A city from one hundred million years ago would hardly still be waiting for us under a few meters of earth, with streets, walls and perhaps a perfectly fossilized car park. Erosion would have done its job, tectonics too, while a large part of the ocean floor would have already been swallowed up again by the earth’s mantle. If there had existed before humans industrial civilizationtherefore, we may have lost almost all of its most obvious traces.
This is the deliberately surprising question posed by Gavin Schmidt, climatologist at NASA’s Goddard Institute for Space Studies, and by astrophysicist Adam Frank of the University of Rochester. In 2018 the two published onInternational Journal of Astrobiology a study entitled The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?. And no: . Indeed, they write that they consider it highly unlikely. Theirs is another operation: to ask what signs we should look for, if it ever existed.
A city can disappear. Carbon is harder to erase
The problem begins with the scarcity of the geological record. Today’s urbanized areas occupy a tiny fraction of the Earth’s surface and, on scales of tens or hundreds of millions of years, erosion, sedimentation and crustal movements greatly reduce the likelihood that a built structure will remain recognisable.
Schmidt and Frank then changed their perspective. Instead of imagining finding fossilized skyscrapers, they tried to figure out which ones global chemical footprints it leaves an industry capable of modifying the atmosphere, oceans and natural cycles.
Ours, despite having been industrial for just a few centuries, is already producing many. The burning of coal, oil and gas alters the ratio of carbon isotopes; fertilizers and the Haber-Bosch process modify the nitrogen cycle; warming and acidification leave signals in the sediments. To these could be added particularly persistent synthetic substances, plastics and radioactive isotopes produced by nuclear activities.
The result is almost paradoxical: a civilization could leave very few recognizable objects and, at the same time, write one’s presence in the chemistry of the entire planet.
The precedent of 56 million years ago
At this point the authors looked back. In the history of the Earth there are episodes which, observed only through some geological indicators, present characteristics vaguely similar to those that the Anthropocene could leave to future generations.
The most spectacular case is the Paleocene-Eocene Thermal Maximumor PETM, occurred approximately 56 million years ago. In a geologically short interval, an enormous amount of relatively poor carbon of the isotope carbon-13 entered the atmosphere-ocean system. Global temperatures increased by approx 5-7 °Cthe oceans changed chemically, erosion increased, and major extinctions occurred among benthic foraminifera.
Schmidt and Frank note that several signals from the PETM point in the same direction as those predicted for the geological footprint of our era: abrupt carbon isotopic anomalies, warming, changes in sedimentation, and alterations in biogeochemical cycles.
This . The explanations studied by paleoclimatology concern large natural processes, including massive carbon emissions associated with magmatic activity and other mechanisms of the Earth system. The authors themselves explicitly invite us to avoid inserting a lost civilization into every slightly strange geological layer.
The “Silurian Hypothesis” isn’t really about the Silurians
The name contributes a lot to the misunderstanding. Silurians are a fictional species of intelligent reptiles that appeared in Doctor Whodescribed as preceding humanity. Schmidt and Frank borrowed the reference with a certain amount of irony.
In fact, in the study they specify that they have not identified artefacts, technological fossils or other indications of an advanced pre-human society.
However, the scientific question remains serious: what combinations of signals would make it possible to distinguish, after millions of years, an industrial transformation of the planet from a major natural climate event?
A single carbon peak would be of little use. It would be much more interesting to find a combination that is difficult to explain through known processes: simultaneous anomalies in the isotopes of different elements, very long-lived synthetic molecules, metals in unusual concentrations, sudden changes in sedimentation and perhaps radionuclides with particular characteristics. The study actually proposes to search multiple signaturesbecause none of these signals, taken alone, would be an industrial identity card.
Then there is an even more curious detail. The more rapidly a civilization consumes fossil fuels and changes the planet, the stronger its geological signature might become. A very long-lived, efficient society powered by sources with reduced impact could instead be much less conspicuous in the rocks.
In short, terrible environmental management could also be an excellent way to be remembered in a hundred million years.
Why look for a civilization that probably never existed
Research is mainly useful for two things. The first concerns us. Understanding what will remain of the industrial era means observing the Anthropocene from an almost geological perspective: what we perceive today as emissions, fertilizers, synthetic materials and ecosystem transformations can become a recognizable layer long after the disappearance of the cities that produced them.
The second concerns astrobiology. When we look for technological civilizations elsewhere we easily imagine radio signals, artificial lights or enormous technological works. Schmidt and Frank suggest that they could also be searched planetary footprints: atmospheric or geochemical alterations produced by industrial activities, even after the demise of the responsible company.
Eight years after publication, the Silurian Hypothesis it therefore remains above all a well-constructed thought experiment. No industrial dinosaur empires appeared in the Paleocene strata. We discovered something perhaps less cinematic: civilizations can disappear much more easily than the consequences they leave on the planet.
