Earth’s climate has remained stable enough to maintain life as we currently understand it for billion years. The likelihood that early conditions would even support basic life forms is low. Therefore, scientists know that the odds that a climate is favorable to the evolution of intelligent life are staggering. A single, unfavorable climatic event would have derailed the course of evolution and Homo sapiens would not be here today.
Exactly how how unlikely this outcome is is not thoroughly understood, but there are two main theories. The first is that Earth has an unknown feedback mechanism between carbon or silicate or both that prevents the temperatures from swinging too far in either direction. Considering the current global warming evidence, we know that Earth’s temperature can change significantly over a few hundred years. Scientists also know through the “faint young Sun paradox” that the Earth should have either sat too cold for the evolution of complex multicellular lifeforms, or the Sun theoretically should have reduced Earth’s oceans to steam.
The mystery of a global thermostat leads to the second theory of Earth’s consistently habitable climate: luck. This scenario has become increasingly credible with the discoveries of over two billion exoplanets with life-supporting characteristics. Perhaps some of these exoplanets have experienced billions of years of climate stability, but with the closest sitting over four light-years away from Earth, supportive evidence is unlikely to be quantified.
This problem of habitability is increasingly interesting considering that Earth’s climate could deteriorate into sterile or deep-frozen conditions within as little as one million years.
To test the sheer odds that inhabitants of Earth have evolved on a habitable planet that has been lucky enough to remain stable for three billion years, a novel simulation was performed. The model considers habitability through thermal changes alone and thus tracked one variable, planetary surface temperature. The model assigned each of the 100,000 generated planets a random set of climate feedbacks. These feedback sets had randomized signs and magnitudes which would increase or decrease planetary temperature following a randomized climate event. These events could be described as a supervolcano eruption, a reduction of reflective ice causing warmer oceans, or even an asteroid impact. The size and frequency of the events were random across the generated planets. The model also assigned a long-term, feedback inducing, climate force to represent factors such as increased sun luminosity of that particular planet. The model performed 100 simulated runs on each planet and the temperature was recorded. If the planet survived for three billion years without temperatures rising or dipping into uninhabitable limits, the planet was deemed a potential location for intelligent life.
The results showed that only one out of the 100,000 planets remained habitable during all of its 100 tests. Even planets that remained stable for some time, much like Mars may have, did not stay that was for the entire three billion year simulation. Planets that were designed to have no feedback set, therefore no way to rebound from a climate event, never lasted long. A planet designed to experience events of magnitudes and frequency of pure chance may have shown an extremely small probability towards habitability but the odds for success over a three billion year duration is highly unlikely.
There were always effects that caused the planet to deteriorate into inhospitable temperatures. Whether that was the sheer size of the event or the fluctuations that the planet could endure, the results show that Earth could not have endured by chance alone. However, as incredible as Earth’s stabilizing mechanism must be, there is a limit to it. The results show that comparably suitable planets were taken down by slightly more intense chance events. Whether an astroid is slightly larger or the planet becomes slightly warmer, it is through an element of luck that Earth’s climate has remained in the hospitable zone.
