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It took 3 or 4 billion years for evolution to produce Homo sapiens. If the climate had failed completely once during this period, then evolution would have stopped abruptly and we would not be here now. So, to understand how we came to exist on planet Earth, we will need to know how Earth managed to stay fit for life for billions of years.
It is not a trivial problem. The current global warming shows us that the climate can change considerably in a few centuries. On geological timescales, it is even easier to change the climate.
Calculations show that it is possible for Earth’s climate to deteriorate to subzero or above boiling temperatures in just a few million years.
We also know that the Sun has become 30% brighter since the first evolution of life. In theory, this should have boiled the oceans by now, given that they weren’t generally frozen on early Earth – this is called the “weak young sun paradox.” Still, somehow, this habitability puzzle has been solved.
Scientists have proposed two main theories. The first is that Earth might have something like a thermostat – a feedback mechanism (or mechanisms) that keep the climate from roaming at deadly temperatures.
The second is that, on a lot of planets, maybe some just succeed by luck, and Earth is one of them. This second scenario is made more plausible by the discoveries in recent decades of many planets outside our solar system – the so-called exoplanets.
Astronomical observations of distant stars tell us that many have planets orbiting them, and that some are of such a size, density, and orbital distance that temperatures are theoretically suitable for life. possible. It has been estimated that there are at least 2 billion candidate planets in our galaxy alone.
Scientists would like to visit these exoplanets to determine if any of them have equaled Earth’s billion years of climatic stability. But even the closest exoplanets, those orbiting the star Proxima Centauri, are more than four light years away. Observational or experimental evidence is hard to find.
Instead, I explored the same question through modeling. Using a computer program designed to simulate climate change on planets in general (not just Earth), I first generated 100,000 planets, each with a different set of random climate feedbacks. Climate feedbacks are processes that can amplify or decrease climate change – think for example of the melting sea ice in the Arctic, which replaces ice reflecting sunlight with an open sea absorbing sunlight, which which in turn causes more heating and melting.
In order to study the probability that each of these various planets will remain habitable over huge (geological) time scales, I have simulated 100 times. Each time, the planet started from a different initial temperature and was exposed to a random set of different climatic events.
These events represent factors of climate change such as super volcano eruptions (like Mount Pinatubo but much larger) and asteroid impacts (like the one that killed the dinosaurs). In each of the 100 tests, the planet’s temperature was tracked until it either got too hot or too cold or survived for 3 billion years, at which time it was considered a possible melting pot. for smart living.
The simulation results provide 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 stabilizing feedbacks so strong that it remained habitable every 100 times, regardless of random climatic events.
In fact, most planets that have remained habitable at least once have done so less than ten times out of 100. On almost every occasion in the simulation, when a planet has remained habitable for 3 billion years, that is, was in part due to luck.
1000 different planets were randomly generated and executed twice. Green circles indicate habitability for 3 billion years. (Toby Tyrrell)
At the same time, luck alone turned out to be insufficient. Planets specially designed to have no feedback at all, have never remained habitable; random walks, shaken by climatic events, never lasted the course.
This overall result, according to which results depend partly on returns and partly on luck, is robust. All kinds of modeling changes didn’t affect her. By implication, the Earth must therefore have climate stabilizing feedbacks, but at the same time, luck 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 happened at a slightly different (more critical) time, we probably wouldn’t be here on Earth today. .
It gives a different perspective on why we are able to look back at the remarkable and extremely extensive history of life on Earth, which is evolving and diversifying and becoming increasingly complex to the point that it gave birth to us. . 
Toby Tyrrell, Professor of Earth System Science, University of Southampton.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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