For decades, scientists have argued that the Earth is entering a new geological phase predominantly shaped by human activity, dubbed the Anthropocene.
But in order to establish a new epoch, scientists need to provide proof that the planet is experiencing irreversible global changes, and that these changes are leaving permanent geological markers. An example of such a marker, or "golden spike," would be the iridium layer left behind by the impact that killed the dinosaurs.
According to a paper published today in Nature, the Anthropocene fits both these criteria. Moreover, the authors argue that officially accepting this new epoch as a scientific fact may spark a paradigm shift in terms of how humans view their relationship to the planet.
"Accepting the Anthropocene acknowledges that humans are a geological force of nature, as powerful as the meteorite strikes or sustained volcanic eruptions that have characterized past periods in Earth's history," lead author Simon Lewis, a geologist at the University College London (UCL), told Motherboard.
Lewis and his co-author Mark Maslin, a fellow UCL geologist, suggest that the year 1610 is the best candidate for the Anthropocene's start date, because it was around this time that humans left a morbidly fascinating golden spike in the geological history of Earth.
It was presaged by the colonization of the Americas by Europeans, which kicked off in 1492 and resulted in the deaths of about 50 million indigenous peoples. This dramatic crash of an agriculturally sophisticated population resulted in mass abandonment of farmland. As wildlife reclaimed these regions, atmospheric carbon dioxide levels noticeably dropped, due to the uptick in carbon storage as a result of reforestation.
"The decline in carbon dioxide led to the coolest part of the Little Ice Age, which had impacts globally, but particularly in Europe and China," said Lewis. "There have been some hints that other population crashes may have had effects on global atmospheric carbon dioxide concentrations."
For example, the Black Death also killed off about 50 million people, but the carbon dioxide dip was not as pronounced as it was in the case of the 16th century New World crash. "I suspect the difference in the 16th century was due to the number and speed of the deaths," said Lewis.
"These deaths were in the tropics, so the vegetation recovered much more quickly than vegetation would in the temperate zone, and often tropical lands stored more carbon when [they] recover compared to temperate lands," he added.
But the European colonization of the New World didn't just leave atmospheric fingerprints. It also represents one of the biggest ecological shakeups in the planet's history.
"Geologically, the repeated jumping of species across oceans is unprecedented, which alongside anthropogenic climate change, and farming reducing the land area available for other species, will have profound evolutionary consequences," Lewis told me. "Collectively these changes have pushed Earth onto a new evolutionary trajectory."
Realizing your own species is the geological equivalent of a meteor impact is a hard pill to swallow, but unfortunately, that's the reality. But Lewis and Maslin don't have a "doom and gloom" approach to the epoch. Rather, the authors argue that acknowledging the Anthropocene will be a crucial step in managing its destructive edge.
"Recognizing the global and long-term impacts of human activity ought to focus political attention on global discussions and agreements on what are acceptable and unacceptable environmental changes," Lewis said, citing increasing geopolitical awareness of climate change as an example.
"Overall thinking about the world in the context of the Anthropocene may help us think about and better prioritise the long-term impacts of our actions today," he said.
The International Commission on Stratigraphy is expected to make a formal decision about recognizing the Anthropocene as a distinct geological epoch in 2016. Regardless of the ruling's outcome, Lewis and Maslin have put forth powerful evidence that the age of the human-shaped planet has been around for centuries.