Humans drive trillions of miles in cars, clear-cut forests for
agriculture and create vast landfills teeming with tin cans, soda
bottles and other detritus of industrialization. There's no doubt that
humans have radically reshaped the planet, and those changes leave
traces in the Earth's geological record.
At the annual meeting of the American Geophysical Union this week, geologists are grappling with how to define the boundaries of that human-centered geologic era, referred to as the Anthropocene. Despite our dramatic impact on the planet, defining our era has proven a difficult task.
"If it's to be a geological period, it has to be visible in the
geological record," said Anthony Brown, a researcher at the University
of Southampton in the United Kingdom, who is trying to define the
Because geology looks at the deep past, such questions would normally
be examined in 100,000, even 1 million years' time, he said.
"In the absence of time travel, we have to work out whether we really do have enough around to define a new geological period."
In one possible way of demarcating the boundary between natural
geologic eras and the human-shaped period, scientists would look at how agriculture changes sediments, Brown told LiveScience.
For instance, when farmers clear-cut forests and plant crops, they
change how sediments and runoff wash into the local rivers, often
creating a thick layer of silty, sandy clay on the flood plain, Brown
said. (Top 10 Ways to Destroy Earth)
But using such geologic clues to date the Anthropocene era runs into a
problem: agriculture began at different times around the globe. Some
areas, such as certain pockets in Africa, may not have had intensive
agriculture until recently.
Alternatively chemical deposits could date the boundary between human
and natural geologic eras. For instance, widespread use of leaded
gasoline and paint has left high levels of lead
in soils throughout the world, said Michael Kruge, a researcher at
Montclair State University in New Jersey. Polycyclic aromatic
hydrocarbons (PAH) could also serve as markers. These are formed from
combustion in natural wildfires, but also come largely from the burning
of fossil fuels.
"In the middle of the 20th century, you see a big spike in these compounds in sediment," Kruge said in a press conference.
Using those measures, the Anthropocene era would begin around
industrialization, thousands of years after humans began reshaping the
planet with agriculture.
Yet another proposal would peg the time of the Anthropocene's birth to
the mass movement of soil, or the accumulation of minerals from coal
burning, cement production for construction, or the massive use of
nitrogen fertilizers. That would date the Anthropocene to the sharp
uptick in the production of these chemicals after World War II.
Other scientists hope to date the Anthropocene's onset using modern-day
fossils, for instance, layers and layers of plastic soda bottles and
tin cans piling up in landfills.
All of these approaches face a challenge, however: combining
human-caused changes with natural, global variations that normally
demarcate different geologic time periods. For instance, our current
geologic time period, the Holocene Epoch, governs our climate and the
extent of our glaciers, and is dictated by eccentricities in the Earth's
orbit (something humans' haven't yet managed to alter). Since the
Earth's orbit isn't going to change any time soon, the Anthropocene
would somehow need to overlap with the Holocene.
"Nobody believes that the astronomical cycle, the 100,000-year cycle
that we're in, is suddenly coming to an end," Brown said. "We have to
combine the anthropogenic with the natural variability in the climate
system. It is a question that geologists never had to face before."
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