Evolution took 3 or 4 billion years to produce Homo sapiens. If the climate had just failed completely once during that time, evolution would have stalled and we wouldn’t be here now. So to understand how we came to be on planet Earth, we need to know how Earth has managed to stay fit for life for billions of years.
This is not a trivial problem. Current global warming shows us that the climate can change significantly over several centuries. It is even easier to change the climate over geological time scales.
Calculations show that the Earth’s climate can deteriorate to temperatures below freezing or above boiling point in just a few million years.
We also know that the sun has become 30 percent brighter since life first developed. In theory, this would have resulted in the oceans being cooked away by now, as they were generally not frozen on the early Earth – this is known as the “faint young sun paradox.” Still, this habitability puzzle was somehow solved.
Scientists have come up with two main theories. The first is that Earth could have something like a thermostat – a feedback mechanism (or mechanisms) that prevents the climate from ever drifting to fatal temperatures.
The second is that of a large number of planets, some may come through luck, and Earth is one of them. This second scenario is made more plausible by the discoveries of many planets outside our solar system – so-called exoplanets – in recent decades.
Astronomical observations of distant stars tell us that many planets revolve around them, and that some are of such size and density and orbital distance that temperatures suitable for life are theoretically possible. It is estimated that there are at least 2 billion such candidate planets in our galaxy alone.
Scientists would love to travel to these exoplanets to find out if any of them match Earth’s billion-year climate stability. But even the closest exoplanets, orbiting the star Proxima Centauri, are more than four light years away. Observational or experimental evidence is difficult to obtain.
Instead, I explored the same question through modeling. Using a computer program designed to simulate climate evolution on planets in general (not just Earth), I first generated 100,000 planets, each with a randomly different set of climate feedbacks. Climate feedbacks are processes that can amplify or mitigate climate change – think, for example, of the melting of sea ice in the Arctic, which replaces sunlight-reflecting ice with sunlight-absorbing open sea, which in turn causes more warming and more melting.
To investigate the likelihood that each of these diverse planets would remain habitable over vast (geological) time scales, I simulated them 100 times. Each time the planet started with a different initial temperature and was exposed to a randomly different set of climate events.
These events represent climate-changing factors such as supervolcanic eruptions (like Mount Pinatubo but much bigger) and asteroid impacts (like the one that killed the dinosaurs). Each of the 100 runs tracked the planet’s temperature until it got too hot or too cold or otherwise survived 3 billion years, at which point it was considered a potential melting pot for intelligent life.
The simulation results provide a definitive answer to this habitability problem, at least in terms of the importance of feedback and happiness. It was very rare (in fact, only once in 100,000) for a planet to have such strong stabilizing feedbacks that it remained habitable 100 times, regardless of random climate events.
In fact, most planets that remained habitable at least once did less than 10 out of 100. On almost every occasion in the simulation, when a planet remained habitable for 3 billion years, it was partly due to luck.
1,000 different planets were randomly generated and executed twice. Green circles show habitability for 3 billion years. (Toby Tyrrell)
At the same time, luck in itself turned out to be insufficient. Planets specially designed to have no feedback at all have never remained habitable; random walks ravaged by climate events never lasted the course.
This overall result, that the results depend partly on feedback and partly on luck, is robust. Various modeling changes did not affect it. Implicitly, therefore, the Earth must have some climate stabilizing feedback, but at the same time there must also have been luck in keeping the Earth habitable.
For example, if an asteroid or solar flare had been slightly larger than it was, or occurred at a slightly different (more critical) time, we probably wouldn’t be here on Earth today.
It provides a different perspective on why we are able to look back on Earth’s remarkable, vastly expanded life history evolving and diversifying and becoming more complex to the point where it gave birth to us. 
Toby Tyrrell, Professor of Earth System Science, University of Southampton.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.