Fig. 19-0: Picture of 160,000-year-old skull of an adult male from Ethiopia, the same species as present humans.
In the absence of “intelligent” life, our planet could continue along more or less the same course it has followed in the past. The sun has enough remaining hydrogen to burn smoothly for several billions of years. The heat produced by the radioactivity of Earth’s mantle will be adequate to drive the crustal plates for similar lengths of time. Evolution would change the assortment of species as they adapted to inevitable planetary changes, but barring catastrophes ecosystems would thrive in harmony with the planetary condition. The appearance of human beings has changed the character and outlook for the planet, although this was not immediately apparent. Our first tool-bearing ancestors appeared some 2 million years ago, and modern man appeared about 160,000 years ago. Throughout this time we competed on a more or less equal footing with our fellow inhabitants and lived without substantial impact on the natural environment. Environmental change at times reduced our numbers to less than 100,000 people. About 10,000 years ago, after the most recent deglaciation, all this began to change. The ability to manipulate fire permitted access to energy, large-scale modification of landscapes and ecosystems, access to metals, and far more efficient tools. The discovery of agriculture and the harnessing of other animals for our personal use gave us a great competitive advantage over plants and our fellow animals. In order to enhance our food supply, we used our ability to harness energy to reroute the flow of water, to till the soils, to eliminate weeds and pests, and to selectively breed and domesticate animals. The development of large communities where individuals developed specialized skills also greatly increased our capabilities as a species. Owing to our success, we began to modify natural systems for our own benefit, which made possible even larger concentrations of population and the great ancient civilizations of Mesopotamia, China, and Egypt.
Starting about 150 years ago, we discovered the great energy potential locked in fossil fuels. This increased energy led to industrialization that greatly accelerated this man-made modification of our planet’s surface. Now each human being uses some twenty times the energy that they can obtain through food, and individuals in the most prosperous nations use a hundred times as much energy. This latest energy revolution, the human energy revolution, enabled us to access all the planetary resources that were built up by billions of years of planetary evolution—Earth’s treasure chest. Some of these resources, such as most metals, are theoretically infinite in amount and readily recycled. Others, such as fossil fuels, are limited in amount and once used are gone forever. In this context, we live in the fossil fuel age, where 500 million years of Earth treasure will be consumed by a single species in a few centuries. Fossil fuels are not the only non-renewable resource. Soils that support our food supply and biodiversity that contains Earth’s biological storehouse of genetic potential are also being rapidly depleted.
The access to energy and the built up planetary resources has permitted a ferocious growth in population, particularly since the dawn of the industrial age, when our energy use so greatly expanded. Extinction of 97% of human beings living on Earth today would reduce us only to the number of people on Earth at the dawn of the scientific revolution some 500 years ago.
Since the demise of the dinosaurs at the Cretaceous/Tertiary boundary, mammals have progressively evolved to dominate terrestrial ecosystems. In very recent times, one particular mammal, Homo sapiens, has changed the entire course of Earth’s history. The appearance of intelligent life and a global civilization is planet changing both in its impact on all the various aspects of the Earth system and in the capacity of a planet in the galaxy. The various energy sources on the planet (fossil fuels, wind, sun, atom) can be harnessed to provide capabilities inaccessible to any other species and never before seen on the planet. Language permits advanced and subtle communication, among people and through generations. Industrialization allows the planet’s exterior to be modified and biological evolution to be engineered. Global data systems permit “sensing” (e.g., temperature, weather, crop health, atmospheric composition, population, biodiversity, etc.) on a planetary scale. Communication of this information permits global relationships and action. Specialization permits enhanced skills and capabilities for the community that vastly exceed those of any individual. And while it is not yet within our reach, we can imagine that such a planet might also have the capacity to communicate with other planets outside our solar system as part of a galactic community, should that be technologically feasible through means we do not yet understand. A visitor who examined our planet a thousand years ago and today could only be astonished by the revolution that has taken place. What is the record of this amazing planetary transformation, and what permitted it to occur as a unique event in planetary history?
The oldest human skull found so far dates back to the time of the penultimate glaciation 160,000 years ago (see frontispiece of this chapter). By “human” we mean Homo sapiens, the primate species with the largest brain capacity. At that time, our numbers were quite small and we likely lived only in East Africa. As recently as 70,000 years ago, genetic studies suggest the total human population may have been as little as 10,000 people. As recently as 50,000 years ago, a small number of people took advantage of the lowered sea level of glacial time and made their way across the land bridge connecting Asia with Australia. Fifteen thousand years ago we crossed the Bering land bridge and populated the Americas. At some time during the last glacial period, our main competitor, the Neanderthal, fell by the wayside.
Until the end of the last glacial period (11,000 years ago), we competed on more or less equal footing with our fellow large mammals, and human population probably did not exceed 1 million. Then, taking advantage of the dramatic warming and increase in habitable area ushered in by the present interglacial period, we transitioned from an existence based on hunting and gathering to one based on agriculture and animal husbandry. No longer forced to roam in our quest for food, we began to build ever more elaborate settlements and eventually cities, giving rise to the ancient civilizations of Mesopotamia, Egypt, and China. As prime agricultural land and reliable water sources were often geographically separated, irrigation became essential to our developing civilization. Trade flourished, permitting materials and technology to be shared over long distances. Written language and monetary currencies grew hand in hand with trade. The refinement and use of fire, providing access to high temperatures, permitted specialized activities such as metallurgy and pottery manufacture to take place in ever more complex societies. These developments also permitted increased specialization in the technical realm, in fields such as literature, music, theater, and painting, and in our capacity for war to destroy each other’s existence. Humans began to specialize and work together so that the entire community had far more power and capability than any individual.
The benign climate and the rise of agriculture and civilization permitted major population growth, leading to about 200 million people at the peak of the Roman empire. Following a substantial decline after the fall of Rome, a medieval warm period led once again to a burgeoning population, followed by a steep ~25% reduction in global population caused by famine and plague. By about 1500 there were no more people than at the time of the Roman Empire. At that point population growth took off. By about 1820, the population had grown to about 1 billion, tripling in some 200 years. Another 2 billion were added by 1960 and in the last 50 years 3 billion more people were added as the population doubled again. As of the year 2012 there were 7 billion, and the estimate for the year 2050, barring catastrophes, is 10 billion.
Figure 19-1 shows population growth over the last 12,000 years, since the end of the last ice age. The vertical axis is logarithmic, so a steady rate of growth would lead to a straight line with a constant “doubling time.” Between 70,000 and 10,000 BC, population appears to have grown at about 0.007% per year on average, or a doubling time of 10,000 years. After the rise of agriculture, that rate may have increased to about 0.03% for a few thousand years, increasing to as much 0.1% during the Roman era, for a doubling time of 700 years. Then for some 1500 years population appears not to have increased, until about year AD 1600, when the rate jumped to 0.5%. After the rise of industrialization, the rate rose to more than 1% per year. It may seem like a small increase, but that changes the doubling time to only 70 years. These statistics show that it is not just the number of people that has increased, it is the rate at which they have increased that has marked the modern era. If global population reaches the estimated 10–12 billion people in the coming century, on the basis of population alone human beings annually will have 40 times more impact on the planet than any year prior to 1600, 4 times more than 1950, and 7,000 times more than the beginning of the modern era 12,000 years ago. Including the magnifying factor of growth in energy, the impact is even larger.
Fig. 19-1: (a) Global population growth versus time. Note that the vertical scale is logarithmic, so a straight line is exponential growth with a constant “doubling time.” The expanded portion of the graph has a linear scale and shows the exponential growth between 1800 and 2010. (b) Growth rate versus time for the last 3,500 years. It is not just the total human population increase but the rate of increase that has changed with time. (Data from U.S. Census Bureau)
The root cause of human-induced planetary change is this population growth and its attendant demands on all aspects of the Earth system. While we see people everywhere we look, population growth is not obvious to the individual. But consider that if a terrible plague visited the Earth and wiped out 97% of the human population, there would still be as many people on Earth as there were in the year 1500. And despite a recent slowing of the rate of population growth, we are still adding more than the equivalent of an AD 1500 population every three years.
This spectacular growth would not have been possible if each of us still had to go out hunting on a daily basis for our food within walking distance of our home. Imagine, for example, if food supply was cut off to a major city for a month and the millions who lived there needed to hunt and forage to survive. Famine and riots would be inevitable.
What has made possible our spectacular growth and success is our ability to work together and access the treasure chest of Earth resources, including most importantly our ability to access energy beyond that provided by metabolizing food. Energy provides our access to all the other resources, most importantly food, but also metals, groundwater, fish, and so on. If, like all other animals, we had to rely on our muscle power alone, civilization would collapse. The rise of human civilization is the latest energy revolution for the planet—the human energy revolution.
Access to energy has been a driving force for human civilization for millennia. Streams were diverted to power mills. Wind was harnessed to push ships through the sea. Wood was burned to fuel furnaces and extract metals. The whale population was decimated to provide oil for lamps. The biggest change occurred with the advent of engines fueled by the combustion of carbon. To access coal, mines were opened and canals and railroads built. With the advent of the automobile, a need for a liquid fuel led to drilling for oil.
The availability of carbon-based energy has allowed human beings to enormously supplement their energy supply. Including all energy sources, the average person globally uses 2,300 watts of power, or the equivalent of twenty-three 100-watt light bulbs burning twenty-four hours a day. The average United States citizen uses five times more, the equivalent of 11,400 watts of power continuously (including all energy sources). The food we eat provides the energy equivalent of merely 100 watts. Our bodies are so efficient that that small amount of power allows all the inner and outer activities of our bodies and makes us an organic electric blanket when we huddle together on a cold night. Access to external energy augments our average power by twenty to a hundred times compared to animal metabolism (Fig. 19-2), and much more energy is available on demand when we want it, such as when we fly, drive a car, or turn on a stove. This energy gives us the capabilities of superheroes. We can fly higher and faster than a bird, easily access building rooftops, punch through walls, excavate mountains, stop speeding bullets, send fire and destruction long distances, and communicate instantaneously with others all over the world.
From a planetary perspective, human access to external energy in the service of our species is the latest energy revolution for Earth, the human energy revolution. The energy revolution of aerobic metabolism discussed in Chapters 15–17 boosted energy by a factor of 18 and allowed the development of multicellular life. Our energy boost of twenty to a hundred times exceeds this even on an average basis, and of course over short times we vastly exceed it. As is obvious from examination of the world around us, this access to energy and the capability to focus our energy use have been able to transform the planetary system in record time.
Fig. 19-2: Per-person energy use by nation. The bar at 2300 watts is the average personal energy consumption globally. The barely visible bar at 100 watts is the average energy we obtain from food. Other animals are similarly restricted. (Data from International Energy Agency, Key World Energy Statistics, 2009)
The increased energy allowed us to extract metals from Earth’s crust, pump water from aquifers and dam rivers, and entirely dominate all other animals and steadily eliminate them. While this increased access to Earth’s resources has major advantages for us, it also has involved sizeable environmental and health costs. Mines destroy ecosystems and scar the landscape. Burning coal releases sulfur and mercury. Denuding land and plowing it eliminates topsoil. Industrial chemicals never produced by nature to which life is not adapted are added to air, water, and life. Groundwater use depletes water supplies. Overfishing depletes large fish from the entire ocean. The waste products of our energy production add CO2, CH4 and other greenhouse gases to the atmosphere and generate nuclear waste. Both the wonders and horrors of modern civilization are made possible by the human energy revolution, largely powered by the burning of fossil fuels.
A central aspect of the energy revolution is that it provides much greater access to and productivity from land, increasing our food supply. Industrial agriculture is highly energy intensive, and meat production even more so. Two pounds of meat from an industrial feedlot for a family dinner requires one gallon of oil to produce. Transporting and killing the animal, packaging and transporting the meat to market and then to the home, requires still more energy. Many of the largest cities are far from agricultural regions and could not exist without food transport using fossil fuels. Our takeover of planetary ecosystems and energy use to produce and transport food from productive to unproductive areas, as well as greatly improved health care, is what has permitted us to expand so greatly in numbers.
The human energy revolution has been possible not from a biological innovation but from our ability to directly access Earth’s fuel cell, discussed in Chapter 15. We take the organic carbon stored over hundreds of millions of years of photosynthesis, and combine it with the oxygenated atmosphere developed by the billions of years of planetary evolution, reversing photosynthesis and releasing the stored energy. Were we able to access all the organic carbon, we would eliminate the O2 in the atmosphere and fully discharge the fuel cell that powers advanced life. Fortunately, most of the organic carbon is in black shales where it cannot be extracted economically.
Fossil fuels have been the most important resource because they concentrate enormously the energy from the sun. This power then gives us the energy to access other resources on a grand scale, moving mountains if necessary. The human energy revolution has enabled us to open and exploit Earth’s treasure chest, the vast resources accumulated gradually over billions of years of Earth’s history.
The abundant resources that Earth provided are what permitted the global rise in population and spread of civilization. Rivers, food, forests, animals, and more and more as time went on, metals, fuels, groundwater, soils, and fertilizer—all were used for the economic and population growth of humans. These resources are the planetary treasure chest.
The accumulation of all these resources has resulted from Earth’s billions of years of evolution. Because of the differential solubility of ions of different oxidation states, changes in the oxidation state of surface environments have been a major factor in the accumulation of high concentrations of certain metals. For example, the early rise in oxygen led to the deposition of BIFs during the period from 3.5 Ga to 1.8 Ga, as Fe was transported from soluble Fe2+ environments to sites of insoluble Fe3+ deposition (see Fig. 16-7). These rocks are the principal source of Fe ore for modern civilization, and because of them iron is so cheap that it forms the basis of modern buildings and industry. The ores of uranium are also controlled by oxidation state. Recall that the reduced form of U is insoluble and the oxidized form soluble. Ancient ores are the detrital uraninite referred to in Chapter 15 and 16 that were able to be deposited in gravels prior to the rise of O2 in the atmosphere. At younger times, uranium was mobilized in oxidized waters, and then precipitated when these waters encountered a reduced environment, such as sediments rich in organic matter, leading to the uranium deposits in Phanerozoic rocks.
Other deposits required particular conditions of the biosphere or the thermal evolution of Earth’s interior. Deposits of phosphorous, an essential ingredient for fertilizer, were largely restricted to Phanerozoic rocks when the ocean became sufficiently biologically productive to create massive deposits of phosphorite rock. Large deposits of chromium and platinum that provide most of these metals for modern civilization were formed when massive basaltic bodies of magma were intruded into the crust during the Archean, when Earth was hotter and produced more magmatism. The 3.5 billion-year-old Bushveld intrusion in South Africa, for example, contains most of the world’s reserves of these two metals.
Other metal deposits are formed as a consequence of plate tectonics. Many deposits of copper, molybdenum, and tin occur in the shallow oxidized portions of hydrothermal systems that formed around granitic plutons emplaced in the crust near convergent margins. Because they occur in mountainous volcanic regions that are recycled rapidly by erosion and require oxidizing conditions, old deposits either did not form or have eroded away, and existing deposits are almost all associated with modern plate margins (Fig. 19-3).
Fig. 19-3: Map of North and South America showing distribution of major ore Cu deposits. Note the preferential presence of these ore deposits near modern convergent margins, and their association with subduction volcanism. Deposits in western Canada formed prior to the subduction of the Farallon ridge when active subduction occurred in that region (see Fig. 11-7). (From Singer, Berger, and Moring (2002); http://purl.access.gpo.gov/GPO/LPS22448. Courtesy of U.S. Geological Survey)
Hydrothermal ore deposits formed at ocean spreading centers have also played a role in the rise of human civilization. Oceanic crust and its ore deposits formed at hydrothermal vents are occasionally faulted and accreted to continents, forming bodies called ophiolites. The ore deposits are very visible and high in grade, making them easy targets for early civilizations. Cyprus contains a large ophiolite that was rich in ore, and similar bodies throughout the Mediterranean served as a source of early metals for humans.
Fossil fuels form from burial and transformation of plant matter, so none were formed prior to the Paleozoic. Billions of years of photosynthesis were required to raise atmospheric O2 levels sufficiently for multicellular plants to evolve. To make a commercial deposit requires still other conditions that permit the concentration of vast amounts of organic carbon. Later metamorphism causes this carbonaceous material to form coal, or to generate more volatile compounds that can flow and be trapped in reservoirs as oil and gas. Coal began to be formed in abundance in the aptly named Carboniferous period, named because of its massive concentration of coal deposits (Fig. 19-4). Oil is more common from younger Mesozoic and Cenozoic rocks.
It is amazing to note the very slow rate of formation of fossil fuels. Earth underwent a very gradual accumulation of these resources over hundreds of millions of years. The horizontal axis of Figure 19-4 gives the annual accumulation rates of the various fuels. Oil has accumulated at a few hundred cubic meters per year—less than the annual output from a single gas station today. Coal has accumulated at about 20,000 tons per year. Annual coal use today is 300,000 times that amount. We are using up this treasure at an exceedingly rapid rate.
Other resources are rapidly rejuvenated, and therefore their age is geologically young. The resources of fresh water and soil obviously depend on the longterm climate stability of the surface, but given that stability they seem to be steadily renewed, as rocks weather, organic matter decomposes, and water cycles on the short timescales of weather and seasons. Even these resources are time dependent, however, because they are influenced on longer timescales by the glacial cycles. Glaciers carve great basins that become lakes filled by the massive amounts of meltwater from receding glaciers, such as the Great Lakes and Finger Lakes of the northern United States. Wetter climates also contributed to rich resources of groundwater in deep aquifers.
Fig. 19-4: Fossil fuel reserves as a function of geological age. Note the massive amounts of coal made in the Carboniferous period, and the young age of most oil formation. (Modified from Pimentel and Patzek, Rev. Environ. Contam. Toxicol. 189 (2007):25–41)
This knowledge of Earth’s history shows us that a highly diverse set of processes operating over billions of years has created the resources on which human civilization depends. Some resources, such as rainwater and rivers, are rapidly recycled and depend on recent climate. Others, such as lakes and groundwater, have longer timescales associated with glacial cycles. And many resources formed under specific conditions that were restricted to short intervals of Earth’s history. These diverse processes have over a very long time led to the accumulation of the planetary treasure chest of resources. No species had ever made use of mineral and fossil fuel resources prior to the appearance of human beings, and upon our arrival we found a well-stocked planet. With no knowledge of geological time and Earth’s history, no sense of a planet of finite size or of geological time, ancient peoples could have no concept that planetary resources were limited in quantity and reflected a long process of planetary evolution and storage. Today we face a fully inhabited planet where understanding the different types of resources in a planetary context has become necessary.
Earth’s resources have different characteristics, depending on their abundance in the earth, whether they are destroyed by human use, and the timescales of replenishment by earth processes. On this basis, we can divide resources into three different classes:
(1) Resources of vast extent and short recycling times (e.g., air and surface water).
(2) Resources that can be recycled and for which there are such vast amounts that availability is simply determined by price (e.g., most metals).
(3) Resources that are finite and not replenished on human timescales. Once used they are gone for thousands to tens of millions of years. “Thousands of years” applies to soils and groundwater. One million years of Earth’s coal production or 15 million years of oil production would be necessary to provide enough fuel for three years of our current use. Biodiversity is perhaps the most precious planetary resource, for which the timescale of replenishment, known from past mass extinctions, is tens of millions of years.
Air and water are vast in amount, essential for all of life, and rapidly recycled. The turnover time of O2 in the atmosphere by life is 5,000 years. Variations in our inputs and outputs are small relative to the size of the reservoir, and we are in no danger of running out of oxygen. Of course, as we burn fossil fuels we use up some of the O2 in the atmosphere, but increasing CO2 from 350–700 ppm on a short timescale only changes O2 by similar amounts, trivial compared to its 210,000 ppm abundance. On more local scales, atmospheric changes are rapid enough that a major cleanup of pollution in an urban environment can have positive results on the scale of years. Air pollution is a tractable problem, given political will, because human management can have rapid results, and the resource continually replenishes itself.
Water is a resource of greater complexity. With the exception of long storage times in glaciers, the recycling time of water is rapid, as evaporation from the oceans to rain to return to the oceans occurs in weeks. On land much of the precipitation evaporates (or is transpired by plants) before seeping into the soil to become groundwater or runoff. The residence time of water in rivers is less than a year.
Even though water is rapidly recycled, it is a vital and limiting resource for human civilization. As population has grown and lifestyle improved, water has been in ever-greater demand. In most of the world, a sizable part of this water is intercepted and used for irrigation of agricultural lands, for processes carried out by industry, and for household use by municipalities. Each of these activities degrades the water’s quality, making it largely unusable for other activities. Evaporation from irrigated farmlands enriches the residual water in salt, industries have traditionally disposed of their spent chemicals in wastewater, and sewage is carried away via municipal sewers. While to an ever-greater extent industrial and municipal waters are purified before being released to rivers, their quality is still usually not up to drinking-water standards. This treated water is generally not “reused.”
Despite nature’s generous supply, in many areas of the world there are shortages. The biggest users are farmers. Of the world’s grain, 40% is now grown on irrigated land. Plants transpire many hundreds of molecules of water for each molecule of CO2 they fix by photosynthesis. A high-technology hectare of farmland produces 100 bushels of grain each year. To do so requires about 300,000 gallons of water. To the extent that is not supplied by rain, it must be supplied by irrigation. Since the 1960s there has been only a 16% increase in arable land, and virtually all of it is irrigated.
Cities developed in arid to semiarid regions are also a significant demand on water. Moving water from areas that have excess to those which have too little has become an integral part of our civilization, and many cities in some countries are built in arid or semiarid regions with insufficient water to support the population. Los Angeles, for example, is a semi-arid region. The city was able to expand in the early twentieth century by diverting water from the Owens valley, causing Owens Lake to dry up and converting fertile farmland to the north into a desert. Subsequent growth caused Los Angeles to take water even farther to the north, including the watershed of Mono Lake, causing Mono Lake to begin to dry up. This desert water body is used as a “fueling” station for a million migrating birds. Each year they fattened up by eating the tiny brine shrimp that teemed in the salty water of the lake. But after the aqueduct was opened in 1941, the lake supply of mountain runoff was largely cut off, and because of this, the lake started to evaporate away. As it did so, its salinity increased, endangering the brine shrimp, and its water level declined, opening a land bridge that threatened the island bird rookeries with plunder by predators. Los Angeles also diverts large amounts of water from the Colorado River and from the Sacramento–San Joaquin delta, where the water removal has threatened the delta smelt. Only 11% of Los Angeles water comes from local groundwater. The rest is imported. Almost two-thirds of the water use in Los Angeles is residential.
Access to surface water can also be problematic for entire nations. When Egypt was ruled by the pharaohs, life depended on the Nile. Fed by the monsoons’ rains in the Ethiopian Highlands, its waters irrigated the croplands, which supplied the food for the few million inhabitants. The solutes carried by this water provided the nutrients needed for plant growth, and the mineral matter it carried was material for the manufacture of bricks. As the population grew, ever more land had to be irrigated. To accomplish this, an increasingly complex series of channels were constructed, to deliver the water to the fields and to carry the salt-loaded effluent to the Mediterranean Sea. In order to permit a second crop to be grown during the dry season, reservoirs were constructed to store the plentiful monsoon runoff.
Eventually it was deemed necessary to build the huge Aswan Dam, whose reservoir could store enough water to supply agricultural needs for several years. It also allowed the flow to the lower Nile to be regulated, providing a year-round irrigation supply. So efficiently was water handled that only a few percent was allowed to go unused into the Mediterranean. Further, initially the electricity provided by the dam’s generators supplied most of Egypt’s needs. All was well.
When the Aswan Dam construction began, Egypt’s population was 27 million. By 2010 it had reached 80 million. Even though Egypt’s crop yields are among the world’s best, only about half the required food can be grown. The rest must be purchased from abroad (paid for by sales of Egyptian petroleum). Also, the nutrients that used to accompany the Nile water are now being consumed by algae in Lake Nasser behind the dam. To have sufficient nutrients for high grain yields, fertilizer has to be manufactured (the energy comes from burning Egyptian petroleum). Further, the demand from electricity now outstrips by twofold that generated at Aswan. The rest has to be generated by burning Egyptian petroleum.
No longer is Egypt able to support its existence solely from the Nile. Egypt’s recent growth depends on its petroleum reserves, which are limited. Further, as is the case for all reservoirs behind dams, over the next few hundred years, Lake Nasser will fill with silt, gradually reducing its storage capacity and eventually eroding the tunnels and turbines used to generate electricity. As the amounts of silt are enormous, dredging is not an option. In this example we see how water use, food, petroleum, fertilizer, and population are all related. While water and petroleum are emerging as limiting resources, the primary driver of the problems is the population increase.
Groundwater is another matter. Groundwater is also a replenished resource, but the timescale of deep ground water replenishment is thousands of years. Much of the deep groundwater at northern latitudes was created by moister climate at midlatitudes from the last ice age and has very long replenishment times because replenishment by rain is very slow. While a time of thousands of years is short from a geological perspective, it is essentially infinite for modern water issues. Some agriculture has been maintained by unsustainable groundwater removal from aquifers. Farmers drill into the deep aquifer and “mine” the water for present use. When extraction exceeds replenishment, the water level declines and deeper wells must be drilled, until the aquifer becomes too poor in water or too saline. For aquifers near coastlines, lowering the water table can also lead to penetration of saline ocean water, making the aquifer unusable. Groundwater can be used indefinitely if it is used at the rate it is replenished. This is feasible in some regions where replenishment rates are rapid. In other areas, however, groundwater formed in wetter conditions thousands of years ago and is not replaced on timescales that permit the agriculture to continue.
Fig. 19-5: Shaded area is the Ogallala Aquifer, which underlies semiarid regions of the western United States. Groundwater withdrawal from the aquifer is what has permitted agriculture to thrive in this region. Large portions of the aquifer are being depleted by extraction rates that exceed replenishment. (Courtesy of U.S. Geological Survey)
Groundwater depletion for irrigation to grow food is an emerging problem throughout the world. The vast Ogallala aquifer in the United States underlies parts of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas (Fig. 19-5). Extracting this water and using it for irrigation in these semiarid regions has led to a high production of corn, wheat, and soybeans. While the aquifer was originally considered to be “infinite,” extraction of water from it has exceeded its replenishment, leading to steady declines in the level of the water table. Groundwater depletion is also occurring in India and China. Large regional scale measurements of groundwater depletion have recently become possible through very precise measurements of changes in Earth’s gravitational field from space. Figure 19-6 shows the decline in water from the world’s most irrigated and populous region in northern India. Groundwater is being depleted at the rate of 50 km3 per year, equivalent to an annual drop of several cm in the water table.
Fig. 19-6: Decline in groundwater in northern India and Bangladesh determined by satellite gravity measurements. Northern India has 600 million people and is heavily irrigated. The loss of some 55 km3 per year is the largest groundwater loss in the world in comparable regions. (Modified from Tiwari, Wahr, and Swensen, Geophys. Res. Lett. 36 (2009), L18401)
A clear example of unreplenished groundwater occurred in Saudi Arabia. Saudi Arabia is arid, and the nation has no year-round rivers or lakes. During the glacial periods, the climate was much wetter, leading to well-stocked aquifers at depth, which are not being replenished by current rainfall. In order to become self sufficient in agriculture, Saudi Arabia initiated a large agricultural development program in the 1970s, making use of their groundwater resources. They increased agricultural land by a factor of 20, requiring an equivalent increase in irrigation with groundwater. Water use and agricultural productivity peaked in the early 1990s, after which the ability to extract water began to decline (Fig. 19-7). In 2008 Saudi Arabia announced that it would end wheat production and depend on wheat imports in order to conserve water.
Fig. 19-7: Water production from Saudi Arabian aquifers. Saudi Arabia has no year-round surface water. Growth of agriculture in the 1970s led to unsustainable use and the permanent decline in groundwater. Owing to lack of water, the Saudis ceased wheat production in 2008. Note the similar form of this curve to the oil depletion curves in Fig. 19-10. (Created from data in Abderrahman, Water demand management in Saudi Arabia, in Water Management in Islam, IDRC (2001); http://www.idrc.ca/cp/ev-93954-201-1-DO_TOPIC.html)
Most metals are present in every rock to some extent, so the amounts theoretically available are essentially infinite. For example, igneous rocks commonly contain 5–10% FeO and tens to thousands of parts per million of manganese, copper, nickel, zinc, and so on. An “ore deposit” is defined by the grade of ore that is necessary for economic extraction. As the price rises, rocks that were not ore become ore, and additional supply becomes available because lower grades of the resource can be economically extracted. Furthermore, if the price rises enough, it becomes economic to recycle the metals, which is straightforward because most metals are not destroyed by use. Mining and recycling also compete in the marketplace, so consumption can go up even if mining does not. For example, about one-half of the copper used in the United States each year is recycled copper, and 88% of steel is recycled after use. These factors permit steady growth in production that has been sustainable over the past century (Fig. 19-8). While there are economic consequences of mining, they are largely limited to the local region, and modern mines are subject to environmental regulations that cause much of the environmental cost to be incorporated into the price. The infinite supply, ability to recycle, and an economic model that includes much of the environmental impact leads to use that is self-regulating.
Fig. 19-8: Steady growth in world copper production, marked by declines after WWI, WWII, and the depression of the 1930s. Note the log scale shows that copper growth has been able to increase exponentially for more than one hundred years. (Data from U.S. Geological Survey)
Phosphorous is more problematic because it is an irreplaceable component of fertilizer and is used in vast amounts (more than 150 million tons in 2008) to enable the harvests that feed the world. Because fertilizer must be low in cost, only very rich deposits of phosphorous can be mined economically. These deposits were emplaced during a few limited periods of Earth’s history, with one peak of deposition near the Cambrian–Precambrian boundary and an even larger peak at the end of the Mesozoic (Fig. 19-9). Growth of population and deterioration of soils increases the need for fertilizer; estimates are that in the next sixty-five years half of Earth’s phosphorous reserves will have been depleted. While some phosphorous can be recycled, it is far more difficult than for metals, because phosphorous is water soluble and is carried by runoff into rivers, lakes, and oceans. Concentrations in these vast bodies of water are then too diluted to be recycled. Metals are concentrated by mining and use and are solid. Phosphorous is deconcentrated by mining and use and is carried away by water.
Fig. 19-9: Accumulation of phosphorous deposits over geological time. Note the discrete intervals of time where most deposits were able to accumulate. (Modified after Yanshin and Zharkov, Intl. Geol. Rev. (1986))
Phosphorous also has deleterious environmental consequences. Because it is a limiting nutrient for life, phosphorous addition to bodies of water causes algal growth, and when the algae die the oxidation of the organic matter uses up all the oxygen in the water, making further life impossible. This creates the eutrophication of lakes and the dead zones in parts of the ocean, e.g., in the Gulf of Mexico near the outflow from the Mississippi River. Poultry manure also contains large amounts of phosphorous, and this phosphorous has led to the gradual eutrophication of Chesapeake Bay. With current practices, most valuable phosphorous is lost and becomes environmentally destructive. There are methods to recover the phosphorous from wastewater, however, so the potential for recycling a portion of phosphorous exists.
In contrast to our first two classes of resources, there are other resources that have a limited supply, and once used are gone “forever” in terms of human timescales. These are the nonrenewable resources. The most obvious of these are the fossil fuels, but soils and biodiversity also fall within this class.
Metals are simply elements, and the whole Earth is made up of them. In contrast, fossil fuels are complex organic molecules largely formed by life only at the surface. Their utility resides not in the elements themselves, but in the energy stored in the molecular bonds. Once this energy is released, it is gone forever, replaced only on timescales of millions of years by further photosynthesis and storage of organic carbon. Fossil fuels are also limited by the amount of organic matter produced over Earth’s history and the need to modify and concentrate it for it to be useful as fuel. Unlike metals, the amount is limited. Once used, the resource is eliminated. There is no recycling.
Fossil fuel resources also have a usage profile very different from metal resources. Metal resources for an individual mine often increase as price increases, because more and more metal becomes profitable to extract. The ore is solid rock that is recovered. In contrast, oil fields are pumped, and at some point the pumps go dry. Gas and oil are much less subject to reserve expansion than metal resources. M. King Hubbert noted that each oil field had a finite lifetime with a characteristic life cycle. When a new field is opened and the resource is abundant, extraction goes through a period of growth. Then the field tops out and goes into a steady decline. This characteristic has been verified repeatedly for individual fields, for North American oil production, for the North Sea, and so on (Fig. 19-10). Discoveries of new fields have also been decreasing, so that new supplies of oil available for their growth phase are diminishing. These facts lead to the concept of peak oil, where global oil production will follow the same trend as U.S. or North Sea production. Given the vast resources expended on oil discovery and the decreasing rate of discovery, oil production is likely to go into a steady decline sometime in the early twenty-first century.
For coal there is generally no fear of imminent shortage on the timescale of a human life because there are likely several hundred years’ supply available. This is also the case for lower-grade or less accessible resources such as oil shales, tar sands, and the abundant gas clathrates on the continental shelves. While several hundred years is long for an election cycle, it is very short on the time frame of human civilization. Figure 19-11 shows the profile of fossil fuel usage on two planetary timescales, one associated with the Phanerozoic era where fossil fuels are gradually accumulated and the other with the 10,000-year timescale of human civilization. We live in the fossil fuel age—the very brief interval in Earth’s history when 500 million years of resources were consumed and destroyed by a single species. We are using up these resources at a rate a million times greater than Earth produced them. Our ancestors are likely to look with wonder at our profligate waste of Earth’s storehouse of organic molecules, because they can be put to so many uses—plastics, artificial joints, lubricants, etc. What were we thinking, simply burning such treasure?
Fig. 19-10: (a) The characteristic life cycle of oil fields identified by M. King Hubbert. Initially the field undergoes rapid growth in production as it becomes fully exploited. The production then peaks and goes into a steady decline. This observation is true for individual fields such as the Alaska North Slope, for larger regions such as the North Sea, and for entire nations such as the United States. This is the concept behind “peak oil” (U.S. Energy Information Agency). (b) The bottom panel shows the discovery rate for new giant oil fields that provide the bulk of the world’s oil. Discoveries have been declining steadily since 1970. Each of these fields will ultimately decline in production similar to the more mature discoveries shown in the top panel. Indeed, global oil production has flattened between 2007and 2010, one of the reasons for the oil price spikes in 2008 (American Association of Petroleum Geologists, Uppsala Hydrocarbon Depletion Study Group).
Fig. 19-11: Time scales of fossil fuel formation (a) and depletion (b). Five hundred million years of accumulation are being depleted in a few centuries in what will come to be known as the fossil fuel age. ((a) data from Pimentel and Patzek, Rev. Environ. Contam. Toxicol. 189 (2007):25–41)
Soils are also a nonrenewable resource on human timescales. Soils form by the slow weathering of rock and the modification of this weathered product by life to create a markedly complex soil ecosystem. Once land is denuded of soil, it can no longer support significant life. Left to itself, Earth builds up soil resources, and humans have made use of this treasure to grow crops for food. Soils naturally do not erode significantly because there is continuous agricultural ground cover, and deep soils are not turned over to reach the surface. Agriculture removes this ground cover and exposes soil to rain and wind, and turns over deep soils, reducing their cohesion and exposing them to the surface. Soil loss in the United States is about ten times the replenishment rate, annual loss is about ten tons per acre, and half of midwestern topsoil has been lost since the 1800s. Some estimates place global topsoil loss at about 1% per year. Loss of soil leads to increased fertilization to maintain productivity, but the ammonia in fertilizer is a fossil fuel product and phosphorous is a limited resource. In some regions, it is less expensive to abandon denuded fields and clear new forest rather than fertilize. The pressure of population, limited new habitat, and modern food needs is leading to progressive deterioration of global soils. This is a problem that is well distributed across both developed and undeveloped nations. The agricultural heartlands of the United States, Europe, and China, for example, are all experiencing topsoil loss that is influencing agricultural potential.
The ultimate nonrenewable resource is biodiversity. All of our food comes from Earth’s total genetic library. Many modern medicines and industrial processes also depend on the genes of organisms, from bacteria to mammals. Ecosystem stability depends on the diversity of life that maintains it, and ultimately the habitability of Earth depends on the viability of ecosystems. Earth’s response to change and catastrophe also depends on the evolutionary potential of life, and this potential scales with the overall genetic diversity. Catastrophes of the past show us that biodiversity recovers only on timescales of tens of millions of years, and some ancient innovations may never be recovered. Destruction of biodiversity, discussed in greater length in the next chapter, is the destruction of billions of years of evolutionary potential.
Human beings appeared and became the dominant species on the planet in a remarkably short period of time. This dominance is reflected in the massive population growth from perhaps only 10,000 people about 70,000 years ago to a population soon to approach 10 billion—an increase of a million times. Human beings dominate every ecosystem, sit at the top of every food web, and claim ownership of every piece of habitable land.
This planetary domination was made possible by the human energy revolution, where one species was able to access energy far in excess of that available to any other species. This revolution in turn was possible only because humans found a planet fully stocked with easily accessible energy, produced by billions of years of planetary evolution. The access to energy also permitted exploitation of all the other planetary resources—water, metals, land with its fertile soils, and a rich, abundant, and diverse biosphere. First destroying other large carnivores that competed for food and land, human beings then proceeded to destroy and adapt habitat for their use, leading to massive extinctions of other species. All resources were treated as equivalent—freely provided by the planet and there for the taking without payment or consequence.
Resources are of different types, however. Some commodities are unlimited in potential supply, can be recycled, and have local environmental impacts that can be rectified. Others, such as phosphorous, are more limited in supply and are much more difficult to recycle. Environmental impacts are often distant from the source, leading to eutrophication of downstream rivers, lakes, and marginal seas. Fossil fuels, soils, and biodiversity are limited, irreplaceable, and once destroyed or lost are gone “forever” with respect to pertinent human timescales. Environmental impacts of fossil fuels and biodiversity are global in scale, such that one nation’s behavior can impact another’s halfway around the globe.
The differences in types of resources are not generally recognized by the modern marketplace. Our attitude toward use of Earth’s bounty encompasses time frames of only a few years and is based on price of immediate extraction and maximizing short-term profit—not on the total amount that is available, nor the potential for recycling, nor environmental impacts. This leads to inevitable rapid extraction and use of irreplaceable treasure. In a few brief centuries, human civilization will have used up billions of years of accumulation of planetary resources. We arrived on the scene to find an eminently habitable world. Our actions in the last two centuries have made the world far more habitable for our own species, allowing us to live in comfort even in arid or frigid regions and permitting vast population growth and concentration in urban centers. At the same time, we have made Earth far less habitable for most of the millions of other species with whom we share the planet. Ultimately, it remains a pressing question whether we are also making it less habitable for our own descendants.
