Yet another measure of our footprint on Earth is the fraction of the total plant growth on the continents that we have appropriated to serve our needs for food, clothing, shelter, and other necessities and amenities of life. Before humans became a presence on Earth, the fraction appropriated was zero. But today, with more than 6.8 billion people occupying the continents, the fraction is certainly not zero. This measure of the human dominance of the land-based ecosystems is variously estimated at between 20 and 40 percent of net primary production,72 an astounding number in spite of the uncertainty. an astounding number in spite of the uncertainty.

The appropriation of resources for human use is not restricted to the land. In the oceans, an estimated 90 percent of the large predator fish present in the oceans a half century ago are gone. Although there are ample dry statistics to make this point, none is quite so visual as an archive of tourist photos from Florida showing the size of the catch worth bragging about, progressing through the twentieth century. Decades ago the catch worth posing for was as big as the fisherman, but through the years photos reveal smaller and smaller fish, with today's trophy catch seldom exceeding three feet.

Three quarters of the marine fisheries around the world are now fished to capacity or overfished. Thirty percent of the fisheries have collapsed, a term defined to indicate a depletion greater than 90 percent of the recorded maximum abundance.73 c.u.mulative catches of the global fishing fleet reached a maximum in 1994, and since have declined by 13 percent, despite large increases in the fleets and their range of operations. A well-recognized indicator of marine resource depletion is the fact that the fish catch is declining despite many efforts to increase it. c.u.mulative catches of the global fishing fleet reached a maximum in 1994, and since have declined by 13 percent, despite large increases in the fleets and their range of operations. A well-recognized indicator of marine resource depletion is the fact that the fish catch is declining despite many efforts to increase it.

Human depletion of other resources can be recognized in the same way. Petroleum production in the United States reached a peak in the mid-1970s and has declined ever since, in spite of the fact that the petroleum industry has intensified exploration and has been motivated by substantial increases in the price of oil and natural gas. Worldwide, the production of petroleum will likely reach a peak in the next decade or two.

LEAVE ONLY FOOTPRINTS.

As tourists, we have been urged to "take only pictures, leave only footprints" in order to preserve the natural and historical sites we visit. But the societal footprint we are leaving on Earth will win us no prizes for environmental stewardship. Species are disappearing from the global ecosphere at rates about a thousand times faster than only a millennium ago. A 2008 status report on the world's 5,487 known species of mammals showed that more than one fifth face extinction, and more than half show declining populations, largely because of declining habitat and hunting on the land, and overfishing, collisions with boats and ships, and pollution in the sea.74 Gus Speth, the dean of the Yale School of Forestry and Environmental Studies, says that "Earth has not seen such a spasm of extinction since 65 million years ago," when the dinosaurs and many other species disappeared following the collision of an asteroid with Earth. Gus Speth, the dean of the Yale School of Forestry and Environmental Studies, says that "Earth has not seen such a spasm of extinction since 65 million years ago," when the dinosaurs and many other species disappeared following the collision of an asteroid with Earth.75 We humans, in addition to manufacturing chemicals that impact the land, air, and waters of Earth, also produce heat, sound, and light, each with its own environmental consequences. It is well known that the buildings, roads, and roofs of a city absorb the heat of the day, and rera diate it through the night, in amounts greater than do the surrounding natural areas. Even the ground beneath the city has warmed substantially, as the warm footprint of heated buildings has replaced the cold air of winter at the ground surface. I recall that when I first started research into underground temperatures, my colleagues and I drilled a borehole near the laboratory for testing our instruments. The very first measurements of the temperatures down this hole surprised us-they showed that the soil next to the building had warmed almost ten Fahrenheit degrees since the building was constructed many decades earlier.

If the gradual warming of cities is a subtle manifestation of urbanization, the generation of light is an obvious one. Driving at night on the highways of America, cities glow on the horizon like giant navigational beacons. And air pa.s.sengers flying cross-country on a clear night see an illuminated panorama of the cities, towns, and isolated rural homes. The activities of people extend well into the hours of natural darkness, all made possible by our relatively newfound ability to "domesticate" light, just as we learned to domesticate fire thousands of years ago. The increase in urban illumination has long been known to impair the work of astronomers, who find once-visible stars gradually disappearing in an ever-brighter sky. And it deprives urban dwellers of one of the most beautiful experiences of all-viewing the millions of stars in the nighttime sky, visible only in areas free of light pollution.

I recall camping in the middle of the Kalahari Desert while engaged in some geophysical fieldwork in Botswana. In the dark of night-and it was truly dark-I found nothing so exquisite as gazing upward to see the majesty of a sky filled with uncountable stars, distant points of illumination in every direction, as far as the eye could see. In the Book of Genesis, G.o.d declares that he will "multiply your descendants into countless thousands and millions, like the stars above you in the sky." What a tragedy that so many people live their entire lives without ever experiencing this image.

But light pollution affects more than astronomers and city dwellers.76 The other living creatures of the night, those we call nocturnal, display evolutionary behavior keyed to the existence of darkness. Some bats have become urbanized because their favorite insects swarm around urban light sources. And nocturnal mammals-many rodents, badgers, and possums-have become more vulnerable to predators because of their increased visibility at night. Some species are tuned to the longer-term variations in light and darkness that accompany seasonality. Many birds breed when daylight reaches a certain duration. If the nights are shortened artificially, and the days thereby apparently lengthened, the birds mistakenly think it is time to breed. Unfortunately, many of the grubs and insects that will nourish the hatchlings have not received the message that they should also advance their breeding cycle. The other living creatures of the night, those we call nocturnal, display evolutionary behavior keyed to the existence of darkness. Some bats have become urbanized because their favorite insects swarm around urban light sources. And nocturnal mammals-many rodents, badgers, and possums-have become more vulnerable to predators because of their increased visibility at night. Some species are tuned to the longer-term variations in light and darkness that accompany seasonality. Many birds breed when daylight reaches a certain duration. If the nights are shortened artificially, and the days thereby apparently lengthened, the birds mistakenly think it is time to breed. Unfortunately, many of the grubs and insects that will nourish the hatchlings have not received the message that they should also advance their breeding cycle.

The biological rhythms of many species are tied to the daily cycles of light and darkness-these are called circadian rhythms. Because daylight provides the setting for work, the human body itself has evolved to use nighttime for sleeping. Alaskan, Scandinavian, and Russian hotels provide dark window shades to simulate darkness for summer tourists. Some geologists working summers at polar lat.i.tudes with twenty-four hours of daylight find that there is a real tendency to work too long, which leads to inadvertent fatigue and a greater susceptibility to accidents and illness.

No one needs to be reminded that populated areas are noisy areas: the sounds of vehicles throughout the day and night; of construction equipment digging holes, driving piles, and moving earth; of airplanes pa.s.sing overhead; of motorboats, jet skis, and snowmobiles; of loudspeakers, radios, and televisions blaring. We go into the woods or the countryside for "a little peace and quiet," to leave behind the noisy urban environment.

But it may surprise you to learn that the waters of the oceans are not quiet sanctuaries free of anthropogenic noise. To be sure, the cacophony of the cities is absent, but sound travels very efficiently through ocean water, so that no place beneath the surface of the oceans is free of industrial sound. The cranes and conveyors that load and unload maritime cargo, the clanking of anchor chains dropping or lifting, the hum of diesel engines, the slow churning of ma.s.sive propellers pushing ships through the sea, the pinging of sonar depth-finder systems, the underwater air guns and explosives used in seismic exploration of the sea bottom, naval training exercises with depth charges-the list of man-made sounds in the ocean is virtually endless. There is no place in the oceans-no place at all-where a sensitive hydrophone on the seafloor will not detect sounds of human origin.

Because so much of the ocean below the surface is dark, marine creatures often rely on sound for communication and navigation. The human noises have become distracting and disorienting, and in some cases of nearby noise, physically debilitating. Migration routes and breeding habits of marine mammals have changed in response to the geography and intensity of the noise-they, too, seek peace and quiet in the ocean. The ma.s.s grounding of whales and dolphins has on occasion been linked to high-intensity military sonar exercises in the vicinity. The U.S. Navy has acknowledged the "side effects" of this activity, but when challenged in court to end the noise-making, the navy argued that the necessity of conducting such exercises overrode the damage to the marine mammal population. In late 2008, the United States Supreme Court decided in favor of the navy.77 A BLANKET IN THE ATMOSPHERE.

The parade of human effects on the land and water that we have just viewed certainly has left big footprints. But the biggest driver of climate change today, without question, is the impact of human industrial activity on the chemistry of the atmosphere. Since the beginning of the industrial revolution in the eighteenth century, there has been an ever-increasing extraction and combustion of coal, petroleum, and natural gas. The burning of these carbon-based fuels, previously sequestered in geological formations for millions of years, has rapidly pumped great quant.i.ties of greenhouse gases into the atmosphere, gases that affect Earth's climate by absorbing infrared radiation trying to escape from Earth's surface, thereby warming the atmosphere.

The industrial pollution of the atmosphere actually began long before the combustion of fossil fuels. Ice cores in Greenland show deposition of lead during Roman times, when that metal was used widely in the plumbing systems of the Roman Empire. The word plumbing plumbing derives from the Latin word for lead- derives from the Latin word for lead-plumb.u.m-and Pb is the symbol for lead in the periodic table of elements. The smelting of lead and its use in manufacturing created lead dust that circulated widely in the atmosphere, some of which fell on Greenland and was incorporated into the acc.u.mulating ice. The deposition of Roman lead in Greenland came and went in tandem with the rise and fall of the Roman Empire. Lead reappeared in the Greenland ice when leaded gasoline made its debut as an automobile fuel, and largely disappeared when it was phased out as a fuel additive.

Another example of industrial atmospheric pollution is tied to the chemicals known as chlorofluorocarbons (CFCs). These inert nontoxic synthetic chemicals were first developed in 1927 as refrigerants, to replace the more combustible and toxic chemicals such as ammonia then in common use in refrigerators. In the decades following World War II, more and more uses for CFCs were discovered-for air-conditioning in homes, commercial buildings, and autos, and in the fabrication of electronics, foam insulation, and aerosol propellants. But the very property that made the CFCs attractive, their relative lack of reactivity with other chemicals, also made them very durable. Over decades they acc.u.mulated in the atmosphere, where they played an important role in the destruction of stratospheric ozone and the opening of the ozone hole in the last two decades of the twentieth century.

Ever since the beginning of the industrial revolution, air pollution has had deleterious environmental impacts, not the least of which have been the effects on public health. In 1948, a five-day-long incident befell the industrial town of Donora, Pennsylvania, on the Monongahela River, some twenty miles southeast of Pittsburgh. In what has been described as "one of the worst air pollution disasters in the nation's history,"78 a combination of unusual atmospheric conditions compounded by smoke-stack discharges of sulfuric acid, nitrogen dioxide, and fluorine from two large steel production facilities led to a stagnant yellowish acrid smog. Respiratory distress for several thousand residents, and death for at least twenty, ensued. a combination of unusual atmospheric conditions compounded by smoke-stack discharges of sulfuric acid, nitrogen dioxide, and fluorine from two large steel production facilities led to a stagnant yellowish acrid smog. Respiratory distress for several thousand residents, and death for at least twenty, ensued.

In 1980, well before the fall of the Iron Curtain that separated the socialist countries of Eastern Europe from their Western European neighbors, I was invited to lecture in Czechoslovakia and East Germany about my geothermal research. On the highway from Prague to Leipzig through some nicely forested areas, I encountered a ten-to-fifteen-mile-long stretch of dead trees-defoliated gray matchsticks pointing mutely to the sky. Soon the cause became apparent: a ma.s.sive chemical factory spewing great clouds of toxic pollution into the atmosphere, snuffing the life out of the forest downwind. The death of the forest was apparently considered acceptable collateral damage. One can only imagine what the airborne pollution did to the people and wildlife living nearby.

The combustion of coal has long been tied to health problems. In the United Kingdom, well known for dense, damp, chilly fogs, the addition of coal dust and tars to the fog forms a toxic brew a.s.sociated with dramatic increases in pulmonary disorders. In the last decades of the nineteenth century, several "smog events" were accompanied by death rates as much as 40 percent higher than the seasonal norms. The infamous five-day London smog in December of 1952, an event that darkened the city at midday, was the product of a cold, dense fog made worse by increased burning of high-sulfur coal by London's chilly residents. It led to more than four thousand coincident fatalities, with another eight thousand to follow in the weeks and months thereafter.

Industrialization and atmospheric pollution usually grow together. Following the rapid industrialization of Asia in the past few decades, severe atmospheric pollution is now commonplace over large parts of Asia. Dense brown clouds of industrial haze regularly blanket many large Asian cities, where automobiles and coal-fired power plants have grown, on a per capita basis, even faster than the population. In rural areas where consumerism has yet to penetrate, the simple aspects of daily living, such as cooking fires fueled by wood or dung, or the seasonal burning of the fields to prepare them for the next planting, also contribute to the haze that dims the Sun, r.e.t.a.r.ds agricultural production, and burns the lungs of millions of urban dwellers. The non-Asian world became aware of this acute pollution as China attempted to "clear the air" for the 2008 Olympic Games in Beijing, by closing factories and restricting automobile usage for weeks in advance of the Olympics.

The rapid post-World War II growth of coal-fired electric power plants in the central industrial states of America-Illinois, Indiana, Michigan, Wisconsin, and Ohio-was also followed by widespread atmospheric pollution that produced what came to be known as acid rain. The combustion of coal with high sulfur content yielded oxides of both sulfur and nitrogen, which reacted with water vapor in the atmosphere to produce potent acids. These acids were then deposited by rain and snowfall on the forests, lakes, and soil hundreds of miles downwind of the midwestern power plants, princ.i.p.ally in the northeastern states and adjacent Canada.

In only a half century, the acid rain and snow led to a decline in the pine forests of the Northeast, and increased the acidity of some lakes in the Adirondacks to a level where fish eggs could not hatch. In the late 1960s and early 1970s, dead lakes surrounded by declining forests and acidic soils appeared with increasing frequency. When the cause of the acid rain was recognized, it provided some of the motivation for pa.s.sage of the Clean Air Act in 1970, which, among other things, led to installation of scrubbers on smokestacks to capture the sulfur and nitrogen compounds that caused the downwind acid rain. The authors of the Clean Air Act, however, did not foresee a more dramatic environmental consequence of the burning of coal-the production of CO2 that would acc.u.mulate in the atmosphere and dissolve in the oceans, altering Earth's climate and producing a more acidic ocean. Let us now take a close look at this unantic.i.p.ated world-changing pollutant. that would acc.u.mulate in the atmosphere and dissolve in the oceans, altering Earth's climate and producing a more acidic ocean. Let us now take a close look at this unantic.i.p.ated world-changing pollutant.

CO2.

In 1958, Charles David Keeling of the Scripps Inst.i.tution of Oceanography in California began making measurements of the CO2 in the atmosphere atop Mauna Loa, in Hawaii. These daily measurements, which have continued to the present day, provide the world's longest instrumental record of the direct consequence of burning carbon-based fossil fuels. This continued monitoring of carbon dioxide levels in the atmosphere is another example of the important Cinderella science that I describe in chapter 4. in the atmosphere atop Mauna Loa, in Hawaii. These daily measurements, which have continued to the present day, provide the world's longest instrumental record of the direct consequence of burning carbon-based fossil fuels. This continued monitoring of carbon dioxide levels in the atmosphere is another example of the important Cinderella science that I describe in chapter 4.

These measurements show an increase in CO2 concentrations in the atmosphere from 315 parts per million (ppm) concentrations in the atmosphere from 315 parts per million (ppm)79 in 1958 to 390 ppm in 2009, an increase of 22 percent over just the past half century. in 1958 to 390 ppm in 2009, an increase of 22 percent over just the past half century.

Riding along on this upward trend of CO2 in the atmosphere is a fascinating annual oscillation-a small seasonal up-and-down that is the signature of photosynthesis occurring in the plant life of Earth each year. Most of the land area on Earth, and therefore most of the land plants, are in the Northern Hemisphere. During each Northern Hemisphere summer the growth of plant life draws CO in the atmosphere is a fascinating annual oscillation-a small seasonal up-and-down that is the signature of photosynthesis occurring in the plant life of Earth each year. Most of the land area on Earth, and therefore most of the land plants, are in the Northern Hemisphere. During each Northern Hemisphere summer the growth of plant life draws CO2 out of the atmosphere; in winter, when much of the vegetation is dormant, the CO out of the atmosphere; in winter, when much of the vegetation is dormant, the CO2 resumes its upward climb. This oscillation represents the actual metabolism of plant life on our planet, the annual "breathing" of Earth's green vegetation. resumes its upward climb. This oscillation represents the actual metabolism of plant life on our planet, the annual "breathing" of Earth's green vegetation.

Carbon dioxide concentrations in the atmosphere over Mauna Loa from 1958 to 2008. Data from Scripps Inst.i.tution of Oceanography

During the 1990s, CO2 continued its climb above preindustrial levels at a rate of 2.7 percent per year, a rate more than twice as great as when Keeling began making measurements in 1958. In the first decade of the twenty-first century the rate has increased to 3.5 percent. Now, more than a half century long, the Keeling graph is the iconic signature of human energy consumption, continued its climb above preindustrial levels at a rate of 2.7 percent per year, a rate more than twice as great as when Keeling began making measurements in 1958. In the first decade of the twenty-first century the rate has increased to 3.5 percent. Now, more than a half century long, the Keeling graph is the iconic signature of human energy consumption,80 acknowledged even by skeptics. acknowledged even by skeptics.

The growth of CO2 in the atmosphere is only part of the story-more than a third of all the CO in the atmosphere is only part of the story-more than a third of all the CO2 emitted by the global industrial economy enters the ocean, with effects that we are only beginning to understand. Oceanic uptake of CO emitted by the global industrial economy enters the ocean, with effects that we are only beginning to understand. Oceanic uptake of CO2 leads to a progressive decline in the pH of the water in the oceans, an indicator that the water is trending toward becoming a weak acid, a process sometimes referred to as ocean acidification, although the seawater is not yet an acid. A lowering of the pH does result in a decline in the production and stability of minerals that calcifying organisms use to build reefs, sh.e.l.ls, and skeletons, an effect likened to the onset of osteoporosis in the marine environment. This is already being observed along the Great Barrier Reef of Australia, where the calcification rate has decreased by about 14 percent since 1990, the largest decline in the past four hundred years. leads to a progressive decline in the pH of the water in the oceans, an indicator that the water is trending toward becoming a weak acid, a process sometimes referred to as ocean acidification, although the seawater is not yet an acid. A lowering of the pH does result in a decline in the production and stability of minerals that calcifying organisms use to build reefs, sh.e.l.ls, and skeletons, an effect likened to the onset of osteoporosis in the marine environment. This is already being observed along the Great Barrier Reef of Australia, where the calcification rate has decreased by about 14 percent since 1990, the largest decline in the past four hundred years.81 IN PERSPECTIVE.

We can place the Keeling measurements into a much longer temporal context provided by microscopic bubbles of air trapped in ice. In chapter 3, while discussing the rhythms and orbital pacemakers of the last several ice ages, I mention that the deep ice core drilled at the Russian Vostok Station in East Antarctica revealed a 100,000 year periodicity in the temperature of precipitation of the annual snowfall. This same ice core, which at its bottom comprises ice as old as 450,000 years, also gives us a remarkable historical record of atmospheric carbon dioxide and methane.

The mechanism of record-keeping in the ice is fascinating-when snowflakes fall they acc.u.mulate into a very fluffy layer of flakes and air, which gets compressed into ice by the weight of subsequent snowfalls acc.u.mulating above. The air that is present becomes trapped as microscopic bubbles in the ice layer, and those bubbles const.i.tute samples of the atmosphere at the time the snow recrystallized into an ice layer. The gases contained in the bubbles can be extracted and chemically a.n.a.lyzed to reveal the year-by-year greenhouse gas concentrations in the atmosphere over the complete time span represented in the ice core.

Changes in temperature and atmospheric carbon dioxide at the Vostok site in Antarctica over the past four hundred thousand years. Data from the Carbon Dioxide Information a.n.a.lysis Center (CDIAC) of the Oak Ridge National Laboratory

These remarkable graphs show not only that the temperature went up and down with a 100,000-year periodicity, but that CO2 did as well, in a pattern that is almost a direct overlay of the temperature record. The fundamental lesson of this overlay is that temperature and CO did as well, in a pattern that is almost a direct overlay of the temperature record. The fundamental lesson of this overlay is that temperature and CO2 are closely related in the processes that produce ice ages. A second observation drawn from these graphs is that atmospheric CO are closely related in the processes that produce ice ages. A second observation drawn from these graphs is that atmospheric CO2, over the course of the four complete ice age cycles shown, ranged roughly between 200 and 300 parts per million (ppm), from the cold of a glacial maximum when ice sheets covered vast expanses of the northern continents, to the briefer warm times that separated glaciations. Ice cores from elsewhere in Antarctica have extended the history of temperature and CO2 back some 800,000 years, but throughout this long record CO back some 800,000 years, but throughout this long record CO2 did not move out of the 200- to 300-ppm range. did not move out of the 200- to 300-ppm range.

But no longer. At the beginning of the industrial revolution in the middle of the eighteenth century, ice bubbles showed a concentration of CO2 at around 280 ppm, near the upper value of 300 ppm that characterized the earlier interglacial periods. By 1958, when Dave Keeling began his measurements of atmospheric CO at around 280 ppm, near the upper value of 300 ppm that characterized the earlier interglacial periods. By 1958, when Dave Keeling began his measurements of atmospheric CO2 atop Mauna Loa in Hawaii, it had already reached 315 ppm, well beyond the upper limit of the past 800,000 years. The concentration in 2009, a half century after Keeling began these measurements, reached 390 ppm, and is increasing by 2 to 3 ppm each year. It will likely cross the 400 ppm threshold in just a few more years. In the absence of any effective mitigation of greenhouse gas emissions in the near future, CO atop Mauna Loa in Hawaii, it had already reached 315 ppm, well beyond the upper limit of the past 800,000 years. The concentration in 2009, a half century after Keeling began these measurements, reached 390 ppm, and is increasing by 2 to 3 ppm each year. It will likely cross the 400 ppm threshold in just a few more years. In the absence of any effective mitigation of greenhouse gas emissions in the near future, CO2 in the atmosphere will reach the 450 ppm mark by 2030, a level that most climate scientists think will be accompanied by increasingly dangerous changes in our climate, which I describe in the next chapter. in the atmosphere will reach the 450 ppm mark by 2030, a level that most climate scientists think will be accompanied by increasingly dangerous changes in our climate, which I describe in the next chapter.

In retrospect, a landmark point was reached in the mid-twentieth century when the concentration of CO2 and other greenhouse gases moved out of the range of natural variability displayed over the previous 800,000 years. And it has become clear that yet another tipping point had been crossed, a point in time when the human influence on climate has overtaken the natural factors that had previously governed climate. Since then, population growth and increasing technological prowess have made the human influence increasingly dominant. Today the atmospheric concentrations of carbon dioxide and methane are substantially above their respective preindustrial levels, due to the contributions by humans and our many machines. This change in atmospheric chemistry is the real signature of human industrial activity, and the most important driver of contemporary global warming. and other greenhouse gases moved out of the range of natural variability displayed over the previous 800,000 years. And it has become clear that yet another tipping point had been crossed, a point in time when the human influence on climate has overtaken the natural factors that had previously governed climate. Since then, population growth and increasing technological prowess have made the human influence increasingly dominant. Today the atmospheric concentrations of carbon dioxide and methane are substantially above their respective preindustrial levels, due to the contributions by humans and our many machines. This change in atmospheric chemistry is the real signature of human industrial activity, and the most important driver of contemporary global warming.

THE PRINc.i.p.aL DIVISIONS of the geological time scale-such as the Paleozoic, Mesozoic, and Cenozoic eras, which together span the last 570 million years or so of Earth's history-have beginnings and ends marked by major changes in the character and distribution of life on Earth. Major extinctions on many branches of the tree of life mark the boundaries between these eras. Trilobites and many other marine invertebrates thrived in the Paleozoic, but did not survive into the Mesozoic. The Mesozoic era was the age of reptiles, with the dinosaurs the prime megafauna. But the saurians and many other taxa met their demise as the result of an asteroid impact sixty-five million years ago.

The Cenozoic is known as the mammalian era, and many of its subdivisions are a.s.sociated with major climatic events: the temporal boundary between the Paleocene and Eocene was a very warm interval, likely caused by releases of the strong greenhouse gas methane from the ocean floor into the atmosphere. The Pleistocene is the period encompa.s.sing the most recent ice ages, followed by the Holocene, representing the eleven thousand years since the end of the last ice age. During the Holocene the climate has been remarkably steady, and h.o.m.o sapiens h.o.m.o sapiens, that amazingly capable mammal, has flourished.

For nearly three million years, representatives of the genus h.o.m.o h.o.m.o, both ancient and modern, have been pa.s.sengers aboard Earth on its annual journey around the Sun. For most of that time, humans were but p.a.w.ns-albeit increasingly clever ones-in the hands of nature, adapting to changes in climate and food and water as best they could. But in the past few centuries, after we domesticated fossil energy to amplify our personal strength, we humans are no longer simply pa.s.sive pa.s.sengers on the planet-we now are the dominant species of the planet. Unwittingly, we have become the managers of the intricately intertwined Earth system of rock, water, air, and life-the lithosphere, hydrosphere, atmosphere, and biosphere. And we are belatedly discovering that we are clumsy managers, woefully unprepared for this endeavor, and undergoing rough-and-tumble on-the job training.

As recounted in this chapter, humans are having a profound impact on the landscape, on the waters, and on the living things of Earth. People have plowed up large areas of the land surface, have moved soil and rock at a rate much greater than natural erosion processes do, have controlled the flow of nearly every major river in the world, and have significantly altered the chemistry of lakes, rivers, the atmosphere, and oceans. We humans have wrested control of the carbon cycle, the nitrogen cycle, and the hydrological cycle away from nature, and have overpowered all other life forms on Earth, driving many to extinction. These clear and deep human footprints signal a major change in life on Earth, a reshuffling of the deck of life, with we humans moving to the top of the deck in only the eleven thousand years of the Holocene. Might not this reordering of life on Earth, driven by human fecundity and ingenuity, qualify for a new epoch in the geological time scale?

THE ANTHROPOCENE.

Some say yes, and have given this epoch82 a provisional name: the Anthropocene. a provisional name: the Anthropocene. 83 83 This name was first proposed informally by Paul Crutzen and Eugene Stoermer, This name was first proposed informally by Paul Crutzen and Eugene Stoermer,84 although several earlier authors have elaborated on the concept of a human-dominated period in Earth's history. In a short essay t.i.tled "Geology of Mankind," although several earlier authors have elaborated on the concept of a human-dominated period in Earth's history. In a short essay t.i.tled "Geology of Mankind,"85 Crutzen, a co-winner of the 1995 n.o.bel Prize in chemistry for his work on the chemistry of ozone depletion, again argued that humans have become the dominant species of the planet, and by the usual standards for defining a new unit of the geological time scale, that a new subdivision was warranted. Crutzen points to the late eighteenth century as the start of the Anthropocene, when fossil fuels began to drive James Watt's new steam engine and the concentration of carbon dioxide in the atmosphere began to increase. William Ruddiman, a climate scientist at the University of Virginia, places the beginning of the human influence on the terrestrial environment five thousand years earlier, when agriculture began to generate the greenhouse gas methane, and deforestation led to more carbon dioxide remaining in the atmosphere. Crutzen, a co-winner of the 1995 n.o.bel Prize in chemistry for his work on the chemistry of ozone depletion, again argued that humans have become the dominant species of the planet, and by the usual standards for defining a new unit of the geological time scale, that a new subdivision was warranted. Crutzen points to the late eighteenth century as the start of the Anthropocene, when fossil fuels began to drive James Watt's new steam engine and the concentration of carbon dioxide in the atmosphere began to increase. William Ruddiman, a climate scientist at the University of Virginia, places the beginning of the human influence on the terrestrial environment five thousand years earlier, when agriculture began to generate the greenhouse gas methane, and deforestation led to more carbon dioxide remaining in the atmosphere.86 By any measure, humans have moved to center stage. We have placed our footprints indelibly upon Earth, and by changing the chemistry of Earth's atmosphere, we have inadvertently begun a planet-wide experiment with the global climate. In the next chapter I examine the consequences of this experiment, in terms both of changes that have already taken place on Earth, and of projected changes that we will face in the future. Today ice is already retreating because of human activities on Earth, and is perhaps on a trajectory to disappearance. Will future generations live on a world without ice?

CHAPTER 7.

MELTING ICE, RISING SEAS.

It is very difficult for someone living in the United States to grasp the fact that if the sea level rises just a few feet, a whole nation will disappear.

-BEN GRAHAM Amba.s.sador to the United States from the Republic of the Marshall Islands

In the far reaches of the South Pacific Ocean sit many small islands-coral-fringed atolls that have formed on the subsiding calderas of extinct volcanoes. The growth of coral is fast enough to maintain the reef surface essentially at sea level, keeping pace with the slow geological subsidence of the ocean floor supporting the base of the volcano. Charles Darwin is best remembered for his compelling formulation of biological evolution, but he also was the first to recognize the role of volcanoes in the formation of coral atolls. Myriad low-lying islands in the South Pacific have been home to Polynesian, Melanesian, and Micronesian communities that have populated the islands for several thousands of years. These islanders have survived many challenges, including brutal occupation and warfare during World War II. But in the face of their newest challenge-rising sea levels from a warmer climate-these communities have no high ground to retreat to. Their only option is evacuation, mostly to places culturally alien to them. As Amba.s.sador Graham says, it is indeed very difficult, and not only for Americans, to grasp the fact that if the sea level rises just a few feet, whole nations will disappear.

Tuvalu is a nation of twelve thousand people living on the atolls of the Ellice Islands, some two thousand miles north of New Zealand. Most of the islands sit only a few feet above sea level. Funafuti, the princ.i.p.al atoll, has an airstrip built by the United States during World War II. Today that lone runway provides the only easy connection with neighbors in Fiji and Samoa. But the airstrip has become increasingly vulnerable to partial inundation at the time of very high tides.87 Tidewa ter in an atoll does not just move inland from the sh.o.r.eline-it seeps upward through the coral and soil from below. The slow rise of sea level due to climate change has given the tides a head start, so that at some time in this century even ordinary tides will begin to force water to the surface to form shallow tidal lakes. Tuvaluans for a while longer will live on a saturated sponge that gets squeezed with regularity. But within this century they probably will have to abandon their home. Tidewa ter in an atoll does not just move inland from the sh.o.r.eline-it seeps upward through the coral and soil from below. The slow rise of sea level due to climate change has given the tides a head start, so that at some time in this century even ordinary tides will begin to force water to the surface to form shallow tidal lakes. Tuvaluans for a while longer will live on a saturated sponge that gets squeezed with regularity. But within this century they probably will have to abandon their home.

CHANGES IN THE ICE, water, landscape, and life of Earth go hand in hand with a change in its climate. In the broadest of terms, a shift is now under way in the balance between ice and water-ice is diminishing, water increasing. In the parlance of Earth's hydrological cycle described in chapter 2, we are witnessing a transfer of H2O from the solid cryosphere to the liquid hydrosphere. Mountain glaciers have been retreating, Arctic sea ice has been diminishing, the Greenland ice cap has been melting, permafrost has been shrinking, and sea level has been rising. And because of the continuing burning of fossil fuels, the greenhouse gas CO2 continues to increase in the atmosphere, bringing yet further warming. What will be the consequences of this continuing climatic trend in the near future? The answers to this question differ from place to place, and from one elevation to another. continues to increase in the atmosphere, bringing yet further warming. What will be the consequences of this continuing climatic trend in the near future? The answers to this question differ from place to place, and from one elevation to another.

It is a mistake, however, to think that climate change is some abstract characteristic of the future. To the contrary: changes in the climate have been taking place for decades, and are continuing into the future at an ever faster pace. Indeed, the many changes already observed in the natural world, along with the millions of temperature measurements in the atmosphere, oceans, and rocks, are what persuaded the IPCC to conclude in 2007 that the warming of Earth is unequivocal.

In most of the continental mid-lat.i.tudes, including the lower slopes of mountains, snow and ice make only seasonal appearances. Permanent snow and ice on mountains, of course, depends on where the mountain is located-on the Antarctic Peninsula permanent snow and ice begins at sea level, but in the contiguous states of America year-round mountain snowpack and glaciers are found only at high elevations-well above ten thousand feet-in Glacier and Rocky Mountain national parks, and atop Mounts Rainier and Olympus in Washington. In the polar regions, ice dominates the landscape at all elevations year-round.

As snow cover lessens and glacial ice melts, it will not be just the scenery that changes. Water for munic.i.p.al systems and agriculture in the foot-hills and plains surrounding high mountains comes from melting of both the annual snowpack and much older glacial ice, residual from colder times of the past. This is the very water that millions of people drink from the tap, that flushes sewage from towns and cities, and that waters the crops in the fields. Melt.w.a.ter derived from this snow and ice is well timed for agricultural purposes, coming in the spring planting and summer growing seasons. But in a warmer world, where instead of snow, precipitation comes as rain that runs off when it falls, the water is not stored for later delivery during the agricultural season. And when the glacial ice of the mountains is finally gone, that source of water will disappear forever.

SEASONAL SNOW AND ICE.

Let us begin a tour of the diminishing domain of snow and ice at its most tenuous geographic margins, at those lat.i.tudes where snow covers the ground for only a few days at least once a year. Where wintertime temperatures are already hovering around the freezing point, a little warming will mean the end of an already short period of snow cover. This occurs not just because there are fewer days sufficiently cold for snow to fall, but also because the dark peripheral ground, covered less and less by snow, progressively absorbs more heat over the year, and impedes the snow that does fall from acc.u.mulating. The southern margin of annual snow occurrence in the United States and Europe is slowly shifting northward a few miles each decade. In the mid-twentieth century some winter snow in Memphis was never welcome, but not uncommon. Now, in the early twenty-first century, a snowless winter in Memphis is not so rare, but ice storms from freezing rain, even less welcome than snow, are more frequent. Over the past fifty years the area covered by snow in North America has diminished in all months except November and December. February used to be the month of maximum snow cover, but that honor now belongs to January.

Wherever precipitation is delicately balanced at the freezing point, freezing rain is just as likely as snow. As a result, the incidence of ice storms is at a maximum, along with the special inconveniences of downed tree branches and power lines, and glazed roadways that promote fender-bender collisions. And as the number of days when the temperature is below freezing declines and snow falls less frequently, there are adverse consequences for outdoor winter sports. Nowhere is this change in the snow regime being felt more than in the Alpine towns and villages of Switzerland, where the local economies are heavily dependent on winter tourism. In mountainous terrain the snowline is creeping upward about seventy feet per decade. Studies of snow depth and duration88 over the past sixty years show that a regime shift began in the late 1980s, when snow days declined by 20 to 60 percent. The reduction in snowfall coincided with an upward shift in the average wintertime temperatures, leaving little doubt about what was behind the diminished snowfall. The higher temperatures and the lesser amounts of snow have continued right through the first decade of the twenty-first century. Some Swiss ski slopes at vulnerable elevations are trying to preserve their wintertime snow and, in some cases the glacial ice beneath, with summertime plastic covers to shield them from the Sun. Ski resorts that make snow artificially because they are already challenged by inadequate natural snow are using their snowmakers more frequently, as they face the financial stress of a shorter natural season. over the past sixty years show that a regime shift began in the late 1980s, when snow days declined by 20 to 60 percent. The reduction in snowfall coincided with an upward shift in the average wintertime temperatures, leaving little doubt about what was behind the diminished snowfall. The higher temperatures and the lesser amounts of snow have continued right through the first decade of the twenty-first century. Some Swiss ski slopes at vulnerable elevations are trying to preserve their wintertime snow and, in some cases the glacial ice beneath, with summertime plastic covers to shield them from the Sun. Ski resorts that make snow artificially because they are already challenged by inadequate natural snow are using their snowmakers more frequently, as they face the financial stress of a shorter natural season.

The warming of the waters in the Great Lakes of North America has led to later freezing and earlier melting of lake ice, as well as a diminished area with ice cover. The shorter period and smaller area of ice cover has had the consequence of increased loss of water due to more evaporation from the open lakes in winter. This has caused a drop of several feet in the lake levels over the past two decades, to a point where the upper Great Lakes-Superior, Michigan, and Huron-are approaching record lows. Lakeside cottages once a few steps away from the sh.o.r.eline now see a beach a half-mile wide in some places, with wetland vegetation taking hold. And the entry channels to some major ports have become so shallow that without frequent dredging, they become impa.s.sable to the big freighters on the Great Lakes. These thousand-foot-long behemoths, were they at sea, would be too large to pa.s.s through the Panama Ca.n.a.l. In 2007, five fully loaded cargo ships ran aground attempting to enter the harbors at Muskegon and Grand Haven on the eastern sh.o.r.e of Lake Michigan. After being tugged free, they had to go across the lake to Milwaukee to off-load some cargo, before returning-floating higher-to unload the rest. The equally costly alternative is to carry a lighter load at the outset.

FROM THE MOUNTAINS.

Long before humans came to the high Sierra Nevada of California, snowmelt fed streams tumbling through quartz veins containing gold, eroding and transporting the precious metal downstream. Where the currents slowed, the gold was dropped in the sand and gravel of the streambeds, later to be discovered by nineteenth-century prospectors. Today, it is the water itself that is the treasure-the snowmelt provides much of the annual agricultural water for California's fertile Central Valley, which stretches from Sacramento to Bakersfield. The winter snows that have for years made Lake Tahoe a popular skiing destination and Squaw Valley the site of the 1960 Winter Olympics, undergo springtime melting to swell the Sacramento River and deliver water to thirsty vegetable and fruit farms in the Central Valley. A little farther south, the Merced River, with its source in the great glacially carved valleys of Yosemite, also heads downhill to irrigate another section of the Central Valley.

But what happens when more of the precipitation comes as rain instead of snow? That H2O is not stored in the winter snowpack-it does not wait for springtime to begin the downward journey to the dryer expanse of the Central Valley and then onward to the sea. Storage reservoirs along the way do not have the capacity to simply hold the water until later. So the water arrives much earlier than needed to help grow the produce of spring and summer, and when the agricultural calendar does call for delivery, the water is less abundant. A late-summer soil moisture deficit is a common result.

The dams and storage reservoirs along the waterways descending from the high Sierra serve another purpose-hydroelectric power generation. When water arrives early at already full reservoirs, it must bypa.s.s the dams via spillways. But every drop of water that bypa.s.ses the dams also bypa.s.ses the electric generators, the result of which is a deficit in power generation to accompany the deficit in soil moisture. If water is held in the reservoirs longer to provide steady hydroelectric power, the downstream flow is in places inadequate for agriculture and to maintain aquatic habitat for sp.a.w.ning salmon. The challenges of water management in an already semi-arid region are many. Within the larger expanse of the United States, water resources are already overallocated among agriculture, urban needs, ecosystem maintenance, hydroelectric energy, and recreation, at a time when demands in each sector are increasing.

In Europe, the Rhine River is fed in part by melting snow in the Alps and in part by rainfall over the low-lying parts of the river basin. A warming climate is changing the discharge of the Rhine to a rainfall-dominated regime-one of increasing winter flow and decreasing summer flow. The longer and more frequent low-flow episodes of summer are already apparent: less water for households, industry, agriculture, river transportation, and hydroelectric power during the time of peak summer demand.

Warming also shortens the snow season at both ends-winter snows begin later, and spring melting begins earlier. Already the peak stream flow from melting snowpack is appearing earlier in the season; by mid-century it is projected to arrive a full month earlier than the historical norms in the western United States.89 The seasonal shift in melting also leads to longer summers and longer dry seasons, with more opportunities for wildfires. Research on the wildfires in the American West has shown that an extended dry season translates into a more intense fire season, with more wildfires that burn longer. The seasonal shift in melting also leads to longer summers and longer dry seasons, with more opportunities for wildfires. Research on the wildfires in the American West has shown that an extended dry season translates into a more intense fire season, with more wildfires that burn longer.90 Contributing to the potential for wildfires are the large areas of forest succ.u.mbing to insect infestations in western North America. Vast areas of the pine forests in the western states and adjacent Canada are being decimated by the pine bark beetle.91 In British Columbia more than thirty-three million acres have already been lost, an area about the size of the state of Louisiana, and the infestation has spread to Montana, Wyoming, and Colorado. The beetle has crossed the Continental Divide into Alberta and is now making an appearance in the forests around the Great Lakes. This beetle is no stranger to the pine forests-it is not a recent invasive species-but the damage to the forests by it has grown dramatically in recent decades. What has happened? In the past, long and deep wintertime freezes controlled the bark beetle population, but now, with warmer winters and shorter deep freezes across the region, the beetle is suffering much less seasonal attrition. Greater numbers of hungry beetles are emerging each spring in search of nourishment, and the pine forests are their diet of choice. The dead trees then become fuel for wildfires. In British Columbia more than thirty-three million acres have already been lost, an area about the size of the state of Louisiana, and the infestation has spread to Montana, Wyoming, and Colorado. The beetle has crossed the Continental Divide into Alberta and is now making an appearance in the forests around the Great Lakes. This beetle is no stranger to the pine forests-it is not a recent invasive species-but the damage to the forests by it has grown dramatically in recent decades. What has happened? In the past, long and deep wintertime freezes controlled the bark beetle population, but now, with warmer winters and shorter deep freezes across the region, the beetle is suffering much less seasonal attrition. Greater numbers of hungry beetles are emerging each spring in search of nourishment, and the pine forests are their diet of choice. The dead trees then become fuel for wildfires.

Much higher on mid-lat.i.tude mountains, beyond the zone of seasonal snowpack, one encounters real glaciers, the streams of ice formed by compression and recrystallization of the snow of centuries past. In the contiguous United States these ice flows are few, princ.i.p.ally atop Mount Rainier and other volcanic peaks in the Cascade Range of California, Oregon, and Washington, and in Glacier National Park in Montana. Most of these mid-lat.i.tude glaciers are remnants of the more extensive mountain glaciers of the last ice age. The extent of these former rivers of ice can be seen in the now-empty U-shaped valleys of the Sierra Nevada and Rocky mountains, as well as high up on the flanks of the Cascade Range volcanoes. All of these remaining patches of ice are shrinking-within the current century, Glacier National Park will lose the very features that give it its name.

In Alaska, a score of long ice streams radiate downward from the twenty-thousand-foot peak of Mount McKinley, in Denali National Park, but surveys of the Denali glaciers show they are thinning and retreating rapidly. At lower elevations, along the coast of the Gulf of Alaska and in Glacier Bay National Park, the glaciers delivering ice to the sea have felt the effects of climate change as well. In Glacier Bay, modern cruise ships today sail into the bay and up the fjords to the glacier fronts. Only two centuries ago, when explorer George Vancouver visited the area, the bay was almost completely frozen over and virtually inaccessible.

Earth is losing ice today even faster than during the warming that followed the Little Ice Age of the seventeenth and eighteenth centuries. Projections for the future indicate that it will likely continue to do so, and at an accelerating pace. Every decade for the foreseeable future will see the loss of more ice. Vulnerable places, such as high mountains in equatorial and temperate lat.i.tudes, will see ice vanish soon.

The Andes mountain range forms an impressive spine running the full length of western South America. The high peaks of the Andes, with summits near twenty thousand feet, are also sites where ancient ice is present and is replenished with annual snowfall. But over much of the extent of this long mountain range, the mountains form a curtain that captures atmospheric moisture at high alt.i.tude, leading to a deficit of rainfall at lower elevations.

For some two thousand miles along the Pacific margin of South America there is a coastal desert, broken only by thin green ribbons where rivers and streams bring water from the high ice fields and snowpack. The villages, towns, and cities on the western slope of the Andes in Peru and Chile are made possible by the water rushing down from melting snow and ice above. Fields of agriculture-the abundant fruit and flowers and the remarkable vineyards of the region-are possible only because of the melt.w.a.ter from the high Andes. La Paz, the administrative capital of Bolivia, draws much of its munic.i.p.al water and all of its electricity from glacial melt. Lima, the capital of Peru, and its port city of Callao rely on snowpack and glacial melt.w.a.ter to flush the munic.i.p.al sewage (much of it untreated) to the sea.

But the warming of the climate is imperiling this source of water. Mountain glaciers from Peru to Patagonia are losing their ice to a warmer world, and are on the path to disappearance within a few decades. The Quelccaya ice cap in Peru, the Chacaltaya Glacier in Bolivia, Perito Moreno of Argentina (flowing eastward on the other side of the Andes), San Rafael Glacier in Chile, the Darwin glaciers flowing out of the South Patagonia ice field into the Beagle Channel-all show an unmistakable loss of ice at a rate even faster than occurred when the region emerged from the last ice age. The extent of Andean ice today is less than the region has known for at least five thousand years, and is undergoing attrition at a pace not seen since humans took up residence along the Andes fourteen thousand years ago. In 2009 the World Bank published a report projecting that the imminent loss of Andean glaciers will affect the water supply of nearly eighty million people and significantly reduce hydroelectric energy production in the region.

Across the Atlantic, glacial ice has a toehold even in equatorial Africa, atop nineteen-thousand-foot Mount Kilimanjaro, well known from Ernest Hemingway's "The Snows of Kilimanjaro." But the toehold is slipping fast-Kilimanjaro has seen a decline in its glacial cap throughout the twentieth century, and will likely be ice-free by 2020.

The Himalaya mountain range of Asia, comprising the long, high, remote boundary between India and Pakistan on the south and the Tibetan Plateau of China to the north, is often called "the roof of the world" for good reason. The highest peak, Mount Everest, stands above twenty-nine thousand feet, and more than one hundred others exceed twenty-three thousand feet. No mountain on any other continent reaches that elevation. In Sanskrit, the word Himalaya Himalaya means "home of the snow," also for good reason. After Antarctica and the Arctic polar region, including Greenland, the ice ma.s.s on this Asian roof is the world's third largest. More than fifteen thousand glaciers flow downward from the high Himalayas, with a combined ice volume equivalent to some three thousand cubic miles of freshwater. means "home of the snow," also for good reason. After Antarctica and the Arctic polar region, including Greenland, the ice ma.s.s on this Asian roof is the world's third largest. More than fifteen thousand glaciers flow downward from the high Himalayas, with a combined ice volume equivalent to some three thousand cubic miles of freshwater.

Each year these glaciers yield melt.w.a.ter that provides a little less than half of the annual flow volume of the Brahmaputra, Ganges, and Indus rivers, the three princ.i.p.al rivers of South Asia that flow to the sea across Bangladesh, India, and Pakistan. The other half of the flow comes from the annual monsoon rains and snowmelt from the high mountains. But the contributions to these rivers are not spread evenly over the year-the monsoon dominates during the rainy season, and glacial melt.w.a.ter is the princ.i.p.al source of water during the dry months.

A long-term warming of the atmosphere is well doc.u.mented in the region, and measurements of the extent of glacial ice leave no doubt that the glaciers of the Himalayas are melting, and at an accelerating rate over the past two decades.92 What message are these data delivering? Something akin to an urgent telegram that says that the volume of freshwater contained in the glacial ice of the Himalayas will last only another two to three decades. When the ice is largely depleted, the dry season flow of the Indus, Ganges, and Brahmaputra will diminish-already the lower Ganges is nearly empty for several months of the year. The lives of more than a billion people are intertwined with these rivers. Ironically, as the melting accelerates, there will be a temporary increase in water availability, until the shutoff arrives abruptly. What message are these data delivering? Something akin to an urgent telegram that says that the volume of freshwater contained in the glacial ice of the Himalayas will last only another two to three decades. When the ice is largely depleted, the dry season flow of the Indus, Ganges, and Brahmaputra will diminish-already the lower Ganges is nearly empty for several months of the year. The lives of more than a billion people are intertwined with these rivers. Ironically, as the melting accelerates, there will be a temporary increase in water availability, until the shutoff arrives abruptly.

Ice on the Tibetan plateau north and east of the Himalaya also supplies the Irrawaddy and Mekong rivers flowing through Burma, Thailand, Cambodia, Laos, and Vietnam, and the Yellow and Yangtze rivers coursing through China. These rivers, too, face the loss of glacial melt.w.a.ter in just a few decades hence. One of the princ.i.p.al glaciers feeding the Yangtze, the largest river in China, has retreated half a mile in a little more than a decade. The loss of the glacial water has grave implications for agriculture, urban drinking water and sanitation systems, and hydroelectric power generation. Seasonal water stress will become a reality on all sides of the Himalayas, a fact of life, or death. Taken altogether, one quarter of Earth's population will within another decade be affected significantly by lesser snowfall and glacial ice loss. That number translates into almost two billion people-and most of them live in Asia.

Just as oil was the crucial resource of the twentieth century, water will be the prized resource of the twenty-first century. Already there is an international compet.i.tion for water, and not just the decades-long tug-of-war between the United States and Mexico over Colorado River water. Nine nations share the Danube, six the Zambezi, four the Jordan. As politically charged a phrase as "the West Bank" is, one must remember that it is literally the west bank of the Jordan River. Water shortages are such a real threat for many nations that the potential for international conflict is also very real. In 2003 the Pentagon published a study of the implications of climate change for national security,93 and pointed to water shortages as a special factor in international instability: "Military confrontation may be triggered by a desperate need for natural resources such as energy, food and water rather than by conflicts over ideology, religion, or national honor." and pointed to water shortages as a special factor in international instability: "Military confrontation may be triggered by a desperate need for natural resources such as energy, food and water rather than by conflicts over ideology, religion, or national honor."

PERMAFROST.

Farther north from the regions of simple seasonal snow cover the annual average temperature sits below the freezing point. There, aside from the summertime thawing of the upper foot or two-the "active zone" described in chapter 4-the ground is permanently frozen. This is the domain of permafrost. It extends over about 20 percent of Earth's land surface, mostly in the sub-Arctic and Arctic regions of Asia, North America, and Europe.

Earth's warming climate is taking a toll on permafrost terrains in many localities. As melting progresses to greater depths, the land surface suffers a disruption, stemming from the fact that ice occupies more volume than its equivalent melted water-the very same property that lets ice float on water. When deeper permafrost melts, the land surface above collapses into a jumbled irregular array of pits and hills. Houses and barns built on stable permanently frozen ground are undermined by ground subsidence; roads across the permafrost are broken up as if by an earthquake. Trees rooted in these slumping blocks are no longer vertical-their tilted orientation has earned them the description "drunken trees." The jumbled terrain that results from the melting of permafrost is called thermokarst. It takes its name from the true karst topography, the landscape caused by slightly acidic groundwater dissolving limestone and producing sinkholes and collapses over subsurface cave systems.

The Alaska pipeline is a long forty-eight-inch-diameter tube, a giant conduit through which oil produced at Prudhoe Bay, on the Arctic coast of Alaska, is transported to the tanker seaport of Valdez, some eight hundred miles south on the Gulf of Alaska. Along many stretches of this long traverse, the pipeline pa.s.ses over permafrost. The design and construction of the pipeline in the 1970s took into account that warm oil, pumped from deep below Earth's surface, would melt the permafrost if the pipeline were buried in it. To avoid the possible buckling or breaking of a buried pipeline that melting permafrost might produce, engineers decided against burial. So for hundreds of miles the pipeline snakes across the tundra and through the boreal forest aboveground, perched on pedestals tall enough to let caribou wander unimpeded beneath.

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