by Adam Frank
from TheAtlantic Website
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.
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.
We never got back to aliens.
Instead, that first conversation launched a new study (The Silurian Hypothesis - Would it be Possible to Detect an Industrial Civilization in the Geological Record?) 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.
But once you roll the
clock back to tens of millions or hundreds of millions of years,
things get more complicated.
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
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.
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.
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.
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
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 geological
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.
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,
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?
During the PETM, the planet's average temperature climbed as high as 15º Fahrenheit (-9.44ºC) above what we experience today. It was a world almost without ice, as typical summer temperatures at the poles reached close to a balmy 70º Fahrenheit (21ºC).
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
ocean without oxygen for many
millennia (or even longer).
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 bill.
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.
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.
So the more sustainable
your civilization becomes, the smaller the signal you'll leave for
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.
pickup-driving Paleocenians, we're only now learning to see how rich
that potential might be...