Astronomers have been telling us for nearly 500 years that humanity is not the most important thing in the universe. Evolutionary biologists established long ago that we're not even the greatest show on Earth.
Now, geologists - the scientists who literally decide what on Earth is going on - may reach the opposite conclusion: Humanity is the most powerful force on the planet, shaping the environment more than water, wind, or plate tectonics.
Fifteen years ago, two prominent researchers suggested that the Earth has formally entered a phase of human domination. Unless there's some unforeseen calamity caused by volcanic activity or a meteor, they argued, "mankind will remain a major geological force for many millennia, maybe millions of years, to come".
Nobel prize-winning chemist Paul Crutzen and University of Michigan biologist Eugene Stoermer called this new episode in planetary history the Anthropocene Epoch. The idea has been gaining steam in the scientific and mainstream press for several years.
This week, the journal Nature dedicates more than nine pages to the next logical question: If we have crossed into the Anthropocene - which "appears reasonable", they write with understatement - when did it begin?
Geologists are quite insistent on physical evidence. Wherever possible, each of the planet's eons, eras, periods, epochs, and ages are distinguished with a "golden spike", a physical marker somewhere in rock, glacier, or sediment that signals evidence of big changes in the Earth's operating system. It needn't be gold, or even a spike, but without satisfying the International Commission on Stratigraphy's requirements (which includes several additional procedural hurdles), there will be no new epoch.
The Nature article, by Simon Lewis of University College London and Mark Maslin of the University of Leeds, evaluates nine possibilities that others have put forward as the starter's pistol of the Anthropocene Epoch. The episodes reach as far back as tens of thousands of years ago, when people hunted large mammals to extinction. Others are as recent as the post-World War II period, when such "persistent industrial chemicals" as plastics, cement, lead, and other fruits of the laboratory started to find their way into nature.
The authors ultimately dismiss all but two of the examples because the events were too local (rice farming in Asia) and happened over too long a time span (the extinction of large mammals), which are two main obstacles to a golden spike. The two dates that meet their standard are 1610 and 1964. Let's look at 1610 first.
The "Columbian Exchange" refers to the worldwide migration of plants, animals, disease, and who knows what else during the age of European colonisation. It was also a time of carnage. The estimated human population of the Americas in 1492 ranges from 54 million to 61 million people. By 1650, that figure had been decimated, to about 6 million, by disease, war, slavery, and famine. The loss of so many people meant a "near-cessation of farming and reduction in fire use". As a result, forests and vegetation regenerated around 1610, drawing massive amounts of carbon dioxide out of the atmosphere, globally. Carbon dioxide levels dropped by 7 parts to 10 parts in every million parts of air. (That's equivalent to the amount of CO2 we're now adding to the atmosphere every three or four years.)
Lewis and Maslin favour this "New-Old World Collision" as the beginning of the Anthropocene. Shifting species are a hallmark of geologic-scale change, as are significant changes in atmospheric composition; 1610 offers both. The authors also like 1610 because it captures the Industrial Revolution, which Crutzen and Stoermer had suggested.
Of the nine possibilities they lay out, the only other one that might pass muster is 1964, when radioactive isotopes from nuclear bomb testing peaked.
Curiously, the authors lament that 1964 technically meets the standards for evidence, but not for typical levels of catastrophe wrought by geologic change: The bombs never went off. "One disadvantage is that although nuclear explosions have the capacity to fundamentally transform many aspects of Earth's functioning," they write, "so far they have not done so."
The Nature authors muse that 1610 and 1964 would have very different cultural and historical meaning. The earlier date blames "colonialism, global trade, and coal" for the Anthropocene. The bomb spike tells the story of a high-tech civilisation that invented the means to destroy the world but successfully avoided it.
At the broadest level, Lewis and Maslin write, "the formal definition of the Anthropocene makes scientists arbiters, to an extent, of the human-environment relationship, itself an act with consequences beyond geology".
But for there to be consequences beyond geology of the geologists' decisions, the rest of the world has to take notice of the scale of the change at hand. It's just not clear this is happening yet.