It took 3 or 4 billion years to develop Homo sapiens. If the climate had completely failed in that time, then development had come to a crashing halt and we would no longer be here. So in order to understand how we exist on planet Earth, we need to know how the Earth fit for life for billions of years.
This is not a trivial problem. Current global warming shows us that climate can change drastically over the course of a few centuries. With geological time, changing the climate is even easier.
Calculations suggest that the Earth’s climate is likely to remain above temperature drop or boiling within a few million years.
We also know that the sun has become 30 percent brighter since life first evolved. In theory, this should have boiled the seas by now, given that they were not usually frozen on early Earth – this is known as the “faint young sun paradox”. Nevertheless, somehow, this habit was resolved.
Scientists have come up with two main principles. The first is that the Earth can hold anything like a thermostat – the reaction mechanism (or mechanism) that prevents the climate from wandering to deadly temperatures.
The second is that out of a large number of planets, perhaps some make it through luck, and Earth is one of them. This second scenario is made more admirable by many discoveries outside our solar system in recent decades – the so-called exoplanet.
Astronomical observations of distant stars tell us that many planets orbit, and some have a size and density and orbital distances such as the temperature appropriate for life are theoretically possible. It is estimated that there are at least 2 billion such candidate planets in our galaxy.
Scientists would love to travel to these exoplanets to test whether any of them matched Earth’s billion years of climate stability. But even the nearest exoplanets, orbiting Proxima centauri, are more than four light years away. It is difficult to come by observation or experimental evidence.
Instead, I explored the same question through modeling. Using a computer program designed to simulate climate evolution on normally (not just Earth) planets, I first generated 100,000 planets, each with a randomly varying climate response. Climate reactions are processes that can exacerbate or reduce climate change – think for example of melting sea-ice in the Arctic, an open sea spot that absorbs sunlight to ice that reflects sunlight. Turns on, which in turn causes more heating and more melting.
To test how likely each of these diverse planets were to be habitable over vast (geological) timelines, I simulated each 100 times. Each time the planet started at a different initial temperature and was exposed to a randomly different set of climatic events.
These events represent climate-changing factors such as the Supervolcano eruption (eg Mount Pinatubo but much larger) and the asteroid impact (such as the one that killed the dinosaurs). On each of the 100 runs, the temperature of the planet was tracked until it became too hot or too cold or survived for 3 billion years, at which point it was believed that it would be a part of intelligent life. Is a possible crucible.
Simulation results give a definitive answer to this habitual problem, at least in terms of the importance of feedback and luck. It was very rare (in fact, just once out of 100,000) for a planet to have such strong stabilization reactions that, despite random climatic events, are all habitable 100 times.
In fact, most planets that were habitable at least once did less than ten times out of 100. On almost every occasion in a simulation when a planet was habitable for 3 billion years, it was partially under luck.
At the same time, fate showed itself to be inadequate. Planets that were specially designed that had no reaction were never habitable; Walking randomly, surrounded by climate events, never stayed the course.
This overall result, that the results depend partly on the response and partly on the fate, is strong. Not all changes in modeling affected this. The implication, therefore, is that the Earth must have some climate-stable feedback, but it must also be included in good luck to remain habitable.
If, for example, the asteroid or solar flare was slightly larger than this, or occurred at slightly different (more important) times, we would probably not be on Earth today.
It gives a different view on how we are able to develop and diversify Earth’s remarkable, highly extended, life history and become more complex to the point that it gave birth to us.
Toby Tyrell, Professor of Earth Systems Science, University of Southampton.
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