Earth has been habitable for billions of years – simulations show it was ‘just lucky’

It took 3 or 4 billion years for evolution to produce Homo sapiens. If the climate had failed completely once during this time, then evolution would have come to a screeching halt 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 usually frozen on early Earth – this is called the “little paradox of the young sun”. Still, somehow, this habitability puzzle has been solved.

Scientists have proposed two main theories. The first is that Earth may have something like a thermostat – a feedback mechanism (or mechanisms) that keep the climate from heading into deadly temperatures.

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The second is that, on many 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 of 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 suitable for life are theoretically 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 whether 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 of sea ice in the Arctic, which replaces ice reflecting sunlight with open sea absorbing sunlight, 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 climate change drivers such as eruptions of super volcanoes (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 returns and luck. It was very rare (in fact, just 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. 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 more and more complex to the point it has given us. birth.

In the following video, Professor Toby Tyrrell discusses his research.

This article by Toby Tyrrell, Professor of Earth System Science, University of Southampton is republished from The Conversation under a Creative Commons license. Read the original article.

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