It took 3 or 4 billion years to occur Homo sapiens. If the climate had completely failed once at that time, evolution would have stopped and we would no longer be here. Therefore, to understand how we came to exist on planet Earth, we will need to know how the Earth managed to stay fit for life for billions of years.
This is not a trivial problem. Today’s global warming shows us that the climate can change considerably over a few centuries. Along geological time scales, it is even easier to change the climate.
Calculations show that the Earth’s climate can deteriorate to temperatures below freezing or above boiling in a few million years.
We also know that the Sun has become 30 percent brighter since life first evolved. In theory, this should have caused the oceans to boil, as they were generally not frozen on early Earth, this is known as the “weak paradox of the young Sun.” Still, somehow, this habitability puzzle was solved.
Scientists have put forward two main theories. The first is that the Earth could possess something like a thermostat, a feedback mechanism (or mechanisms) that prevents the climate from going to deadly temperatures.
The second is that, of a large number of planets, perhaps some only reach luck and Earth is one. This second scenario is made more plausible by the discoveries of the last decades of many planets outside our solar system, the so-called exoplanets.
Astronomical observations of distant stars indicate that many have planets that orbit them and that some have a size and density and an orbital distance such that theoretically suitable temperatures are possible for life. It has been estimated that there are at least 2 billion candidate planets of this type in our galaxy alone.
Scientists would love to travel to these exoplanets to investigate whether any of them have coincided with the billion years of Earth’s climate stability. But even the nearest exoplanets, orbiting the star Proxima Centauri, are more than four light-years away. It is difficult to obtain observational or experimental evidence.
Instead, I explored the same question through modeling. Using a computer program designed to simulate the evolution of climate on planets in general (not just on Earth), I first generated 100,000 planets, each with a set of different random comments. Climate feedback is processes that can amplify or decrease climate change; think, for example, of melting sea ice in the Arctic, which replaces ice that reflects sunlight with open sea that absorbs sunlight, which in turn causes more warming and more melting.
In order to investigate the probability that each of these various planets remained habitable on huge (geological) time scales, I simulated every 100 times. Each time the planet started from a different initial temperature and was exposed to a set of randomly different climatic events.
These events represent climate-altering factors, such as the eruptions of the supervolcano (such as Mount Pinatubo but much larger) and the impacts of asteroids (such as the one that killed the dinosaurs). In each of the 100 races, the planet’s temperature was tracked until it became too hot or too cold or it survived for 3 billion years, at which time it was considered a possible melting pot for life. smart.
The simulation results give a definitive answer to this habitability problem, at least in terms of the importance of feedback and luck. It was very rare (in fact, only once in 100,000) for a planet to have such strong stabilizing feedback that it remained habitable every 100 times, regardless of random climatic events.
In fact, most planets that remained habitable at least once did so less than ten times out of 100. On almost every occasion in the simulation when a planet remained habitable for 3 billion years, it was in part good luck.
1,000 different planets were generated at random and run twice. Green circles show habitability for 3 billion years. (Toby Tyrrell)
At the same time, luck alone proved insufficient. Planets specially designed to have no comment, were never left habitable; random walks, affected by climatic events, never lasted the course.
This overall result, that the results depend partly on the comments and partly on luck, is robust. All sorts of changes in modeling have not affected him. Consequently, the Earth must have some climate-stabilizing comments, but at the same time, good fortune must also have been involved for it to remain habitable.
If, for example, an asteroid or solar flare had been slightly larger than it was or had occurred at a slightly different (more critical) time, we probably wouldn’t be here on Earth.
It gives a different perspective on why we are able to look back at the remarkably enlarged history of the Earth, of life evolving and diversifying and becoming increasingly complex to the point that it gave rise to us. 
Toby Tyrrell, Professor of Earth System Science, University of Southampton.
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