Nobody knows which day of the week a six mile-wide asteroid crashed into what would someday be the Yucatan Peninsula. What people do know is that day was around 65 million years ago, and that the days that came after were colder, darker, and filled with fewer and fewer dinosaurs.

The collision reconfigured Earth's life support systems by kicking up huge amounts of dust, vaporizing massive volumes of water, and triggering hundreds of earthquakes and volcanic eruptions. The strike—and ensuing mass extinction—marks one of the most well-known geologic divisions, between the Cretaceous twilight and the Paleogenic dawn. In terms of global impact, humans are like a scattershot version of that asteroid; they have changed the planet so much that many scientists believe modern society deserves its own geologic epoch—the Anthropocene. And while nobody knows which day humans became a force of nature, a pair of scientists believe they have an equation that can pinpoint the year.

This planet is about 4.5 billion years old. For at least three quarters of that time, it has supported life. "The Earth is generally in a state of balance, with feedback loops that keep things like the atmosphere and temperature at equilibrium for deep spans of time," says Owen Gaffney, a writer and co-author of the new study, published in the Anthropocene Review. During those times of equilibrium, lifeforms evolve slowly, extinction is rare, and biodiversity increases. Then along comes an asteroid strike or megavolcano eruption. Or the Earth tilts half a degree on its axis. Each cataclysm alters the atmosphere, temperature, ocean composition, and dozens of other processes that determine what is fit enough to survive.

Item: In 1700, human-used land covered about 5 percent of the Earth. In 2000, it was about 55 percent.

Item: Human greenhouse gas emissions are causing the ocean to acidify at nearly the rate it did prior to the Permian extinction—the planet's largest—about 300 million years ago.

Item: At a lowball estimate, the current rate of extinction is 10 times higher than during times of ecological equilibrium.

“You could make such a long list of these things that it gets boring talking about all the ways that humans are changing the planet,” says Erle Ellis, landscape ecologist at the University of Maryland. What’s controversial, he says, is this paper’s attempt to pinpoint a date when human activity came to dominate the biosphere.

Ellis is a member of the Anthropocene working group, tasked with doing the preliminary work that would eventually define the Anthropocene as a formal geological epoch. Formalizing it would be a huge deal, and is hotly controversial among earth scientists. At issue is not whether human activity is changing life on Earth. "There's no controversy there," says Ellis. The real problems vexing the working group are practical: Will all this human activity compress into a stratigraphic smear large enough for future geologists to study? And what is the point of naming a layer of rock that hasn't even formed yet? "Putting a new epoch in now really does nothing useful for stratigraphers and geologists working in bodies of rock to understand formation processes," says Ellis.1

Gaffney's co-author, climate scientist Will Steffen of Australian National University, is also a member of the working group. In the pair's new paper, they describe an equation that compares the effects of the last few hundred years to the baseline conditions of the Holocene, which spans the last 11,700 years. "We looked across the board at key Earth processes, and the speed at which they had changed," says Gaffney. For most of that time, the systems were in equilibrium—held stable by phlegmatic solar activity, a 26.5 degree tilt to the Earth's axis, and an absence of island-sized rocks falling from the sky.

According to their calculations, human activity eclipsed the sun, Earth, and errant falling stars as the dominant process shaping life on this planet around 1950. "This coincides with things like the first nuclear bombs, which put traceable radiation in the atmosphere, which is visible in the sedimentary record," says Gaffney. They aren't the first to fix on the post-war period. The 1950s mark what many researchers call the Great Acceleration, when the booming middle class caused spikes in global GDP, agricultural land use, paper production, dam building, personal vehicle ownership, international tourism, and other markers of consumption. Steffen and Gaffney's equation just adds more oomph to the argument that the Anthropocene began in the same era as color TVs.

Gaffney doesn't shy away from the implications here—that a relatively small subset of humans are responsible for the Anthropocene and all it entails. "Our key conclusion is that the key driver of global change is the production and consumption of goods by the world's upper and middle classes," he says. "Defining the Anthropocene could have broader societal impacts, and the potential to shift world views, similar to how Darwin's theory of evolution, or Copernicus' heliocentric astronomy caused huge paradigm shifts."

Humans aren't the first organisms to reshape the world—nor will they likely be the last. About 2.3 billion years ago, single-celled organisms called cyanobacteria developed photosynthesis, inhaling carbon dioxide and flooding the atmosphere with oxygen. Their collective outgassing is sometimes called the Oxygen Holocaust, as it suffocated much of the anaerobic life that had dominated Earth. Cyanobacteria were so successful that their dead little bodies eventually congealed into vast oil reserves. And when the Anthropocene layer does form, the carbon burned from that oil will be why it has such a nice, dark hue.

1 For what it's worth, Ellis is among a minority of Anthropocene working group members who believes the epoch's roots go far deeper into human history. "The Industrial Revolution happened way before 1950," he says. "You can also consider the incredible effects that agriculture, which is thousands of years old, has had on the biosphere."