Living in the Anthropocene: Earth in the Age of Humans

As the name Anthropocene suggests, the impact of human activities has reached global proportions. The physical and biological transformations now taking place may be equivalent in magnitude to the major environmental transitions that marked significant geologic turning points in the distant past, such as the start of the Eocene, the Paleocene, and the Holocene. The biogeochemical consequences of anthropogenic change are both far reaching and profound, as are its social, cultural, political, and economic effects. Gaining a full understanding of the complex causes and implications of these planetary alterations therefore requires a blending of perspectives from many fields of study. The start of the Anthropocene remains a matter of debate, although the mid-twentieth century—which witnessed a phenomenal escalation of a wide spectrum of environmental indicators (the so-called Great Acceleration)—is widely acknowledged as a significant turning point in human-induced environmental change.

The most obvious transformations have taken place within terrestrial habitats, but the oceans have also been intensely affected by acidification, warming, mass extinctions of marine mammals, depletions of wild fish, industrialization of undersea landscapes, and intensification of plastic pollution. Natural environments, whether terrestrial or marine, may never be restored to anything resembling their prehuman conditions. Our evolutionary success as the most recent bipedal hominid species has been fostered by our ancestors’ exceptional ability to adapt to changing environments, often by altering our surroundings through the use of technology and social organization. Since the Industrial Revolution, economic growth has become an end in itself for much of the world, and the ramifications of this worldview can be seen in the environmental challenges characteristic of the Anthropocene.

THE ADVENT OF THE ANTHROPOCENE

J. R. McNEILL

In 1944, the Hungarian-born social scientist Karl Polanyi published a turgid, difficult, and detailed book entitled The Great Transformation . In it he showed that markets, as the chief means of distributing goods and services among populations, had not dominated any societies before 1700. They were not in any sense natural or the reflection of innate human desires. Alternative arrangements had flourished in the past, even if, by 1944, it was hard for most people to conceive of a functional economy not based overwhelmingly on markets. Polanyi showed that the prevalence of markets was comparatively recent and had required certain changes in societies and politics to triumph.

An homage to Polanyi, the term Great Acceleration refers to the sharp mid-twentieth-century uptick in the rate of ecological change around the globe. To most of us, it now seems normal, almost natural, that humans should exert vast influence over the biosphere and basic biogeochemical systems. That is because we cannot remember a world in which this was not true. But in fact that condition is novel and a bizarre departure from the arrangements that governed the human place in the biosphere for the first two hundred thousand years of Homo sapiens’s career. The concepts of the Great Acceleration and the Anthropocene can help put into high relief the uniqueness of modern times and remind us that what we easily misunderstand as normal and natural is indeed anything but.

A few examples will underscore the reality of the Great Acceleration. Our forebears took many millennia to reach a population of one billion, which occurred around 1800 or 1820. It then took more than a century for human numbers to reach two billion (around 1930). Soon, in the middle of the twentieth century, an unprecedented frenzy of survival and reproduction began, and the human population tripled in the span of one lifetime. It reached three billion by 1960 and four billion by 1975, and thereafter it added a new billion every twelve to thirteen years. Meanwhile, total energy use quintupled between 1950 and 2015. In those sixty-five years, people burned a quantity of fossil fuels that had taken 150 million years to accumulate. More than three-quarters of the anthropogenic greenhouse gas emissions in human history occurred during those sixty-five years. The world’s motor vehicle fleet grew like kudzu, from forty million to nine hundred million, over the same time span. These and many more accelerating trends made the post-1950 world very different from all that had come before it. Collectively, they vaulted us into a new time period in both human history and the history of Earth: the Anthropocene.

Figures 1–8. These eight graphs collectively show that the middle of the twentieth century was a transitional moment for several indicators, or driving forces, of global environmental change. For those who embrace the concept of the Anthropocene and endorse a mid-twentieth-century birthday for it, these graphs provide persuasive evidence. Several more such charts appear on the website of the International Geosphere-Biosphere Programme. (OECD: Organisation for Economic Co-operation and Development; BRICS: Brazil, Russia, India, China, and South Africa.) From W. Steffen et al., The Trajectory of the Anthropocene: The Great Acceleration, Anthropocene Review 2 (2015): 81–98. Used with permission of Sage Publishers.

The Anthropocene at present means all things to all people. Arguments simmer about how old it is. The weight of the evidence suggests that the Anthropocene began in the mid-twentieth century, but some scientists argue for a much older Anthropocene, beginning with the harnessing of fire, the late Pleistocene extinctions, or the dawn of agriculture. Others prefer an Anthropocene dating to the Columbian Exchange (beginning in 1492), when sailors carried ark-loads of species of animals, plants, and microbes from one continent to another, leaving a lasting mark in the paleontological record. Still others see the Industrial Revolution (ca. 1780–1850) as the decisive period that sets the Anthropocene apart, thanks to the rapid adoption of fossil fuels and the subsequent greenhouse gas emissions that were generated on an ever-larger scale in those years.

Arguments about the antiquity (or novelty) of the Anthropocene are also arguments about its essence. For most of those who regard atmospheric chemistry as the most relevant marker of global change, an Anthropocene that begins with industrialization makes the most sense. For those who require that the Anthropocene leave a signal in the paleontological record and who see the history of life on Earth as the key variable in demarcating intervals of time, the late Pleistocene extinctions or the Columbian Exchange make more sense.

If one tries to mesh all the relevant variables and to avoid privileging any particular discipline’s outlook and preferred forms of evidence over others, the Great Acceleration seems to be the best birth date for the Anthropocene. Taking a basket of variables approach, as the International Geosphere-Biosphere Programme recently did, underlines the many respects in which the mid-twentieth century marks a point of inflection on rising curves. Beyond the upticks in rates of population growth and energy use, which together may lie at the heart of the matter, a long list of relevant variables describe a similar trajectory: carbon emissions, methane emissions, fertilizer consumption, ocean acidification, and nitrogen loading of coastal waters, among others. It is the collective weight of all these variables that separates the Anthropocene from what came before, rather than any single one. Their simultaneous and interwoven acceleration trajectories, beginning about 1950, blasted us into the Anthropocene.

The Great Acceleration is doomed. The remarkable trends that most of us have lived with all our lives will not last long. In some cases, finite supply is the issue. The world does not have enough fresh water to allow another quadrupling of water withdrawals (as occurred from 1950 to 2010). There are not enough good sites left to support another sextupling of the world’s large dams. There are not enough fish to permit another quintupling of the marine fish catch. In other cases, saturation or equilibration is the issue. The proportion of the global population living in cities, now at more than 50 percent, cannot more than double (as happened from 1950 to 2010). Total human population could in theory triple once more, as it did in that same period, but no one imagines it will, because urbanization, formal female education, and other social changes have sharply reduced couples’ reproductive ambitions. Globally, human fertility is only a little more than half of what it was in 1970. So for all these reasons and several others, the Great Acceleration will come to a close. Indeed, many of its trends have already leveled off (dam building, marine fish catch) or begun to decline.

The Anthropocene, however, will live on. Even if the human population starts to fall some fifty or sixty years hence (as some speculate it will), even if by 2075 we have banished fossil fuels to the margins of a low-carbon energy system, even if green parties win every election, the Anthropocene will live on. That is because, for a long time to come, there will still be billions of people using the global environment as a source of materials and a sink for wastes, even if at restrained rates, and for a long time to come, the carbon already emitted into the atmosphere will continue to trap heat and warm Earth’s surface and its oceans. Less certainly, increasing skill in manipulating human and other genomes could give a new tint to the Anthropocene, raising the efficiency with which we eliminate some species (pesky mosquitoes, perhaps) and alter others.

Thus, of all the possible understandings of the Anthropocene, the one that best matches the evidence is the post-1950 Anthropocene, born of the Great Acceleration. Historians may leave it to others to reflect upon whether the Anthropocene is on balance a good or a bad thing and how long it might last. Those questions require a clear look into the future, and historians have trouble enough seeing into the past.

THINKING LIKE A MOUNTAIN IN THE ANTHROPOCENE

SCOTT L. WING

Aldo Leopold’s posthumously published book A Sand County Almanac (1949) includes an essay entitled Thinking Like a Mountain. In it, he recounts shooting a wolf as a young man. Watching the fierce green fire in her eyes die helps him to consider the deeper meaning of the wolf’s existence—or, as he puts it, to think like a mountain. In this essay of only 878 words, Leopold gave twentieth-century conservationists two powerful but distinct metaphors. The fierce green fire speaks of the emotional effect of losing the wildness that wolves represent. Thinking like a mountain suggests a more detached point of view but one that appreciates the interplay of the wolf and its ecosystem. In the decades after he shot the wolf, Leopold writes that he has watched the face of many a newly wolfless mountain, and seen the south-facing slopes wrinkle with a maze of new deer trails…seen every edible bush and seedling browsed, first to anaemic desuetude, and then to death. What he has learned, and what the mountain has always known, is that extirpating predators leads to explosive growth of herbivore populations, overbrowsing, reduced vegetation, and loss of soil—in short, a cascade of unintended, long-term consequences considered undesirable by the humans who initiated them. Leopold’s essay stretches the minds of readers unaccustomed to thinking about the biological and landscape changes that can be precipitated by removing a keystone predator from an ecosystem.

The thinking mountain of the essay’s title recognizes ecological complexity, but its spatial frame is regional rather than global and its time scale more human than geologic. Looking back, though, we see that A Sand County Almanac was published near the onset of the dramatic upward inflection in human resource use and global effects often referred to as the Great Acceleration, making it one of the first conservationist texts of the Anthropocene. Three score and eight years is not a long time to watch a new epoch unfold, but the changes in the environment that have taken place in those years may justify a reevaluation of Leopold’s classic metaphor.

The idea behind the Anthropocene is conceptually simple: the changes we are now making to Earth are on a par with the shifts in the global environment that mark the beginnings of previous geologic time periods. (By convention, stratigraphers define each period of the geologic time scale by its beginning, or base, with the end of one being defined by the beginning of the succeeding period. No gaps or interregna are allowed in geochronology.) Although there is plenty of scholarly debate about naming a formal Anthropocene epoch, there is no doubt that the changes humans have been causing to Earth systems since the mid-twentieth century are comparable in type and magnitude to past changes that we use to recognize the beginnings of new phases of Earth history. Indeed, some of these earlier changes reflect perturbations of the very same processes we are now altering, even though their rates and ultimate causes are different.

The beginning of the Eocene epoch (approximately fifty-six million years ago), for example, was accompanied by a major release of carbon, with attendant global warming and ocean acidification, an event called the Paleocene-Eocene Thermal Maximum, or PETM. Although the amount of carbon released at the onset of the PETM was greater than the amount that humans are likely to generate except in the most extreme future scenarios, the rate of carbon release today is probably ten times faster. The rapid shift in carbon chemistry (the ratio of 12C to 13C) at the onset of the PETM now forms a convenient marker for the base of the Eocene, as do the changes in the composition of fossil faunas and floras that it caused. The effects of the PETM on Earth’s climate and biota lasted for more than a hundred thousand years. In a parallel fashion, the change in the ratio of carbon isotopes caused by burning fossil fuels will form a permanent sedimentary marker for recognizing an Anthropocene epoch, as will the massive changes in biodiversity, sedimentation, nutrient supply, and ocean chemistry that are resulting from human activities.

Since 1949, we have gained much greater insight into how the integrated Earth-life system works. Geologists have contributed to this understanding by reconstructing past global environmental changes and the interaction of environmental change with life. Earth system scientists have developed the ability to monitor environmental change globally in real time—giving us a sense of how, and how fast, the Earth’s system is shifting. Powerful computers can now simulate the interconnected processes that influence the global environment, producing predictions of future change. As a result, we know not only that we are increasing carbon dioxide (CO2) in the atmosphere every year but also that much of that carbon dioxide will still be in the atmosphere in the year 3000, that most of the resulting increase in temperature and sea level will still be in force in the year 12,000, and that a full return to background carbon dioxide won’t occur for more than a hundred thousand years. Human effects on the global environment are large and unprecedented and are producing a welter of unintended consequences. Even more important, though less widely appreciated, is that these human-induced changes will persist for far longer spans of time than the historical, or even archaeological, record that shapes our thinking.

The revolution in Earth system science that has taken place since 1949 should have radically altered our sense of who we are. The new insights haven’t undone the Copernican revolution, which removed us from the center of