Ancient industrial civilizations would be very hard to detect

Centauri Dreams and The Atlantic have articles that consider if would we know if an early mammal built a civilization in the last 60 million years? Let’s assume a civilization lasting no more than 100,000 years, which turns out to be 500 times longer than our own civilization to this point. You would think that specific markers of industry like plastics live forever but they do break down eventually.

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.

We also might detect spikes in fossil fuel air pollution. We do see ‘spikes’ in the geological record, though none that are ‘spiky enough’ to fit into the hypothesis of a Silurian civilization. Using our own ‘Anthropocene’ era as a guide, we are seeing a huge increase in atmospheric carbon levels much unlike the slower spikes of the Paleocene-Eocene Thermal Maximum (PETM), when the planet’s average temperature rose well above what we have today. Those much earlier spikes (56 million years ago) took hundreds of thousands of years to play out. A civilization’s signal in terms of carbon output is, at least judging from our own, much more sudden, though we have yet to learn how it will end.

The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record?

If an industrial civilization had existed on Earth many millions of years prior to our own era, what traces would it have left and would they be detectable today? We summarize the likely geological fingerprint of the Anthropocene, and demonstrate that while clear, it will not differ greatly in many respects from other known events in the geological record. We then propose tests that could plausibly distinguish an industrial cause from an otherwise naturally occurring climate event.

Schmidt, G., & Frank, A. (2018). The Silurian hypothesis: Would it be possible to detect an industrial civilization in the geological record? International Journal of Astrobiology, 1-9. doi:10.1017/S1473550418000095

The 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.