Fire ranks among humanity's oldest and most powerful tools. Now the residue from all the oil and coal burned to power modern civilization may provide the best marker for the start of a new geologic epoch that highlights Homo sapiens’s world-changing impact, known as the Anthropocene, or "new age of humans."

"We're actually changing and continuing to change how the Earth system functions and leaving markers that could still be found in a million years time," says Earth scientist Karen Bacon of the University of Leeds in England. "That's quite incredible to think about."

The proposed marker is billions of microscopic black balls found from the high arctic to the bottoms of lakes in Chile. When a flame eats into the hydrocarbons known as coal or oil, not all the carbon atoms pair up with the oxygen that makes fire and carbon dioxide emissions possible. Some of that carbon remains bonded tightly to its fellow carbons, unburned, forming a little black sphere. These spheres are known as spheroidal carbonaceous particles (SCP) to environmental scientists and incomplete combustion products to fire engineers; the general public knows them best perhaps as soot.

That soot, fly ash or whatever you want to call it is borne aloft by the hot gases of combustion before settling gently back to Earth in lungs, into lakes and the sea or onto the landscape. The more coal and oil burned, the more black spheres formed, making such soot the perfect record of a swelling fossil-fuel pyromania. As CO2 changes the climate, one of humanity's biggest impacts on the planet as a whole, these pitted carbon balls mark the potentially irrevocable change. "In the very top of these sediment cores we take, we get these fly ash indicators," explains Leeds geologist Graeme Swindles, lead author of a paper published in Scientific Reports on May 28 (Scientific American and Scientific Reports are part of Nature Publishing Group.)

Swindles argues the SCPs are a marker for the start of the Anthropocene. For example, long boggy brown peat cores that record millennia of deposits suddenly turn black as night at their tops from the recent addition of sooty carbon. "These things are all over the place—they're everywhere," he adds, noting that he has found SCPs even in the far reaches of the Amazon rainforest, remote from any industrial activity, albeit at lower concentrations. "We have indicators that are basically like fossils that can be used to pinpoint the boundary."

Courtesy of Graeme Swindles

Environmental scientist Neil Rose of University College London has been collecting such SCP records from lakes around the world for 25 years. On every continent more fly ash starts falling into the geologic record around 1950, Rose notes in a paper published by Environmental Science & Technology on March 19, even though we have been adding SCPs to the environment for roughly 170 years now.

Such fly ash is not typically produced by natural phenomena like volcanic eruptions—except those that set off coal combustion like the massive ones that followed a space rock's impact 66 million years ago to end the Cretaceous period. Volcanic SCPs are different—much lighter in color and easy to distinguish from the dark black spheres formed by high-temperature fossil-fuel burning in coal-fired power plants and vehicles. "If SCPs survive as long as the particles from the [Cretaceous–Paleogene] boundary, then they will outlive many radionuclides with half-lives—and probably most ice records on the planet," Rose notes.

That makes for a marker that will be preserved in the more than four-billion-year-long geologic record. Similar soot particles can still be found from the volcanic eruptions that aided the mass extinction at the end of the Permian period around 252 million years ago. The dark balls also appear at roughly the same time everywhere—circa 1950—exactly what geologists look for in determining the boundaries of geologic ages, like the newly proposed Anthropocene.

Soot is not the only potential marker. Other proposals include the radionuclide spike in the wake of the first atom bomb explosion in 1945 to a dip in CO2 levels around 1610 occasioned by the mass deaths that followed European arrival in North and South America a century or so earlier. But none of these alternatives are directly tied to a change in the entire planet. Soot, on the other hand, is the aftermath of the coal and oil burning that is also responsible for changing the global climate.

Swindles even has a spot that might make for a good location to host a golden spike in the geologic timescale: Malham Tarn, the highest glacial lake in England situated in the Yorkshire Dales. The lake and adjacent bog record some 8,000 years of human activity in the vicinity, from the advent of farming millennia ago to the industrial revolution, and remains largely unchanged throughout its history But in the last 50 years, "everything changes," Swindles says. "Traces of pollution, fly ash particles coming in, evidence of recent climate change—all the records from that site basically look like hockey sticks."

That half-century has been dubbed the "Great Acceleration," and includes everything from plastics to the boom in population that put more than seven billion people on the planet today. This further strengthens the case for a shift in geologic epoch around 1950. "Any potential [spike] would have both the radionuclide record as well as the SCP one—as well as a few invasive species, bits of microplastic and stable carbon isotopes," explains geologist Jan Zalasiewicz of the University of Leicester in England and chairman of the working group that is evaluating whether or not to add the Anthropocene to the geologic timescale. "It adds to the evidence that the mid–20th century is not only important as regards Earth system change, but that it left durable markers in strata."

A shift in geologic epoch signifies an irreversible change in this planet's history, whether the icy ages of the Pleistocene or the long summer of the Holocene. "Geologists simply try to do the best they can to define a boundary that works reasonably well reasonably often," Zalasiewicz says. Humanity's quest for fire has left a consistent trace, remarkable even in the vast expanse of geologic time—and that trace is, in large part, a dirty, black smudge that covers the globe as well as higher concentrations of atmospheric CO2 that may set Earth’s temperature for millennia.