The eruptions of Tambora in 1815 and Krakatoa in 1883 were the signature volcanic events of the nineteenth century. The twentieth century was barely under way when the Santa Maria volcano in Guatemala erupted in 1902. This eruption blasted away much of the 12,000-foot summit of the mountain, sending some 1.3 cubic miles of volcanic ash high into the stratosphere and from there around the world. Although somewhat smaller than Tambora and Krakatoa, Santa Maria was estimated by volcanologists as probably one of the five biggest eruptions of the past few centuries. It was followed a decade later by the even bigger eruption of Novarupta, on Mount Katmai, in Alaska, which delivered more than four cubic miles of ash to the atmosphere.

The eruptions of Krakatoa, Santa Maria, and Novarupta in short order kept the atmosphere murky and the climate cooler for the better part of three decades. But after the 1912 eruption of Novarupta, there were no significant explosive volcanic events for a half century, which allowed the atmosphere to clear. The absence of volcanic dust also contributed to the climb in the global average temperature during the period 1910-50. But in the last half of the twentieth century, explosive volcanism returned. The eruptions of Agung in Indonesia in 1963, El Chichon in Mexico in 1982, and Pinatubo in the Philippines in 1991 kept the atmosphere dustier and the Sun dimmer than usual.

If these natural factors were the only ones at work in the last half of the twentieth century, a quieting Sun trying to penetrate a murkier atmosphere would have led to a slight cooling of Earth's surface. But in fact the temperature has continued to climb nearly one Fahrenheit degree since the mid-twentieth century, indicating that natural factors alone were not in control of Earth's climate. Indeed, for the first time in the history of Earth, other factors affecting climate-human factors-were growing in importance and beginning to overshadow the natural mechanisms.

Climatologists make a useful (albeit somewhat artificial) separation of the factors that cause changes in the climate, into natural and anthropogenic. Natural causes are those that are independent of human activity, whereas anthropogenic causes arise from human activity. It is safe to say that for most of Earth's history the causes of climate change were entirely natural, simply because there were no humans present on the planet. Our human predecessors, various species of the genus h.o.m.o h.o.m.o, first appeared on Earth some three million years ago. As their numbers grew and their technology improved, their impact on Earth and the climate has become increasingly apparent.

In their 2007 Fourth a.s.sessment Report, the IPCC scientists concluded with 90 percent certainty that the rise in temperature in the latter several decades of the twentieth century was attributable mainly to human activities. This ascendancy of the anthropogenic component of climate change, surpa.s.sing the natural drivers, was a subtle and unheralded tipping point in the history of our planet.

THE SECOND TRENCH OF DENIAL.

In the previous chapter I note that there are people skeptical of the instrumental record of Earth's warming over the past century. These skeptics a.s.sert that the record misrepresents the true state of climatic affairs. They have argued that you can't believe the thermometers, or the scientists who deploy them and interpret the readings. This rejection of the instrumental record of rising temperatures was the first trench of denial the climate contrarians dug. They defended that trench tenaciously, but one by one they abandoned it, slowly retreating in the face of overwhelming evidence from human and natural thermometers that Earth was indeed warming.

But these climate contras soon set up camp in a second defensive trench-grudgingly accepting that Earth may be warming, but then arguing that humans have had nothing to do with it. If Earth is warming, they argue, then it must be due to the Sun, or to cyclical changes in Earth's climate a.s.sociated with long-term variability in atmospheric and oceanic circulation. Even though the sunspot count and the direct measurements of solar radiation by satellites during the last half of the twentieth century both trend toward cooling, not warming, the skeptics have marshaled other arguments to support their belief that all climate change is solar in origin. Let's pause to examine some of these contrarian arguments.

Other planets are warming. One argument the contras put forward as "proof" that solar activity is driving climate change on Earth stems from changes observed on other bodies in the solar system. If several bodies are indicating a warming, then surely, according to this line of reasoning, the common cause must be the central element of the solar system-the Sun. The favorite example that the contras cite is the apparent warming of the (former!) planet Pluto by about 3.5 Fahrenheit degrees over the past two decades. The evidence of warming comes from an observed tripling of the atmospheric pressure of Pluto, which implies that some of the nitrogen at the surface of Pluto has evaporated and returned to the atmosphere. But if Pluto-the most distant large body in our solar system-has warmed by 3.5 Fahrenheit degrees because of increased radiant energy from the Sun, then planets closer to the Sun should have warmed even more. In particular, Earth-forty times closer to the Sun than Pluto-should have warmed more than 18 Fahrenheit degrees, an amount clearly far greater than Earth has experienced. If such a solar explanation for the warming of Pluto were true, there would be no ice left on Earth. A better explanation of the warming of Pluto can be found in seasonal effects in Pluto's 250-year orbital journey around the Sun, or possibly changes in Pluto's albedo that have led to less sunshine being reflected from its surface. Similarly, an apparent warming of Mars is almost surely due to fewer dust storms and a more transparent Martian atmosphere. One argument the contras put forward as "proof" that solar activity is driving climate change on Earth stems from changes observed on other bodies in the solar system. If several bodies are indicating a warming, then surely, according to this line of reasoning, the common cause must be the central element of the solar system-the Sun. The favorite example that the contras cite is the apparent warming of the (former!) planet Pluto by about 3.5 Fahrenheit degrees over the past two decades. The evidence of warming comes from an observed tripling of the atmospheric pressure of Pluto, which implies that some of the nitrogen at the surface of Pluto has evaporated and returned to the atmosphere. But if Pluto-the most distant large body in our solar system-has warmed by 3.5 Fahrenheit degrees because of increased radiant energy from the Sun, then planets closer to the Sun should have warmed even more. In particular, Earth-forty times closer to the Sun than Pluto-should have warmed more than 18 Fahrenheit degrees, an amount clearly far greater than Earth has experienced. If such a solar explanation for the warming of Pluto were true, there would be no ice left on Earth. A better explanation of the warming of Pluto can be found in seasonal effects in Pluto's 250-year orbital journey around the Sun, or possibly changes in Pluto's albedo that have led to less sunshine being reflected from its surface. Similarly, an apparent warming of Mars is almost surely due to fewer dust storms and a more transparent Martian atmosphere.

A cosmic ray connection. Another suggestion advanced by the climate contras relates to how the Sun might interact, via an intermediary mechanism, to change the amount of cloud cover over Earth, and thereby change Earth's albedo. This very complex scenario runs along these lines: Earth is perpetually being showered with cosmic rays from s.p.a.ce-streams of charged particles that emanate from the Sun and other nearby stars. The particle stream from the Sun is called the solar wind. Most charged particles are deflected around Earth by our planet's magnetic field, or that of the Sun. But a few leak through the magnetic shield and are thought by some to promote clouds by serving as a "seed" around which water vapor will adhere and nucleate clouds. When the Sun is more active and generates a stronger solar wind, the magnetic shield contracts more tightly around Earth and becomes a better shield. Fewer particles leak into the atmosphere, and therefore there are fewer clouds. Thus a more radiant Sun would lead to lesser cloud cover over Earth, thereby allowing more sunshine to warm Earth's surface. Conversely, when the Sun is quieter, Earth's magnetic field relaxes a bit and allows more charged particles to enter the atmosphere and nucleate more clouds These reflect incoming sunlight back to s.p.a.ce, which in turn will cool Earth's surface. The net result, were this complex scenario actually taking place, is that Earth would warm when the Sun is more active, and would cool when the Sun is quieter. Earth's temperature would rise and fall, tracking the ups and downs in solar activity. To the contras, this represents a possible mechanism that would rea.s.sert solar control of Earth's climate. Another suggestion advanced by the climate contras relates to how the Sun might interact, via an intermediary mechanism, to change the amount of cloud cover over Earth, and thereby change Earth's albedo. This very complex scenario runs along these lines: Earth is perpetually being showered with cosmic rays from s.p.a.ce-streams of charged particles that emanate from the Sun and other nearby stars. The particle stream from the Sun is called the solar wind. Most charged particles are deflected around Earth by our planet's magnetic field, or that of the Sun. But a few leak through the magnetic shield and are thought by some to promote clouds by serving as a "seed" around which water vapor will adhere and nucleate clouds. When the Sun is more active and generates a stronger solar wind, the magnetic shield contracts more tightly around Earth and becomes a better shield. Fewer particles leak into the atmosphere, and therefore there are fewer clouds. Thus a more radiant Sun would lead to lesser cloud cover over Earth, thereby allowing more sunshine to warm Earth's surface. Conversely, when the Sun is quieter, Earth's magnetic field relaxes a bit and allows more charged particles to enter the atmosphere and nucleate more clouds These reflect incoming sunlight back to s.p.a.ce, which in turn will cool Earth's surface. The net result, were this complex scenario actually taking place, is that Earth would warm when the Sun is more active, and would cool when the Sun is quieter. Earth's temperature would rise and fall, tracking the ups and downs in solar activity. To the contras, this represents a possible mechanism that would rea.s.sert solar control of Earth's climate.

However, almost every element of this complex series of feedbacks is conjectural and unsubstantiated. Indeed, the nucleation effect of cosmic rays has not been demonstrated under realistic conditions in the laboratory, and cloud cover over Earth has not been observed to have a strong correlation with variations in the solar wind or cosmic rays in general. This mechanism gets high marks for imagination, but has not earned a pa.s.sing grade in the real world of observations. It is an interesting idea, but there is no evidence to suggest that it actually works.

Natural cycles. The skeptics frequently a.s.sert that the current warming of Earth is the result of "natural cycles." They know that the geological record indicates swings of climate long before humans came to populate the Earth, and they suspect that maybe nature is again at work in the current warming episode. "Isn't today's climate change just one more example of these natural processes at work?" But the logic of this avenue of thinking is partially flawed, because the statement has an implicit but unfounded premise: the only factors influencing climate today are the same ones that have influenced climate in the geologic past. The skeptics frequently a.s.sert that the current warming of Earth is the result of "natural cycles." They know that the geological record indicates swings of climate long before humans came to populate the Earth, and they suspect that maybe nature is again at work in the current warming episode. "Isn't today's climate change just one more example of these natural processes at work?" But the logic of this avenue of thinking is partially flawed, because the statement has an implicit but unfounded premise: the only factors influencing climate today are the same ones that have influenced climate in the geologic past.

This logical flaw can be easily seen with a simple a.n.a.logy. Ask yourself: Did forest fires ever occur before there were people on Earth? The answer, of course, will be yes. Lightning strikes did start forest fires in the distant past. Then ask if that means that all forest fires today occur only because of lightning strikes. At that point the flaw in logic becomes clear: today, in addition to lightning, forest fires also result from arsonists, careless campers, and thoughtless smokers tossing cigarette b.u.t.ts from their cars. The takeaway lesson is that in addition to natural processes there is a new player in the forest fire arena today, the human population.

Credible climate scientists do not limit their inquiry into causes of climate change to those factors active in the prehuman past-they should and indeed do consider the possibility that over time, the causes of climate change may vary. Their task is to understand what has caused climate changes of the past, and what is causing contemporary climate change. The causes may or may not be the same, but scientists must evaluate the role of all possible causes, old and new, to decide which are the most important at a given time. And as the evidence does in fact indicate, human activities overtook natural factors in the twentieth century, to become the dominant force driving climate change today. Nature, long the conductor of the climate orchestra, has been displaced by the human population. In the next chapter we will see the great array of footprints we have placed on planet Earth.

CHAPTER 6.

HUMAN FOOTPRINTS.

I will bless you . . . and multiply your descendants into countless thousands and millions, like the stars above you in the sky, and like the sands along the seash.o.r.e.

-GENESIS 22:17

IPCC scientists in their 2007 a.s.sessment Report concluded that "most of the observed increase in globally averaged temperature since the mid-20th century is very likely (90% probability) due to the observed increase in anthropogenic greenhouse gas concentrations." In other words, according to the IPCC scientists, there are nine chances out of ten that we humans, through our burning of fossil fuels, have been the dominant factor in the warming of the last half century. Ninety percent certainty is an extraordinary statement of confidence in the conclusion-were you to go into a casino and be offered the opportunity to win at any game nine times out of ten, you would surely play with great confidence, and very likely leave with a bundle of cash.

But as certain as the scientists are about the role of humans in the recent climate change, the American public remains less persuaded. In a 2008 Gallup poll, only three out of five Americans believed that the climate was changing, let alone that humans had anything to do with it. The reasons why the American public has been slow to grasp the realities of climate change are many and complex, but certainly include the decades of disinformation and propaganda put out by the fossil fuel industry. Add to that eight years of the George W. Bush administration in Washington, which deliberately fostered additional doubts about climate change by exaggerating the scientific uncertainty and discouraging government climate scientists from speaking out about the causes and consequences of climate change. And there are a number of people simply distrustful of scientists because of the widespread scientific embrace of biological evolution that conflicts with their religious beliefs. So when scientists make p.r.o.nouncements about Earth's changing climate, these same people dismiss the climate science because they don't trust scientists in general. They dismiss the message because of the messenger.

Certainly these industrial, governmental, and philosophical impediments have made it hard to persuade people that we humans have become big players in the climate system. However, other reasons also make it difficult for some people to recognize that the large human population has been driving Earth's climate away from the environmental background in which human society developed and thrived over the past ten thousand years.

Some find it hard to grasp the very concept that the global average temperature of Earth has changed over the past century. Most of us are unaccustomed to thinking at global spatial scales and intergenerational time scales. Whatever setting we are born into is imprinted upon us as normal and unchanging, even if it has experienced something different from the worldwide average and may be in the middle of rapid social, economic, and environmental change. We are not born with global vision or a sense of history. Sensing change over a time interval longer than the characteristic human lifetime requires a well-honed historical awareness and memory, attributes that no one is born with, and which therefore must be acquired.

A second reason why some people have difficulty recognizing a human role in climate change is that in our daily lives we tend to focus on contemporary local concerns. Some of this narrow focus stems directly from our evolutionary history. Only a thousand generations ago, our ancestors' chief occupation was the daily business of finding food in their immediate environment. Successful hunting of wild game and harvesting of natural fruits and grains were skills rewarded by natural selection. Humans did not have the need to know what the local climate would be like a century into the future, or whether there might be an intense drought developing halfway around the world due to an El Nino event in the Pacific Ocean. They were much more concerned with the necessities of the here and now, and had little time or inclination to ponder the abstract world.

Yet another reason why many people do not recognize their role in climate change is that their daily activities are separated from the subsequent effects those activities have on the climate, by both s.p.a.ce and time. It is an abstraction to connect the simple act of increasing the setting of a thermostat in one's home, or driving alone to work each day, to the reality that these activities slowly but steadily increase the absorption of infrared radiation in the atmosphere and warm the planet.

But there is an even more fundamental reason that impedes recognition of the human role in climate change. In the face of hurricanes, tornadoes, tsunamis, earthquakes, and volcanic eruptions, all natural phenomena that can kill thousands of people very quickly, people feel very insignificant and powerless compared to the forces of nature. And indeed, as individuals, we do have very little power. But what people do not appreciate is that, collectively, the almost seven billion people on Earth today, with millions of big machines, are staggeringly powerful and becoming more so every year. It is the sum of activities of billions of individuals-a collective human force far greater than Earth has previously experienced-that is indeed changing Earth's climate.58 Robert F. Kennedy understood this collective power when, in a social context, he said, "Few will have the greatness to bend history itself, but each of us can work to change a small portion of events and in the total of all those acts will be written the history of this generation." Robert F. Kennedy understood this collective power when, in a social context, he said, "Few will have the greatness to bend history itself, but each of us can work to change a small portion of events and in the total of all those acts will be written the history of this generation."

In the remainder of this chapter, I guide you on a tour of our planet, and show you how completely we humans have taken control of Earth-its land, oceans, ecosystems, and, most certainly, its climate.

WHAT DO PEOPLE DO?.

What on Earth have people been doing to push the climate out of equilibrium? One answer can be found in the way humans alter the land they live on, and in so doing change the planetary albedo-the amount of sunshine that is reflected back to s.p.a.ce from Earth's surface. These changes to the land began long before the twentieth century.

The princ.i.p.al human activity that has directly led to changes in reflected sunlight is deforestation, whereby dark forest canopy is replaced by more open, lighter-colored, more reflective agricultural lands. Deforestation had a big head start over other human activities as a climate factor-with a beginning traceable to the human use of fire.

Fire was, and remains, nature's own agent of deforestation. Long before humans appeared on Earth, lightning strikes routinely set forests afire, and the flames burned until a lack of fuel or natural extinguishers-princ.i.p.ally rainfall-eventually limited their spread. The arrival of humans did nothing to slow the burning; quite to the contrary, early humans valued fire as a mechanism to drive and concentrate game, and a way to produce clear s.p.a.ce where they could more easily become aware of nearby predators, and eventually to use it for agriculture. Once early humans discovered the advantages of fire for light, warmth, cooking, and protection, they worked hard to maintain and preserve fire rather than extinguish it. Fire was a friend, not foe, of the early hunter-gatherers.

Humans later "domesticated" fire-they learned to make fire with tools, to have fire when and where they wanted it. Fire, or the heat it generated, found new uses that eventually powered the industrial revolution. Controlled fires boiled water to produce steam for engines, and fires in confined s.p.a.ces gave rise to the internal combustion engine-the burning of fuels within closed cylinders that made gas expand and push pistons to produce mechanical energy. Eventually, however, people began to think of fire not only as a friend, but also as a hazard, to the cities in which they lived and to the forest resource that provided construction material and fuel. Since about the middle of the nineteenth century, our attention has turned to extinguishing fire wherever it occurs unintentionally.

As Earth warmed following the end of the last ice age, the global population was much smaller than today. An estimated few million people were scattered over all the habitable continents, with a population density less than one person per square mile, only 10 percent of the density of present-day Alaska. But farmers these people were not. Everywhere, they remained dependent on hunting skills. And so formidable were these hunting skills that even the small human numbers were able to push the giant woolly mammoths, the mastodons, and the great Irish elk toward extinction.

The warming that followed the Last Glacial Maximum was episodic, but by ten thousand years ago the climate had become similar to what we, the present-day representatives of the human race, have known. Over the next ten millennia, almost to the present day, the climate remained remarkably stable at this new level, an equable condition that fostered fundamental changes in the way of life of humans. Climatic stability enabled the establishment of sustainable agriculture, which in turn provided sufficient food to allow population growth and urbanization, along with the specialized skills that develop in the urban setting. When a subset of the population can produce more food than they personally require, not everyone needs to be a hunter, or gatherer, or farmer.

USING THE LAND.

Following the retreat of the continental glaciers, forests reoccupied the newly exposed terrain, eventually covering about one third of Earth's land area. With the establishment of agriculture, humans began to leave another big footprint on the landscape, cutting or burning the forests, plowing soil, and diverting water.

As successful agriculture supported a larger population, the many uses of timber accelerated deforestation. Not only did forests succ.u.mb to land clearing for agriculture, but increasingly timber also became an industrial commodity used in dwellings, urban construction, and even for roadbeds. There are still places in the world today where wheeled vehicles, motorized or otherwise, roll across the washboard-like surface of tree trunks laid side by side on the ground, mile after mile. Just a few years ago, in a visit to the temperate rain forest of Chile, I experienced such a roadbed, with its rhythmic staccato vibrations similar to those that accompany travel on a corrugated gravel road.

Another use of timber led to equally dramatic deforestation. The recognition of the simple fact that wood floated on water stimulated the large-scale building of sailing ships for exploration, colonization, trade, piracy, and political and military advantage. The Phoenicians, Romans, and Vikings sailed long distances in substantial wooden ships. The European powers of the Middle Ages became the first pract.i.tioners of globalization, sending vessels around the world to disseminate Christi anity and acc.u.mulate wealth. The 1571 eastern Mediterranean battle of Lepanto, between the European Holy League and the Ottoman Turks, and the failed invasion of England by the Spanish Armada near the end of the sixteenth century both involved hundreds of naval vessels constructed of prime timber, each requiring thousands of mature trees. The forests of Europe no longer seemed infinite.

When the Europeans arrived in North America, about 70 percent of the land east of the Mississippi River was forested. By the end of the nineteenth century that had been reduced to around 25 percent. Much of the landscape was literally stripped naked. Wood was used for nearly every endeavor in the growing nation-for the barges and channels and locks of the inland ca.n.a.l system; for the ties, trestles, and rolling stock of the national railways; for the fences that demarked property; for the telegraph and telephone poles that enabled early telecommunication; and eventually for paper. Photos of the state capital in Vermont toward the end of the nineteenth century show the extent that the hills surrounding Montpelier had been denuded. Such scenes were widespread across North America-almost the entire forest cover of Michigan succ.u.mbed to logging, and to fires that often followed close on the heels of careless lumbering practices.

Today, deforestation remains very active in many parts of the world. The tropics are being particularly hard hit, with large areas of Brazil, Indonesia, and Madagascar being subjected to relentless clear-cutting. Half of the world's tropical and temperate rain forests are now gone, and the current rate of deforestation exceeds one acre per second. That is equivalent to cutting down an area the size of the state of Mississippi each year. But in places such as the eastern states of America, where deforestation was rampant in the nineteenth century, the forests are returning, as other materials have replaced wood in much of the modern economy of the region. The recovery in the eastern states, however, is far from complete-today's second-growth forests cover only 70 percent of the pre-colonial distribution.

How do changes in the forest cover affect climate? Deforestation generally changes the color of Earth's surface from dark green to lighter brown, thus causing more sunshine to be reflected back to s.p.a.ce rather than warming the planet's surface. Countering this slight cooling, however, is the much more significant effect of the cutting and burning of trees itself. In the natural state of affairs, living trees pull the greenhouse gas carbon dioxide (CO2) from the atmosphere in the process of photosynthesis, and dead trees decay, liberating CO2 and returning it to the atmosphere-an atmospheric equilibrium established by pulling out and pumping back equal amounts of CO and returning it to the atmosphere-an atmospheric equilibrium established by pulling out and pumping back equal amounts of CO2. But rapid and large-scale deforestation upsets that equilibrium-the loss of trees decreases photosynthesis, leaving more CO2 in the atmosphere. And when deforestation occurs by burning, it returns CO in the atmosphere. And when deforestation occurs by burning, it returns CO2 to the atmosphere far faster than new trees can grow and remove it. In sum, deforestation leads to warming of the atmosphere. to the atmosphere far faster than new trees can grow and remove it. In sum, deforestation leads to warming of the atmosphere.

HUMAN NUMBERS GROW.

The past ten millennia have generally been good times for us humans, and we have multiplied at a breathtaking pace. At 6.8 billion and growing, the human population today is more than a thousand times bigger than it was at the end of the last ice age, some 10,000 years ago. But the growth of population has not been steady over that time-it has accelerated dramatically in recent centuries.

Multiplying a number by one thousand is almost the same as doubling that number ten times. The concept of ten doublings is a good approximation of the growth of Earth's human population from the last gasps of the ice age, when the population was around four million people, up to almost the present day. The growth began slowly-the first, second, and third doublings together required more than six thousand years, an interval of time that began when humans first began to congregate in villages and ended not long after the construction of the Great Pyramids of Egypt. The fourth doubling required a thousand years, and the fifth, only five hundred. The sixth doubling began when Rome ruled the West and the Han Dynasty the East, and ended as Europe entered the Dark Age. The seventh doubling took place in the seven hundred years between 900 and 1600, slowed by the Black Plague, which killed a quarter of the global population in the fourteenth century. The seventh doubling ended just as European explorers were circ.u.mnavigating the globe and claiming colonial territory in the New World. The eighth doubling, occurring in the two hundred years between 1600 and 1800, encompa.s.sed the creation of the United States of America, and carried the global population to the landmark statistic of one billion human inhabitants.

An extraordinary change in technology also occurred during the eighth doubling: the discovery of how to access the fossil energy contained in coal. No longer would humans rely solely on wood for heat or flowing water for industrial power. Spurred on by the abundant energy in coal, the world population underwent its ninth doubling in only 130 years, to reach two billion by 1930, in spite of the Napoleonic Wars, World War I, and a virulent flu pandemic. The tenth doubling occurred between 1930 and 1975, overcoming the effects of World War II and three subsequent Asian wars. Those ten doublings took Earth's population from four million around ten thousand years ago to four billion in 1975, and the doubling interval shrank from twenty or thirty centuries to fewer than five decades. The eleventh doubling, now under way, from four to eight billion, will be achieved around 2025.

As I write in early 2009, the global population is at 6.8 billion. Were a person to be born each second, and if no one ever died, it would take more than 215 years to populate Earth with 6.8 billion people. The current rate of population growth is more than a million people each week, the result of more than 4 births, offset by fewer than 2 deaths, each second. At that rate, Earth's population grows by the addition of a Philadelphia or a Phoenix each week, a Rio de Janeiro each month, and an Egypt each year.

A doubling of Earth's population, a process that once required a few thousand years, today takes place in less than fifty. The human footprint on the planet is increasingly apparent simply due to the sheer number of people on Earth today. One cannot fully understand the changes in the global environment under way outside the context of the dramatic population growth of the last few centuries.

PEOPLE AND MACHINES.

Since the end of the last ice age, humans have grown not only in numbers, but also in technological skill and resource consumption. In only a thousand generations, they have moved from human power to horsepower, at first literally and later with machines that amplified the strength of humans and their domesticated beasts of burden. These machines have enabled us to travel far faster than we or horses can run, carry far more than the capacity of backpacks or saddlebags, dig far deeper in the soil than shovels, hoes, or plows can reach, and kill far more people faster than clubs, spears, or arrows could ever accomplish.

For much of the industrial revolution, the rate at which humans use energy was measured in horsepower-a throwback to one of the animals that humans domesticated for agriculture and transportation. That unit of energy expenditure remains in common use in the automobile industry, where the power of engines is still rated in horsepower. James Watt, the developer of one of the first commercial steam engines, wanted a way to compare the work his engine could accomplish to the power output of the more familiar workhorse. Watt estimated the lifting that one horse could accomplish in bringing coal out of a mine. He determined that a horse could, using ropes and pulleys, lift a ton of coal up a mineshaft fifteen feet each minute, which, when expressed in the more common terms for the rate of energy use, is about 750 watts.59 This is about the power required for a small microwave oven or s.p.a.ce heater. The kilowatt-hour is the common unit of electricity consumption, which translates into using electricity at a rate of a thousand watts for one hour, or a little more than one horsepower for an hour. In my home, my family and I consume around twenty-four kilowatt-hours of electrical energy each day, which is the equivalent of having a horse working around the clock. This is about the power required for a small microwave oven or s.p.a.ce heater. The kilowatt-hour is the common unit of electricity consumption, which translates into using electricity at a rate of a thousand watts for one hour, or a little more than one horsepower for an hour. In my home, my family and I consume around twenty-four kilowatt-hours of electrical energy each day, which is the equivalent of having a horse working around the clock.

Of course, we use much more energy in our daily lives than just electrical energy. There is natural gas used to heat my home, gasoline used in the car I drive to and from work, and energy used in my workplace. Energy is also used for manufacturing, bulk transport of goods, agriculture, and much more. Effectively we all have many more horses working for us. Worldwide, the per capita rate of energy consumption is about 2,600 watts-that is, about 3.5 horsepower for every man, woman, and child on the planet, or the energy equivalent of a global population of workhorses numbering almost 25 billion. And the rate of energy consumption is hardly stable-to the contrary, it increased sixteenfold during the twentieth century alone.

Surely many people in developing countries would welcome the news that they have three and a half horses working for them. But of course the global average rate of energy consumption is deceiving-many people have no horses at all working for them, and some others have a stable full. In the United States, the 300 million residents, about 4 percent of the world's population, account for 20 percent of the global energy expenditure. That amounts to more than fifteen horses working for every single American.

Richard Alley, a well-known climate scientist at Penn State University, has carried the horse a.n.a.logy further. He points out that the carbon emitted from the combustion of fossil fuels is in the form of the greenhouse gas carbon dioxide (CO2). This gas is colorless, odorless, and tasteless, and so its presence in the atmosphere is not easily detected with our human sensory organs. But Alley asks us to imagine how different our att.i.tude toward this important source of global warming would be if the carbon were emitted not as an invisible gas but rather as horse manure that acc.u.mulated ankle deep over the entire land surface. That would certainly get our attention in ways that CO2 in the atmosphere does not. in the atmosphere does not.

PLOWING AND BUILDING.

Deforestation was only the beginning of human interactions with the natural Earth. Once people cleared the land for agriculture and towns, they put sticks, bones, spades, plows-and later tractors, bulldozers, steam shovels, and ma.s.sive excavators-to work. As earth-movers, humans showed what they could do, and they could do a lot,60 century after century. century after century.

The seeds of agriculture were first sown some nine thousand years ago, as villages became established and nomadic life gave way to a more rooted, sedentary social structure. About 2.5 acres of crop and pasture-land were required to feed a person for a year then, and it is not much less even today. Every year, the loss of topsoil a.s.sociated with tilling the land, at least until the adoption of soil conservation measures in the mid-twentieth century, amounted to about ten tons for each person, or about the volume of ten human graves for each person fed by agriculture. What has changed dramatically, of course, is the number of people to feed. With the global population nearing seven billion people, we lose on average about three inches of soil to erosion every century over all the farm and pasture land of the world, an area close to 40 percent of Earth's ice-free land surface.

As people developed quarrying and mining, both for raw materials and energy, they dislodged more and more earth. With urbanization also came the need for water for the growing population, thus leading to the excavation of ca.n.a.ls and the construction of aqueducts. Political and economic control required road and wall building-the Romans paved nearly two hundred thousand miles of roads and highways, and built Hadrian's Wall seventy-five miles across the north of England as a defense against the unwilling-to-be-governed Scots. The Chinese built the Great Wall-actually a series of walls-stretching for some four thousand miles across northern China to defend against Mongol raiders. Great monuments, such as the pyramids of Egypt, and less grandiose but widespread burial mounds const.i.tuted ma.s.sive construction projects.

In the modern world, the scale of our human a.s.sault on the landscape is no less profound. Coal mining, always a hazardous operation underground, surfaced with the discovery of widespread coal deposits with just a thin veneer of soil covering them. Surface strip-mining increased the volume of coal, rock, and soil moved by a factor of ten or more compared with underground operations. Today in the Powder River Basin of Wyoming, gigantic machines claw into the thick coal seams, delivering load after load of coal to waiting railway hopper cars. Mile-long railway trains leave the mines every twenty minutes. Over a year the trains could form a belt that completely encircles Earth. These trains snake across the prairies of Nebraska in an unending stream, slowly diverging to other tracks that fan the delivery of coal to electrical power plants in the eastern and southern states. And the pits of Wyoming-the residual scars of strip mining-grow larger.

In the east, coal mining continues in Pennsylvania and West Virginia. The long-ago geological collision of Africa with North America folded the coal seams, along with the other sedimentary layers, into the beautiful valley-and-ridge topography of the Appalachians. Erosion along the crests of the ridges has brought the coal seams closer to the surface, but not quite unburdened them to full exposure. But that is no insurmountable problem to coal mining today-just move in with dynamite and earthmovers, sc.r.a.pe off the mountaintops until the coal is exposed, and strip the coal away.61 The overburden, as geologists call the rock that sits between the coal and the surface, is unceremoniously dumped into the adjacent valleys, where it destroys forests on the mountain slopes and causes flooding in the streams occupying the valleys below. The very name of this process-mountaintop removal-expresses both the power and hubris of this human endeavor. The overburden, as geologists call the rock that sits between the coal and the surface, is unceremoniously dumped into the adjacent valleys, where it destroys forests on the mountain slopes and causes flooding in the streams occupying the valleys below. The very name of this process-mountaintop removal-expresses both the power and hubris of this human endeavor.

Giant earth excavator62

ALL OF THESE changes brought to the landscape by industrious people are best understood and more fully appreciated only when they are compared with natural processes that move earth around. When compared to nature's ability to move sediment and rock, is the human impact trivial or enormous? This question fascinated Bruce Wilkinson, one of my geology colleagues at the University of Michigan for many years. Bruce is not a dapper tweed-attired professor-he is a gritty field geologist, always in Levi's; with his big voice, he is never afraid to speak truth to power or call attention to a naked emperor. Not surprisingly, Bruce approached the question of whether human earth-moving was significant with geological logic. He reasoned that the long-term record of sediment erosion and transport can be found in the sediment deposits that have acc.u.mulated on the ocean floor, and in the sedimentary rocks of past eras on the continents. He calls this the "deep-time perspective."63 It involves the careful estimation of sediment volumes: in the deltas of all the rivers of the world, on the continental shelves, on the deep ocean floor, and in the ancient sedimentary rocks now stranded on the continents. It involves the careful estimation of sediment volumes: in the deltas of all the rivers of the world, on the continental shelves, on the deep ocean floor, and in the ancient sedimentary rocks now stranded on the continents.

Wilkinson's calculations showed that over the past five hundred million years, natural processes of erosion have on average lowered Earth's land surface by several tens of feet each million years. When he next calculated the present-day rate of erosion, the result was startling-humans are moving earth today at ten times the rate that nature eroded the planetary surface over the past five hundred million years. Perhaps more alarming is the erosion rate in the places where the erosion is actually occurring. On land used for agriculture, soil loss is progressing at a rate almost thirty times greater than the long-term worldwide average of natural erosion.

Not only is soil erosion much more rapid than in the geological past, but the rate of soil loss also far exceeds the rate at which new soil is produced. 64 64 In the same way that our consumption of petroleum far exceeds the pace at which nature makes it, and our withdrawal of groundwater far exceeds the rate at which nature recharges aquifers, the human practices that lead to the loss of agricultural soil are effectively "mining" the soil, using up a resource of finite extent. As humorist Will Rogers once noted, "They're making more people every day, but they ain't makin' any more dirt." David Montgomery of the University of Washington estimates that almost a third of the soil capable of supporting farming worldwide has been lost to erosion since the dawn of agriculture, with much of it occurring in the past half century. In the same way that our consumption of petroleum far exceeds the pace at which nature makes it, and our withdrawal of groundwater far exceeds the rate at which nature recharges aquifers, the human practices that lead to the loss of agricultural soil are effectively "mining" the soil, using up a resource of finite extent. As humorist Will Rogers once noted, "They're making more people every day, but they ain't makin' any more dirt." David Montgomery of the University of Washington estimates that almost a third of the soil capable of supporting farming worldwide has been lost to erosion since the dawn of agriculture, with much of it occurring in the past half century.65 Once the land surface is plowed for agriculture, or opened to livestock grazing, wind has easier access to dust to blow around. The blowing dust is dropped eventually, and some falls into lakes and the ocean. The amount of dust acc.u.mulating in lakes of the western United States has increased by 500 percent during the past two centuries, an increase attributable to the expansion of livestock grazing following the settlement of the American West.66 Blowing dust travels the world. Satellite photos show huge clouds of dust billowing out of the Sahara Desert in northern Africa, in giant plumes that spread westward over the Atlantic Ocean. From China, similar clouds head eastward across the Pacific, but unlike the clouds from the uninhabited Sahara, the clouds emanating in China are not just dust-they include industrial pollution that the winds carry all the way to the western coast of North America. The atmosphere, by distributing industrial waste and unintentional erosion from agriculture, is an effective agent of globalization-the globalization of pollution.

Blowing dust and soot from diesel engines, cooking fires in rural undeveloped areas, and the burning of gra.s.slands and forests are also having their effects on climate, both regionally and globally. In the same way that ash from large volcanic eruptions blocks sunshine from reaching Earth's surface by making the atmosphere less transparent, dust and soot also dim the Sun, at least as it is seen from Earth. But the dark soot particles in the atmosphere also absorb some of the sunlight reflected back to s.p.a.ce from Earth's surface, thereby trapping energy in the visible wavelengths of the solar spectrum just as greenhouse gases absorb some of the infrared wavelengths. The dust and soot also lead to accelerated melting of snow and ice around the world, by darkening the white surface ever so slightly. The darkening causes less sunlight to be reflected back to s.p.a.ce and more solar heat to be absorbed by the dust and soot, thereby further increasing the melting of the snow and ice.

FLOWING WATER.

It is not just the land that people have changed-they have had equally dramatic effects on the water. The development of agriculture and urbanization could not have proceeded without the parallel development of water resources. The roots of hydraulic engineering date back almost six thousand years, and these special skills developed independently in many locations. In ancient Persia, large underground conduits called qanats carried water from the highlands to the arid plains. People built levees to stabilize river channels, and ca.n.a.ls to carry water to fields for irrigation along the banks of the Nile in Egypt, the Indus in Pakistan, and the Yellow River in China. In the Fertile Crescent of Mesopotamia, water management along the Tigris and Euphrates reached high levels of sophistication thousands of years ago.

Following the retreat of the last continental ice sheets, many areas of North America and Europe were dotted with small lakes, marshes, and swamps. And as sea level rose following deglaciation, the estuaries of rivers extended farther inland, creating additional wetlands. Agriculture demands, land development pressures, and public health concerns led to the draining of wetlands. The District of Columbia, seat of the United States government, was originally a malarial swamp, as was much of southern Florida. The eradication of malaria in the United States was a singular public health achievement resulting from the draining of wetlands.

Today, half of the wetlands that existed around the world only ten thousand years ago are gone. Although there have been some genuine benefits a.s.sociated with wetland drainage, there have also been losses. When wetland drainage began in earnest, little was known of the many services wetlands provided-their importance in the ecology of wildlife and the role they played in water purification and as a buffer against storm surges in coastal areas. And the a.s.sault on wetlands is not over-in the United States, wetland loss continues at a rate of one hundred thousand acres every year.

We have left our imprint on the lakes and rivers of the world as well. The Aral Sea, a once huge inland body of water situated along the border between Kazakhstan and Uzbekistan, in central Asia, was only half a century ago high on the list of the world's largest lakes, surpa.s.sed in area only by the upper Great Lakes of North America and Lake Victoria in Africa. Today the Aral Sea has almost disappeared, reduced to a scant 10 percent of its former area.67 The shrinking has not been due to long-term climatic cycles that sometimes lead to lake fluctuations elsewhere. No, the Aral Sea has been the victim of water diversion from the two princ.i.p.al rivers that feed it, water diverted to irrigate cotton planted in the desert of Uzbekistan. Diversion ca.n.a.ls began taking water away from the Aral Sea following World War II, and by 1960 the lake level began to fall-almost a foot each year in the 1960s, but tripling by the end of the century as withdrawals grew. The shrinking has not been due to long-term climatic cycles that sometimes lead to lake fluctuations elsewhere. No, the Aral Sea has been the victim of water diversion from the two princ.i.p.al rivers that feed it, water diverted to irrigate cotton planted in the desert of Uzbekistan. Diversion ca.n.a.ls began taking water away from the Aral Sea following World War II, and by 1960 the lake level began to fall-almost a foot each year in the 1960s, but tripling by the end of the century as withdrawals grew.

As the lake level fell and the water volume diminished, the remaining water became more saline, in much the same way that Great Salt Lake of Utah became saltier as it shrank from its glacial melt.w.a.ter maximal extent. Today what remains of the Aral Sea has a salt concentration ten times its pre-diversion salinity, and three times the salinity of ocean water. The fishing industry of the Aral Sea, formerly producing one sixth of the fish of the entire Soviet Union and employing tens of thousands of people, disappeared with the water. Abandoned fishing boats now sit motionless on a sea of sand.68

RIVERS HAVE FROM the earliest days of human settlement been favored places for villages. They provided domestic and agricultural water, avenues of transportation, and power for industrialization. The human tendency to "control and improve" nature led to the construction of dams on many rivers. Today, some fifty thousand large dams and many smaller ones have altered the natural flows of rivers virtually everywhere. No major river in the United States has escaped damming somewhere along its course; not a single one flows unimpeded from headwaters to the sea.

In the spring of 1953, when I was in high school in Omaha, a major flood began to build farther up the Missouri River as heavy snow melt and ice dams raised this great river to flood stage. As the crest approached Omaha, it became apparent that it might top the levees and floodwall that protected the lower levels of the city, including the airport. The call went out for volunteer manpower to sandbag the levees. Cla.s.ses be d.a.m.ned-this was an opportunity for students to get involved in some excitement, and so many of my friends and I were soon patrolling and reinforcing the levees as the water crept upward. When the crest was only hours away, it became obvious that it might top the floodwall, so emergency construction began to place flashboards atop the floodwall-and they proved essential. The river crested two feet above the floodwall, but was held back by the flashboarding. Omaha was saved from a major flood, ironically the last flood ever to roll down the upper Missouri River Valley. Soon after, six large dams were constructed upriver, in the Dakotas and Montana, that have ended the Missouri River's free flowing days.

Later in my professional career as a geologist, I rafted the length of the Grand Canyon of Arizona, a chasm cut by millions of years of erosion by a turbulent Colorado River. To every serious geologist, the Grand Canyon is an obligatory pilgrimage, a geological Mecca. When first explored by John Wesley Powell, an early director of the U.S. Geological Survey, in the 1860s, the Colorado River was free-flowing, with treacherous rapids and large seasonal changes in volume. Today the river that carved the greatest canyon in the world is just a controlled-flow connection between the huge Glen Canyon and Hoover dams, two of the five large dams on the Colorado. In the Grand Canyon, the Colorado River predictably rises and falls on a daily basis, as water is released from the Glen Canyon Dam each day. Ironically, even as the flow has steadied, some of the rapids have become even fiercer. Within the canyon, where tributaries join the Colorado, they continue to deliver debris that no longer is moved downstream by an annual spring flood on the Colorado. The result is a growing fan of boulders spilling into a narrowing river channel-a sure recipe for enhancement of rapids.

A few years ago I traveled on a small boat down the Douro River in Portugal.69 The Douro arises in northern Spain (where it is called the Duero) before crossing into Portugal on its way to the Atlantic. The stories of the early navigators of the Douro tell of cataracts, turbulence, and danger. Much to my surprise, the Douro today is little more than five narrow flat lakes behind five large dams. Whatever current exists today is due to controlled flow through the locks and spillways of the dams. The Douro arises in northern Spain (where it is called the Duero) before crossing into Portugal on its way to the Atlantic. The stories of the early navigators of the Douro tell of cataracts, turbulence, and danger. Much to my surprise, the Douro today is little more than five narrow flat lakes behind five large dams. Whatever current exists today is due to controlled flow through the locks and spillways of the dams.

Dam construction is hardly a thing of the past. The Itaipu Dam, on the Parana River of South America, became operational in 1984. When it opened it was the largest in the world in terms of electrical generating capacity. Itaipu's electricity output has since been overtaken by the ma.s.sive Three Gorges Dam on the Yangtze River in China, already impounding water and scheduled for full operations in 2012. Three Gorges is only the latest of major dam construction projects on all the princ.i.p.al rivers of southern Asia, and China has plans for another dozen in the upper reaches of the Yangtze.

Dams and diversions so diminish the flow of major rivers that some rivers barely reach the sea. The apportionment of Colorado River water between the United States and Mexico (the river's mouth is in Mexican territory) was decided on the basis of measured river volumes of the early 1920s. Unappreciated at the time was the fact that the river volumes then were at historic highs, not to be seen again in the rest of the twentieth century. Today, after the allotted withdrawals from the Colorado River in the United States, there is little water left for Mexico, and in some years not a drop of water flows out of the mouth of the Colorado into the Gulf of California.

The situation is not very different on the Ganges or the Nile, where the flows at the river mouths have been reduced to just a trickle. The fertile Nile Delta, long the princ.i.p.al source of food for much of Egypt, is being slowly reclaimed by the Mediterranean Sea because so little sediment is carried by the weakened Nile to replenish the soil of the delta. Farther upstream, where the Nile now flows slowly, and in the giant reservoir behind the great dam at Aswan, where it hardly flows at all, quiet backwaters have been created that have allowed schistosomiasis, a debilitating parasitic infection, to flourish in places it never appeared before. Schistosomiasis is second only to malaria in terms of tropical diseases that afflict humans.

The sediment load is not the only burden moved along by rivers. They also carry heavy chemical loads, picked up from industrial pollution, inadequate sewage treatment systems, increased runoff from the impervious surfaces of urban areas, and the fertilizers and pesticides used in agriculture. More manufactured nitrogen fertilizer is spread over agricultural lands than is provided by the entire natural ecosystem. Although the sediment load is slowed in its pa.s.sage to the sea by the presence of dams, the chemical load carried by rivers is virtually unaffected by those barriers. Effectively, the chemical waste streams of entire continents are flushed into the sea.

This flux of chemicals to the sea is increasingly producing "dead zones" in the oceans, regions of the ocean floor devoid of all but microbial life forms. The delivery of fertilizers to the sea promotes the growth of algae in the surface waters, but when these organisms die, they fall to the seafloor, where they fuel microbial respiration. The dissolved oxygen in the bottom water is depleted by the microbes, and is therefore unavailable for other bottom-dwelling marine creatures, princ.i.p.ally fish and mollusks, that need oxygen.

Dead zones are appearing all over the globe-in Chesapeake Bay; the Gulf of Mexico; the Adriatic, Baltic, and Black seas of Europe; and along the coast of China. The number of dead zones, now in excess of four hundred, has roughly doubled every decade since the 1960s. In aggregate they now cover an area approaching one hundred thousand square miles,70 about the size of the state of Michigan. about the size of the state of Michigan.

WATER UNDERGROUND.

Large numbers of people in the United States drink water that is pumped from underground. And not all of these people live in rural areas-many cities also draw some of their munic.i.p.al water from wells. Almost 40 percent of the public water supply in the United States is pumped from subsurface aquifers, and nearly all residential water in rural areas is drawn from a well. But domestic use of groundwater represents the lesser call on underground water-twice as much is pumped for agricultural irrigation in the areas of the United States where precipitation is inadequate, at least for the crops selected for cultivation. The regions most dependent on groundwater for agriculture are in California and the Southwest (where there is a great use of the surface water as well) and in the vast Great Plains of Montana, the Dakotas, Nebraska, Kansas, Oklahoma, Texas, New Mexico, and Colorado.

On the Great Plains, the 100 meridian of west longitude is the dividing line for irrigation-to the east there is usually sufficient winter snow and summer rain to keep the soil moist enough to support agriculture naturally. West of longitude 100 the crops need some help from irrigation. Fortunately, thick layers of sand and gravel, representing millions of years of waste and wash from the eroding Rocky Mountains, lie beneath the surface of the Great Plains. The never-ending compet.i.tion between tectonic uplift of the mountains and the erosive power of rain, snow, and ice produces vast amounts of debris, carried eastward by streams and rivers meandering across the plains. And within the buried deposits of sand and gravel, water fills the pore s.p.a.ces between the grains, water that had not seen the light of day for many thousands of years-until mechanized agriculture came to the Great Plains in the twentieth century.

The prairie pioneers overturned the sod and dug wells to withdraw water from the saturated aquifers beneath them. The first mechanical boost to pumps came from windmills, which took advantage of the wind that came "sweeping down the plain." Rural electrification provided steady energy independent of the vagaries of the wind, and soon pumps of higher capacity were pulling more water from the aquifers below.71 The best known of these buried aquifers is called the Ogallala, the name of a small town in western Nebraska. More than a quarter of the irrigated land in the United States sits atop this aquifer. The best known of these buried aquifers is called the Ogallala, the name of a small town in western Nebraska. More than a quarter of the irrigated land in the United States sits atop this aquifer.

Withdrawals of water from the Ogallala aquifer have been at a far faster rate than nature has been able to recharge it-leading to a drop in the subsurface water table. In areas of intense irrigation, such as in southwestern Kansas and west Texas, the water table has been dropping several feet each year, requiring wells to be deepened just to reach the water. In some places the water has been exhausted. Effectively, the water that had been in place since the end of the last ice age has been "mined out." With the draining of this aquifer at a rate equal to some fifteen Colorado Rivers each year, natural recharge cannot keep pace-the groundwater in reality is a non-renewable resource.

WILL WE TAKE IT ALL?.