The atmosphere recorded the mass death, slavery and war that followed 1492. The death by smallpox and warfare of an estimated 50 million native Americans—as well as the enslavement of Africans to work in the newly depopulated Americas—allowed forests to grow in former farmlands. By 1610, the growth of all those trees had sucked enough carbon dioxide out of the sky to cause a drop of at least seven parts per million in atmospheric concentrations of the most prominent greenhouse gas and start a little ice age. Based on that dramatic shift, 1610 should be considered the start date of a new, proposed geologic epoch—the Anthropocene, or recent age of humanity—according to the authors of a new study.
 
"Placing the Anthropocene at this time highlights the idea that colonialism, global trade and the desire for wealth and profits began driving Earth towards a new state," argues ecologist Simon Lewis of the University of Leeds and University College London. "We are a geological force of nature, but that power is unlike any other force of nature in that it is reflexive, and can be used, withdrawn or modified."
 
Lewis and his U.C.L. colleague, geologist Mark Maslin, dub the decrease in atmospheric carbon dioxide the "Orbis spike," from the Latin for world, because after 1492 human civilization has progressively globalized. They make the case that human impacts on the planet have been dramatic enough to warrant formal recognition of the Anthropocene epoch and that the Orbis spike should serve as the marker of the start of this new epoch in a paper published in Nature on March 12. (Scientific American is part of Nature Publishing Group.)

orbis-spike

The Anthropocene is not a new idea. As far back as the 18th century, the first scientific attempt to lay out a chronology of Earth's geologic history ended with a human epoch. By the 19th century, the idea was commonplace, appearing as the Anthropozoic ("human life rocks") or the "Era of Man" in geology textbooks. But by the middle of the 20th century, the idea of the Holocene—a word which means "entirely recent" in Greek and designates the most recent period in which the great glacial ice sheets receded—had come to dominate and incorporated the idea of humans as an important element of the current epoch but not the defining one.
 
That idea is no longer sufficient, according to scientists ranging from geologists to climatologists. Human impacts have simply grown too large, whether it's the flood of nitrogen released into the world by the invention of the so-called Haber–Bosch process for wresting the vital nutrient from the air to support agriculture or the fact that civilization now moves more earth and stone than all the world's rivers put together.
 
Researchers have advanced an array of proposals for when this putative new epoch might have begun. Some link it to the start of the mass extinction of large mammals such as woolly mammoths and giant kangaroos some 50,000 years ago or the advent of agriculture around 10,000 years ago. Others say the Anthropocene is more recent, tied to the beginning of the uptick of atmospheric CO2 concentrations after the invention of an effective coal-burning steam engine.
 
The most prominent current proposal connects the dawn of the Anthropocene to that of the nuclear age—long-lived radionuclides leave a long-lived record in the rock. The boom in human population and consumption of everything from copper to corn after 1950 or so, known as the "Great Acceleration," roughly coincides with this nuclear marker as does the advent of plastics and other remnants of industrial society, dubbed technofossils by Jan Zalasiewicz of the University of Leicester, the geologist in charge of the group that is advocating for incorporating the Anthropocene into the geologic time scale. The radionuclides can then serve as what geologists call a Global Stratotype Section and Point, more commonly known as a "golden spike." Perhaps the most famous such golden spike is the thin layer of iridium found in rock exposed near El Kef, Tunisia, that tells of the asteroid impact that terminated the reign of the dinosaurs and thus marked the end of the Cretaceous Period about 65 million years ago.

nuclear-bomb-spike
 
Lewis and Maslin reject this radionuclide spike because it is not tied to a "world-changing event"—at least not yet—although it is a clear signal in the rock. On the other hand, their Orbis spike in 1610 reflects both the most recent CO2 nadir as well as the redistribution of plants and animals around the world around that time, a literal changing of the world.
 
Much like the golden spike that marks the end of the dinosaurs, the proposed Orbis spike itself would be tied to the low point of atmospheric CO2 concentrations around 1610, as recorded in ice cores, where tiny trapped bubbles betray past atmospheres. Further geologic evidence will come from the appearance of corn pollen in sediment cores taken in Europe and Asia at that time, among other indicators that will complement the CO2 record. Therefore, scientists looking at ice cores, mud or even rock will find this epochal shift in the future.

columbian-exchange
 
The CO2 drop coincides with what climatologists call the little ice age. That cooling event may have been tied to regenerated forests and other plants growing on some 50 million hectares of land abandoned by humans after the mass death brought on by disease and warfare, Lewis and Maslin suggest. And it wasn't just the death of millions of Americans, as many as three quarters of the entire population of two continents. The enslavement (or death) of as many as 28 million Africans for labor in the new lands also may have added to the climate impact. The population of the regions of northwestern Africa most affected by the slave trade did not begin to recover until the end of the 19th century. In other words, from 1600 to 1900 or so swathes of that region may have been regrowing forest, enough to draw down CO2, just like the regrowth of the Amazon and the great North American woods, although this hypothesis remains in some dispute.
 
Whether in 1610, 1945 or 50,000 B.C., the new designation would mean we are living in a new Anthropocene epoch, part of the Quaternary period, which started more 2.5 million years ago with the advent of the cyclical growth and retreat of massive glaciers. The Quaternary is part of the Cenozoic ("recent life") era that began 66 million years ago and is, in turn, part of the Phanerozoic ("revealed life") eon, which started 541 million years ago and encompasses all of complex life that has ever lived on this planet. In the end, the Anthropocene might supplant its old rival, the Holocene. "It is only designated an epoch, when other interglacials are not, because back in the 18th century geologists thought humans were a very recent species, arriving via divine intervention or evolving on Earth in the Holocene," Lewis argues, but scientists now know Homo sapiens arose more than 200,000 years ago in the Pleistocene epoch. "Humans are a Pleistocene species, so the reason for calling the Holocene an epoch is a relic of the past."
 
Maslin suggests downgrading the Holocene to a stage within the Pleistocene, like other interglacial spans in the geologic record. But Zalasiewicz disagrees with this bid to get rid of the Holocene. "I don't see the need," he says. "Systematic tracing of a Holocene–Anthropocene boundary globally would be a very illuminating process in all sorts of ways."
 
The changes wrought by humans over the course of the last several centuries, if not longer, will echo in the future—whether in the form of transplanted species, like earthworms or cats; crop pollen in lake sediments; or even entire fossilized cities. Still, whether the Anthropocene started tens, hundreds or thousands of years ago, it accounts for a minute fraction of Earth's history. And this new epoch could end quickly or endure through millennia, depending on the choices our species makes now. "Embracing the Anthropocene reverses 500 years of scientific discoveries that have made humans more and more insignificant," Maslin notes. "We argue that Homo sapiens are central to the future of the only place where life is known to exist."