One day in 2017 Adam Frank, professor of physics and astronomy at the University of Rochester, was visiting NASA’s Goddard Institute for Space Studies (GISS) in New York. His goal was to learn more about global warming from an astrobiological perspective. “That meant – Frank commented – asking if some industrial civilization that appeared on any planet would trigger its own version of climate change.” Then he entered the office of Gavin Schmidt, director of the GISS, and while he was telling him his ideas, the latter said to him: “Wait a second,” he said , “how do you know that we are the only civilization that has ever existed on our own planet? ”. What if an industrial civilization had existed hundreds of millions of years ago on Earth? It is certainly a provocative question, but the really important question is: How could we know that it existed?
This was the beginning of a thought experiment that ended in an article that both scientists published in the Journal of Astrobiology : “The Silurian Hypothesis : Would it be possible to detect an industrial civilization in the geological record?”. The first peculiar thing is the title, which refers to the doyen of science fiction series on television, Doctor Who . The Silurians are a race of this long-running British series that first appeared in 1970: prehistoric and scientifically advanced humanoid reptiles that preceded us and hibernated to survive a great geological cataclysm caused by the arrival of the Moon.
The underlying question is whether an advanced civilization can leave remains that survive tens of millions of years into the future. How can we know if an ancient civilization modified the landscape, built cities? The evidence for the existence of past civilizations becomes scarcer as we go further back in time. Paleontology is also proof of this rule: the closer in time the remains of our ancestors are, the greater the number of fossils . It is obvious that we are not going to find ancient civilizations, so what kind of evidence should we expect?
For Frank and Schmidt, the first thing to do is define what characterizes a civilization and for this they have resorted to the same idea that in the 1960s the Russian astrophysicist Nikolai Kardashev used to classify the technological level of extraterrestrial civilizations that could exist in the universe: depending on their energy expenditure. In fact, every civilization is defined by three factors: the energy it consumes, the information it handles, and the waste it generates .
Can we find a civilization lost by its dumpsters ? It is not unreasonable: the entire world produces more than 1 billion tons of garbage a year. What can be left of all that? Consider a material we have declared war on: plastic. Studies have shown that increasing amounts of plastic on the seafloor, from coastal areas to deep basins and even in the Arctic.
Wind, sun and waves destroy large plastic objects, leaving the seas full of microscopic plastic particles that will eventually settle to the bottom of the ocean, creating a layer that could persist for a very long time. What’s more, some researchers suggest that this plastic could end up producing a new type of rock, which will undoubtedly be detectable by future archaeologists. Even something as seemingly innocuous as synthetic steroid residues -used in bodybuilding or for medical purposes- it has become so pervasive that it will be detectable in geological strata within 10 million years. Moreover, according to Frank and Schmidt, we are leaving evidence of our existence that will last 100 million years . Just consider what we do to feed 7 billion souls: fertilizers have completely altered the nitrogen cycle, much of which is being deposited on the seabed and on top of mountains.
On the other hand, the development of technology is changing the physiognomy of our planet in a more subtle way, by altering the distribution of minerals in the earth’s crust. Thus, our desire for rare earths – such as dysprosium, which is used in hybrid cars, or neodymium, whose use ranges from welding goggles to high-power permanent magnets – has meant that those elements that should be buried deep in the earth are now on the surface.
And what about the changes produced in the carbon cycle by the burning of fossil fuels? If there is a fact that is at the center of the geological footprint that humans are going to leave for the future on Earth, it is this. We started burning fossil fuels more than 300 years ago and this emission into the atmosphere has changed the present proportion of heavy isotopes of carbon (C13 and C14): it is the so-called Suess effect in honor of the Austrian chemist who discovered it, Frank Suess, seeing that this fact was modifying the accuracy of the C14 dating.
If these are traces that our civilization is destined to leave for the future, could we find these same “signs” in the Earth’s geological record? For Frank and Schmidt, a ‘suspicious’ event is the Paleocene-Eocene Thermal Maximum (PETM), which occurred 55 million years ago. Then the Earth experienced the largest and fastest temperature rise ever recorded , increasing the average temperature of the planet by six degrees in just 20,000 years: all the ice on Earth melted and the temperate forests reached the same poles. The thermal maximum lasted about 150,000 years and it was the marine species that suffered the most from this climate change . In contrast, before the MTPE the world’s mammals were limited to rodents and some large carnivores and herbivores, but shortly after the thermal maximum the main families of mammals – including horses and primates – rapidly emerged. and spread all over the planet. The cause of the PEMT is a mystery: some event released 2 gigatons of carbon per year for an entire millennium. By comparison, our current industrial activity releases 4 times more per year.
By looking at the ratios of carbon and oxygen isotopes back then, scientists have found that they skyrocketed exactly the same as they do now because of our industrial activity. Are we facing proof of the existence of non-human industrial civilizations prior to ours? Almost certainly not and for one simple reason: the timescale of those changes. What makes the present epoch, the Anthropocene, so remarkable in Earth’s history is the rate at which we are dumping fossil carbon into the atmosphere. There have been geological periods when CO2 has been as high or higher than it is now, but never before in the history of the planet has so much been released into the atmosphere so quickly . “The isotopic peaks we see in the geological record may not be sharp enough to fit the Silurian hypothesis,” Adam Frank comments, “but there is a puzzle here: if the industrial activity of an earlier species is short-lived, it is possible We can’t see it easily.”
Schmidt, G. A.; Frank, Adam (2019). “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?”. International Journal of Astrobiology. 18 (2): 142–150. doi:10.1017/S1473550418000095