The Anthropocene, for the uninitiated, is a proposed (albeit still informal) geological epoch, characterized by the global impact of human activity. But the question these days, at least among researchers concerned with such things, doesn't have to do with what the Anthropocene is, but when.

Above: Bikini Atoll atom bomb tests. Image credit: US Navy

If you're late to the Anthropocene festivities, you should start here with our explainer on the subject. When you're finished, head over to The New Yorker, where Michelle Nijuis reports on the latest twist in the ongoing debate over whether humanity's impact on Earth is significant enough to warrant codifying the designation – and, if it is, when, exactly, this new epoch began. Here's a tidy summary:

In a paper published today in the journal Nature, Simon Lewis, of the University of Leeds, and Mark Maslin, of University College London, propose that the Anthropocene's "golden spike"—the line between it and the Holocene—be set at either 1610 or 1964... The year 1610 is distinguished in Antarctic ice cores by a dip in atmospheric carbon dioxide. In the decades after the Europeans—and their germs—arrived in the Americas, some fifty million people died; huge swaths of abandoned farmland reverted to forest, and the trees absorbed more carbon than the crops. The year 1964, meanwhile, is discernible in rock layers by its high proportion of radioactive isotopes—fallout from nuclear-weapons testing.

It's worth pointing out that these are not the only start dates that have been put forward in an effort to define when the Anthropocene began. In fact, what's notable about Lewis and Maslin paper is that it reviews several of these proposed dates, including the one given in the so-called "Early Anthropogenic Hypothesis," a popular explanation, first proposed by paleoclimatologist William Ruddiman in 2003, that posits a start date some 8,000 years ago, at the advent of early agriculture.

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Ruddiman's critics have said that human populations were likely too small to support his hypothesis. Lewis and Maslin, for their part, assess Ruddiman's and several other theories "against the requirements of a golden spike [i.e. a global marker of a human event in a stratigraphic material like rock, sediment, or glacier ice] " by reviewing "the major events in human history and pre-history and their impact on stratigraphic records." A summary of their analysis is presented in the table below:

In the end, Lewis and Maslin conclude that most of these proposed Anthropocene start dates "can probably be rejected because they are not derived from a globally synchronous marker." Notably, Lewis and Maslin acknowledge Ruddiman's hypothesis as the earliest date with "gold spike" potential, but note its lack of corroborating geologic evidence – a shortcoming they claim makes it a weak candidate for the beginning of the Anthropocene.

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According to Lewis and Maslin, what we're left with are the two aforementioned markers:

  1. The collision of the New World with the Old, as indicated by the 1610 minima in CO2 readings in glacial ice, and
  2. The bomb spike 1964 peak in 14C

In their review, Lewis and Maslin carefully consider the pros and cons of each date. Nijuis does a good job summarizing their analysis at The New Yorker, but I encourage you to read it for yourself. For one thing, it's just plain interesting – taking into consideration, for example, the social significance of each option. It's also quite readable.

In the end, however, the authors conclude that "choosing between the [1610 and 1964 dates] is challenging," before presenting a third option, viz. an alternative, GSSA date, based on stratigraphic evidence, to be agreed upon by a committee.

So which will win out – 1610, 1964, or some as-yet unagreed upon date? We'll have to wait for 2016, when the Anthropocene Working Group delivers its recommendations to the International Commission on Stratigraphy.

Read Lewis and Maslin's paper in today's issue of Nature.

Ht The New Yorker