2018-04-21 22:39:28 UTC
A look at the available evidence
(as done in "Boundary" & "Inherit the Stars")
Partial skeleton remains from an ancient burial site
Regis Duvignau / Reuters
ADAM FRANK APR 13, 2018 SCIENCE
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It only took five minutes for Gavin Schmidt to out-speculate me.
Schmidt is the director of nasa’s Goddard Institute for Space Studies
(a.k.a. GISS) a world-class climate-science facility. One day last year,
I came to GISS with a far-out proposal. In my work as an astrophysicist,
I’d begun researching global warming from an “astrobiological
perspective.” That meant asking whether any industrial civilization that
rises on any planet will, through their own activity, trigger their own
version of a climate shift. I was visiting GISS that day hoping to gain
some climate science insights and, perhaps, collaborators. That’s how I
ended up in Gavin’s office.
Just as I was revving up my pitch, Gavin stopped me in my tracks.
“Wait a second,” he said. “How do you know we’re the only time there’s
been a civilization on our own planet?”
It took me a few seconds to pick my jaw off the floor. I had certainly
come into Gavin’s office prepared for eye rolls at the mention of
“exo-civilizations.” But the civilizations he was asking about would
have existed many millions of years ago. Sitting there, seeing Earth’s
vast evolutionary past telescope before my mind’s eye, I felt a kind of
temporal vertigo. “Yeah,” I stammered, “Could we tell if there’d been an
industrial civilization that deep in time?”
We never got back to aliens. Instead, that first conversation launched a
new study we’ve recently published in the International Journal of
Astrobiology. Though neither of us could see it at that moment, Gavin’s
penetrating question opened a window not just onto Earth’s past, but
also onto our own future.
We’re used to imagining extinct civilizations in terms of the sunken
statues and subterranean ruins. These kinds of artifacts of previous
societies are fine if you’re only interested in timescales of a few
thousands of years. But once you roll the clock back to tens of millions
or hundreds of millions of years, things get more complicated.
When it comes to direct evidence of an industrial civilization—things
like cities, factories, and roads—the geologic record doesn’t go back
past what’s called the Quaternary period 2.6 million years ago. For
example, the oldest large-scale stretch of ancient surface lies in the
Negev Desert. It’s “just” 1.8 million years old—older surfaces are
mostly visible in cross section via something like a cliff face or rock
cuts. Go back much farther than the Quaternary and everything has been
turned over and crushed to dust.
And, if we’re going back this far, we’re not talking about human
civilizations anymore. Homo sapiens didn’t make their appearance on the
planet until just 300,000 years or so ago. That means the question
shifts to other species, which is why Gavin called the idea the Silurian
hypothesis, after an old Dr. Who episode with intelligent reptiles.
So, could researchers find clear evidence that an ancient species built
a relatively short-lived industrial civilization long before our own?
Perhaps, for example, some early mammal rose briefly to civilization
building during the Paleocene epoch about 60 million years ago. There
are fossils, of course. But the fraction of life that gets fossilized is
always minuscule and varies a lot depending on time and habitat. It
would be easy, therefore, to miss an industrial civilization that only
lasted 100,000 years—which would be 500 times longer than our industrial
civilization has made it so far.
Given that all direct evidence would be long gone after many millions of
years, what kinds of evidence might then still exist? The best way to
answer this question is to figure out what evidence we’d leave behind if
human civilization collapsed at its current stage of development.
Now that our industrial civilization has truly gone global, humanity’s
collective activity is laying down a variety of traces that will be
detectable by scientists 100 million years in the future. The extensive
use of fertilizer, for example, keeps 7 billion people fed, but it also
means we’re redirecting the planet’s flows of nitrogen into food
production. Future researchers should see this in characteristics of
nitrogen showing up in sediments from our era. Likewise our relentless
hunger for the rare-Earth elements used in electronic gizmos. Far more
of these atoms are now wandering around the planet’s surface because of
us than would otherwise be the case. They might also show up in future
sediments, too. Even our creation, and use, of synthetic steroids has
now become so pervasive that it too may be detectable in geologic strata
10 million years from now.
And then there’s all that plastic. Studies have shown increasing amounts
of plastic “marine litter” are being deposited on the seafloor
everywhere from coastal areas to deep basins and even in the Arctic.
Wind, sun, and waves grind down large-scale plastic artifacts, leaving
the seas full of microscopic plastic particles that will eventually rain
down on the ocean floor, creating a layer that could persist for
The big question is how long any of these traces of our civilization
will last. In our study, we found each had the possibility of making it
into future sediments. Ironically, however, the most promising marker of
humanity’s presence as an advanced civilization is a by-product of one
activity that may threaten it most.
When we burn fossil fuels, we’re releasing carbon back into the
atmosphere that was once part of living tissues. This ancient carbon is
depleted in one of that element’s three naturally occurring varieties,
or isotopes. The more fossil fuels we burn, the more the balance of
these carbon isotopes shifts. Atmospheric scientists call this shift the
Suess effect, and the change in isotopic ratios of carbon due to
fossil-fuel use is easy to see over the last century. Increases in
temperature also leave isotopic signals. These shifts should be apparent
to any future scientist who chemically analyzes exposed layers of rock
from our era. Along with these spikes, this Anthropocene layer might
also hold brief peaks in nitrogen, plastic nanoparticles, and even
synthetic steroids. So if these are traces our civilization is bound to
leave to the future, might the same “signals” exist right now in rocks
just waiting to tell us of civilizations long gone?
Fifty-six million years ago, Earth passed through the Paleocene-Eocene
Thermal Maximum (PETM). During the PETM, the planet’s average
temperature climbed as high as 15 degrees Fahrenheit above what we
experience today. It was a world almost without ice, as typical summer
temperatures at the poles reached close to a balmy 70 degrees
Fahrenheit. Looking at the isotopic record from the PETM, scientists see
both carbon and oxygen isotope ratios spiking in exactly the way we
expect to see in the Anthropocene record. There are also other events
like the PETM in the Earth’s history that show traces like our
hypothetical Anthropocene signal. These include an event a few million
years after the PETM dubbed the Eocene Layers of Mysterious Origin, and
massive events in the Cretaceous that left the ocean without oxygen for
many millennia (or even longer).
Are these events indications of previous nonhuman industrial
civilizations? Almost certainly not. While there is evidence that the
PETM may have been driven by a massive release of buried fossil carbon
into the air, it’s the timescale of these changes that matter. The
PETM’s isotope spikes rise and fall over a few hundred thousand years.
But what makes the Anthropocene so remarkable in terms of Earth’s
history is the speed at which we’re dumping fossil carbon into the
atmosphere. There have been geological periods where Earth’s CO2 has
been as high or higher than today, but never before in the planet’s
multibillion-year history has so much buried carbon been dumped back
into the atmosphere so quickly. So the isotopic spikes we do see in the
geologic record may not be spiky enough to fit the Silurian hypothesis’s
But there is a conundrum here. If an earlier species’s industrial
activity is short-lived, we might not be able to easily see it. The
PETM’s spikes mostly show us the Earth’s timescales for responding to
whatever caused it, not necessarily the timescale of the cause. So it
might take both dedicated and novel detection methods to find evidence
of a truly short-lived event in ancient sediments. In other words, if
you’re not explicitly looking for it, you might not see it. That
recognition was, perhaps, the most concrete conclusion of our study.
It’s not often that you write a paper proposing a hypothesis that you
don’t support. Gavin and I don’t believe the Earth once hosted a
50-million-year-old Paleocene civilization. But by asking if we could
“see” truly ancient industrial civilizations, we were forced to ask
about the generic kinds of impacts any civilization might have on a
planet. That’s exactly what the astrobiological perspective on climate
change is all about. Civilization building means harvesting energy from
the planet to do work (i.e., the work of civilization building). Once
the civilization reaches truly planetary scales, there has to be some
feedback on the coupled planetary systems that gave it birth (air,
water, rock). This will be particularly true for young civilizations
like ours still climbing up the ladder of technological capacity. There
is, in other words, no free lunch. While some energy sources will have
lower impact—say solar vs. fossil fuels—you can’t power a global
civilization without some degree of impact on the planet.
A person holds a globe against a background of Earths hit by meteors,
crumbling, and colliding with rockets.
Why Earth's History Appears So Miraculous
A Foreboding Similarity in Today’s Oceans and a 94-Million-Year-Old
Once you realize, through climate change, the need to find lower-impact
energy sources, the less impact you will leave. So the more sustainable
your civilization becomes, the smaller the signal you’ll leave for
In addition, our work also opened up the speculative possibility that
some planets might have fossil-fuel-driven cycles of civilization
building and collapse. If a civilization uses fossil fuels, the climate
change they trigger can lead to a large decrease in ocean oxygen levels.
These low oxygen levels (called ocean anoxia) help trigger the
conditions needed for making fossil fuels like oil and coal in the first
place. In this way, a civilization and its demise might sow the seed for
new civilizations in the future.
By asking about civilizations lost in deep time, we’re also asking about
the possibility for universal rules guiding the evolution of all
biospheres in all their creative potential, including the emergence of
civilizations. Even without pickup-driving Paleocenians, we’re only now
learning to see how rich that potential might be.
ABOUT THE AUTHOR
ADAM FRANK is a professor of astrophysics at the University of
Rochester. His work has appeared in Scientific American, The New York
Times, and NPR. He is the author of Light of the Stars: Alien Worlds and
the Fate of the Earth.