JMG had an interesting observation/estimate regarding the scale of human life relative to the time span of the planet's life bearing capacity.
Let’s imagine, by contrast, a metaphor that maps the entire history of life on earth, from the first living thing on this planet to the last, onto a single year. We don’t know exactly when life will go extinct on this planet, but then we don’t know exactly when it emerged, either; the most recent estimate I know of puts the origin of terrestrial life somewhere a little more than 3.7 billion years ago, and the point at which the sun’s increasing heat will finally sterilize the planet somewhere a little more than 1.2 billion years from now. Adding in a bit of rounding error, we can set the lifespan of our planetary biosphere at a nice round five billion years. On that scale, a month of thirty days is 411 million years, a single day is 13.7 million years, an hour is around 571,000 years, a minute is around 9514 years, and a second is 158 years and change. Our genus, Homo,* evolved maybe two hours ago, and all of recorded human history so far has taken up a little less than 32 seconds.
(*Another gender-nonspecific word for “human being,” this one comes from Latin, and is equally distinct from vir, “man,” and femina, “woman.” English really does need to get its act together.)
That all corresponds closely to the standard metaphor. The difference comes in when you glance at the calendar and find out that the present moment in time falls not on December 31 or any other similarly momentous date, but on an ordinary, undistinguished day—by my back-of-the-envelope calculation, it would be September 26.
I like to imagine our time, along these lines, as an instant during an early autumn afternoon in the great year of Earth’s biosphere. Like many another late September day, it’s becoming uncomfortably hot, and billowing dark clouds stand on the horizon, heralds of an oncoming storm. We human mayflies, with a lifespan averaging maybe half a second, dart here and there, busy with our momentary occupations; a few of us now and then lift our gaze from our own affairs and try to imagine the cold bare fields of early spring, the sultry air of summer evenings, or the rigors of a late autumn none of us will ever see.
With that in mind, let’s put some other dates onto the calendar. While life began on January 1, multicellular life didn’t get started until sometime in the middle of August—for almost two-thirds of the history of life, Earth was a planet of bacteria and blue-green algae, and in terms of total biomass, it arguably still is. The first primitive plants and invertebrate animals ventured onto the land around August 25; the terrible end-Permian extinction crisis, the worst the planet has yet experienced, hit on September 8; the dinosaurs perished in the small hours of September 22, and the last ice age ended just over a minute ago, having taken place over some twelve and a half minutes.
Now let’s turn and look in the other direction. The last ice age was part of a glacial era that began a little less than two hours ago and can be expected to continue through the morning of the 27th—on our time scale, they happen every two and a half weeks or so, and the intervals between them are warm periods when the Earth is a jungle planet and glaciers don’t exist. Our current idiotic habit of treating the atmosphere as a gaseous sewer will disrupt that cycle for only a very short time; our ability to dump greenhouse gases into the atmosphere will end in less than a second as readily accessible fossil fuel reserves are exhausted, and it will take rather less than a minute thereafter for natural processes to scrub the excess CO2 from the atmosphere and return the planet’s climate to its normal instability.
Certain other consequences of our brief moment of absurd extravagance will last longer. On our timescale, the process of radioactive decay will take around half an hour (that is to say, a quarter million years or so) to reduce high-level nuclear waste all the way to harmlessness. It will take an interval of something like the same order of magnitude before all the dead satellites in high orbits have succumbed to the complex processes that will send them to a fiery fate in Earth’s atmosphere, and quite possibly longer for the constant rain of small meteorites onto the lunar surface to pound the Apollo landers and other space junk there to unrecognizable fragments. Given a few hours of the biosphere’s great year, though, everything we are and everything we’ve done will be long gone.
Beyond that, the great timekeeper of Earth’s biosphere is the Sun. Stars increase in their output of heat over most of their life cycle, and the Sun is no exception. The single-celled chemosynthetic organisms that crept out of undersea hot springs in February or March of the great year encountered a frozen world, lit by a pale white Sun whose rays gave far less heat than today; the oldest currently known ice age, the Cryogenian glaciation of the late Precambrian period, was apparently cold enough to freeze the oceans solid and wrap most of the planet in ice. By contrast, toward the middle of November in the distant Neozoic Era, the Sun will be warmer and yellower than it is today, and glacial eras will likely involve little more than the appearance of snow on a few high mountains normally covered in jungle.
Thus the Earth will gradually warm through October and November. Temperatures will cycle up and down with the normal cycles of planetary climate, but each warm period will tend to be a little warmer than the last, and each cold period a little less frigid. Come December, most of a billion years from now, as the heat climbs past one threshold after another, more and more of the Earth’s water will evaporate and, as dissociated oxygen and hydrogen atoms, boil off into space; the Earth will become a desert world, with life clinging to existence at the poles and in fissures deep underground, until finally the last salt-crusted seas run dry and the last living things die out.
And humanity? The average large vertebrate genus lasts something like ten million years—in our scale, something over seventeen hours. As already noted, our genus has only been around for about two hours so far, so it’s statistically likely that we still have a good long run ahead of us. I’ve discussed in these essays several times already the hard physical facts that argue that we aren’t going to go to the stars, or even settle other planets in this solar system, but that’s nothing we have to worry about. Even if we have an improbably long period of human existence ahead of us—say, the fifty million years that bats of the modern type have been around, some three and a half days in our scale, or ten thousand times the length of all recorded human history to date—the Earth will be burgeoning with living things, and perfectly capable of supporting not only intelligent life but rich, complex, unimaginably diverse civilizations, long after we’ve all settled down to our new careers as fossils.
This does not mean, of course, that the Earth will be capable of supporting the kind of civilization we have today. It’s arguably not capable of supporting that kind of civilization now. Certainly the direct and indirect consequences of trying to maintain the civilization we’ve got, even for the short time we’ve made that attempt so far, are setting off chains of consequences that don’t seem likely to leave much of it standing for long. That doesn’t mean we’re headed back to the caves, or for that matter, back to the Middle Ages—these being the two bogeymen that believers in progress like to use when they’re trying to insist that we have no alternative but to keep on stumbling blindly ahead on our current trajectory, no matter what.
There's more discussion in the essay but the above pertinent to the mania that drives us to build war machines that can end all life on the planet many times over, yet be utterly inconsequential in The Grand Scheme of Things.
