1.2: The Ocean's and Earth's Environment

Only very recently have we come to realize that humans have already caused profound changes, not just in local environments but in the global ocean, atmosphere, and terrestrial environments as a whole. Too little is known about the global or regional consequences of environmental changes already caused by human activities. Even less is known about the future consequences if our civilization maintains its current exponential growth and development. The urgent need to assess the unknowns has been felt throughout the environmental science community. The oceanographic community has been particularly affected because most global environmental problems involve the oceans and ocean ecosystems, and our knowledge of the oceans is much poorer than our knowledge of the terrestrial realm. Changes in the marine environment caused by human activities are many and varied. They include many forms of pollution (Chap. 16) and physical changes in the coastal environment (Chaps. 11, 13). Many pieces of information must be obtained by different oceanographic disciplines before human impacts on the ocean can be identified, assessed, and effectively managed. Throughout this text, we refer to how oceanographic findings, principles, or studies can be applied to the practical problems of ocean management. The intent is to facilitate understanding of similar problems reported almost daily in the media.

Among contemporary environmental problems associated with human activities, three related impacts stand out as the most important and complex: global climate change due to enhancement of the greenhouse effect (CC9), acidification of the oceans (Chaps. 5, 13, 16), and deoxygenation of the oceans (Chaps. 12, 13, 16). Each of these problems are caused largely by the release of large quantities of carbon dioxide to the atmosphere, primarily as a result of fossil fuel burning.

A greenhouse maintains a higher internal temperature compared to the outside temperature because the windows allow more solar energy into the greenhouse than they allow radiated out, warming the interior. The Earth’s atmosphere acts in a similar way to control temperatures at the Earth’s surface, with several gases, especially carbon dioxide, functioning like the glass in a greenhouse. The greenhouse effect is crucial for maintaining Earth’s climate, but an increase in greenhouse gas concentrations increases its efficiency, leading to rising global temperatures. Since the Industrial Revolution, the burning of fossil fuels has steadily increased the concentration of carbon dioxide in the atmosphere, as shown in Figure 1-2. The concentrations of other greenhouse gases, such as methane and chlorofluorocarbons, are also increasing as a result of human activities. If there are no other changes to compensate for the consequent increase in greenhouse efficiency, the Earth’s temperature will rise as concentrations of greenhouse gases rise.

Figure 1-2. Change in the concentration of carbon dioxide in the atmosphere since 1880. Data for the smooth part of the curve were obtained from ice cores. Data from 1960 were obtained from direct measurements at Mauna Loa, Hawaii, and show both seasonal oscillations and a long-term upward trend. The upward trend continues.

The trend in atmospheric carbon dioxide concentration has accelerated from about 0.7 ppm per year in the 1960s to 2.6 ppm per year in the 2020s and is expected to continue increasing. The concentration is now more than 50% above the pre-industrial concentration of about 280 ppm and has been consistently above 420 ppm since 2023. The data has not shown any significant slow down of the rate of increase due to the COVID-19 pandemic or the emissions reductions of Western nations, including the United States.

Since 1950, Earth’s temperature has risen 1–2 degrees Celsius and is projected to rise by an additional several degrees Celsius in the next two to three decades.

If the predicted increase in the global temperature does indeed occur, it will cause dramatic climate changes throughout the world. These changes are likely to be devastating to agriculture, the environment, and our entire civilization. In addition, if the predicted warming occurs, sea level will rise as a result of thermal expansion of the warmed ocean water and melting of ice from glaciers and polar ice sheets in Greenland and Antarctica. Some experts predict that sea level will rise a meter or more during the next several decades, inundating large areas of low-lying coastal land and entire low-lying island chains.

More than 30% of the carbon dioxide released by fossil fuel burning has been absorbed by the oceans. However, the added carbon dioxide reacts to form a weak acid. Consequently, although ocean waters are still weakly alkaline, with an average pH of approximately 8.1, the acidity of the oceans is rising. The acidity of a substance is measured by its pH, with lower pH values indicating more acidity. The oceans have been more acidic in the ancient past than they are today, but the rate at which acidification is currently taking place is believed to be many times faster than has ever occurred before. Because the rate of acidification is far too fast to allow many species to evolve and adapt, it is anticipated that ocean acidification will result in the extinction of many marine species and a drastic alteration of marine ecosystems. Species that are especially at risk include those that create hard parts (skeletons, shells, and other structural materials) of calcium carbonate. These include many of the most important groups of marine organisms, such as shellfish and, perhaps more critically, the pteropods that are the base of many marine food chains that provide food for fishes and marine mammals.

The effects of ocean acidification on marine ecosystems are already significant. For example, the acidity of coastal waters of the Pacific Northwest has risen in recent decades to a level that prevents oyster larvae from growing to maturity. This has significantly affected the large oyster industry in the region. Oysters must now be hatched and grown in remote locations such as Hawaii, where the water is less acidic, or in tanks with controlled pH water. Due to its cold water and high biological productivity, the Bering Sea off Alaska is the most acidic part of the world ocean, with pH values of 7.7 already observed there and continuing to decline. At these low pH values, there is strong evidence that the Alaska King Crab population will decline and that populations of the small animals called pteropods, a principal food source for Bering Sea fishes, including salmon, will also decline since the water is too acidic for the pteropods to make their calcium carbonate shells.

Research suggests that future ocean acidification will cause major changes in the species composition of diverse marine food chains and is likely to severely damage coral reefs before the end of the century. However, some species that rely on calcium carbonate skeletal material do appear to be capable of adapting to this predicted increase in acidity levels. If we are going to mitigate or minimize the ecosystem effects of ocean acidification, we must not only reduce the amount of carbon dioxide we release to the atmosphere but actually remove much of the anthropogenic carbon dioxide already in the atmosphere.

Another consequence of anthropogenic releases of carbon dioxide is expected to be widespread deoxygenation of the oceans. This threat is caused by a combination of anthropogenic releases of carbon dioxide and civilization’s releases to the oceans of large amounts of nitrogen and phosphorus, primarily from agriculture and in treated sewage discharges. This threat is a little complicated as several of the physical, chemical, and biological processes that are explained in later chapters of this book are involved.

For now, it is important to understand the following points that are illustrated in Figure 1-3. The oceans are essentially separated into a warm surface layer and colder deep water, and oxygen concentration in ocean water is controlled by both physical and biological processes. The surface layer is constantly mixed and in contact with the atmosphere, allowing it to exchange gases; therefore, its oxygen concentration is primarily determined by the atmospheric oxygen concentration. The much thicker deep-water layers below the warm surface layer are colder and mix with the surface layer only slowly (Fig. 1-3a, CC1, Chaps. 5, 8). In the deep-water layer, respiration (including microbial decomposition of detritus) removes oxygen from the water and replaces it with carbon dioxide (Fig. 1-3b, Chaps. 5, 12). If water in the deep layers of the oceans was not continuously replaced by oxygenated surface waters, the deep layers would lose all oxygen and would become void of all life, except for certain microbial species (Chaps. 12, 13). However, surface layer oxygen-rich water does sink at certain locations in high latitudes where it is cooled and becomes dense enough to sink. This water moves slowly through the deep oceans, continuously losing oxygen and gaining carbon dioxide due to respiration, then eventually mixes back into the surface layers to be re-oxygenated (Fig. 1-3c, Chap. 8). This circulation results in a vertical profile of dissolved oxygen concentration with high oxygen concentrations in the surface layer and in the deepest water layers with an oxygen minimum at depths in or just below the pycnocline zone (Fig. 1-3, Chap 8). The concentration of dissolved oxygen at this oxygen minimum depth varies from location to location but, in some areas, is too low for animal life to survive. Areas that exhibit such low oxygen concentrations are referred to as Oxygen Minimum Zones (OMZ). OMZs are usually found at depths of between about 200 and 1000 m (Fig. 1-3, Chap. 8), and at least some such oxygen-deficient areas are currently expanding.

Figure 1-3. Oxygen concentration in ocean water is controlled by both physical and biological processes. The oceans consist of two main layers, a warm surface layer and a cold deep layer, with a transition zone, called the pycnocline zone, between them. (a) Surface layer water is well mixed and in contact with the atmosphere, and its oxygen concentration is primarily controlled by gas exchange with the atmosphere. (b) Photosynthesis by marine life in this layer converts carbon dioxide to organic matter and releases oxygen, but this is offset by respiration, which converts oxygen to carbon dioxide. Respiration also occurs in the deep layer, fueled by animals that migrate vertically or live in the deep and decomposition of organic material (detritus), but there is no photosynthesis and no contact with the atmosphere, so respiration continuously consumes oxygen in this deep layer, and, therefore the longer the water remains in the deep layers, the less oxygen it contains. (c) The deep layer water is continuously replenished by sinking of cold water in some near-polar regions and then circulates through the oceans while being slowly mixed upward, eventually into the surface layer again. The pycnocline provides a barrier that inhibits mixing, so the upward mixing process is slow.

For many centuries, the balance between the supply of oxygen-rich water to the deep layers and the rate of respiration by organisms in the deep layer of the ocean has maintained enough dissolved oxygen for the organisms that live there. However, that has not always been true in several episodes of Earth’s past when the deep ocean layers were devoid of oxygen. No animals or any other marine life, except for some species of microbes, can exist in water devoid of oxygen.

There are two ways that the deep oceans can lose all their oxygen and become anoxic. This can occur if the rate at which water circulates through the deep oceans is reduced, allowing a longer time for respiration to consume the oxygen, or if the rate at which respiration takes place in the deep oceans increases, or a combination of these. Respiration of living organisms in the deep layers of the oceans is fueled by the decomposition of organic matter (food) just as it is elsewhere. However, since no light penetrates down to the deep layer to support phototrophy, most of the food supply in the deep layers comes from the surface layer, as falling detritus or vertically migrating organisms (Fig. 1-3b). This food supply is limited, which limits the total respiration and, therefore, oxygen loss as water flows through the deep oceans. If the supply of food from the surface layer to the deep ocean water is increased, the additional food supports more deep-layer marine life, which means more respiration, and the oxygen concentrations in deep water decline.

The Earth has experienced at least 5 known mass extinctions of species, including the one that occurred when the dinosaurs became extinct (Fig. 1-4). While deoxygenation of the oceans is thought not to have been the sole cause of these extinctions, both acidification and deoxygenation did occur during most of these extinctions and are thought to have contributed to the extinction.

Now, we can consider why scientists believe that deoxygenation of the oceans is a major long-term threat. First, less oxygen can dissolve in seawater at higher temperatures, so climate warming will reduce oxygen concentrations in the surface layers of the oceans that are in contact with the atmosphere. Second, warming of the oceans is likely to enhance the temperature differences between deep ocean layers and surface layers, which will inhibit vertical mixing of the two. Warming of ocean surface waters is also expected to reduce the rate at which cold surface waters near the poles sink into the depths. Each of these effects will lengthen the time that deep ocean water spends out of contact with the atmosphere before returning to the surface layers to exchange gases and re-equilibrate with the atmosphere. Third, warmer ocean water is expected to increase the rate of production of food by phototrophy in the ocean’s surface layer, which will increase the supply of detritus to the deep layer, accelerating oxygen consumption. Current research indicates that each of these three changes have already begun to occur. Moreover, photosynthetic food production in most of the oceans is limited by the low concentrations of nutrients, especially nitrogen and phosphorus. Humans release large amounts of these nutrients, especially in agricultural runoff and treated sewage wastes. This is expected to enhance the rate of photosynthesis and production of organic matter, which will increase the production of detritus and the amount of detritus transported below the surface layer. Decomposition of this detritus is expected to increase, which would increase the rate of oxygen loss in the deep ocean layers.

Our understanding does not yet allow us to assess the future of ocean acidification and deoxygenation in the global ocean with any certainty. However, we do have observations of the ecological damage that they can cause in limited regions. For example, as mentioned earlier, acidification of the coastal waters off the west coast of North America has already been observed to have exceeded the ability of oysters to reproduce successfully (Chaps 5, 16). Also, there are numerous coastal and estuarine areas, including the Chesapeake Bay and the coastal waters west of the Mississippi Delta, that experience low or totally depleted oxygen in their deeper water layers (below a shallow pycnocline), either periodically or permanently, due primarily to excess anthropogenic nutrients. These areas have been called “dead zones” because living organisms that are unable to move out of these areas as they develop die due to a lack of oxygen (Chaps. 13, 16). Also, water from an OMZ in the North Pacific Ocean has periodically encroached on the Oregon continental shelf, causing a periodic “dead zone” to appear.

The oceans and atmosphere act together as a complex system that regulates our climate and the concentrations of carbon dioxide and other gases in both the oceans and atmosphere. An essential role of the oceans is to capture, store, and redistribute the sun’s heat energy to the atmosphere (CC5), so the oceans are integral to weather and climate. Within the ocean-atmosphere system, numerous complicated changes and feedbacks occur as a result of increases in atmospheric concentrations of carbon dioxide and other gases. Some changes add to the predicted global warming, whereas others reduce or even negate it. This is also true for ocean acidification and deoxygenation. However, our current understanding of the ocean-atmosphere system is poor, especially those segments of the system that are associated with ocean processes. To predict Earth’s future with less uncertainty so that we can take appropriate actions, we must improve our understanding of the oceans.

In addition to studying contemporary ocean processes, oceanographers study the oceans to uncover important information about past changes in the world’s climate. Such historical information, found primarily in ocean seafloor sediment and sedimentary rocks, can reveal how the Earth’s climate has changed over tens of millions of years and help us assess how it might change in the future, either as a result of natural changes or as a result of the enhanced greenhouse effect.

Climate change, ocean acidification, and deoxygenation are complex processes involving many concepts referenced in the preceding paragraphs. You might ask why they are summarized in this introductory chapter. The reason is simple. Each chapter contains information needed to fully understand how these processes work. If climate change, acidification, and deoxygenation continue unchecked, the ultimate likely consequences include global mass species extinction, a far greater threat than the damage that may be caused by any other environmental issue. Keep this in mind as you learn about the oceans, and you will realize that what you are learning is not just science but information about things that are vital to your life and the lives of all other humans and living creatures. We will return to the subjects of ocean acidification and deoxygenation in Chapter 16, when we discuss human impacts on the oceans. By then, you will have learned about the fundamental physical, chemical, and biological processes operating in the oceans and will be able to much better understand how acidification and deoxygenation are developing and that they represent a major long-term threat to both ocean ecosystems and human civilization.

Many important decisions must be made by current and future generations about human use and the protection of our environment. It is important for all of us to participate in these decisions with an understanding of the complexity of the ocean-atmosphere system and the uncertainties inherent in all scientific studies and predictions of such complex environmental systems (CC10, CC11). We must also recognize that science cannot provide definitive answers to even the most intensively studied environmental problems, as all environmental systems include non-linear relationships, making them chaotic. This means that predictions based on even the best science and modeling will always have high uncertainty.