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You Are An Epoch: Defining the Anthropocene

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The notion that the planet has entered a new geological epoch informally known as the Anthropocene — so named because it is linked to the planet-altering activities of the human species — has been percolating for more than a decade. But it has remained a contentious idea among scientists, given that other geological time periods, including the current, post-Ice Age Holocene, are typically identified by dramatic changes in rock layers and other unmistakable physical evidence that something — a meteorite strike, massive volcanic activity — cast the planet in a new direction, altering its climate and its floral and faunal mix.

Is the rise of homo sapiens the epochal equivalent of an interstellar fireball slamming into the planet? Have we so fundamentally and irreversibly changed the Earth that it warrants a new and official notch on the geologic time spiral? Given the overwhelming evidence that carbon dioxide emissions from the burning of fossil fuels is heating up the planet, many observers suggest that this is a question with an obvious answer. A new study appearing this week in the journal Nature furthers the argument for the Anthropocene — but it uses a curious dip in atmospheric carbon dioxide to stake its claim.

Led by researchers at University College London, the study suggests that the Anthropocene essentially kicked off when Europeans began colonizing the New World of the Americas. The event launched an explosive modern era of growth and trade that, among other things, shuttled plant and animal species — intentionally and otherwise — from one continent to another, resulting in what the scientists describe as a "global re-ordering of life on Earth."

The study points to fossilized maize, a Latin American plant, in European seabed sediments dating as far back as 1600 — just one example of the ecological tossed salad that got underway when Old and New Worlds collided.

Such a swift and sustained exchange of species across the oceans is without precedent in the history of the planet, the researchers suggest — and they argue that one signal of the impacts is preserved in the geologic record: A steep decline in levels of atmospheric carbon dioxide, occurring in the early 17th century and recorded in samples of Antarctic ice dating to that period. The researchers attribute the drop to Europeans' arrival in the Americas a century earlier, which among other things delivered smallpox to the hemisphere and resulted in the rapid decimation of tens of millions of Native Americans – and an abrupt end to their farming practices.

The continental collapse of farming took just a few decades, and the re-emergence of forests, particularly in Latin America, caused a detectable plunge in atmospheric carbon carbon dioxide.

The scientists considered other potential epoch-shifting (and human-generated) markers, like the nuclear fallout now detectable in soils, or even the steady rise in atmospheric carbon dioxide since the Industrial Age. But the former seemed too small-bore, while the latter struck the researchers as a trend too long and smooth to offer a clear landmark on which to plant an epochal milestone. But the combination of colonial-era species juggling and an associated, planet-wide impact preserved in ice-cores, the study team reasoned, satisfy the traditional requirements for officially designating a new epoch.

"In a hundred thousand years, scientists will look at the environmental record and know something remarkable happened in the second half of the second millennium," said UCL geologist and the study's lead author, Simon Lewis, in a statement accompanying the release of the analysis. "They will be in no doubt that these global changes to Earth were caused by their own species. Today we can say when those changes began and why."

Whether that argument or the study's findings will persuade the official gatekeepers for naming new geologic time periods is an open question. The process for such a designation is long and involved, and would ultimately require ratification by the International Union of Geological Sciences. A working group of stratigraphers — those who study rock layers and related geological phenomena — is actively weighing the question.

Meanwhile, a variety of critics — some of them active at the fringes of the climate change debate — suggest that officially recognizing the Anthropocene, which was first coined in 2000 by atmospheric chemist and Nobel laureate Paul Crutzen, is wrongheaded.

Others suggest it amounts to acceptance of something humanity ought to be working to correct.

"Is it a productive concept? It is an extremely dangerous one," Eileen Crist, an associate professor in the Department of Science and Technology in Society at Virginia Tech, told one interviewer. "Geological epochs last thousands of years: If humanity informally endorses this one, and/or it is formally vetted by the relevant scientific bodies, we are likely stuck with it forever." This, she added, would suggest that "our collective goal is not to drastically scale back our overwhelming presence, but rather to inscribe it in the annals of geological time."

Whether or not that's true, it seems likely that the notion of the Anthropocene is here to stay. From the web site of the official Anthropocene working group at the Subcommission on Quaternary Stratigraphy:

"The Anthropocene has emerged as a popular scientific term used by scientists, the scientifically engaged public and the media to designate the period of Earth's history during which humans have a decisive influence on the state, dynamics and future of the Earth system. It is widely agreed that the Earth is currently in this state."

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