There's no question that humans are reshaping the Earth. We've replaced vast forests with cropland. We've dammed mighty rivers, nurtured algae blooms, wiped out other species, and noticeably raised the planet's temperature.

These changes are so drastic, in fact, that many scientists say we're now living in an entirely new geological epoch, which they call the "Anthropocene." The term was first proposed in 2000 by atmospheric chemist Paul Crutzen and ecologist Eugene Stoermer. They argued that humans have replaced nature as the dominant environmental force on Earth, and we should mark the occasion.

The phrase soon began to surge in popularity. But it created a new and surprisingly contentious debate: When, exactly, did the Anthropocene begin? When did humans become this planet-altering force? Was it the Industrial Revolution in the 1700s? Was it when we exploded the first atomic bomb in 1945? Was it with the first rice farm 5,000 years ago? Or was it way earlier still — say, 50,000 years ago, when we first started killing mammoths?

As it turns out, there are pros and cons to each of these starting points. So here's a guide to this messy but often fascinating debate:

Why defining the "Anthropocene" is so tricky

Geologists usually divide up the planet's 4.5-billion-year history into eons, eras, periods, epochs, and ages. Typically, the division between different geological stages is marked by an agreed-upon boundary — physical evidence in the rocks or sediment, such as fossils, that indicate a major change in the Earth's system. (These boundaries are often known as "golden spikes.")

Officially, the International Union of Geological Sciences still says we are living in the Holocene epoch, which began 11,700 years ago after the last ice age receded. If scientists wanted to revise that and say that, actually, the Holocene has ended and we're now living in the Anthropocene, then they'd need to find a clear geological boundary between the two.

Two potential start dates for the Anthropocene: 1610 or 1964

In a big paper for Nature back in March, Simon Lewis of University College London and Mark Maslin of the University of Leeds tried to locate just such a boundary. Using the strict criteria from the International Commission on Stratigraphy, they looked at nine possible start dates for the Anthropocene.

The scientists looked for evidence of a "golden spike"

One possibility is that the Anthropocene began around 50,000 years ago, when early human migration to North America began wiping out huge megafauna such as hairless mammoths and saber-toothed cats. Or perhaps it started around 11,000 years ago, at the dawn of agriculture. Or in 1492, when Christopher Columbus and subsequent European explorers initiated a massive exchange of plants and animals and diseases between the Old and New Worlds. Or in 1945 when the first atomic bomb was tested.

This table from the paper shows all the different proposed start dates, along with physical evidence that might help demarcate a geological boundary:

Lewis and Maslin end up dismissing most of these potential start dates. Some, they said, simply unfolded over too unclear a timeframe (like the megafauna extinctions). Others, they said, were too localized (like rice farming, concentrated in Asia). Instead, they settled on two promising contenders:

1610: The first was 1610, a time at which greenhouse gas concentrations in the atmosphere dipped by 7 to 10 parts per million — a drastic change.

This dip was driven by sweeping man-made activities: At that point, the Columbian Exchange was in full swing. New species were being introduced irreversibly into both continents. European diseases like smallpox had killed tens of millions of people in North America. Agriculture was collapsing, and forests were making a comeback, absorbing more carbon dioxide.

That subsequent dip in atmospheric carbon dioxide, centered on 1610, was a potential "golden spike" — a clear global signal that could define a new geological epoch. We could call 1610 the year that, as a University of Leeds press release put it, "the collision of the New and Old Worlds a century earlier was first felt globally." Using this as a start date would also encompass things like the dawn of the industrial age.

1964: The other worthy contender for the start of the Anthropocene, Lewis and Maslin note, is the year 1964. This was the year that traces of radioactive fallout from nuclear testing reached a peak worldwide. Using this as a boundary would likely pass muster with geologists — it's quite visible in the record.

1964 also helps mark the start of the "Great Acceleration"

More usefully, using 1964 as a start year would help denote what some scientists have dubbed the "Great Acceleration" — that period over the last 50 years when economic and population growth really took off and technological advances accelerated. These changes are likely to be extremely significant to Earth's history going forward. We're now in an era when, as the authors put it, "the future of the only place where life is known to exist is being determined by the actions of humans."

The downside to using 1964? Nuclear fallout doesn't feel quite as weighty as the Columbian Exchange: "Although nuclear explosions have the capacity to fundamentally transform many aspects of Earth's functioning, so far they haven't done so," the authors conceded.

Other geological groups have also suggested atomic testing as a good marker, though they'd tweak the date slightly. One working group of the Quarternary Stratigraphy of the Geological Society in London recently recommended July 1945 as a good starting point. That was the year of the first atomic bomb test in Alamogordo, New Mexico. Those isotopic byproducts create a distinctive market in ice cores, marine sediment, and soils.

So there we go. The start of the Anthropocene was in 1610 or 1945 or 1964. Case closed? Well, not quite.

Other experts insist we shouldn't ignore ancient farming

That Nature paper, however, hasn't yet quieted the debate. In a new essay for Science this week, four researchers — William Ruddiman, Erle Ellis, Jed Kaplan, and Dorian Fuller — argue that creating a bright line in, say, 1945 doesn't really make much sense.

After all, the atomic bomb was a big deal, but starting the Anthropocene there would still miss the thousands of years before that in which humans were causing massive large-scale changes to the Earth's system through agriculture.

These authors had previously found that, for instance, the clearing of forests for agriculture 7,000 years ago led to meaningful rises in carbon dioxide emissions. Likewise, the spread of rice farming 5,000 years ago appears to have led to rises in methane emissions. (It's worth noting, though, that some climatologists have downplayed these changes as insignificant.)

True, the authors of the Science essay argue, it's hard to find a clear "golden spike" in the geological record to mark these agricultural revolutions. While the changes were globally significant, they unfolded erratically over time. Still, the authors ask, "Does it really make sense to define the start of a human-dominated era millennia after most forests in arable regions had been cut for agriculture?"

The lead author of the essay, William Ruddiman, has been arguing for quite some time for an earlier start date to the Anthropocene. He told me that much of the research on ancient forest clearances and landscape transformation was still fairly new and not yet universally accepted. But, he argued, it should change our view of how significant early human activity was.

"A lot of people still think most forest clearance happened in the industrial era," he says. "But scientists working in archaeology and landscape ecology have shown that there was an enormous amount of clearance going on long before the last few centuries."

At the same time, Ruddiman says, he's also not quite convinced that defining a single clear start date for the Anthropocene is as useful as it sounds. The identification of geological epochs, he says, was a tremendous achievement. But ever since the 1950s, scientists have been able to use geochemistry and radiometric dating to identify time frames more precisely. We can refer to things that happened thousands of years ago or millions of years ago without always having to use these geological divisions.

Indeed, the Science essay concludes by asking whether "Anthropocene" might be more useful as an informal term to describe human influence on the planet rather than a hard-and-fast geological epoch:

Despite differing views, the term "Anthropocene" is clearly here to stay. One way forward would be to use the term informally (with a small "a"). This approach would allow for modifiers appropriate to the specific interval under discussion, such as early agricultural or industrial.

"In this way," they conclude, "we could avoid the confinement imposed by a single formal designation, yet acknowledge the long and rich history of humanity's environmental transformations of this planet, for better and for worse."

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