How do we really know there weren’t previous civilizations on our planet that rose and fell long before humans appeared? That’s the question posed in a thought experiment by University of Rochester’s Professor Adam Frank and Dr. Gavin Schmidt of NASA’s Goddard Institute for Space Studies. Their paper was published this week in the International Journal of Astrobiology (arXiv.org preprint).
If an industrial civilization had existed on Earth many millions of years prior to human era, what traces would it have left and would they be detectable today? Image credit: Michael Osadciw, University of Rochester.
“We have not seen any evidence of another industrial civilization,” Professor Frank said. “But by looking at the deep past in the right way, a new set of questions about civilizations and the planet appear: What geological footprints do civilizations leave? Is it possible to detect an industrial civilization in the geological record once it disappears from the face of its host planet?”
“These questions make us think about the future and the past in a much different way, including how any planetary-scale civilization might rise and fall.”
In what they deem the ‘Silurian Hypothesis,’ Professor Frank and Dr. Schmidt define a civilization by its energy use.
Human beings are just entering a new geological era that many researchers refer to as the Anthropocene, the period in which human activity strongly influences the climate and environment.
In the Anthropocene, fossil fuels have become central to the geological footprint humans will leave behind on Earth.
By looking at the Anthropocene’s imprint, the team examines what kinds of clues future scientists might detect to determine that human beings existed.
In doing so, they also lay out evidence of what might be left behind if industrial civilizations like ours existed millions of years in the past.
Human beings began burning fossil fuels more than 300 years ago, marking the beginnings of industrialization.
“The emission of fossil fuels into the atmosphere has already changed the carbon cycle in a way that is recorded in carbon isotope records,” the authors said.
Other ways human beings might leave behind a geological footprint include:
(i) global warming, from the release of carbon dioxide and perturbations to the nitrogen cycle from fertilizers;
(ii) agriculture, through greatly increased erosion and sedimentation rates;
(iii) plastics, synthetic pollutants, and even things such as steroids, which will be geochemically detectable for millions, and perhaps even billions, of years;
(iv) nuclear war, if it happened, which would leave behind unusual radioactive isotopes.
“As an industrial civilization, we’re driving changes in the isotopic abundances because we’re burning carbon,” Professor Frank said.
“But burning fossil fuels may actually shut us down as a civilization. What imprints would this or other kinds of industrial activity from a long dead civilization leave over tens of millions of years?”
Illustrative stable carbon isotopes and temperature (or proxy) profiles across three periods. Top left: the modern era (from 1600 CE with projections to 2100). Top right: the Paleocene-Eocene Thermal Maximum (55.5 million years ago). Bottom: Oceanic Anoxic Event (about 120 million years ago). Image credit: Gavin A. Schmidt & Adam Frank, doi: 10.1017/S1473550418000095.
These questions are part of a broader effort to address climate change from an astrobiological perspective, and a new way of thinking about life and civilizations across the Universe.
Looking at the rise and fall of civilizations in terms of their planetary impacts can also affect how astrobiologists approach future explorations of other planets.
“We know early Mars and, perhaps, early Venus were more habitable than they are now, and conceivably we will one day drill through the geological sediments there, too. This helps us think about what we should be looking for,” Dr. Schmidt said.
“If a civilization is able to find a more sustainable way to produce energy without harming its host planet, it will leave behind less evidence that it was there,” he added.
“You want to have a nice, large-scale civilization that does wonderful things but that doesn’t push the planet into domains that are dangerous for itself, the civilization. We need to figure out a way of producing and using energy that doesn’t put us at risk,” Professor Frank said.
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Gavin A. Schmidt & Adam Frank. The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record? International Journal of Astrobiology, published online April 16, 2018; doi: 10.1017/S1473550418000095
