Evolution took 3 or 4 billion years to produce Homo sapiens. If the climate had failed only once during that time, then evolution would have stopped the catastrophe and we would not be here now. So to understand how we existed on planet Earth, we will need to know how the Earth has survived for billions of years.
This is not a minor problem. The current global warming shows us that the climate can change significantly for even several centuries. It is even easier to change the climate during geological periods. Calculations show that the Earth’s climate is likely to deteriorate to temperatures below zero or above boiling in just a few million years.
We also know that the Sun has become 30% brighter since life evolved. In theory, this should cause the oceans to return by now, given that they were not usually frozen in early Earth ̵
Scientists have come up with two main theories. The first is that the Earth may have something like a thermostat – a feedback mechanism (or mechanisms) that prevents the climate from wandering to fatal temperatures.
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The second is that of the many planets, some may just be lucky and Earth is one of them. This second scenario is made more plausible than the discoveries made in recent decades on many planets outside our solar system – the so-called exoplanets. Astronomical observations of distant stars tell us that many of them have orbits around them and that some are large in size and density and orbital distance, so 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 like to travel to these exoplanets to see if any of them have matched a billion years of Earth’s climate stability. But even the closest 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 issue through modeling. Using a computer program designed to simulate the evolution of the planet’s climate as a whole (not just Earth), I first generated 100,000 planets, each with an arbitrarily different set of climate responses. Climate feedback is a process that can intensify or reduce climate change – consider, for example, the melting of sea ice in the Arctic, which replaces sun-reflecting ice with sun-absorbing high seas, which in turn causes more warming and more melting.
To study how likely each of these diverse planets is to remain habitable on a huge (geological) time scale, I simulated each of them 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 are climate-changing factors, such as super-volcanic eruptions (such as Pinatubo, but much larger) and asteroid impacts (such as the one that killed the dinosaurs). At each of the 100 runs, the planet’s temperature is tracked until it becomes too hot or too cold, or otherwise survives for 3 billion years, at which point it is considered a possible crucible for intelligent life.
The simulation results provide a definite answer to this habitability problem, at least in terms of the importance of feedback and luck. It was very rare (actually only once in 100,000) for a planet to have such strong stabilizing feedback that it remained inhabitable for all 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. In almost every case in the simulation, when a planet has remained habitable for 3 billion years, this is partly due to luck. At the same time, luck alone proved insufficient. Planets that are specially designed to have no feedback have never remained habitable; occasional walks, suppressed by climatic events, never continued the course.
This overall result, that results depend in part on feedback and in part on luck, is strong. Any changes in the modeling did not affect him. By default, the Earth must therefore have feedback to stabilize the climate, but at the same time, luck must be involved in order to remain habitable. If, for example, an asteroid or a solar flash were a little bigger than they were, or had happened at a slightly different (more critical) time, we probably wouldn’t be here on Earth today. This gives a different perspective on why we can look back on the remarkable, incredibly extended history of life, which is evolving and diversifying and becoming increasingly complex to such an extent that it has given birth to us.
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This article by Toby Tyrell, Professor of Earth Systems Science, University of Southampton, has been republished by The Conversation under a Creative Commons license. Read the original article.
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