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When did the Anthropocene Begin?

This was a basic tenet of geological science: that human chronologies were insignificant compared with the vastness of geological time; that human activities were insignificant ompared with the force of geological processes. And once they were. But no more.

—NAOMI ORESKES¹

In 2008, Anthropocene was accurately described as “a vivid yet informal metaphor of global environmental change.”² Although it was proposed as a new interval of geological time, it had not been defined in geological terms. It is noteworthy, for example, that the scientists who used the word described it variously as a new age, epoch, or era, even though those terms have distinct meanings in geology. Some academic papers treated it as little more than an informal label for the period since the Industrial Revolution, without reference to qualitative changes in the Earth System.³ None of the principal authors of Global Change and the Earth System were geologists, and the IGBP does not seem to have formally or informally submitted the concept to geological organizations for consideration.

As long as it remained informal, Anthropocene was convenient shorthand for a wide variety of phenomena, but its scientific usefulness was limited by the lack of a specific definition based on objective criteria. Loosely defined and even undefined words are widely used in casual conversation, but in science lack of clear definitions can cause confusion.

Fortunately, some geologists set out on their own to determine whether a prima facie case could be made for formally defining the Anthropocene as a new geological period, using appropriate geological criteria. And that led to a question that has implications far beyond geology: When did the Anthropocene begin?

The Geological Time Scale

Geologists divide Earth’s 4.5 billion–year history into a hierarchy of time intervals—eons, eras, periods, epochs, and ages—called the geological time scale. We live in the Quaternary Period, the most recent subdivision of the Cenozoic Era, which began 65 million years ago. The Quaternary in turn is divided into two epochs—the Pleistocene, which began 2.58 million years ago, and the Holocene, from 11,700 years ago to the present.

The divisions are not arbitrary: they reflect major changes in the dominant conditions and forms of life on Earth, as revealed in geological strata—layers laid down over time in rock, sediment, and ice. The Cenozoic Era is marked by the rise of mammals, following the mass extinction of dinosaurs and most other plants and animals at the end of the Mesozoic. The Pleistocene Epoch was characterized by the repeated expansions and contractions of continental ice sheets in the Northern Hemisphere that are popularly called Ice Ages. The last glacial retreat marks the beginning of the Holocene, which has been characterized by a stable, relatively warm climate: all human history since shortly before the invention of agriculture has occurred in Holocene conditions.

So for geologists, formally approving the Anthropocene is not like applying a faddish label to a current trend, comparable to the Jazz Age or the Gay Nineties. It would mean declaring, on clear scientific criteria, that the present is as different from the Holocene as the Holocene was from the Pleistocene before it.

The subdiscipline of geology that studies and sets standards for geological strata is stratigraphy, and it was the Stratigraphic Commission of the Geological Society of London, the world’s second-largest organization of geologists, that decided to initiate a review. In late 2007, after a year of investigation, the commission submitted a paper to the journal of the world’s largest geological association, the Geological Society of America, which featured it on the cover of the February 2008 issue. The title was a question: “Are We Now Living in the Anthropocene?”⁴

Although “increasing levels of human influence” can be seen in thousands of years of Holocene strata, the authors concluded that before the Industrial Revolution, “human activity did not create new, global environmental conditions that could translate into a fundamentally different stratigraphic signal.” Since then, however, “the exploitation of coal, oil, and gas in particular has enabled planet-wide industrialization, construction, and mass transport,” producing a wide range of changes that leave traces in strata around the world. The commission focused on four areas of current and expected change that might leave traces for future geologists.

• Increased erosion now exceeds natural sediment production by an order of magnitude.

• Carbon dioxide and methane levels are significantly higher than at any time in nearly a million years, and are rising much faster than in any previous warming period.

• Mass extinctions, species migrations, and replacement of natural vegetation with agricultural monocultures are changing the nature of the biosphere.

• Sea level rises may reach ten to thirty meters for each 1°C increase in temperature, and acidification of ocean water will have severe effects on coral reefs and plankton.

The combined impact of these changes “makes it likely that we have entered a stratigraphic interval without close parallel” in the Quaternary Period, but “it is too early to state whether or not the Quaternary has come to an end.” The authors conservatively concluded that the Anthropocene should be evaluated as a new epoch, not a new period.

Earth has endured changes sufficient to leave a global stratigraphic signature distinct from that of the Holocene or of previous Pleistocene interglacial phases, encompassing novel biotic, sedimentary, and geochemical change. These changes, although likely only in their initial phases, are sufficiently distinct and robustly established for suggestions of a Holocene-Anthropocene boundary in the recent historical past to be geologically reasonable….

Sufficient evidence has emerged of stratigraphically significant change (both elapsed and imminent) for recognition of the Anthropocene … as a new geological epoch to be considered for formalization by international discussion.

This tentative “yes, there is enough evidence for the subject to be considered” received a remarkably quick response. Within months, the International Commission on Stratigraphy (ICS), the committee of the International Union of Geological Sciences (IUGS) that has responsibility for the geological time scale, asked Jan Zalasiewicz, chair of the London Society’s Stratigraphic Commission, to convene an international Anthropocene Working Group to investigate and report on whether to formally define the Anthropocene as a geological epoch.

To recommend such a change, the AWG must find that there have been major, qualitative changes to the Earth System, and that geological evidence preserved in rock, sediment, or ice uniquely differentiates layers laid down in the Anthropocene from earlier times. To define when the Holocene/Anthropocene transition occurred, they must propose either a specific stratigraphic marker (often called a “golden spike”) or a specific date, or both.⁵

The Anthropocene Working Group includes some 38 volunteer members from 13 countries on five continents. About half are geologists; the rest have backgrounds in other earth sciences, archaeology, and history. They hope to make recommendations during the 35th International Geological Congress in South Africa in August 2016, but formalization of the Anthropocene is not a foregone conclusion. The recommendation might be that the term should remain informal, or that a decision should be delayed. If the AWG recommends formalization, the geological time scale still will not be changed unless 60 percent majorities in the ICS and the IUGS agree.

As paleontologist Anthony Barnosky says, if the Anthropocene gets through all those hoops, “it would not only be a very big deal for earth scientists—the academic equivalent of, say, adding a new amendment to the United States Constitution—but it would also underscore that people have become a geological force every bit as powerful as the kinds of forces that turned an ice-covered Earth into a warm planet, or that wiped out the dinosaurs.”⁶

In his first articles on the Anthropocene, Paul Crutzen suggested that the new epoch may have begun at the time of the Industrial Revolution, when large-scale burning of coal launched a long-term rise in atmospheric concentrations of greenhouse gases. That led some observers to conclude that the issue had been prejudged, and many words have been wasted criticizing or praising Crutzen and his co-thinkers for supposedly believing (as some green theorists do) that industrialization as such is the source of all environmental problems. Actually, Crutzen was opening a discussion, not declaring a conclusion: he clearly stated that “alternative proposals can be made.”⁷

And in fact a dozen or more proposals for dating the Anthropocene have been made to the AWG. Though they differ substantially from one another, the starting dates under serious consideration fall into two broad groups that can be labelled Early and Recent, depending on whether the proposed starting date is in the distant past, or relatively close to the present.

An Early Anthropocene?

The first Early Anthropocene proposal was advanced by American geologist William Ruddiman, who argues that the Anthropocene began when humans began large-scale agriculture in various parts of the world between eight and five thousand years ago. Those activities, he believes, produced carbon dioxide and methane emissions that raised global temperatures just enough to prevent a return to the Ice Age.⁸

Other Early Anthropocene arguments suggest dating the Anthropocene from the first large-scale landscape modifications by humans, from the extinction of many large mammals in the late Pleistocene, or from the formation of anthropogenic soils in Europe. One widely discussed proposal focuses on the intercontinental exchange of species that followed the European invasions of the Americas, and proposes 1610 as a transition date. Some archaeologists propose to extend the beginning of the Anthropocene back to the earliest surviving traces of human activity, which would take in much of the Pleistocene, and others have suggested that the entire Holocene should simply be renamed Anthropocene, since it is the period when settled human civilizations first developed.

This outpouring of proposals reflects humanity’s long and complex relationships with Earth’s ecosystems—many of the proposed beginnings are significant turning points in those relationships, and deserve careful study. But the current discussion is not just about human impact: “The Anthropocene is not defined by the broadening impact of humans on the environment, but by active human interference in the processes that govern the geological evolution of the planet.”⁹ None of the Early Anthropocene options meet that standard, and none of them led to a qualitative break with Holocene conditions.

Even if Ruddiman’s controversial claim that the agricultural revolution caused some global warming is correct, that would only mean that human activity had extended Holocene conditions. The recent shift out of Holocene conditions, to a no-analog state, would still need to be evaluated and understood. Noted climatologist James Hansen and his colleagues write:

Even if the Anthropocene began millennia ago, a fundamentally different phase, a Hyper-Anthropocene, was initiated by explosive 20th-century growth of fossil fuel use. Human-made climate forcings now overwhelm natural forcings. CO₂, at 400 ppm in 2015, is off the scale…. Most of the forcing growth occurred in the past several decades, and two-thirds of the 0.9°C global warming (since 1850) has occurred since 1975.¹⁰

The idea of an Early Anthropocene has been promoted by anti-environmentalist lobbyists associated with the Breakthrough Institute, because it supports their claim that there has been no recent qualitative change and thus there is no need for a radical response. In their view, today’s environmental crises “represent an acceleration of trends going back hundreds and even thousands of years earlier, not the starting point of a new epoch.”¹¹

As Clive Hamilton and Jacques Grinevald explain, the Early Anthropocene argument is attractive to conservatives because it minimizes recent changes to the Earth System:

It “gradualizes” the new epoch so that it is no longer a rupture due principally to the burning of fossil fuels but a creeping phenomenon due to the incremental spread of human influence over the landscape. This misconstrues the suddenness, severity, duration, and irreversibility of the Anthropocene, leading to a serious underestimation and mischaracterization of the kind of human response necessary to slow its onset and ameliorate its impacts.¹²

A Recent Anthropocene?

The various Early Anthropocene proposals have been considered carefully and rejected by a substantial majority of the Anthropocene Working Group. In January 2015, over two-thirds of the members signed an article titled “When Did the Anthropocene Begin?: A Mid-Twentieth-Century Boundary Level Is Stratigraphically Optimal.”

Humans started to develop an increasing, but generally regional and highly diachronous, influence on the Earth System thousands of years ago. With the onset of the Industrial Revolution, humankind became a more pronounced geological factor, but in our present view it was from the mid-20th century that the worldwide impact of the accelerating Industrial Revolution became both global and near-synchronous.¹³

They rejected the Early Anthropocene proposals because they only address one aspect of the case for a new epoch, human impact on terrestrial ecosystems. “The significance of the Anthropocene lies not so much in seeing within it the ‘first traces of our species’ (i.e., an anthropocentric perspective upon geology), but in the scale, significance, and longevity of change (that happens to be currently human-driven) to the Earth System.”¹⁴

In January 2016, the AWG majority published a particularly strong statement on whether changes to the Earth System have been sufficient to justify declaring a new epoch, and if so, when the new epoch began. The title of their paper, published in Science magazine and signed by twenty-four AWG members, is unequivocal: “The Anthropocene is functionally and stratigraphically distinct from the Holocene.”

Interviewed by The Guardian, Colin Waters, lead author of the paper, described the global shift as “a step-change from one world to another that justifies being called an epoch. What this paper does is to say the changes are as big as those that happened at the end of the last ice age. This is a big deal.”¹⁵

The January 2016 paper summarized recent research that identifies major ways in which Holocene conditions no longer exist:

• Atmospheric concentrations of CO₂ have exceeded Holocene levels since at least 1850, and from 1999 to 2010 they rose about 100 times faster than during the increase that ended the last ice age. Methane concentrations have risen further and faster.

• For thousands of years global average temperatures were slowly falling, a result of small cyclical changes in the Earth’s orbit. Since 1800, increased greenhouse gases have overwhelmed the orbital climate cycle, causing the planet to warm abnormally rapidly.

• Between 1906 and 2005, the average global temperature increased by up to 0.9°C, and over the past 50 years the rate of change doubled.

• Average global sea levels began rising above Holocene levels between 1905 and 1945. They are now at their highest in about 115,000 years, and the rate of increasing is accelerating.

• Species extinction rates are far above normal. If current trends of habitat loss and overexploitation continue, 75 percent of species could die out in the next few centuries. This would be Earth’s sixth mass extinction event, equivalent to the extinction of the dinosaurs, 65 million years ago.

A particularly frightening observation: even if emission levels are reduced, by 2070 Earth will be the hottest it has been in 125,000 years, which means it will be “hotter than it has been for most, if not all, of the time since modern humans emerged as a species 200,000 years ago.”

Much of the paper focused on a key question for geologists: Has human activity produced a stratigraphic signature in sediments and ice that is distinct from the Holocene? It turns out, contrary to the doubts some expressed at the beginning of this process, that future geologists will have a wealth of indicators to choose from:

Recent Anthropogenic deposits contain new minerals and rock types, reflecting rapid global dissemination of novel materials including elemental aluminum, concrete, and plastics that form abundant, rapidly evolving “technofossils.” Fossil fuel combustion has disseminated black carbon, inorganic ash spheres, and spherical carbonaceous particles worldwide, with a near-synchronous global increase around 1950.

Anthropocene ice and sediments are also marked by unique concentrations of chemicals, such as lead from gasoline, nitrogen and phosphorus from fertilizers, and carbon dioxide from burning fossil fuels. But “potentially the most widespread and globally synchronous anthropogenic signal is the fallout from nuclear weapons testing.” Residues from hydrogen bomb explosions that began in 1952 peaked in 1961–62, leaving a clear worldwide signature.

Each of these stratigraphic signatures is either entirely new or outside of the Holocene range of variability—and the changes are accelerating. The paper recommended that the International Commission on Stratigraphy accept the Anthropocene as a new epoch.

On the question of when the Anthropocene began, the authors’ analysis was “more consistent with a beginning in the mid-20th century” than with earlier proposed dates. They did not make a specific midcentury recommendation beyond noting that a number of options have been suggested, ranging from 1945 to 1964.

Finally, they left open the question of “whether it is helpful to formalize the Anthropocene or better to leave it as an informal, albeit solidly founded, geological time term, as the Precambrian and Tertiary currently are.”

This is a complex question, in part because, quite unlike other subdivisions of geological time, the implications of formalizing the Anthropocene reach well beyond the geological community. Not only would this represent the first instance of a new epoch having been witnessed firsthand by advanced human societies, it would be one stemming from the consequences of their own doing.

It is still possible that the usually conservative International Commission on Stratigraphy will either reject, or decide to defer, any decision on adding the Anthropocene to the geological time scale, but as the AWG majority writes, “The Anthropocene already has a robust geological basis, is in widespread use, and indeed is becoming a central, integrating concept in the consideration of global change.”

In other words, failure to win a formal vote will not make the Anthropocene go away.