There are no more woolly mammoths, woolly rhinos or giant ground sloths. Around 50,000 years ago the biggest land animals in the world began to disappear. The number-one suspect: Homo sapiens. Hunting combined with the burning of landscapes in places like Australia seem to be the main reason there are no more giant kangaroos, along with these other big animals.
The lethal pairing of hunting and burning is just one of the ways humans have been changing the world for millennia. Another is planting crops such as corn or wheat, which now cover most of the world's arable land. Chickens, cows and pigs have become the dominant megafauna, thanks to ranching and herding. Forests have been cleared to make room for agriculture and the mass expansion of the rice paddy may have led to enough greenhouse gas emissions to stave off a long cool-down into an ice age starting 5,000 years ago.
Each of these world-changing actions should be considered when choosing a start date for the Anthropocene—a potential new geologic epoch that begins when humankind started significantly altering Earth—according to a new report published in Science on April 3. So should more recent human inventions, such as widespread burning of coal or detonation of the atomic bomb. Given the long spans of time separating each of these possibilities, "we suggest simply using the term 'anthropocene' informally," says William Ruddiman, a "semiretired" paleoclimatologist at the University of Virginia and lead author of the new report. That would "allow room to recognize the millennia-long, rich history of anthropogenic changes," he says.

Courtesy of William Ruddiman

But the scientists in the Working Group on the Anthropocene are currently considering which of a number of proposals might best define a more precise start date for a formal Anthropocene epoch. The ideas range from the nadir in atmospheric carbon dioxide concentrations caused by the regrowth of CO2-absorbing forests in Africa and the Americas following mass deaths from human-introduced smallpox around 1610 to when scientists exploded the first atomic bomb in the New Mexico desert on July 16, 1945. The latter time frame, around 1950, is perhaps the most popular choice. "Sometime around the mid-20th century seems on current evidence to be where the stratigraphic markers are clearest, most widespread and most nearly synchronous," explains Jan Zalasiewicz, a geologist at the University of Leicester in England and chairman of the working group that is evaluating whether or not to incorporate the Anthropocene into the geologic time scale. That includes new data showing that fly ash from coal burning spiked around 1950, accelerating a steep rise in atmospheric CO2 levels. The spike of ash is preserved in sediment cores pulled from lakes around the world, although mostly in Europe.
Similarly, the 20th century’s two world wars introduced new geologic features to the landscape, shaped by bombardment and other military actions, that are likely to be preserved for millennia, along with all those mines, oil and gas wells and even the cracked rock, plutons and radioactivity that will mark where nuclear bombs exploded underground. Atmospheric CO2 concentrations surpassed 310 parts per million around 1950, too—roughly the highest levels that occurred during the entire duration of Homo sapiens’s existence on the planet. Concentrations have now reached 400 ppm, higher than any time in the last 800,000 years, at least.
A more recent start date to a formal Anthropocene would omit perhaps the largest changes humanity has wrought on a global scale, however: cutting down forests and plowing grasslands for farms. University of Maryland Baltimore County ecologist Erle Ellis, a co-author on this new report, has found that humans have been impacting most land on the planet for at least 3,000 years, mostly by farming. As Ruddiman asks: Would historians tie the start date of the settling of the North American West to the building of the Sears Tower in Chicago in 1973? "Not likely—it would omit too much of the actual history," he says.
Cutting down forests is reversible, however; witness the regrowth of trees in the eastern U.S. and Europe as well as the massive reforestation after Europeans invaded the Americas, precipitating massive loss of people from smallpox and other diseases. "The impact of farming could only be clearly seen as a global change to the Earth system when it stopped across a continent," argues Simon Lewis, an ecologist at the University of Leeds and co-proposer of a 1610 start date.
A shift in geologic epoch is meant to signify an irreversible change. For example, the Pleistocene endured for more than 2.5 million years despite a cycle of general freezing and thawing that represents swings of 6 degrees Celsius in global average temperatures, 120 meters of sea level rise and fall and at least 80 ppm changes in CO2 concentrations, along with the repeated growth and retreat of glaciers. "We must treat human changes in exactly the same way as natural ones so we can really understand the extent and magnitude of our impacts," argues Mark Maslin, a professor of physical geography at University College London and co-author with Lewis of an earlier paper proposing 1610 as the best start date. "Defining the Anthropocene means officially recognizing that humans are a major geological superpower and that, at this moment in time, we are the most important force shaping and changing the global environment."
In the end, whether the Anthropocene is formally designated as a new geologic epoch or not, the concept "helps us understand this rather remarkable interval of recent Earth history by putting it in a deep-time perspective," Zalasiewicz says. It shows "what is really novel in planetary evolution and what is déjà vu, and gives us some sense of scale and tempo." Humans may have been transforming parts of the planet for millennia but fossil fuels, nuclear weapons and other modern marvels helped us pick up the pace.