How habitable is the Earth ?  

Charlie Stross has a look at how little of the earth is habitable for humanity - and how is was often even less so as we go back in time - How habitable is the Earth?.

We H. Sapiens Sapiens appear to be an infestation on this planet. After the slow-burning evolution of hominins in Africa, our ancestral populations erupted out into Eurasia in a geological eye-blink, spread into the Americas by way of the Bering land bridge (sea levels being somewhat lower during the ice ages) and finally reaching even the remotest islands of oceania around twelve thousand years ago. Today we're ubiquitous. Even our pre-industrial ancestral cultures, from those resembling the inuit to the antecedents of the tuareg, occupied a slew of geographical environments that put cockroaches to shame.

So you'd think that, to a first approximation, the Earth is inhabitable by human beings. And this tends to colour our approach the prospects of finding extrasolar planets that might be hospitable to human life (if we could ever get there from here).

Actually, I think this is not quite the case. In fact, to a first approximation, from the perspective of prospective interstellar colonists, the Earth is uninhabitable. That we could imagine otherwise bespeaks a profound cognitive bias on our part (and a degree of relativism: because when all's said and done, the Earth is a lot less hostile than, say, the surface of Venus or the cloud base of Jupiter).

Why is the Earth uninhabitable?

Let's play a thought-experiment ...

I want you to imagine that, instead of being a perplexed mostly-hairless primate reading a blog, you're the guiding intelligence of an interstellar robot probe. You've been entrusted with the vital mission of determining whether a target planet is inhabitable by members of your creator species, who bear an eerie resemblance to H. Sapiens Sapiens. To gauge the suitability of the target world you've been given an incubator that can generate decorticated human clones — breathing meat-machines with nobody home up top. When you get to the destination you're going to transfer them to the surface and see how long they survive. If it can make it through 24 hours (or one diurnal period), congratulations! — you've found a potential colony world; one so hospitable that a naked and clueless human doesn't die on their first day out.

Your first destination planet is the cloud-whorled third planet out from an undistinguished G2 star, orbited by an airless, tidally-locked moon with roughly 1.3% of the planet's own mass. (Sound familiar? It should be.) You start sending down meat-machines to probe the surface at random. What conclusions do you draw about the inhabitability of Earth?

Let's start with Earth in its current configuration.

78% of the surface area is seawater. Drop a naked meat puppet there and it's going to go glug glug glub ... tritely, this is Not A Good Start.

Of the remaining 22%, about one third is either mountain ranges, deserts, or ice caps. It's reasonable to say that, in the absence of protective equipment, the meat probes are going to die of exposure in less than one diurnal period — possibly in as little as an hour if they're unlucky enough to land in the middle of the Antarctic winter.

We're down to about 15% of the planetary surface — 15% that isn't lethal without life support equipment such as boats, tents, and clothing. Our meat probes can breathe the air without their lungs freezing or dessicating. They aren't going to drown rapidly. And they aren't going to roll off a cliff. They might get a tad sunburned or hypothermic depending on the weather, and they might be eaten by a mountain lion or bitten by a rattlesnake, but they stand a reasonable chance of making it through 24 hours on the surface without dying.

Triumph! We have confirmed that a small part of this planet is inhabitable. Except ... I cheated. I pulled a fast one on you. Because I picked Earth in its current configuration — as it is today. ...