Why is Earth still habitable after billions of years? In part, we're just lucky.

When you look around you, almost anywhere on Earth, you see life. Earth seems exquisitely supportive of life: We see it in the air, in the water, in the land, and even deep underground.

But was that inevitable? We know that there have been mass extinction events in the past, some taking out the majority of life on Earth. However, since life got started and spread around on Earth there hasn't been any event that completely eradicated life. Of course! Else we wouldn't be here to ponder it.

Still, it's interesting. That means, despite some seriously incapacitating temporary events, Earth's climate has remained relatively stable for 3 – 4 billion years.

Artwork depicting the last day of the Cretaceous Period. Credit: Davide Bonadonna via Imperial College London

Photo: Davide Bonadonna via Imperial College London

That's also weird. We know that stars like the Sun grow hotter as they age, and that long ago the Sun was about 30% fainter. That either means the ancient Earth should have been frozen solid, or, assuming it was clement, the Earth should be boiling hot now. Neither is true, which is a mystery.

This is called the Faint Young Sun Paradox, and has prompted many scientists to presume that the Earth has a kind of thermostat, a set of conditions that tend to rebalance a system that gets out of whack so it doesn't get too hot or too cold. This would be a negative feedback system, where if a condition arises to, say, heat up the Earth, things will change in a way to cool it back down.

But we know positive feedback conditions exist, too. If you release too much carbon dioxide into the air the oceans will warm up, which releases more CO₂, and you get a feedback loop that ends badly. As we're seeing now. And if there's too little CO₂ in the air the Earth would be frozen solid.

So maybe we're just lucky, and our environment just happened to stay stable for all those eons life has been around.

So is it by chance, or by mechanism? Or both?

Artwork depicting exoplanets similar to Earth, with an example of temperature versus time found in a simulation. Credit: Kate Davis

To find out, a scientist ran a clever experiment. He created a simulation of 100,000 planets (!!) where each was given a set of random climate feedbacks, some negative and some positive, and tracked their temperatures for 3 billion years — no other variables (water content, for example, or breathable atmosphere) was simulated. For simplicity he just wanted to see if a planet could keep a habitable temperature for a long period of time, as Earth has.

To be clear the simulate feedbacks weren't based on real ones like CO₂ in the air; instead he assigned the planets random mathematical feedbacks, strictly numerical situations to see what would happen. He also threw in bigger random changes at random times to simulate external forcing of temperature, similar to things like asteroid impacts or supervolcano eruptions.

Each planet sim was then run 100 times, with the variations in it changed a bit, to see what happens to the temperature.

The point here wasn't to create a full-up climate simulation but to see how big a role chance plays in a planet's habitability. He was testing two hypotheses. Hypothesis 1 is that feedback has no affect, so random fluctuations rule the day; it's just pure chance if a planet stays in a habitable temperature range for billions of years. The second hypothesis is that having feedback, either negative or positive, guarantees either success or failure, with chance playing no role.

In other words, he was hoping to see if climate feedbacks really are why Earth has remained habitable for so long, or if we're just lucky. A planet was considered habitable if its temperature stayed relative stable over the 3 billion year simulation.

Artwork depicting Earth without water (left) versus the actual planet observed from space (right). Credit: David Gallo/WHOI and NASA/NOAA

What he found is interesting. Out of 100,000 planets, 9% were successful at least once (and 1,400 were successful on the very first run out of 100 runs). Some planets were successful twice, some three times… and in fact, looking over all 100,000 planets, he had every number between 1 and 100 successful runs.

But, only 1 planet had 100 successful runs out of 100. That's a robust planet, indicating that nothing was able to prevent it from being a nice place to live (and least in temperature).

Overall, looking at the range of outcomes and how they occurred, his conclusion is that both feedbacks and random chance play a role in a planet's ability to stay in a livable temperature range. While the success rate varied from model to model, changing the factors over the 100 runs still supported the idea that both mechanism and chance played a role.

Apparently, fortune favors the prepared planet.

So can we extrapolate this to Earth, saying it's both the feedbacks we have and random chance that have kept our fair world, well, fair? If we rewind the tape and vary the circumstances a bit, we'd still have a habitable world to live on?

I wouldn't go that far. This seems to support that idea, but as the author himself said in the paper, “The simplifications and uncertainties in the model design mean that it must be unrealistic in some respects. Caution is therefore required in extrapolating from model results to reality.”

In other words, this is a very simplistic test, and much more complicated ones should be done. After all, Earth has come close a few times to the tipping point, so it's not hard to imagine a whopping great asteroid impact or some other factor doing us dirty. But still, this simulation is an interesting first step!

It does make a prediction: Most exoplanets that are similar to Earth will be uninhabitable, since that happened in the majority of his test runs. Planets like Earth were the exception. If we find that's true it doesn't prove the hypothesis, but supports it. And if we find most really are habitable, well, that'll be interesting, won't it?

And this serves as a cautionary tale. We don't really know how robust the Earth is, how well it can take a hit and keep on keeping on. It's been slammed in the past, staggered, and shook things off, but not without some degree of long-term environmental impacts. And our own species, our civilization, balances on a razor's edge right now. It wouldn't take such a huge hit to cause untold calamity to us, even if the Earth's ecosystem somehow managed to survive.

I have a lot of issues with humans, but I'd prefer we don't go extinct. Uncontrolled fiddling with the feedbacks already in place strikes me as a pretty terrible idea. The Earth may be robust, but we aren't.

We need to be more careful. There's a reasons these things are called cautionary tales.