The paradox gets its name from a question posed by Enrico Fermi over lunch with Emil Konopinski, Edward Teller, and Herbert York around 1950, although the question had been asked by others before. Referring to the compelling calculations made by astronomer Frank Drake and others that intelligent life must exist around billions of other stars, Fermi asked, “Where are they?” His equally sound reasoning was that if there were indeed billions of other intelligent civilizations who were billions of years older than we are, why hadn’t they already colonized Earth, or at least made their presence known?
Many explanations have been offered over the years, most are dismal, some are sinister, and others image that we humans are a special snowflake cherished and watched over by our adoring intellectual superiors. Another, more worrying explanation, comes from Earth’s geologic record and the chemistry of carbon-based life.
Life apparently evolved on Earth in an atmosphere rich in carbon and seas rich in organic compounds. The first photosynthetic organisms began trafficking in a small amount of carbon from the atmosphere for their own purposes, and returning a small amount of O2 – probably recycling only a small portion of the CO2. At that time the planet was warmer and the atmosphere was richer in CO2.
As the story is typically told, life converted the CO2 rich atmosphere to the O2 abundance we now enjoy, leaving behind carbon as fossil fuels and our present CO2 levels that are probably few percent what the were when hard shelled animals first evolved, the kind that leave fossils. This explanation seems unlikely given that the loss of carbon to deposits we know of as fossil fuels can only account for a small fraction of atmospheric O2 that was produced. The chemistry of photosynthesis/respiration limits the amount of O2 released to a 1:1 ratio with CO2 consumed. Thus the total amount of oxygen that can ever have been released by photosynthesis is limited by the amount of carbon dioxide on the primordial earth. It’s not known just how much that was – some researchers say a lot and some say only a little. If the carbon was largely methane at Earth’s formation, that further limits the amount of oxygen that biology can be credited with.
Although we as carbon-based life are very familiar with carbon, it is a trace element on Earth, 1000 times rarer than oxygen – even rarer than titanium. We are awash in carbon in our day to day lives thanks to fossil fuels and the uses to which we put them. Much of what we know as coal was laid down during the Carboniferous period, about 300-350 million years ago, when plants had evolved the ability to produce lignin to toughen their stalks, giving rise to wood. It took about 50 million years for microorganisms to employ the necessary enzymes to digest lignin, and during that interval much carbon was laid down and trapped on the forest floors. Atmospheric CO2 dropped precipitously. Luckily that era ended and CO2 levels returned before life went extinct from carbon starvation.
Oil, natural gas, peat, and kerogen, the other major fossil carbon forms, result from the continual deposition of organic sediments, which over geologic time are transformed by heat and pressure. The quantity of carbon trapped in oil is dwarfed by that stored in coal. In total, all the trapped carbon on Earth is thought to be in the ballpark of the amount of oxygen in the atmosphere, but there is growing evidence for the Great Oxidation Event over two billion years ago, which is believed to have consumed far larger quantities of oxygen, transforming the mineral makeup of Earth’s crust.
The amount of carbon lost into the crust since Earth’s formation thus can’t really account for the creation of our oxygen atmosphere. There are various theories to explain the source of this oxygen, one being the escape of hydrogen to space and the eventual liberation of oxygen from water. The conversion of only a few feet of water from the oceans (over billions of years) is needed to account for our present abundance of O2. Other theories involve geophysical processes.
So if burning all fossil fuels can’t return CO2 to the 5000ppm levels of 500+ million years ago in the Cambrian, what happened to our carbon? The answer is that most of the carbon is trapped in rock. Much CO2 has ended up as carbonate rock – limestone and dolomite – thanks largely to seashells and corals, structures that evolved during the Cambrian. This process transforms CO2 to -CO32- (carbonate) compounds, and in the process consumes rather than liberates oxygen. Some carbonates also naturally precipitate out of seawater but at a slower rate.
What’s clear from the geologic record is that over hundreds of millions of years, carbon sequestration has gradually been removed CO2 from the atmosphere – today we retain only a few percent of the original CO2. Following this trend down would see the complete disappearance of atmospheric CO2 in a mere 20-30 million years – less than 0.5% of Earth’s history. We arrived at the play just in time to lament the death of Romeo and Juliet.
Since plant and animal life will die off long before then from lack of CO2, this sequestration of carbon will stop before reaching zero – some traces of atmospheric CO2 persisting. If not reversed, the only remaining life on earth will likely be such simple organisms as extremophiles and lichens – the end of animal life. Volcanic processes slowly return CO2 to the atmosphere as they melt carbonate rock, thus eons after animal life has disappeared, CO2 may return on its own. We won’t be around to see it.
Death by carbon sequestration could doom any sophisticated carbon-based life that might have arisen on other planets, unless intelligence evolves first. If carbon-based life is the only kind that can exist, the fact that it is self-limiting may help solve the Fermi Paradox by explaining why intelligent life rarely has the time required to evolve. If intelligence does evolve on a planet in time, it has a chance to reverse the carbon loss. But only a chance.
If life on Earth is to persist, Man must eventually work to liberate carbon from the rocks, keeping up atmospheric CO2 levels. There is hope. We presently liberate some CO2 from rock in the process of making cement. Because breaking the chemical bonds binding the CO2 requires energy, we actually liberate about as much CO2 from burning fossil fuels to make the cement as we recover from the rock. Fossil fuels will soon run out, so new and much more plentiful sources of energy must be found.
Preventing this carbon starvation requires more than the evolution of intelligence. It also requires the knowledge that the problem exists, the will to do something about it, and the resources and technological infrastructure to reverse it. Without Man, ten million years from now dolphins might evolve intelligence comparable to our own, but would it do any good? Would dolphins be in a position to develop the infrastructure to deal with it? Or would it already be too late for them? We’ve been lucky enough to find the abundant energy and mineral resources that allowed the industrial revolution. We have the tools and productive momentum to tackle this problem, but will we?
Our sophisticated telescopes hunt for exoplanets and are finding many more than Drake et al. predicted. The paradox worsens. Even if we do find wet planets in the Goldilocks zone that have oxygen-rich atmospheres, we shouldn’t get our hopes up. We may only find the fossil remains of life that perished of carbon starvation billions of years ago.