Chapter 11: The Water Cycle and Ocean Salinity

There’s a fairy tale told to Norwegian school children about how the sea came to be salty. In this tale, a greedy man steals a magic (and lucrative) salt grinder from his poor brother and escapes in a ship with the salt grinder on board. The magic salt grinder requires a secret spell to stop it, but the greedy brother ran off too quickly to learn the spell. Soon the ever-churning salt grinder fills the ship with salt and the ship sinks. All hands are lost, including the greedy brother. But the salt grinder—still waiting for the secret code—carries on at the bottom of the ocean. Over the centuries, the grinder churned and churned and churned, spilling salt into the deep, dark ocean. It still churns to this day. And that’s how the ocean became salty. 

People have long been curious about salt and the salty nature of seawater. In fact, salts—at least the dried kind found on land—have occupied the attention of humans for millennia. According to Mark Kurlansky, author of Salt: A World History (2004), human use of salt began in an arid region of China around 6000 BCE. People gathered salt from a seasonally dry lake bed and stored it in pots with fish and soybeans, producing what came to be known in modern times as soy sauce. Next time you sit down to a meal with soy sauce, consider that you are partaking in a tradition more than 8,000 years old. 

Native Americans apparently used salt for various purposes, too. A historical marker in Redondo Beach, California, commemorates the location where native people gathered to collect salts for cooking, food preservation, trading, and other uses. In fact, as the sign points out, there are times in human history when salt was more valuable than gold. (I have trouble imagining pirates demanding the location of a treasure chest of salt, but that’s another story.) 

Salt remains an integral part of modern human cuisine. On your next grocery store visit, stop by the spice aisle and check out the different varieties of sea salts. My favorite is Himalayan sea salt. When I eat it, I feel like I am on top of the world. (You know, Himalayas . . . Mount Everest . . . highest mountain in the world? Oh, never mind.) 

But where do those salts come from? And how does their concentration change from place to place and time to time in the world ocean? The answers lie ahead.

11.1 A Useful and Simple Model

Before we explore the comings and goings of salt in the ocean, it’s helpful to formalize a conceptual model introduced in Chapter 6, what I call the reservoir model. It helps us understand beach sand budgets, salinity, the water cycle, and a whole lot more. 

The reservoir model has three features:

• a reservoir, a place where something is stored
• a source, a process that increases the mass or volume of a reservoir
• a sink, a process that decreases the mass or volume of a reservoir

A fish tank, a swimming pool, and the ocean are all places where water is stored; they are reservoirs. The atmosphere is a reservoir for various gases. The solid Earth contains reservoirs of fossil fuels (oil, coal, and natural gas), diamonds, and other kinds of rocks and minerals. Energy reservoirs exist too. Chemical energy is stored in food and wood. Heat energy is stored in the ocean, the atmosphere, and land surfaces. Even human systems feature reservoirs. Your refrigerator is a reservoir of food. Your bank account is a reservoir of money.

The money metaphor illustrates an important property: reservoirs are not static. Practically speaking, a single number represents your money reservoir, but this reservoir may be subdivided into smaller reservoirs, such as checking, savings, loans, or retirement funds. If your checking account is anything like mine, the balance changes almost daily. Reservoirs represent a snapshot of a quantity at one place in a given moment of time. 

Reservoirs may grow in volume, shrink, or remain the same. And that’s where the various sources and sinks come into play. Sources, also known as inputs, add to a reservoir. Sinks, also known as outputs, subtract from a reservoir. The volume of material, energy, or some other quantity in a reservoir at any moment in time may be described by the following simple mathematical formula:

Vt_reservoir = Vt_sources – Vt_sinks

(Eq. 11.1)

where Vt_reservoir is the volume of the reservoir at time, t, Vt_sources is the sum of all additions to the reservoir at time, t, and Vt_sinks is the sum of all subtractions from the reservoir at time, t. 

Consider the implications of this formula. What happens to the reservoir volume when sources are greater than sinks? What happens when sinks are greater than sources? What happens when sources and sinks are equal? 

When sources exceed sinks, the volume of a reservoir may grow. More is added than is taken away. When sinks exceed sources, the volume of a reservoir may contract. More is being removed than is being added. But when the rate of addition is the same as the rate of removal, then the size of the reservoir remains the same.

Note that even though a reservoir may not be changing in size, it can still have active sources and sinks. Imagine you are adding water to a bathtub from a faucet but you’ve left the drain partially open. If the source (water in) exceeds the sink (water out), the bathtub will fill. If the water drains faster than it’s being added by the faucet, the volume of water will fall. What happens when the faucet and drain flow at equal speeds? The volume in the bathtub will not change. Just because a reservoir is not visibly changing, it doesn’t mean nothing is happening. The sources and sinks of a reservoir may be quite active and maintain a steady volume.

Let’s use this model to think about exchanges of water in the ocean, the main reservoir for water on Earth’s surface. If we combine sources and sinks, we get a simple equation that looks like this: 

• Volume of ocean water = sources – sinks
• (Eq. 11.2)

Now this model may seem obvious and not very useful. But when we start talking about phenomena such as sea level rise, this simple model proves useful. The most profound scientific ideas often start with simple models.

11.2 The Global Water Cycle

To understand the nature of salt in the ocean, we must talk about water, the substance that dissolves the salt. The global water cycle represents a model of the movement of water between various reservoirs on Earth’s surface. Over days to decades, the global water cycle drives variations in salinity throughout the world ocean. Over longer periods, other processes account for salinity variations.

The water cycle describes where water resides, the processes that move water, and the rate at which these exchanges occur. The water cycle serves as the main vehicle by which the hydrosphere interacts with the atmosphere, cryosphere, geosphere, and biosphere. It represents water’s journey from the ocean to the atmosphere to the land and back to the ocean again. It also serves as the means by which salts (and a variety of other things) move from land to sea. Fundamentally, it explains why some parts of the ocean are saltier than others.

11.2.1 Earth’s Water Reservoirs

Seven important reservoirs for water exist on our planet. The largest reservoir exists in Earth’s interior, the geosphere—perhaps as much as 18 oceans’ worth (Peslier et al. 2017). But most of Earth’s interior water is chemically bound to rock and cycles over scales of millions of years (e.g., Bodnar et al. 2013). So it doesn’t factor into our discussion here. The water we consider here can be found just beneath, upon, and above Earth’s surface, Earth’s surface reservoirs.

As you undoubtedly know, the ocean basins hold the most water on Earth’s surface. This includes the parts of the world ocean that freeze over in winter as sea ice. It also means that most of the water on our planet is salty. Solid water constitutes the second-largest surface reservoir. Snow, ice, and glaciers are the most familiar forms. Beneath our feet we find the third-largest reservoir of water on our planet’s surface—groundwater. This reservoir proves most critical to humans as it provides much of our drinking water. The land surface water reservoir ranks fourth. Freshwater lakes, rivers, streams, and ponds make up this reservoir.

While people don’t usually think about it as a place with water, the atmosphere is the fifth most plentiful reservoir. Water vapor, an invisible gas, surrounds us and makes our air humid. Clouds, a visible form of suspended liquid or solid water (i.e., ice crystals), provide an endless form of entertainment on a summer day. My favorite clouds look like whales. What are your favorites?

And we can’t forget about one other important reservoir: the water contained in living matter—the biosphere. The amount is small—0.0003 percent, according to one estimate (Bodnar et al. 2013)—but life manipulates water in ways that the physical world doesn’t. Life influences rates of evaporation, the formation of clouds, the severity of floods, and the impacts of waves and tides on coastal erosion. Of course, life cannot exist without water, and the availability of water determines when and where life may flourish. The intersection of water and life figures prominently in discussions of the biology of the ocean and climate change, so we’ll place the biosphere sixth on our reservoir list. 

11.2.2 Pathways of Exchange between Reservoirs

These reservoirs all connect to each other, so each represents a potential source or sink to the others. Movement of water in or out of a given reservoir defines the pathways and rates of flow, or fluxes, between reservoirs. But let’s make one thing clear from the outset: the global water cycle is primarily driven by interactions between the ocean and the atmosphere. More water evaporates from the ocean and more rain falls into the ocean than anywhere else on Earth. In fact, the amount of rain that falls on the ocean equals about 71 percent of the rain that falls on Earth (e.g., Boldnar 2013). This should come as no surprise. The ocean covers 71 percent of the planet’s surface after all. So naturally the ocean will be more involved in the water cycle than the land. I stress this point because most illustrations of the water cycle emphasize the land. Now you know it’s the ocean where most of the global water cycle takes place.

Let’s consider the ways water enters and leaves the ocean. Precipitation, the gravity-driven descent of liquid or solid water out of the sky, moves water from the atmosphere to the ocean. Gravity drives the flow of liquid water from rivers and streams (and everything they carry) into the ocean. Gravity also causes water to flow through soil and porous rocks, a process called infiltration. This water then flows underground through aquifers, porous subsurface sediments that serve as reservoirs of water. These ultimately drain to the ocean. Gravity makes glaciers flow downhill, where they may break off directly into the ocean, a process called calving. Seaborne chunks of glaciers—icebergs—eventually melt. Glaciers also release meltwater—water that orignates from frozen sources—to the ocean, a contribution that is rapidly increasing due to global warming. And each day humans discharge hundreds of billions of gallons of wastewater into the ocean—water into which human waste and chemicals from homes and businesses have been discharged. Globally, at least half of this wastewater enters the ocean untreated (Jones 2021). Surface runoff—water that flows over the land surface—originates from rain, snowmelt, and even human activities, such as landscape irrigation. This water often carries nutrients and chemicals that enter the ocean.  

The main pathway by which water leaves the ocean is evaporation, the conversion of liquid water into water vapor. Though hard to observe (water vapor is invisible to our eyes), you may see water vapor from a warm surface (a lake or a field) condense when colder air passes over it, a phenomenon known as evaporation fog, or steam fog. Water also returns to the atmosphere via the process of transpiration, the uptake of water by roots and its subsequent evaporation through the stems, leaves, and flowers of plants. Globally, trees release an average of 39 percent of the precipitation they receive back to the atmosphere (Schlesinger and Jasechko 2014). Finally, small amounts of ocean water may be temporarily stored in sea ice, glaciers, coastal groundwater, and even Earth’s crust. The water cycle has more twists and turns than a Los Angeles freeway.

11.2.3 The Water Cycle Is Solar- and Gravity- Powered

In the global view, heat from the Sun and gravity supply the energy that drives the water cycle. The transformation of liquid water to water vapor and its subsequent transport by winds (also solar- powered) distribute water across the planet. Precipitation of water onto elevated land surfaces supply the gravity-driven flows. 

The transformations of water between its three physical states prove important for understanding how heat moves around our planet. An understanding of these transformations can help you understand a lot about the world ocean and your everyday life.

11.3 The Three States of Water

In our everyday experience, heating or cooling something results in a straightforward change in its temperature. But for water, heating and cooling may bring about changes in its physical state; that is, whether it’s a solid, liquid, or gas. 

By definition, solids maintain their own form. Liquids take the shape of the containers in which they are held. Gases also take the shape of their container, but they will expand to fill the container, while liquids will not. A fourth state of matter, plasma, consists of free-moving electrons and ions. Plasmas occur primarily in the upper atmosphere and outer space, although lightning and St. Elmo’s fire—an electrical discharge from the masts of ships—also qualify as plasmas. We won’t deal with plasmas here. 

As far as we know, water is the only substance on Earth to occur in all three physical states at the same time: 

•  ice, the solid form of water 
•  liquid water, simply referred to as water
•  water vapor, the gaseous form of water

Each of these states plays an important role in Earth and ocean processes. The transformations between these states occur at specific temperatures. However, as we shall see, it’s a little more complicated than heating water to its boiling point or cooling it to its freezing point.

11.3.1 Specific Heat: Heat We Can Sense

Water’s enormous importance in global heat exchange comes from its ability to absorb a high amount of heat. In fact, water ranks first in heat-absorbing capability among all liquids at environmental temperatures. 

Scientists define the heat-absorbing abilities of a substance as specific heat, the amount of heat required to change the temperature of a given mass of a substance by 1 degree Celsius (°C; scientists use the symbol ° as a shorthand for degrees). Because specific heat changes the temperature of a liquid, we sometimes refer to it as sensible heat, heat that can be detected by instruments (or touch). We use the Celsius scale here to make calculations simple. By definition, the specific heat of water is one calorie per gram per degree Celsius. (Note that “calorie” is spelled with a little c to differentiate it from “Calorie,” used for food, which is 1,000 times greater but often also styled as lowercase). In an honest-to-goodness physics course, you would use official scientific units—the International System of Units, or SI units, also known as the metric system—in which case you would learn that the specific heat of water is 4.186 joules per gram per degree Celsius. But given that it’s hard enough for nonmajors (and even beginning science majors) to grasp the math, we’ll stick to calories. My apologies to the physicists.

To help you visualize specific heat, imagine a spoon holding one cubic centimeter of water—roughly a fifth of a teaspoon—which has a weight of one gram. Warming it (carefully) over a flame and recording its temperature with a thermometer, a temperature increase of 1°C would indicate that you have added one calorie of heat. Another degree increase, another calorie. For each degree of temperature rise, you’ve added one calorie. 

If you cool your gram of water, you can watch the temperature fall. One calorie of heat will be lost for every 1°C drop in temperature. If you start with a gram of water at room temperature—22°C (72 degrees Fahrenheit, °F)—and remove one calorie, its temperature will be 21°C. Remove another calorie, and its temperature will be 20°C. The math follows easily if you understand what is happening: remove heat, lower the temperature; add heat, raise the temperature. 

11.3.2 Latent Heat: “Hidden” Heat

Specific heat applies to changes in temperature that occur when liquid water gains or loses heat. But what about ice and water vapor? How do the solid and gaseous forms of water interact with heat?

Here’s where water gets really interesting. While the freezing and boiling points of water are set at specific temperatures—0°C (32°F) and 100°C (212°F), respectively—water doesn’t just instantaneously transform into a different state. Additional heat must be removed or added to “rearrange” water molecules into their solid or gaseous form. Because the heat involved in transforming water into its solid or gaseous state does not register as a change in temperature, these forms of heat are referred to as latent heat. The term “latent” means “hidden,” so we can think of latent heat as a kind of “invisible” heat, undetectable with a thermometer.

Two kinds of latent heat must be considered: the latent heat of fusion—the heat required to transform liquid water to a solid and vice versa—and the latent heat of vaporization—the heat required to transform liquid water to water vapor and back again. Before ice or water vapor can form, heat must be removed or added, respectively, to rearrange the water molecules into their new physical states, a solid or a gas. 

What’s important to remember is that to make ice—to turn liquid water into solid water—not only must you remove enough calories to lower the temperature to the freezing point, but once you reach freezing, you must remove even more calories of heat to change the liquid to a solid. Similarly, to boil water—to turn liquid water into water vapor—you must add enough heat to raise its temperature to the boiling point and then add even more heat to turn the liquid water into water vapor.

How much heat? A lot! The latent heat of fusion for water is 80 calories per gram. The latent heat of vaporization is even more extraordinary: 540 calories per gram. That’s 5.4 times the amount of heat required to take the liquid temperature of water from 0°C to 100°C. That means you have to remove 80 calories of additional heat after reaching 0°C (32°F) to turn one gram of liquid water into ice. At the other end of the temperature scale, you would need to add 540 calories of additional heat once you reach the boiling point to turn one gram of liquid water into water vapor.

Let’s return to our fifth of a teaspoon of room temperature water. With a temperature of 22°C, you would need to remove 22 calories of heat to lower its temperature to 0°C (32°F). But to turn it into ice, you would need to remove another 80 calories. So, in total, you would need to remove 22+80 calories, or 102 calories, to turn one gram of room temperature water into ice.

To go the other direction, you would need to add 78 calories to reach the boiling point (100 degrees minus 22 degrees is a difference of 78 degrees). Then, you’ll need to add 540 more calories to transform that gram of water into water vapor. That’s a total of 78+540 calories, or 618 calories, to turn one gram of room temperature water into ice.

Why is latent heat important? When tropical water vapor carried by winds condenses at higher latitudes, it releases heat, keeping temperate and polar regions warm. Latent heat proves important in the formation and intensification of storms and hurricanes. It’s integral to the integrity of polar ice caps, which can absorb a tremendous amount of heat. Once gone, the heat that once went into melting ice goes directly into the atmosphere. Finally, latent heat cools your body. When your body heats up during exercise, you sweat. When those sweat molecules gain enough heat, they vaporize, carrying latent heat with them. Heat is transferred more effectively from your body to the surrounding air because of latent heat transfer during sweating.  Pretty cool, huh?

11.4 Water, the Dissolver of Rocks

We now return to the main topic of our chapter. In addition to moving water, precipitation and evaporation of water drive the movement of salts into the ocean. Precipitation delivers water to Earth’s surface, where it dissolves the soluble parts of rocks—a process called chemical weathering (discussed in Chapter 6). Once dissolved, the salts go where the water flows. Eventually, rivers, streams, and underground flows deliver those salts to the ocean. Over time, without processes to remove them, the salts build up.

The best evidence that salts in the modern ocean originated from continental rocks comes from an analysis of their chemical composition. If we compare the elements in continental rocks, the dissolved elements in rivers, and the elements dissolved in seawater (i.e., the major constituents), we discover that they are virtually identical. Their concentrations vary, but their major constituents—Na, Cl, and others—are largely the same.

11.5 Salinity over Long Timescales

Why don’t salts build up in the ocean? Why isn’t the ocean becoming saltier? Just how has ocean salinity changed over long timescales—centuries to millennia?

To understand the answers to these questions, we must go back in time. We begin in a period of geologic time known as the Archean, an eon that stretched from 4 to 2.5 billion years ago. Earth’s first ocean—presumably freshwater—formed from outgassing of water vapor from the interior. Subsequent cooling of that water vapor brought torrential rains that covered Earth’s basaltic crust. Higher elevations—probably volcanoes—stuck up like islands, but there were no continents. At some point, Earth’s crust began to cool, and an early form of plate tectonics took hold. The sinking, melting, and reworking of Earth’s oceanic crust gave rise to a smallish amount of continental crust. The first continent was born—Ur, named by geologist John Rogers in 1996. At a billion and a half years old, the relatively new Earth had one enormous ocean and one small continent (Rogers 1996).

While this was happening, water began to dissolve the soluble components of Earth’s crust. Eventually, whatever was soluble became dissolved in the ocean, and the ocean became salty. Hydrothermal vents—known to be present at least 3.2 billion years ago—added and subtracted their share of minerals, mostly NaCl.  Some research suggests that the early ocean may have been twice as salty as the current ocean (e.g., Knauth 2005). Other evidence points to an Archean ocean remarkably similar to our modern one (e.g., Marty et al. 2018). We’ll have to wait for further studies to get a definitive answer.

Over time, through plate tectonic conversion of oceanic crust, other continents appeared: Arctica, Baltica, and Atlantica. About a billion years ago, they collided and formed a supercontinent called Rodinia. While this supercontinent-building was going on, parts of the coastal ocean were cut off.  Seawater in the resultant isolated basins was trapped. The trapped water evaporated, the salts were left behind, and vast salt deposits called evaporites were formed. 

Through tectonic uplift of the land, the evaporite deposits became permanent parts of the continents. At certain periods during Earth’s history, tectonic and climatic conditions were such that huge deposits of evaporites were formed, called mega-evaporites. You can find these vast deposits of salt in places like Australia, Oman, Iran, Pakistan, and beneath the Gulf of Mexico. During periods of mega-evaporite formation, the oceans got less salty. 

The latest episodes of salinity reduction appear to have occurred in the Mesozoic during the breakup of the supercontinent Pangea (~250 million years ago) and later when the ice caps formed (~35 million years ago). Prior to the Mesozoic, salinity values may have ranged as high as 50 parts per thousand (ppt). By the early Miocene (~23 million years ago), salinity values had fallen to 37 to 39 ppt (e.g., Hay et al. 2006). Eventually, the sources of salt into the ocean balanced out the sinks. The salinity of the world ocean was established at the value that it is today, about 35 ppt. Though the salinity of the world ocean continues to change (the plates haven’t stopped moving, and sea level is rising due to melting of glaciers and ice caps), it’s reasonable to assume that over time periods of a few million years, the salinity of the ocean is constant.

11.6 Salinity over Short Timescales 

At shorter timescales—weeks to decades—oceanographers have a much clearer picture of the factors that change salinity in the ocean. For much of the ocean, two processes dominate: precipitation and evaporation. Because these processes occur at the surface of the ocean, they have their greatest effect on sea surface salinity, designated as SSS. 

Unlike changes over geologic timescales, changes in SSS don’t involve addition or removal of salts. Salinity varies by addition or removal of water at the ocean’s surface. Precipitation and evaporation dilute and concentrate, respectively, the salts in seawater. Within any span of time, the ocean may experience both precipitation and evaporation. For this reason, oceanographers (and meteorologists, as it turns out) express the change in SSS as the difference between these two processes, or

∆ SSS = E – P

(Eq. 11.3)

where SSS is sea surface salinity, E is evaporation, and P is precipitation.

Now, here’s where it gets interesting. We can imagine three different scenarios:

E > P

E < P, or

E = P.

What happens to SSS in each of these?

If E > P, then SSS will increase.

If E < P, then SSS will decrease.

If E = P, then SSS will remain the same.

From this analysis, we may predict that differences in rates of precipitation and evaporation control surface salinity at a given location in the ocean. Observational data support this prediction. Recently compiled data on ocean salinity measured from ships, buoys, Argo floats, and satellite sensors demonstrate that the “freshest” regions of the ocean occur where precipitation dominates—that is, where E < P. Evaporation dominates in the saltiest regions, where E > P.

Of course, freshwater runoff modifies ocean salinity as well. For example, the Amazon River—the world’s largest river by volume of discharge at about 55 million gallons per second (e.g., Giffard et al. 2019)—lowers SSS to a distance some 100 miles out to sea. Meltwater from glaciers and ice caps can decrease SSS too, though these effects tend to be highly localized.

Seasonal changes in sea ice extent also modify SSS. When seawater freezes in fall and winter (at a temperature of about -2°C), the salts come out of solution as brine—super-salty seawater. This process, called brine rejection, increases SSS. Of course, when sea ice melts in spring and summer, it releases freshwater into the surrounding ocean, lowering SSS. 

Hydrothermal vents play a role in salinity variations, though their effects are not well established. Circulation of water within the region beneath the seafloor—the subseafloor environment—removes some elements and adds others. Black smokers and white smokers provide visual evidence of precipitation of elements as the warmer vent fluids interact with the cooler surrounding seawater. Seafloor observatories (Chapter 4) have begun to reveal the extent to which hydrothermal vents contribute heat, salts, and gases to the water column. These new findings suggest a more dynamic and significant role for vents than once thought (e.g., Spietz et al. 2018; Seyfried et al. 2022; Evans et al. 2023).

Finally, SSS data over the past half century reveal trends associated with a warming climate. Because warming amplifies certain climate signals, wetter regions have been getting wetter and drier areas have been getting drier. Ocean salinities are decreasing and increasing, respectively, in response to these changes (e.g., Durack and Wijffels 2010).

Though we’ve focused on the surface here, changes in SSS affect the entire ocean. Increases in surface salinity cause an increase in seawater density. And if surface waters become more dense than the waters beneath them, the surface waters sink, in some cases, all the way to the bottom. These sinking motions contribute to the abyssal circulation of the ocean.

11.7 Airborne Sources of Salts

Other processes can also contribute elements to the ocean. Transport of aeolian dust, wind-suspended particles of silt, clay, and microorganisms, contributes a small but significant percentage of the substances found in the ocean. These inputs prove especially important to biogeochemical and sedimentary processes in the open ocean, where terrestrial influences are minimal.

British naturalist Charles Darwin (1809–1882) was among the first to publish observations of dust while aboard ship off North Africa. He was also the first to note the presence of microorganisms—“infusoria”—in the material (Darwin 1846):

From the several recorded accounts it appears that the quantity of dust which falls on vessels in the open Atlantic is considerable . . . Lieut. Arlett found the water so discolored that the track left by his ship was visible for a long time. . . . Professor Eherenberg . . . finds that it is in considerable part composed of Infusoria, including no less than sixty-seven different forms.

With the advent of Earth-observing satellites, we now know that billions of tons of dust take to the sky from the major deserts of the world, especially the Sahara Desert in Africa,  the Gobi Desert in Mongolia and China, and the Chihuahan Desert in Mexico (e.g., Goudie and Middleton 2006; Rivera Rivera et al. 2009; Ginoux et al. 2012; Wu et al. 2020). Emissions from automobiles, industrial activities, wildfires, and fallout from nuclear testing also contribute contaminants and radioactive particles to the atmosphere, where they may ride with the wind before settling into the ocean.

Dust from the Sahara can influence the development of hurricanes (e.g., Dunion and Velden 2004; Shu and Wu 2009) Iron in Saharan dust may fertilize the Amazon rain forest (e.g., Bristow et al. 2010). Clouds of Saharan dust may even cause regional climate variability (e.g., Evan et al. 2016). Some studies suggest that aeolian iron may contribute to increased productivity of phytoplankton in iron-poor, low-latitude regions of the ocean (e.g., Conway and John 2014). We’re just beginning to understand the ways in which aeolian dust affects processes in the Earth system (e.g., Tagliabue et al. 2017; Liu et al. 2022; Adebiyi et al. 2023).

11.8 Biological Sources and Sinks

Numerous biological processes alter the concentration of substances in the ocean. The first involves microscopic drifting photosynthetic microbes, the phytoplankton, introduced in Chapter 4. In the presence of sufficient sunlight and appropriate biologically important nutrients, phytoplankton carry out photosynthesis, the light-driven manufacture of organic compounds from inorganic carbon. 

Different types of photosynthesis exist, with different pathways for different elements—like oxygen. So we have to be specific when talking about photosynthesis in the ocean. Photosynthetic reactions that yield oxygen are called oxygenic photosynthesis—oxygen-yielding photosynthesis. Contrast that with anoxygenic photosynthesis, which does not yield oxygen. Both use carbon dioxide, but only oxygenic photosynthesis releases oxygen as a byproduct.

Organisms that carry out oxygenic or anoxygenic photosynthesis are autotrophs, organisms capable of using external energy sources to manufacture their own cellular components from inorganic materials. Through photosynthesis autotrophs turn inorganic carbon—namely carbon dioxide—into organic carbon, the stuff of which all life is made. To be classified as a true autotroph, an organism must get at least 50 percent of its cellular carbon from carbon dioxide, according to microbiologists (e.g., Schönheit et al. 2016). 

The autotrophs with which you are most familiar are plants—the green, multicellular organisms with leaves, trunks, and roots that you eat, smoke, pluck, sit under, weave into clothing, and use to build houses. Phytoplankton are autotrophs too and carry out photosynthesis at rates roughly equivalent to land plants.

Hard as it may be to fathom, phytoplankton supply about half of the oxygen in the atmosphere. Put another way, every other breath you take comes from phytoplankton. Other photosynthetic oceanic autotrophs, such as seaweeds, seagrasses, mangroves, and corals (with their photosynthetic symbionts), also contribute to the production of oxygen, about 3 percent globally (e.g., Malone et al. 2017). Of course, they also transform dissolved inorganic carbon into organic carbon. Because all of these autotrophs use light energy, they may be referred to as photoautotrophs, organic matter–making organisms that use sunlight as an energy source.

Another type of autotroph is the chemosynthetic autotroph, or chemoautotroph. Like plants and phytoplankton, chemoautotrophs fix inorganic carbon into organic carbon. However, they don’t require light, and they don’t produce oxygen. Instead, chemoautotrophs use chemical energy, such as methane or hydrogen sulfide. This process, called chemosynthesis, removes these dissolved gases from the water column and uses them as an energy source. While we commonly associate chemosynthesis with hydrothermal vents, exploration of the seafloor in recent decades has revealed several types of chemosynthetic communities, including whale falls, wood falls, and cold seeps (e.g., Dubilier 2008).

In addition to photosynthesis and chemosynthesis, a third biological process changes the concentrations of gases in the ocean. Respiration, the metabolic breakdown of organic matter to obtain energy, occurs in all living organisms. Sparing you the details, respiration produces carbon dioxide as a byproduct. Knowing whether the ocean acts as a net sink or source for carbon dioxide hinges on our understanding of respiration in the ocean.

Oxygen may be affected by respiration too. Aerobic respiration—the most common form—consumes oxygen as part of the metabolic process. Thus, aerobic respiration acts as a source of carbon dioxide and a sink for oxygen. Anaerobic respiration—breakdown of organic matter in the absence of oxygen, which occurs primarily within the seafloor—releases carbon dioxide but has no effect on oxygen concentrations. 

A fourth process involves removal of dissolved substances as organisms construct their cellular materials. In making their cell walls, different groups of phytoplankton may use silica or calcium carbonate. Australia’s Great Barrier Reef is the ultimate expression of the way in which organisms used dissolved materials to build homes. Of course, the different varieties of mollusks, from clams and oysters to any of the prized varieties of seashells, represent sinks for dissolved elements. In truth, nearly every organism in the ocean at one time or another may serve as a source, sink, or both. 

11.9 Residence Time

Just like relatives who come to visit, it’s useful to know how long a substance remains in a given reservoir. (Will they ever leave?) For that, we turn to calculations of residence time, the average time an element (or anything, really) spends in a reservoir. In essence, we’re figuring out how long a given element spends in the ocean from the time it arrives by air or river to the time it leaves by way of a geological or biological process. Our reservoir model once again serves as a convenient way of understanding this idea.

Just like water enters your bathtub, hangs around while you splash and clean, then exits the tub via the drain, dissolved substances in the ocean have a residence time. Scientists define residence time as the amount of time a molecule spends in a reservoir. This concept has a wide variety of applications from engineering to medical to environmental. Residence time helps us understand the movements of various chemicals, drugs, or contaminants in human and biological systems.

A simple way to describe residence time mathematically is to consider what happens at steady state—that is, when the sources and sinks are in balance. In this case, we can define the average residence time (Rt) as:

Rt (years) = Amount of element (kilograms)/Rate of input or output (kilograms/years)

(Eq. 11.4)

According to the Periodic Table of Ocean Elements website (Chapter 10), the residence time of chloride in the ocean is about 87 million years. A given sodium molecule spends about 55 million years from source to sink. Other elements have shorter residence times. Iron, for example, spends from 200 to 500 years in the ocean before being cycled out. 

This formula can be used to compute the residence time of water too. On average, a water molecule spends 3,100 years floating around in the world ocean reservoir before it’s evaporated (Bodnar et al. 2013).

11.10 Measuring Salinity

As noted in Chapter 10, chemical oceanographers were thrilled when they discovered that by using the Principle of Constant Proportions, they only had to measure one major constituent in the ocean to obtain the concentrations of the rest. But advances in the 20th century gave oceanographers a tool that provided an instantaneous readout of seawater salinity.

In 1729, Stephen Gray (1666–1736), a little-appreciated British chemist, discovered that electricity could flow through a wire, a property called electrical conductivity. By the 19th century, the power of that discovery would be fully harnessed. The Transatlantic Cable, completed in 1858, and the “wireless telegraph,” such as the one used aboard the Titanic, serve as two historic applications of electrical conductivity. Another application was driven by a need to measure the salinity of water in boilers aboard ships. A buildup of salt deposits can damage boilers. To solve this problem, engineers developed the first salinometers, instruments for measuring salinity. 

Driven by an interest to use salinometers in the ocean, American physicist Frank Wenner (1873–1954) and oceanographers Rear Admiral Edward “Iceberg” Smith (1889–1961) and Captain Floyd Soule (1901–1968) published in 1930 Apparatus for the Determination aboard Ship of the Salinity of Sea Water by the Electrical Conductivity Method (Wenner et al. 1930). In doing so, they set the stage for one of the most important measurements in the ocean today. 

Salinometers, also known as conductivity meters, measure the “strength” of electrical conductivity—that is, how quickly a material conducts electricity. Because seawater contains charged molecules—cations and anions—it acts as a weak conductor of electricity. In a salinometer, electrical conductance is measured as the flow of electricity through a wire connected at each end to pieces of metal called electrodes. Because the electrodes are separated by a gap, no electricity will flow through the wire unless a conductor, such as seawater, is present in the gap. By submersing the electrodes in seawater, a current is generated that is proportional to the concentration of salts in the seawater: the greater the conductance, the greater the salinity. 

But simply knowing that a solution conducts electricity doesn’t get you very far. The key to making a salinometer useful lies in calibration, a method used to determine the quantitative relationship between conductivity and salinity. When you place a conductivity probe into a sample of seawater, you want to know that the number you read on the meter matches the real salinity of the sample. To do that, oceanographers calibrate their conductivity meters using solutions with a known salinity. Placing the conductivity meter into a solution with a known salinity permits the instrument to be adjusted so it displays and records an accurate salinity.

The most common solution used for calibration by oceanographers is something called standard seawater. It’s obtained from the North Atlantic Ocean, modified to precise specifications, and sold by a single company. That single source ensures that oceanographers around the world can calibrate their instruments with identical seawater. Standard seawater has an accuracy of ±0.0003, meaning any one bottle of standard seawater may differ from others by ±0.0003. 

For decades the salinity of the ocean was reported in parts per thousand. Thus, if a sample of water has a salinity of 10 ppt, it’s made up of 990 parts water and 10 parts salt. Those units remain popular among some oceanographers and in other fields of study that require measurements of salinity. However, in 1978, oceanographers adopted the practical salinity scale, which reports salinity as a ratio of the conductivity of standard seawater to that of a precise concentration of potassium chloride (e.g., Millero et al. 2008). The practical salinity scale, unlike the traditional salinity scale, has no units. Thus, you can choose to report the average salinity of the ocean as 35 ppt, if you’re old school, or 35, if you’re new school.

11.11 The Global Water Crisis

Despite the seeming abundance of water, as many as four billion people experience severe water shortages at least one month out of the year. From one to four billion people struggle to find water at any time of the year. At least 193 cities in 16 countries representing a third of the world’s population lack sufficient water for drinking, agriculture, and industry (Mekonnen and Hoeskstra 2016; He et al. 2021). Major cities such as Cape Town, South Africa; São Paolo, Brazil; and Chennai, India, face what has come to be known as Day Zero, the day that a city’s municipal water supply runs dry. An assessment of water supply and demand in a dozen megacities—cities whose populations exceed 10 million—revealed that 11 of them currently use more water than they can supply (Ahmadi et al. 2020). The lack of water for billions of people across the globe has been called the global water crisis, a term that emerged in the 1990s (Bruns and Frick 2014). 

11.11.1 “All Water Problems Are Local”

Though important, the notion of a global water shortage “is of little practical utility,” according to Gleick and Palanniappan (2010). That’s because supply and demand aren’t matched on global scales (Gleick 2018). With few exceptions, humans obtain their water near a local source: the ground, a lake, a river, a reservoir. As Charles Fishman, author of The Big Thirst: The Secret Life and Turbulent Future of Water (2011), puts it, “All water problems are local.” 

The availability of water in a region depends on three major factors (following Gleick 2018):

• The operation of the water cycle—the relative rates of evaporation and precipitation that remove water from and add it to a reservoir.
• The demand for water—a complex function of population size, primary uses of water (e.g., household, agricultural, or industrial), social factors (e.g., landscape types, rural-to-urban transition), and economic factors (i.e., adoption of water reclamation and conservation technologies).
• The availability of technology for improving water use or supply. 

Though highly simplified, this conceptual model of water supply helps frame the problems facing management of water resources.

11.11.2 Water Scarcity

The mismatch between supply and demand on a regional basis drives water scarcity—a lack of sufficient water in a region for any number of purposes (e.g., United Nations 2023). Shortages of water raise issues of water security, the ability of a community to protect access to sustainable quantities of water for practical and peaceful purposes (e.g., United Nations University 2013). In places where water scarcity is prolonged or severe, disputes over water—water conflicts—may occur. 

Though other sociopolitical issues contribute to tensions between countries, states, or groups, scarcity has arguably led to an increase in water conflicts since the 1990s (e.g., Levy and Sidel 2011). Most disputes can be settled peacefully, but some—such as the conflicts in Syria and Yemen affecting tens of millions of people—underscore the ways in which water scarcity can ignite tensions and fuel violence (e.g., Gleick 2014). In such conflicts, water and water-generated energy supplies (i.e., dams) become centerpieces of military tactics for gaining an advantage over an enemy. Unfortunately, civilian populations suffer the most in these conflicts. Despite the challenges, there is hope for “peaceful sharing and management of water” in at least some parts of the world (e.g., Boretti and Rosa 2019;  Angelakis et al., 2021; United Nations 2022).

11.11.3 Desalination: Freshwater from the Ocean

A full exploration of the ways in which governments and municipalities are grappling with water shortages is beyond the scope of our discussion here. But one strategy in particular has gained attention in recent decades. Because it involves salts and the ocean, I include it here.

With water supplies increasingly vulnerable to climate change, or where other water resources are scarce or limited, desalination—the removal of salts from seawater to produce freshwater—has emerged as one solution (e.g., Darre and Toor 2018; Eke et al. 2020). In extremely arid countries—such as Qatar, Kuwait, the United Arab Emirates, and Saudi Arabia—desalination provides more than half of the freshwater needs (e.g., Darwish et al. 2012; Mannan et al. 2019; Tariq et al. 2022).

More than 20,000 desalination plants—currently operational or under construction—exist in 181 countries around the world, including the United States (e.g., Eke et al. 2020). In fact, the US generates more than 11 percent of the freshwater produced globally. Florida, California, and Texas account for most desalination (68 percent), but plants can be found in 32 other states as well (as of 2018; see Mickley 2018). California hosts a dozen existing and four proposed desalination plants (California Water Boards 2023b). The largest plant can be found in the US in Carlsbad, California, which supplies 400,000 San Diego County residents with about 50 million gallons of water daily (California Water Boards 2023a). Though not without controversy, two additional seawater desalination facilities are planned for Southern California: the West Basin Ocean Water Desalination Project (El Segundo) and the Doheny Ocean Desalination Project (Dana Point). On December 9, 2022, the California State Lands Commission approved a permit for construction of the Doheny plant. Once operational, the facility will supply five million gallons of water daily (South Coast Water District 2023).

Theoretically, the supply of water available for desalination is as big as the ocean. Unlike technologies that recycle wastewater, desalinated water is “new” water. It doesn’t depend on evaporation and precipitation via the water cycle. It’s also relatively climate-independent. Desalination can be maintained regardless of changes in climate as long as an energy source is available (e.g., Jones et al. 2021; Ghazi et al. 2022).

Desalination technologies generally fall into one of two categories: (1) thermal-based desalination, heating and evaporating seawater and recovering the water vapor; and (2) membrane-based technologies, using pressure to push seawater through a semipermeable membrane, a kind of filter which separates the salts from the water. Thermal technologies—the oldest form of desalination—work well in locations, such as North and East Africa and the Middle East (e.g., Xu et al. 2013; (Reif and Alhalabi 2015). Thermal-based desalination plants are often built adjacent to power plants to ensure a steady supply of energy. Membrane-based desalination plants have a lower energy requirement—though still high compared to conventional ways of obtaining water (e.g, Voutchkov 2018). This makes them more popular outside of the Middle East. The highest percentage of existing and planned desalination plants use membrane technologies (e.g, Eke et al. 2020).

Among membrane-based technologies, reverse osmosis systems (RO)—moving water against a concentration gradient (the opposite of osmosis)—are the most popular. In fact, RO systems exist for household, industrial, and military water purification. As described in Darre and Toor (2018), RO desalination plants typically require pretreatment of the intake seawater to remove particles, microbes, and other substances that may foul the membrane filter. The pretreated water is then pumped at high pressure through a membrane with pores large enough to permit water molecules to pass but too small for salts. The process results in production of both freshwater and brine. 

A number of economic, technical, and environmental challenges complicate the acceptance of desalination as a competitive and sustainable source of water (e.g., Xu et al. 2013; Darre and Toor 2018). Its high energy requirements have raised concerns over costs in a future with rising energy prices. Plants may also produce greenhouse gas emissions in a world increasingly seeking to reduce them. Aende et al. (2020) note that the energy used by the Carlsbad desalination plant could power 20,000 homes. While solar-powered desalination plants hold promise, a number of factors, including intermittent availability of the Sun and seasonal and latitudinal variations in solar intensity, continue to hamper their implementation. 

Disposal or reuse of brine—the salty syrup that remains once water has been extracted—has also been raised as a concern (e.g., Jones et al. 2021). In high-energy coastal environments, brine may be piped through diffusers, which help to mix and dilute it in the surrounding seawater.  But this option is not available in low-energy environments, where toxins in the brine may harm marine organisms (e.g., Darre and Toor 2018; Delgado et al. 2020). In some cases, brine may be converted to rock salt and other products, but the volume of brine produced may exceed demand (e.g., Xu et al. 2013). 

Finally, pumping seawater from the ocean may unintentionally remove large quantities of plankton and larvae that form the base of marine food webs. Prior to construction of the Carlsbad plant, scientists raised concerns about potential negative effects on the productivity of a nearby marine protected area (e.g., Darre and Toor 2018).

Future proposals for desalination plants will continue to raise concerns about cost effectiveness, energy demand, greenhouse gas contributions, brine disposal, and harm to the marine environment, especially in California. The Huntington Beach desalination plant—first proposed in 1998—met with considerable opposition from environmental groups, citizens, and taxpayer groups. They argued that the plant would raise water costs, displace Latine neighborhoods, and harm marine life (Symon 2020). Proponents argued that desalination is necessary to avert water shortages (Alvarado 2020). Ultimately, the Coastal Commission rejected the project in May 2022 (Becker 2022).

Despite the negative perceptions of desalination, some researchers argue that these challenges can be overcome. As renewable energy sources develop and brine disposal methods improve, the costs and environmental harm of desalination will be reduced (e.g., Pistocchi et al. 2020). Like any new technology, desalination will benefit from ongoing critical review and additional research to overcome its challenges (e.g., Darre and Toor 2018). However, like any technological fix, desalination may be a temporary solution. Ultimately, humans will need to make difficult choices about how we create a sustainable water supply in the 21st century (e.g., Pistocchi et al. 2020).

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