14.9: How Do We Measure Sea Level?

To know how much sea level has risen in recent years, you have to know something about how sea level has changed in the past. Measurements of sea level date back to the second century BCE when Greek Stoic philosopher Posidonius (135–51 BCE) reported observations in the Mediterranean Sea. Knowledge of tides and their heights undoubtedly existed in the logbooks, charts, and minds of sailors in ancient times, but systematic measurements of the tides did not begin until the mid-1700s (Cartwright 2000). Daily recording of tides nearly 200 years old can be found for a few locations, such as Boston Harbor (Talke et al. 2018). Unfortunately, many of these measurements are unusable because they remain in analog form (i.e., on a piece of paper) or because they’ve been stashed away somewhere and forgotten (Talke and Jay 2017). Tide gauges (described above) have proliferated in the last 50 years, and they remain the principal source of information on sea level rise at a local level.

At least 1,355 stations out of more than 2,000 worldwide provide usable records—in terms of climate-relevant analyses—but most of these are coastal and some countries lack many or any stations (e.g., Woodworth et al. 2017). At the same time, local tide stations are subject to land motions and other sources of bias that limit their usefulness for determining global rates of sea level rise (e.g., Thompson et al. 2016). Thankfully, the 1970s ushered in the era of measurement of sea level globally, using, of all things, satellites.

In Chapter 4, we briefly explored the pioneering missions of TOPEX/Poseidon and the Jason satellites. In November 2020, the European Space Agency—in cooperation with NASA and other European partners—launched the first of two satellites to replace the Jason satellites, the Sentinel-6A. Recently renamed after the former director of NASA’s Earth Science Division, Michael Freilich (1954–2020)—a “passionate advocate” of measurements of Earth from space (Cook 2020)—the now-Sentinel-6 Michael Freilich will be able to track sea level rise with an error of less than one millimeter per year (Scharroo et al. 2016). The satellite will also map at both high and low resolution to provide a more detailed picture of the sea surface and to permit comparisons with sea surface topography from earlier lower resolution missions. The second satellite, the Sentinel 6B—to be launched in 2026—will extend measurements of sea level until 2030 (NASA 2023b).

One other set of satellite measurements deserve mention here with regard not only to our understanding of sea level rise but also to the distribution of water on our planet and the water cycle. The Gravity Recovery and Climate Experiment (GRACE) and the Follow-On mission (GRACE-FO) measure the variations in gravity that result from changes in the distribution of mass across Earth’s surface, especially those that result from changes in the volume of water or ice. Launched in 2002, GRACE established “a new field of spaceborne remote sensing” (Rasmussen 2017), the field of satellite gravimetry—the study of mass transport on Earth’s surface using gravity measurements (Chen 2019).

GRACE and GRACE-FO accomplish these measurements using a deceptively simple but ingenious method. Immense land features—like mountains—generate a larger gravitational force than flat regions of the world. Smaller features, too, exert a gravitational field, all of which exert an influence on the speed of Earth-orbiting satellites. As a satellite approaches a feature, it speeds up (being attracted by its gravitational pull), and as it passes, it slows down (as the gravitational pull tugs at it from behind). To observe these changes in speed, GRACE uses two satellites in identical orbits separated by a distance of about 137 miles (220 km). As the twin satellites pass over different features, the distance between them changes slightly as one satellite is accelerated and the other is slowed down. Pulses of microwaves between the two satellites provide highly accurate measurements of the distance between them (like a radar gun). Using this information (and well-known physical laws), scientists can produce a map of Earth’s gravitational field every month. Differences in the distribution of gravity from month to month reveal movements of water above and below Earth’s surface, changes in ice mass, and changes in sea level.

GRACE has revolutionized our understanding of water on our planet. During the 2011 La Niña, scientists used data from GRACE to observe water that evaporated over the tropical Pacific. Sea level dropped as a result, but the water appeared as precipitation over Australia, South America, and Asia (Boening et al. 2012). GRACE and GRACE-FO have also been vital for observing reductions of ice mass in Greenland and Antarctica (e.g., Velicogna et al. 2020). Ultimately, predictions for future impacts of sea level rise—and especially extreme sea level events—will depend on scientists’ ability to better measure and quantify factors that cause sea level to change. New approaches and technologies promise to improve our understanding of sea level variability—globally and locally—and to better prepare and implement plans to mitigate sea level rise in the coming decades.