March 11--Man's tenure on Earth is brief on the astronomical time scale, but the changes wrought by Homo sapiens have been measurable and profound.
Now, a growing number of scientists propose renaming the present geologic epoch to reflect that human influence: the Anthropocene.
The revision of geology's official time scale would be more than nominal. On a philosophical level, it would place mankind back at the center of his world, though cast as a villain who has manipulated the course of natural history by pushing other species to extinction and altering the chemical and physical composition of the atmosphere and oceans.
"Formally defining the Anthropocene would state that humans are a force of nature at a geological scale, equivalent to plate tectonics or a meteorite strike, or sustained volcanic eruptions," said Simon Lewis, an ecologist at University College in London and co-author of a paper published online Wednesday in the journal Nature. "It changes almost 500 years of scientific understanding, which has pushed humans to become more and more insignificant."
Lewis and his co-author, University of Leeds geographer Mark Maslin, propose ditching or demoting "Holocene," a Greek-derived label that translates as "wholly recent" to describe the present epoch, which began at the end of Earth's last glacial period about 11,650 years ago.
They suggest a tiny dip in the amount of carbon dioxide in Earth's atmosphere, which can be measured in ice cores, marks 1610 as an inflection point of an ongoing era of intensive human influence on Earth's biology, geology and chemistry.
The date would mark the broader trend of the human-aided spread of plant species across continents, which can be found in pollen deposits across many continents, the authors argue.
By 1610, the effect of European contact with the so-called new world was evident in the death of some 50 million people from disease and conflict in the Americas. As a result, less carbon dioxide was emitted as burning declined, and more carbon was taken up by forests, savannas and woodlands that were regenerated after agricultural fields were abandoned.
That decline in carbon dioxide, about 7-10 parts per million, can be measured in Antarctica's Law Dome ice core, and has been dated to 1610, the authors said. After that year, increased trade made the industrial revolution possible. Population growth resumed, and humans began burning fossil fuels at unprecedented scale. The amount of carbon dioxide in Earth's atmosphere has gone up by more than at least a third since then.
The term Anthropocene has been around at least since the 19th century, but acquired pop culture cachet as concern over climate change mounted. It gained momentum among scientists around 2000, when Nobel laureate Paul Crutzen, an atmospheric chemist, suggested that the start of the Industrial Revolution in the Western world could be a suitable milestone for the new epoch.
Several other scientists have suggested the acceleration in fossil fuel use since 1950 as a boundary, while still others -- including the authors -- have considered the fallout from nuclear bomb testing in the 1960s.
The International Union of Geological Sciences has convened a working group to make a recommendation on whether to call an end to the Holocene and how to define the beginning of the Anthropocene.
Its chairman, University of Leicester paleobiologist Jan Zalasiewicz, called the proposed 1610 benchmark "intriguing" and worth consideration by the full panel.
"It's clearly an important historical event," said Zalasiewicz, who added that he was not speaking for the entire working group. "Whether it can provide an effective stratigraphic boundary needs more evidence and further research, I suspect."
The tiny dip in carbon dioxide, he noted, may only be detectable in ice cores, but not in other types of strata.
Indeed, critics have effectively cried: Show us the rocks. Modern strata, they argue, have not fully formed, and there is some doubt whether there will be lasting mineral, chemical and fossil evidence to mark the rock boundary.
"We do not develop stratigraphies of concepts, processes, or hypothetical inferences of what might happen in the future," said geologist Autin Whitney of the State University of New York College at Brockport. "Stratigraphy is the science of defining and delineating layers and rock bodies within Earth materials."
There's also another time issue. Changes in the Anthropocene are spread over years, decades and even centuries and can produce significant discrepancies between geological deposits.
"It would be desirable to have some definition, so some people aren't using the Anthropocene and talking about 8,000 years ago, and other people are using it and talking about the 1960s," said Lewis.
Ideally, hierarchies in the geological time scale are defined by a lower boundary that can be found and measured in rock, sediment or ice, sometimes called a "golden spike." A classic example is the extinction of dinosaurs and the rise of mammals about 65 million years ago, caused by an asteroid impact.
"That's the long-term change, but the actual marker is the peak in iridium that's captured in the rocks, which came from the meteorite strike," Lewis said. "So we need to have some equivalent of those two things."
Lewis said he suspected the international working group will endorse adoption of the term Anthropocene. But such a proposal still would have to work its way through many layers of bureaucracy before a vote takes place. The last time that happened, a proposal to change the term "quaternary period," one level up from an epoch, was scuttled.
In the meantime, nothing is likely to stop increased academic use of the term Anthropocene, nor end the debate over its definition, Lewis said.
"It's a good discussion to have, because it helps us step back and think about the really large-scale, long-term impact that our activities can have," he said. "That's really important to think about."
Spend some of the Anthropocene with me, on Twitter: @LATsciguy
UPDATE
12:45 p.m.: This article has been updated with commentary from geologist Autin Whitney of the State University of New York College at Brockport.
This story was first published at 12:00 p.m.
