How Habitable is the Earth?


Charlie Stross has an interesting post on his blog that asks the question “How habitable is the Earth?” He goes on to conclude, through a great discussion of the evolution of our planet, that the fraction of time that the earth has been habitable to humans is a tiny sliver of the time the Earth has been around, and that furthermore, much of the earth is not habitable for humans because it is water or ice or mountains. If much of our planet, even now, is not habitable, he argues, what hope is there of finding other habitable worlds out there in space?

It’s an interesting discussion, but I find it somewhat misleading. It makes the rather large assumption that for a world to be considered habitable, a human would have to be able to survive for 24 hours, naked, on the surface. Ok, that’s one definition of habitable. But if you are postulating that these humans are capable of interstellar travel, it seems like you might make allowances for the use of clothing and the ability to build shelter. After all, we’ve known about those ones for a while. You could go even further and suggest that these humans might be able to alter the air they breathe, either through individual gas masks, or on a planetary scale. We used CO2 scrubbers on the Apollo missions to make the air breathable, maybe that would work on a planet with otherwise unbreathable air?

I think he’s fundamentally right in terms of human habitability: the likelihood of a planet being perfectly attuned to humans is extremely low. We evolved to live on Earth and nowhere else. We are going to have to make some adjustments to ourselves or our environment to live anywhere else.

The problem is that he then extrapolates and suggests that this might explain the Fermi paradox (aka. if there are so many stars and planets out there, why haven’t we heard from any little green men?). But that is completely off-base! He is essentially saying that, because humans evolved to live on Earth and nowhere else, it is unlikely for anything else to be living out there because there are likely few earth-like places. That does not follow. There could be aliens out there that are completely happy on their planets that would be instantly lethal to us. And it’s entirely possible that if they set foot on Earth they would find it a very hostile and uninhabitable environment (and not just because of the terrified earthlings).

Anyway, it’s an interesting article. Go take a look.

And speaking of interesting articles, have you gone and voted for my MSL: Mars Action Hero article over at scientificblogging? I’m one of the finalists for their science writing competition, so take a look and vote for me if you like it. To see the other entries, click here. Feel free to vote for as many as you like, and remember you can vote daily until the 23rd!


Explore posts in the same categories: Astrobiology, Humans in Space, Planets in General

3 Comments on “How Habitable is the Earth?”

  1. Lucy Says:

    I just noticed this got blogged by the NYTimes “Idea of the Day” — surprising, since I didn’t think the article was all that exciting. The “naked meat puppet” that’s dropped on a totally random landing site? What alien civilization is going to send a probe all the way to another solar system without a better landing site selection process than that? Plus, just a you were saying, I think any aliens that can travel to Earth can figure out how to build a boat…

    The part about Earth’s history and the oxygen catastrophe were more interesting, but of course that happened because life was *already* present and abundant enough to pump all that oxygen into the atmosphere.

    • Ryan Says:

      Yeah… I mean, I understand the “meat puppet” was just a thought experiment used to point out that we are not all that hardy by ourselves, but that’s not such a novel idea. Plus, it takes away our biggest tool for survival: our brains. Drop thinking humans in various locations on earth and I think your success rate would be somewhat higher.

      As I said, I think he’s right about the Earth being mostly uninhabitable for humans most of the time, but that’s not that profound a statement, and extrapolating it to imply that life is rare in the universe is just wrong. The one doesn’t follow from the other at all.

  2. Joseph Says:

    Huh. I would draw exactly the opposite conclusion from the apparent “uninhabitability” of the Earth. That is, for most of the Earth’s history, an observer who takes a snapshot of a randomly selected fraction of the Earth’s surface would conclude that the planet is unsuitable for life. (Again, of course, making an anthropocentric assumption. Man, I wish we had a better statistical sample.) Yet the Earth is inhabited by life, therefore, those observations are insufficient to draw a conclusion about habitability. Which means that we may already have found inhabited worlds, we just don’t know it yet because we don’t have enough information. That would open up a lot more worlds to speculations of life.

    Liquid water exists on the Earth, existed in the past on Mars, is most likely to exist on Europa, Callisto, and possibly Ganymede, and probably comes in pockets on Enceladus. Water ice exists throughout the Solar System, from the permanently shadowed craters of Mercury to the Kuiper Belt, and I recall a Discover magazine article that the first amino-acid reactions we call life may have happened in tiny bubbles in solid ice, not tide pools. Then there are all the non-water-based chemical processes available on Titan, or in the atmosphere of a gas giant. So I’m pretty much convinced at this point that some kind of life could plausibly exist just about anywhere in our own Solar System. That would have profound implications for notions of “habitability” like this one.

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