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As regular readers know, scientists are in a bit of a tussle over what date marks the dawn of Earth’s “age of us” — a.k.a. the Anthropocene — even as other scientists and scholars question whether it’s hubris to think a geological epoch, as strictly defined, can result from human activity.

Candidates for the starting point for the age of humans range from the dawn of agriculture to the age of plastics and fallout and the “Great Acceleration” of greenhouse gas growth and other environmental impacts.

Now a new candidate is in the mix. In a paper in Nature, Simon L. Lewis and Mark A. Maslin of University College London* point to the year 1610, marked by, of all things, a sharp but brief dip in carbon dioxide concentrations (revealed in ice cores).

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The greenhouse-gas decline, they say, is thought to have been the result of the implosion of civilizations in the Americas as European-carried diseases killed off tens of millions of inhabitants of the “New” World. The collapse of agriculture would have resulted in enormous regrowth of forests, and thus the uptake of CO2.

They say the only other candidate that meets the criteria for defining a geological epoch and setting another “golden spike” is 1964. A release from the researchers’ university, University College London, nicely explains their thinking:

Defining an epoch requires two main criteria to be met. Long-lasting changes to the Earth must be documented. Scientists must also pinpoint and date a global environmental change that has been captured in natural material, such as rocks, ancient ice or sediment from the ocean floor. Such a marker – like the chemical signature left by the meteorite strike that wiped out the dinosaurs – is called a golden spike. The study authors systematically compared the major environmental impacts of human activity over the past 50,000 years against these two formal requirements. Just two dates met the criteria: 1610, when the collision of the New and Old Worlds a century earlier was first felt globally; and 1964, associated with the fallout from nuclear weapons tests. The researchers conclude that 1610 is the stronger candidate.

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The scientists say the 1492 arrival of Europeans in the Americas, and subsequent global trade, moved species to new continents and oceans, resulting in a global re-ordering of life on Earth. This rapid, repeated, cross-ocean exchange of species is without precedent in Earth’s history. They argue that the joining of the two hemispheres is an unambiguous event after which the impacts of human activity became global and set Earth on a new trajectory. The first fossil pollen of maize, a Latin American species, appears in marine sediment in Europe in 1600, becoming common over subsequent centuries. This irreversible exchange of species satisfies the first criteria for dating an epoch – long-term changes to Earth.

I’ve been tracking all of this quite closely, both because I’m on the Anthropocene Working Group of the international geological organization that is pondering the official scientific question and because I proposed we were entering a “geological age of our own making” back in 1992.

On the technical question, my main concern is about the challenge in trying to define an Earth epoch just as the starting gun is going off. Here’s how I posed this question to Jan Zalasiewicz, a geologist at the University of Leicester who leads the working group and was the lead author on the recent paper pointing to nuclear tests and plastics:

Most geology is retrospective…. How can you know something has begun and will be significant when we don’t know how long it will last?

His reply:

That’s one of the many novelties of the Anthropocene geologically. We don’t know how the Anthropcoene will pan out. What we do know is that sufficient change has already taken place to say that the course of Earth history has changed and the strata have changed. How much they will change will depend on numerous feedbacks, not the least the kinds that we ourselves as humans will introduce in years and decades and centuries to come. I suspect it’s a little bit like standing on the Earth 65 million years ago, a few years after the impact of the meteorite on the Yucatan peninsula in Mexico and seeing that the world has changed but not yet being able to see how it is going to evolve from then on. I think we are somewhere in that kind of position.

Our longer Skype Discussion (from January) is here:

Eric Holthaus at Slate got in touch this morning seeking my reaction to the new paper. Here’s what I wrote back:

Their proposal of 1610, focusing on that fascinating dip in CO2 connected to the collapse of American indigenous societies, is provocative and compelling. Their other alternative, the bomb-testing signature, is more in line with the views of those seeing the Great Acceleration as humanity’s geological coming of age. I’d give that proposal the highest odds. But there’s still a high hurdle to cross just in convincing the broader geology community to give a thumbs up to the basic idea of “human geology,” as they put it, no matter where the stratigraphers propose putting the Anthropocene “golden spike.” In the end, there are really two Anthropocene debates — one over the formal geological question and the other over the choices we now face, as I wrote in 1992, in confronting “a geological age of our own making.” To me, that is the bigger discussion in the end.

That’s why, for me, the final lines in the new Lewis-Maslin paper really resonated:

Past scientific discoveries have tended to shift perceptions away from a view of humanity as occupying the centre of the Universe. In 1543 Copernicus’s observation of the Earth revolving around the Sun demonstrated that this is not the case. The implications of Darwin’s 1859 discoveries then established that Homo sapiens is simply part of the tree of life with no special origin. Adopting the Anthropocene may reverse this trend by asserting that humans are not passive observers of Earth’s functioning. To a large extent the future of the only place where life is known to exist is being determined by the actions of humans. Yet, the power that humans wield is unlike any other force of nature, because it is reflexive and therefore can be used, withdrawn or modified. More widespread recognition that human actions are driving far-reaching changes to the life-supporting infrastructure of Earth may well have increasing philosophical, social, economic and political implications over the coming decades.

Lewis builds on this in a fine essay on The Conversation.

On the Bloomberg Business website, Eric Roston doesn’t show many signs he agrees with me that humans can chart a good path in this turbulent age. Here’s his headline: “The Year Humans Started to Ruin the World.”

Footnote | * Lewis also has a position at the University of Leeds.