If you know how to read it, the face of a cliff can be as compelling as the latest bestselling novel. Each layer of rock is a chapter in Earth’s history, telling stories of birth and death, winners and losers, that help scientists understand the evolution of the planet over the past 4.6 billion years.
While humans arrived only recently on geologic time scales, our species already seems to be driving some major plot developments. Agriculture occupies about one-third of Earth's land. The atmosphere and oceans are filling up with chemical signatures of our industrial activity. Whole ecosystems have been reshaped as species are domesticated, transplanted or wiped out.
These changes have become so noticeable on a global scale that many scientists believe we have started a new chapter in Earth’s story: the Anthropocene. Atmospheric chemist Paul Crutzen popularized the term in the early 2000s, and it has become engrained in the scientific vernacular. But don’t ask what the Anthropocene technically means unless you’re in the mood for some drama.
“It’s not research, it is diplomacy. It’s not necessary for geologists,” says Lucy Edwards, a researcher with the U.S. Geological Survey. Others think there is a case to be made for at least trying to codify the Anthropocene, because it is forcing the global community to think about the true extent of human influence. "It focuses us on trying to work out how we measure the relative control of humans as opposed to nature," says Tony Brown, a physical geographer at the University of Southampton in the United Kingdom.
"For example, is human activity altering the rate of uplift of mountains? If you had asked that question 20 years ago, geologists would have looked at you as if you were mad," says Brown. "But we know some faults are lubricated by precipitation, so if we are altering global precipitation patterns, there is a slight chance of a link. If that is the case, that is quite a profound potential interaction between humans and their environment."
The International Commission on Stratigraphy—the ruling body that sets formal boundaries on geologic ages—has set up a working group to study the case for making the Anthropocene official. The crux of the debate is where to place the starting boundary line, or base. Geologists continue to tinker with the bases for well-established epochs, eras and ages, and there is usually a relatively wide margin of error. "Even the most precisely defined, the end of the Cretaceous 66 million years ago, is plus or minus 3,000 years. This is minute in geological terms but very big in humans terms," says Brown.
In the reference text "The Geologic Time Scale 2012", Crutzen and colleagues lay out three main options for the start of the Anthropocene. It's possible to set the boundary in the early part of the current epoch, called the Holocene, which began about 11,700 years ago. The idea is that the dawn of agriculture in the early Holocene kicked off a steady rise in carbon dioxide that has altered Earth's natural climate cycles. But that potential base is controversial, in part because agriculture spread to various locations at different times, and a formal interval of geologic time should be recognizable globally.
The next option, and the one preferred by Crutzen, is to put the base near the Industrial Revolution, which the book authors argue became a global phenomenon in the early 19th century. "This is … where the combination of industrialization and the acceleration of population growth created a clear step change in the human signal," the scientists write. But like agriculture, industrial activity didn't start everywhere at once—China was smelting iron in the 11th century, for instance—so not everyone may be happy with the choice.
Still others have proposed linking the base to a global spike in a signal that is unquestionably caused by humans: radioactive isotopes from atomic bomb detonation in the 1950s. Distinctive levels of radioactive substances from bomb use and testing were distributed widely and will linger in the rock record for millennia. But they are not a perfect solution either, as radioactive decay means that the signal will eventually be lost.
Another way to approach the problem is to consider when human influence became the dominant force of change on a combination of Earth systems. Natural cycles and cataclysmic events have affected the environment over deep time, and some of those forces are still at work. But in addition to the signal from atomic bombs, the mid-20th century saw an acceleration in a variety of human impacts, with a doubling of population size, a massive increase in vehicle use and a rapid shift from mostly rural to urban living, which triggered an increase in construction and large infrastructure projects such as dams.
"Probably in the late part of the last century, humans became responsible for moving more soil or rock than natural agencies," says Brown. "We’ve increased erosion rates in most parts of the world, but we've also trapped a lot of sediments, because we've dammed most of world's really big rivers."
“For geologists, there are lots of features on the present-day planet that are human-made or distorted,” says James Ogg, a stratigrapher with Purdue University and the China University of Geosciences. But he believes the best strategy may be to keep the term unofficial. "The Anthropocene is a very useful term, because it helps show the dramatic impacts we’ve had on all aspects of the planet," he says. "But on the geologic time scale, you need a place and time that can be correlated around the world, so that people are speaking the same language. For the Anthropocene, is there actually a time level that we can correlate?"
Brown agrees: "The majority of scientists who engage with the question will say, 'yes we are in Anthropocene'. And it's OK if you just say that. My view is, at moment, we're better off not formalizing it, partly because we will get into very long and not very productive argument about where the boundary should be."
Edwards adds that another problem with making the Anthropocene official is deciding when it might end, and thus how large of a time interval to assign it. The use of the "cene" suffix signals to geologists that it is an epoch (tens of millions of years). But it's also sometimes referred to as an age (millions of years) within the Holocene, and some people say it should be an even smaller unit, a stage.
Given the term’s complexity, if you really just have to have a formal definition, you better be prepared to wait, Edwards says. "Geologists have learned from the Pluto experience," she says, referring to the 2006 vote by the International Astronomical Union to take away Pluto's official status as a planet. "We're not just going to show up at a union meeting and have a decision with all these glaring errors that makes us a laughingstock. Unfortunately, the decision to take it slowly and work it out bothers some people. But to geologists, what's a million years?"