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Chapter 20: Ocean Tides and Sea Level Rise

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Long before clocks were invented, long before people were invented, and long before the first microbe snuggled up to a scalding vent of water, the Earth and the Sea heaved up and down at precise intervals, like clockwork. The heavings continue to this day, rising and falling, sometimes higher than the time before, sometimes lower, but timed to precision.  Of course, we’re talking about daily changes in sea level—the theoretical height of the ocean at rest. The rising and falling occur as a result of the ocean tides—the changes in sea level caused by the gravitational forces of the Earth, Sun, and Moon. 

The tides have existed since the moment that the Moon fell into orbit around the Earth, about 4.53 billion years ago, some 20 million years after the Earth gained most of its mass. Subsequently, organisms evolved adaptations and behaviors synchronized with the tides. They tolerate exposure to the air during low tides, when sea level reaches its lowest extent during the day. They carry out metabolic and reproductive activities when submerged during high tides, when sea level reaches its highest extent. 

The tides also play a part in the everyday lives of people and have done so since antiquity. The Indians, Greeks, Romans, Babylonians, and Chinese described and predicted the tides. As early as 325 BCE, the height of the tides was proposed to depend on the phases of the Moon (Cartwright 2000). Tides made navigation of shallow seas perilous. Certain places were to be avoided at low tides. Swiftly rising tides created dangerous currents and ferocious whirlpools that could wreck an unsuspecting captain. The success of military operations in coastal waters hinged on the navigability of the waters and beaches at high or low tide. 

Knowledge of tides assisted the hunting and gathering of seafood along shorelines, where abundant beds of oysters and mussels, or buried clams, might be found at low tide. The people of the Republic of Palau in the western North Pacific express their relationship with the tides in this way:

The rhythm of our life is the rhythm of the sea. We are born on a certain tide; we die on another. The tide carries our boats out to the fish and brings us back to feed our families. The seasons of our lives depend on different currents that guide us to the schools of feeding fish. (MacGillivray 1995)

Our study of tides here has two aims: (1) to help you better appreciate the importance of tides for human activities; and (2) to help you better understand discussions about changes in sea level. We’re especially concerned about sea level rise—caused by global warming—which already affects coastal cities around the world. Will your favorite seaside hangout be affected? Let’s find out.

20.1 The Tide-Causing Forces

If tides may be considered Mother Nature’s clock, then the Moon is the tides’ clockkeeper, with a little help from the Sun. The gravitational forces of the Moon and the Sun, primarily, and other planetary bodies, to a lesser extent, cause the tides. Textbooks and the internet abound with explanations of the tide-causing forces, many of which are confusing or conceptually incorrect. Here we follow the reasoning provided by Dutch physical oceanographer Theo Gerkema, who has written a highly readable text on ocean tides (Gerkema 2019). (See also Donald Simanek’s website and Steve Hurley’s Explaining Science website for good treatments.)

We start with an Earth completely covered by ocean without friction and where the ocean responds immediately to gravitational forces. We’ll also ignore the Sun for the moment and only consider the Moon. The Earth will remain motionless in this model, other than its rotation on its axis. This simplified model of ocean tides is known as the equilibrium model of tides, and it gives us an intuitive understanding of the tide-causing forces without resorting to mathematics. We will not consider the more realistic dynamic model of tides here (but see Gerkema 2019).

From Newton’s law of gravity, we know that the gravitational force between two objects varies with their mass and distance. This is why the Earth and the Moon orbit the Sun, which is 333,000 times more massive than Earth (and 27 million times more massive than the Moon). However, because the Moon is closer to the Earth than the Sun (238,855 miles versus approximately 93 million miles; e.g., NASA 2023a), the Moon exerts a greater influence on Earth’s tides, nearly twice as much as the Sun. Let’s be clear: Earth and the Moon orbit the Sun. It’s massive and exerts the strongest gravitational force of any object in our solar system. But the Moon’s close proximity to Earth gives it an edge when it comes to the tide-causing forces. 

Now, the important part of this discussion comes down to this: the Moon’s gravitational attraction varies for different points on Earth’s surface. Because gravitational attraction depends on distance between two bodies, points nearer the Moon experience a stronger attraction while points farther away experience less of an attraction. These variations give rise to a non-uniform force (a force that varies) across Earth’s surface. Oceanographers refer to these variations in the Moon’s gravity across Earth’s surface as the the tidal forces (Gerkema 2019; Simanek 2022).

The tidal forces cause movements of water. Tidal forces have a radial component—acting vertically—and tractive component—acting horizontally. Radial forces pull upward or downward. Tractive forces pull back and forth. Because Earth’s gravity acts downward (vertically) with a force about 10 million times greater than the upward gravitational force of the Moon, the Moon’s vertical forces are dwarfed by Earth’s gravity. Thus—and this is important—only the tractive (horizontal) forces cause tides. Earth’s tides result from water moving toward or away from different  locations depending on the magnitude of the tractive forces. The vertical component of the tidal forces has about as much effect on the ocean as does a seagull landing on or taking off from a cruise ship.

I want to emphasize that the ocean is not lifted by the gravitational attraction of the Moon (or Sun). The tidal forces are horizontal forces. They act tangential to the Earth’s surface (i.e., horizontally), not perpendicular (i.e., vertically). The “bulges” featured in many textbook illustrations of tides represent points toward which the ocean flows. They are not real bulges and they certainly don’t result from water being “pulled up” (on the near-Moon side) or “flung” off (on the opposite-Moon side) of the Earth. 

The horizontal movements of water caused by tidal forces are called tidal currents. These tidal currents—water flowing toward or away from different points on Earth—cause the periodic rise and fall of sea level that we experience as tides. The ocean flows toward certain points and away from others at regular intervals. Along a shoreline, we observe sea level rising—the tide coming in—or falling—the tide going out.

20.2 The Tides Follow the Moon

Because Earth rotates on its axis, the locations beneath (and opposite to) the Moon (and Sun) rotate with the Earth. Every 24 hours, the Earth rotates on its axis, but a point on Earth’s surface directly beneath the Moon will take 24 hours and 50 minutes to rotate directly beneath the Moon the following day. That’s because the Moon orbits the Earth, and advances in its orbit by 50 minutes each day. 

If the Earth was not orbiting the Sun, the Moon would complete one orbit around the Earth in 27 days, what’s called a sidereal month. But the Earth does move and by the time the Moon makes a complete circle around the Earth, the Earth has moved forward in its orbit, too. For the Moon to reach the same point with respect to the Sun and Earth (i.e., to move into the same configuration as 27 days earlier), an additional 2.5 days are required. So, the complete cycle of the tides requires 29.5 days, what’s known as the synodic month, the time it takes for the Moon to transit around the Earth and return to its starting point relative to the Earth and Sun.

The phases of the Moon mark different waypoints in the Moon’s orbit during the synodic month. When the side of the Moon facing Earth is fully illuminated by the Sun, the Moon is said to be full; it’s a full Moon. When the Moon is not visible at all, the Moon is said to be new; it’s a new Moon. The point halfway between new and full is called the first quarter. The right half of the Moon is illuminated during the first quarter (in the Southern Hemisphere, it’s the left half). Following the full Moon, at the halfway point between the full Moon and the new Moon, the left half of the Moon is illuminated at the third quarter. From the new Moon to the full Moon, the Moon grows brighter each night; the Moon is waxing. When the Moon grows dimmer, as it does during the transition from full to new Moon, the Moon is waning. 

Though not necessary for an understanding of tides, we can divide the Moon’s orbit even further, and it’s fun to do so because the Moon is the most visible object in the night sky. The sliver of Moon between the new Moon and first quarter, and between the third quarter and new Moon, is called a crescent Moon. We distinguish the two crescent moons as a waxing crescent—from new to first quarter—and a waning crescent—from third quarter to new. Similarly, the three-quarters-full Moon is called a gibbous Moon. From first quarter to full, it’s a waxing gibbous, and from full to third quarter, it’s a waning gibbous. Feel free to use these terms next time you are out beneath the Moon with someone special. They are sure to be impressed.

20.3 Measuring and Predicting Tides

Tides and their measurement represent “the oldest and longest oceanographic records” in existence (Cartwright 2000). To keep track of tides, oceanographers employ an instrument known as a tide gauge, an instrument that tracks and records sea level continuously usually with a float and recording device (e.g., a paper chart recorder pre-1970s and a computer since the 1970s). Tide gauges developed in the 1830s (e.g., Matthäus 1972) permitted measurement and recording of the position of sea level 24 hours a day, seven days a week. 

In their most basic form, tide gauges record the position of a float at regular intervals throughout the day. The characteristic shape that emerges from measurements of tides, a tide graph, illustrates the principles of tides. The x-axis of the graph represents time, while the y-axis of the graph represents the tide height. The “peak” of the curve represents the time and height of the high tides. The “valley” of the curve represents the time and height of the low tide.

20.4 Classification of Tides

The phases of the Moon mark the monthly period of the tides. We also need to define some aspects of the daily period of the tides. 

A tidal day refers to one complete revolution of the Earth beneath the Moon, or 24 hours and 50 minutes. The term tidal period refers to the time between one high tide (or low tide) and the next high tide (or low tide). In many locations around the world ocean, the tides rise and fall twice daily, meaning that there are two high tides and two low tides each day. These locations are said to experience semidiurnal tides, twice daily tides. Other locations experience only one high tide and one low tide per day. These locations are said to experience diurnal tides, once daily tides. The tidal period for semidiurnal tides is half of a tidal day, or 12 hours and 25 minutes. On the other hand, the tidal period is equal to a tidal day for diurnal tides. In some locations, such as the west coast of North America, including Southern California, the height of the two daily high tides differs as does that of the two daily low tides. Regions where successive tides differ in height experience what are called mixed semidiurnal tides, or simply mixed tides. Because their heights differ, the high tides and low tides are identified according to their relative heights. The highest of the daily high tides is called the high high tide. The lowest of the daily low tides is called the low low tide. The lower of the two daily high tides is called the low high tide. The higher of the two daily low tides is called the high low tide. 

20.5 Tidal Height

The height of tides, the tidal height, refers to the vertical distance of sea level above or below a standard baseline. The National Oceanic and Atmospheric Administration (NOAA) is the agency responsible for establishing the definition of this baseline, the tidal datum, also known as the zero tide height. For reasons beyond our discussion here, NOAA takes a 19-year average of the low water (i.e., low tide height), or low low water (i.e., low low tide height) to set the tidal datum for a location. Tide heights for the mean low water (MLW) at semidiurnal and diurnal tide locations or mean low low water (MLLW) for mixed tide locations establish the National Tidal Datum Epoch (e.g., NOAA 2023a). An actual metal marker, called a tidal datum benchmark, provides a reference (with known elevation) for determination of zero tide heights. For all practical purposes, the tidal datum, MLW (semidiurnal and diurnal tides) or MLLW (mixed tides), and zero tide height are all the same thing: they establish the location from which the heights of tides may be determined.

Tides that rise above the zero tide height are called plus, or positive, tides, while tides that fall below the zero tide height are known as minus, or negative, tides. For example, a high tide with a height of six feet is six feet above the zero tide height. A low tide with a height of minus two feet is two feet below the zero tide height. The difference between the height of the high and low tides defines the tidal range. It can be simply defined as follows:

Tidal range = Height of the high tide – Height of the low tide

(Eq. 20.1)

If the high tide is 6 feet and the low tide is –2 feet, the tidal range would be 8 feet. Tidal range can refer to a high tide and the next low tide or it can refer to the maximum difference in tide heights in a day.

20.6 Spring and Neap Tides

Tides also exhibit a monthly cycle that corresponds with the positions of the Earth, Moon, and Sun. Although the tidal forces from the Sun are half those of the Moon, they are still significant, and tidal bulges from the Sun affect the tides. When the Sun and Moon are on the same side of the Earth, or on exact opposite sides, the effect of the lunar and solar tidal forces is additive. Tides during these times are more extreme: the highs are higher and the lows are lower. Tidal range is maximal during spring tides. This period of time is called the spring tides because the tides appear to spring up faster than normal. Spring tides occur during the new and full moons when the Earth, Moon, and Sun are aligned. Alternatively, when the Earth, Moon, and Sun are at right angles to each other, the tides are less extreme. These are the neap tides. Tidal range is minimal during neap tides. Neap tides occur during the first and third quarters of the Moon. Because spring tides occur every time there is a full or new Moon, and neap tides occur every time there is a first quarter or third quarter Moon, there are two spring tides and two neap tides every 29.5 days.

Finally, you may have read or heard about king tides, periods when high tides are exceptionally high. At certain times of the year, the positions and alignment of the Earth, Sun, and Moon create a little extra force giving rise to king tides. It’s a non-scientific term, but one that has become popular in recent years. Now you know what it means.

20.7 Practical Applications of Tide Knowledge

Knowledge of tides has enormous practical use. Tide predictions are as important to ship’s captains as weather reports. You best know the tides before you set sail! Knowing the height of a tide beforehand is critical for safe navigation of watercraft. During times of extreme minus tides, rocks that may be submerged during normal times can appear above the surface. Many shipwrecks, ferryboat groundings, and other boating accidents happen during extreme low tides.

Tides also affect those of us who spend time near the shore. If you ever do any hiking along the Northern California–Oregon–Washington coasts, tide charts tell you when a particular stretch of beach is passable. For example, along the Lost Coast in Humboldt County, hikers have to wait for a low tide to pass around some of the headlands. Timing your hike with the tides is critical, otherwise you may have to wait several hours, or worse, be tempted to try something stupid like swimming around a point in pounding surf. Some have tried. I don’t have to tell you what happened to them.

Fishermen religiously follow the tides. Low tides along sandy coasts mean good clamming. On the Washington coast, people “dig” for razor clams using a clam gun, a hollow metal pipe with a handle at one end. By shoving the pipe into the sand at the location of a clam siphon, and pulling it out of the sand, you capture the clam inside a column of sand. All you have to do next is spill the sand onto the beach and grab the clam before it digs back into the sand.

Tide predictions let you know the best times to observe marine organisms in the rocky intertidal zone, the region of a rocky shore alternately exposed and submerged due to the tides. You’ll also find tidepools, the shallow water-filled depressions in rocks that appear when the tide is low. During minus tides, a dazzling variety of organisms may be observed, including sea anemones, hermit crabs, and sea stars. 

Of course, the best reason to own a tide chart is so you know when to see the most fabulous nighttime beach event of all, the California grunion run. From March through August, on beaches from Morro Bay, California, to Baja California, these anchovy-sized fish swarm the beach to perform their mating dance. Grunion runs typically occur two to four days after the new Moon or full Moon (i.e., at the time of the spring tide) and one to three hours after the high high tide, which occurs at night in Southern California. Though the entire event may unfold over hours on a given night, the actual mating of a pair (or trio) of grunion takes less than 30 seconds. Because the height of the high high tide following the new or full Moon is lower each night (from spring to neap), the eggs remain covered in sand on the beach for 10-14 days until the next spring tide. When the incoming waves wash up on the beach and soak the eggs, out pops a little larval grunion, not even an inch long. The larval fish swims into the surf, never to be seen until it reaches adult size and is ready for mating.

20.8 An Ever-Changing Sea Level

It’s hard to imagine that the dry ground over which you drive and walk on a daily basis was once covered in hundreds of feet of seawater. Enormous changes in sea level across geologic time have dramatically reshaped coastlines. Sea level changes produced the shallow seas that gave rise to the world’s petroleum reserves and enabled (or prevented) human migration across Polynesia. Next time you are in downtown Los Angeles, look up at the US Bank Tower, the tallest building in California at 1,018 feet tall. That’s about 198 feet taller than the height of the sea level at its highest extent nearly 100 million years ago (Haq 2014).

Changes in sea level over geologic time are why we find whale bones in Southern California. Only a few million years ago, Southern California was submerged beneath the Pacific Ocean. Sedimentary rocks just a few miles from the Fullerton College campus contain shells of ancient marine organisms and other fossils. The Interpretive Center at Ralph B. Clark Regional Park in Fullerton hosts hundreds of marine and terrestrial fossils, including a gray whale skeleton and a giant sloth, all former residents of the Orange County as sea level rose and fell over geologic time. (See also Lozinsky 2010.)

While natural cycles contribute to sea level changes now and in the past, we focus here on sea level change as a result of human-caused warming of our planet. Sea level rise presents an immediate danger to people and structures along the coastlines of the world. Coastal-dwelling people will be forced to migrate elsewhere. Local, state, and federal governments will bear the cost of measures to adapt to or retreat from sea level rise, costs which are ultimately borne by the taxpayer. Sea level rise affects all of us, whether we live in the path of the rising sea or not.

20.8.1 A Closer Look at the Definition of Sea Level

At first glance, the height of the ocean—sea level—seems a simple enough affair. But upon closer inspection, we realize that a wide number of processes can cause the sea surface and sea level to change. Waves, tides, winds, atmospheric pressure gradients, ocean currents, Earth’s rotation, and even Earth’s gravitational field, which varies with the bumps and dips of the seafloor (such as seamounts, ridges, trenches), can affect sea level. The ocean is constantly in motion. At the same time, shifts in the height of the land caused by geologic forces—upward or downward—cause vertical motions in shorelines. As the ocean cools down or warms up due to climate change—causing seawater to contract or expand—and as ice caps and glaciers form or melt—changing the amount of water in the ocean—the volume of the ocean changes. At the peak of the last ice age about 20,000 years ago, sea level was lower by nearly 410–440 feet (125–134 m). All that extra seawater was frozen on land as glaciers (e.g., Lambeck et al. 2014). These many processes determine the sea level at any given moment and the rate at which sea level changes over time.

The first problem in estimating true sea level arises from Earth itself: it’s lumpy, way lumpy. As you know from Chapter 7, the seafloor boasts majestic mountains, deep and narrow trenches, and a whole host of other features that all impact sea level. And these variations in the shape (and chemical composition) of Earth’s crust cause differences in Earth’s gravitational field. Gravity, of course, is the restoring force for sea level. So variations in gravity cause variations in sea level. 

Scientists define a theoretical surface known as the geoid (JEE-oyd; more fun to say than study, I’m afraid), the isosurface (equal magnitude surface) of Earth’s gravity. NOAA’s National Geodetic Survey defines the geoid as “the equipotential surface of the Earth’s gravity field which best fits global mean sea level.” The United States Geological Survey is a little less technical, describing the geoid as “the irregular-shaped ball” that serves as “an imaginary sea level surface.” Computer-generated images of Earth’s geoid resemble the kind of lopsided biscuits my mom used to bake (not the greatest cook but an incredible woman in every other way). The geoid works very well for measurements of sea level from space and computer models. But for establishing local variations in sea level, some oceanographers prefer the reference ellipsoid, essentially, “the surface of an ellipsoidal volume that approximates the geoid” (Gregory et al. 2019). 

Thankfully, most people (including you and me) don’t ever have to think about the geoid or the reference ellipsoid. But it is important that you appreciate the technical difficulty of defining sea level. And important that you understand how scientists go to great lengths to establish criteria for determining the magnitude of changes in sea level and the potential for rising sea level to flood coastlines.

20.8.2 Defining Sea Level Change

Because a number of processes can cause sea level to change, it’s important to establish clear benchmarks to assess how much sea level changes at different places and times. A few more definitions help in this endeavor. According to Gregory et al. (2019), the sea surface can be defined as “the time-varying upper boundary of the ocean” whose height is calculated with respect to the reference ellipsoid. As with tide heights, the height of the sea surface can be positive or negative relative to the ellipsoid. From this definition, they establish mean sea level as “the time-mean of the sea surface”—the average of sea level, as it were—with a time period sufficient to average out the effects of waves and atmospheric pressure (among other things) that may cause short-term variations in sea level. 

In general, we think of changes in sea level as caused by changes in the volume of the ocean (discussed below). Increases in the ocean’s volume cause sea level rise, while decreases cause sea level fall. However, sea level can change because of movements of Earth’s crust: coastlines can move up or down vertically in response to geologic and tectonic forces (the concept of isostasy referred to in Chapter 7). Because they’re most concerned about how changes in sea level will affect the integrity of beaches and structures at individual locations along coasts, oceanographers define the term relative sea level rise (also relative sea level change) as “the change in local mean sea level relative to the solid surface, i.e., seafloor or land.” When referring to the global increase in volume of the ocean—either through the thermal expansion of seawater as it warms or increases in the amount of water due to the melting of ice caps and glaciers into the ocean—oceanographers use the term global mean sea level rise (or global mean sea level change), defined as “the increase in the volume of the ocean divided by the ocean surface area” (Gregory et al. 2019). Most of the “sea level rise” that you hear about in the news refers to global mean sea level rise. For variations in sea level along a specific coastline, the term relative sea level rise is more appropriate. 

Consideration is also given to the local effects of waves, tides, and storm surge—the change in sea level caused by synoptic-scale weather systems (occurring over hundreds of miles), such as hurricanes. Gregory et al. (2019) define extreme sea level rise as events with “an exceptionally high or low local sea-surface height” (emphasis mine).

20.9 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.

20.10 What Causes Sea Level Change

Fundamentally, sea level rises or falls as the volume of the ocean changes. That’s the simple view. However, on any given shoreline, sea level may change due to changes in the elevation of the land. As we have seen, sea level rise depends on any of a number of factors at any one location.

The global rise in sea level results primarily from two principal processes that increase the volume of the ocean: (1) thermosteric sea level rise; and (2) barystatic sea level rise. Thermosteric sea level rise results from the thermal expansion of seawater when heated. Increases in the temperature of seawater cause its volume to expand. Barystatic sea level rise comes from the addition of water to ocean basins—an increase in ocean mass—from melting land ice (e.g., ice caps and glaciers), changes in land storage of water (e.g., water formerly stored behind dams), and the atmosphere (e.g., precipitation; Gregory et al. 2019).

Local sea level rise also experiences thermosteric and barystatic sea level rise, but they’re also subject to vertical land movements, defined as “the change in height of the seafloor or land surface” (Gregory et al. 2019). Upward vertical motions are called uplift (or emergence), while downward vertical motions are called subsidence (or submergence). Any number of natural and manmade processes may cause uplift or subsidence. What’s most important are the effects of vertical land motions on relative sea level rise. Where a coast is uplifted, relative sea level rise may be slower than if the land exhibits no vertical motion. On the other hand, a subsiding coastline may experience an accelerated relative sea level rise because the land is losing height while sea level is increasing. 

Understanding the individual contributions of thermal expansion, ocean mass gains, and vertical land movements on sea level rise is important for predicting future changes and impacts. For example, the rate of thermal expansion of seawater increases with increasing temperature. Thus, as the ocean warms, thermal expansion will accelerate (e.g., Widlansky et al. 2020). Similarly, increases in the rate of melting of glaciers and ice caps will accelerate contributions to ocean water mass (e.g., The IMBIE Team 2020). Indeed, thermal expansion and increased ice melt explain the acceleration in global mean sea level rise that has been observed since the 1970s (Frederikse et al. 2020). 

Most of the differences in relative sea level along US coastlines and around the world can be explained by vertical land motions. Tectonic compression (pushing land together) may cause uplift of a coastline—essentially reducing the rate of sea level rise—while tectonic extension (pulling land apart) may cause subsidence—making the rate of relative sea level rise faster. At the same time, many coastlines (along the United States and around the world) are still adjusting to the retreat of glaciers at the end of the last ice age some 16,000 years ago. When present, glaciers depress land masses. Once removed, the land masses return to their former position. This movement of land masses in response to the presence or absence of glaciers is known as glacial isostatic adjustment (NOAA 2023b). There’s also a kind of edge effect with glaciers: the land along their southern terminus—the unglaciated land at the edge of a glacier—experiences an uplift as the land under the glacier sinks. This uplift—known as a forebulge—disappears when the glacier retreats. Forebulge collapse—the subsidence of land following retreat of a glacier—explains a large part of the subsidence that is happening along the north and central US East Coast. Glacial isostatic adjustments may last tens of thousands of years (Whitehouse 2018).

Human activities also contribute to subsidence. Extraction of underground resources—especially groundwater and hydrocarbons—can cause land to subside. In the 1940s, so much water, gas, and oil had been pumped out from beneath Long Beach, California, that 20 square miles of land sank. Some spots subsided nearly 30 feet (Waldie 2015). Long Beach became popularly known as the “Sinking City” in the ’50s. Fortunately, legislation and monitoring have brought the city’s subsidence under control.

The same cannot be said for Louisiana, which may now be called the “Sinking State.” A recent study along coastal Louisiana revealed land subsidence rates of 0.35 inches (9 mm) per year—not including sea level rise—due to natural and human activities (Nienhuis et al. 2017). Disruptions of natural processes of sediment transport that supply and trap sand—including dams, surface hardening, and sand mining—and any number of other human activities may also contribute to subsidence.

The degree to which any of these processes contribute to local sea level rise will vary across different parts of the world. As noted by the Intergovernmental Panel on Climate Change (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate, “responses to sea level rise are local and hence always based on relative sea level experienced at a particular location. . . . Extreme sea level events at the coast that are rare today will become more frequent in the future. . . . One important response for preparing for future sea level rise is to improve observational systems” (e.g., Oppenheimer et al. 2019).

20.11 What Have We Learned About Global Sea Level Rise?

Measurements of sea level over more than a century have revealed two trends: (1) global mean sea level is rising; and (2) the rate at which it is rising is accelerating (Oppenheimer et al. 2019). Over the period from 1901 to 1990, global mean sea level increased at about 0.055 inches (1.4 mm) per year, as measured from tide gauges. Over the period from 1993 to 2015—using data from tide gauges and satellites—scientists measured a rate of global mean sea level rise of about 0.126 inches (3.2 mm) per year, more than double the historical rate. Using data from tide gauges and satellite altimetry with independent estimates based on GRACE and Argo data from 2006 to 2015, scientists determined a rate of rise of global mean sea level of 0.14 inches (3.6 mm) per year (Oppenheimer et al. 2019). If we use these data to calculate how much global mean sea level has risen from 1901 to 2020 (using 0.055 in y-1 for 1901–1992, 0.126 in y-1 for 1993–2005, and 0.14 in y-1 for 2006–2020), we come up with a global mean rise of sea level of about 8.6 inches (218 mm). This roughly matches the eight inches of sea level rise from 1880 to 2009 reported by Church and White (2011). If we assume the fastest rate (no further acceleration) of sea level rise until 2050, we add another four inches (108 mm). And if we project that out to 2100, we get an additional seven inches (180 mm) for a total projected sea level rise of about 19 inches (about 0.5 m).

However, scientists don’t expect a constant rate of sea level rise; they expect it to continue to accelerate as the data above show (e.g., Nerem et al. 2018). Assuming a slow-but-steady acceleration of sea level rise, scientists project a range of two to four feet (0.61–1.1 m) of global mean sea level rise by 2100 (Oppenheimer et al. 2019; Palmer et al. 2020). Of course, projected global sea level rise depends on whether we begin to reduce atmospheric greenhouse emissions or not. If we start on a path toward reductions, sea level rise may be more modest. If not, then it could be even worse. Under the worst-case scenario, sea level could rise by as much as 6.5 to 10 feet (2–3 m; e.g., Jevrejeva et al. 2014; Le Bars et al. 2017).

To give these numbers a little perspective, imagine that you’re on the freeway going 40 mph to a special someone’s house for Thanksgiving. If your destination is 120 miles away, it will take you three hours to get there at 40 mph. That’s a steady speed with no acceleration, like what would happen if sea level simply rose eight inches every 100 years or so. But imagine that your rate of speed doubles every hour. In your first hour you’ll travel 40 miles, but in your second hour—traveling at 80 mph (double your previous speed)—you’ll make 80 miles and be done with your trip (40 mi + 80 mi = 120 mi). What would have taken three hours only takes two hours when your speed is accelerating. Imagine the surprise of that special someone when you arrive an hour early. This is what happens when a process accelerates with time. Acceleration makes things happen faster than we might otherwise have expected. Faster-than-expected sea level rise falls into this category.

20.12 What Have We Learned About Local Sea Level Rise?

In the world of sea level rise—as in the world of real estate—it’s all about location, location, location (a phrase in use since at least 1926; e.g., Safire 2009). While the average global rate of sea level rise is about 0.11 inches per year (3 mm y-1), the relative sea level rise at different locations around the world varies from 0.02 to 0.5 inches per year (0.5 to 12.5 mm y-1; e.g., Brown et al. 2012). Not everywhere experiences the same degree of sea level rise.

Among the world’s 33 megacities—defined as urban regions with more than 10 million people—at least 15 occur along coastlines (e.g., Blackburn et al. 2019; Institute for Economics & Peace 2022) Moreover, these coastal megacities include lands with an elevation of less than 10 meters—the so-called Low Elevation Coastal Zone (e.g., McGranahan et al. 2007). This is the region considered most vulnerable to impacts from sea level rise. An estimated 600 million (Neumann et al. 2015) and perhaps as many as one billion people (e.g., Kulp and Strauss 2019) live in the Low Elevation Coastal Zone. 

In the United States, an estimated 3.7 million people live within one meter of mean high tide and 22.9 million within six meters, all of which may be impacted by sea level rise, especially when other factors (storm surge, waves, subsidence, etc.) are considered. The US East and Gulf Coasts face the greatest risks—especially Florida, New York, New Jersey, and Louisiana—but parts of California may also be vulnerable, especially Southern California (Strauss et al. 2012). 

South of Cape Mendocino (about 200 miles north of San Francisco), tectonic-driven subsidence and human activities are causing coastlines to sink at an average rate of 0.04 inches per year (1 mm y-1). Coastlines subject to sea level rise of 0.012 inches per year (3 mm y-1) will experience 0.016 inches (4 mm) of additional sea level rise annually because the land is sinking at the same time the sea is rising. Talk about a double whammy. By 2100, relative sea level rise could exceed five to six feet for much of California (National Research Council 2012). If anyone is interested in glass bottom boat tours of sunken California cities, hit me up in 2100.

20.13 It’s Not Just Sea Level Rise

Unfortunately, sea level rise—even when accounting for vertical land motion—isn’t the only factor to affect flooding of coastlines. Short-term events lead to temporary and extreme sea level rise. Factors that cause rapid sea level rise include climate cycles (e.g., El Niño), tidal cycles (e.g., king tides), storm surges (e.g., hurricanes), and large ocean waves (e.g., storm swell). Each of these factors may add additional and significant increases to sea level over timescales from days to months and longer. If they overlap—for example, if a king tide coincides with the arrival of storm surge and wave swell from a hurricane—significant damage is a greater possibilty.

Sea level rise has already contributed to the phenomenon known as nuisance flooding—a (usually) tidally driven rise in sea level that causes seawater to inundate coastal areas. NOAA defines nuisance flooding as water levels that exceed the mean high high water by 1.6 to 2.1 feet (0.5–0.65 m; e.g., Sweet et al. 2020). Though considered minor in terms of harm to citizens or damage to properties, nuisance flooding can disrupt daily activities, damage property and infrastructure, and threaten human health (Mofktakhari et al. 2018). In Miami, Florida—which has become a kind of poster child for nuisance flooding—city streets and sidewalks become impassable, parking garages and landscapes flood, and septic systems leak into the streets during king tides (e.g., Loria 2018). By NOAA’s definition, Miami experienced nine days of nuisance flooding in 2019. In Southern California, many locations—especially Newport Beach (Orange County) and Imperial Beach (San Diego)—regularly experience nuisance flooding during king tides (Jones-Bateman 2019). Researchers expect the frequency of nuisance flooding to double every five years for locations already vulnerable to nuisance flooding (Taherkhani et al. 2020). NOAA’s Margaret Davidson (1950–2017) put it this way: “Today’s flood will become tomorrow’s high tide” (e.g., Sweet et al. 2018).

Definitions based on fixed tide heights don’t always reflect conditions on the ground. One neighborhood in Key Largo—about an hour’s drive south of Miami—experienced more than 90 days of nuisance flooding in 2019 as a result of king tides (e.g., Mazzei 2019). To protect their cars from saltwater corrosion, residents drive on top of lawn sprinklers to wash away the salts (e.g., The Weather Channel 2019). The challenge of adequately describing hyperlocal impacts of nuisance flooding has led one group of researchers to turn to social media. Using number of tweets and geographic information, the researchers found that users generally did a good job of tracking flooding events (Moore and Obradovich 2020). Some researchers have begun to promote citizen science—reporting and collection of scientific data from the public—to expand flood monitoring and even flood model calibration (e.g., Reineman et al. 2017; Loftis et al. 2019; Golparvar and Wang 2020). 

Oceanographers are also concerned about extreme events. When high tides coincide with other phenomena that increase sea level, potentially catastrophic sea level rise may occur. Two of the three strongest El Niños on record—1982–1983 and 1997–1998—resulted in millions of dollars of damage through loss of homes, businesses, and infrastructure, in part, due to increases in sea level (e.g., Cayan et al. 2008; NRC 2012). 

The descriptions here only briefly touch on the various causes and consequences of sea level rise globally and locally. While efforts are underway to develop tools, strategies, and solutions for mitigating and adapting to sea level rise, it is unclear whether the public’s sense of urgency has yet been sufficiently awakened. In an October 16, 2020, commentary in the San Diego Tribune, Scripps oceanographer Richard Norris writes:

A recent state report notes that, in 10 years, sea level will be up to a foot higher than it is now. That is not enough to flood much, but add to that the seven additional feet of king tides every spring, and the foot or so of flooding from an El Niño, and even several feet of ocean swell from an ill-timed storm surge, and we step into dangerous territory. In 30 years, the high school fields, the golf course and Campland on the Bay could well be repeatedly underwater along with areas of Pacific Beach, Mission Beach Park, and Marina Village. The time to fix this costly situation is now, so we can make considered plans, instead of being faced with an emergency.

Underscoring these concerns, a recently published paper reported more than half of US coastal communities underestimate the extent of future sea level rise (e.g., Garner et al. 2023). With the ocean already knocking on the door, it’s more important than ever for citizens and policymakers to adopt strategies that ensure the well-being of those who live and work along our coastlines. The coastline is changing, and we’re going to have to change with it.

20.14 Chapter References

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