice more than twice the size of Delaware broke off the Larsen C ice shelf in July, spawning one of the largest icebergs on record.
ESA via Getty Images

But how do you even measure the level of the oceans? You can't just stand on the shoreline with a ruler — the levels fluctuate constantly in part due to waves, tides, and planetary and solar orbit. Not to mention there are hills and valleys, mountains and canyons — under the oceans as well as on land. And water in deep ocean basins also changes based on climate. To complicate matters even more, some coastal areas, such as New Orleans, Louisiana, and Venice, Italy, are sinking, while others, such as Alaska, are rising.

To measure this constant rise and fall of the oceans, scientists use a variety of tools, including tidal gauges, which are placed around the world in harbors, breakwaters and piers. In the United States the job falls to National Water Level Observation Network (NWLON), which has 210 permanent observing systems throughout the United States and its territories.

Before computers, the process was pretty rudimentary. Tidal gauges were placed inside tide houses that contained analog data recorders attached to floats. The floats were in "stilling wells" (long metal tubes that minimized waves) beneath the tide houses and the gauges recorded changes in water level. Nailed to one of the pier pilings was essentially a large measuring stick that scientists used to manually observe tidal levels and then compare the readings taken by the floating data recorder.

Today the process is much more advanced and uses computerized tidal gauges. Unlike older tide-measuring stations that simply used floats and recorders, most of NWLON's 210 stations use acoustics and electronics placed inside a "sounding tube." Sensors send an audio signal down the tube and measure the time it takes the signal to return. The data is collected every six minutes and timing is controlled by a Geostationary Operational Environmental Satellite (GOES) network. The computerized system is more accurate and it measures, among other things, tidal heights, wind speed, wind direction, barometric pressure, along with air and water temperature.

NWLON is continuing to improve the ways it measures sea level. Its newest tools use microwaves to measure the distance from a fixed point above the water to its surface. The agency is in the process of updating most of its monitoring stations with microwave radar water level sensors, which are superior to acoustic sensors. About 40 of the 210 stations use this technology. Scientists also use a variety of satellites orbiting the planet to measure changes in sea level, as well as the reasons causing them to do so.