This is not a trivial problem. Current global warming shows us that the climate can change considerably over the course of even a few centuries. Over geological timescales, it is even easier to change climate. Calculations show that there is the potential for Earth’s climate to deteriorate to temperatures below freezing or above boiling in just a few million years.

 

We also know that the Sun has become 30 per cent more luminous since life first evolved. In theory, this should have caused the oceans to boil away by now, given that they were not generally frozen on the early Earth – this is known as the “faint young Sun paradox”. Yet, somehow, this habitability puzzle was solved.

 

Scientists have come up with two main theories. The first is that the Earth could possess something like a thermostat – a feedback mechanism (or mechanisms) that prevents the climate ever wandering to fatal temperatures.

 

In order to investigate how likely each of these diverse planets was to stay habitable over enormous (geological) timescales, I simulated each 100 times. Each time the planet started from a different initial temperature and was exposed to a randomly different set of climate events. These events represent climate-altering factors such as supervolcano eruptions (like Mount Pinatubo but much much larger) and asteroid impacts (like the one that killed the dinosaurs). On each of the 100 runs, the planet’s temperature was tracked until it became too hot or too cold or else had survived for 3 billion years, at which point it was deemed to have been a possible crucible for intelligent life.

 

The simulation results give a definite answer to this habitability problem, at least in terms of the importance of feedbacks and luck. It was very rare (in fact, just one time out of 100,000) for a planet to have such strong stabilising feedbacks that it stayed habitable all 100 times, irrespective of the random climate events. In fact, most planets that stayed habitable at least once, did so fewer than 10 times out of 100. On nearly every occasion in the simulation when a planet remained habitable for 3 billion years, it was partly down to luck. At the same time, luck by itself was shown to be insufficient. Planets that were specially designed to have no feedbacks at all, never stayed habitable; random walks, buffeted around by climate events, never lasted the course.

 

This overall result, that outcomes depend partly on feedbacks and partly on luck, is robust. All sorts of changes to the modelling did not affect it. By implication, Earth must therefore possess some climate-stabilising feedbacks, but at the same time good fortune must also have been involved in it staying habitable. If, for instance, an asteroid or solar flare had been slightly larger than it was, or had occurred at a slightly different (more critical) time, we would probably not be here on Earth today. It gives a different perspective on why we are able to look back on Earth’s remarkable, enormously extended, history of life evolving and diversifying and becoming ever more complex to the point that it gave rise to us.

EurasiaDiary © Must be hyperlinked when used.

Follow us:
Twitter: @Eurasia_Eng
Facebook: EurasiaEng
Telegram: @eurasia_diary