The Once and Future Earth
Every now and then, when seeing fresh examples of the world's problems, local or global, I take a deep breath, sigh, and think, "In a million years, what difference will it all make?" It may sound fatalistic, and of course current events do matter to our short-timer existences on Earth, but the thought gives me an odd sense of peace and gets me to thinking about the future—the far distant future—of the Earth. It's hard to imagine what the future will bring in ten, a hundred, or even a thousand million years. Where will evolution take life on Earth—including us? How far will human civilization stretch, and what turns will it take? What exciting twists and cliffhangers are in store for the climate? What will be on television?
Some things are a bit easier to predict: what the Sun will do and how the Earth and the Earth-Moon relationship will change.
I ran across a web version of the H.G. Wells novel "The Time Machine" a couple of weeks ago, and re-reading Chapter 11 I was reminded how insightful the story is with regard to visualizing future possibilities. In this chapter, the Time Traveler probes forward in time, going millions of years into the future and arriving in a tidally-locked Earth under a bloated, reddened Sun, with no Moon in the sky. The ocean was calm and cold, sporting only gentle, lazy swells, and the air was considerably less stocked with oxygen than today. Snow peppered the land and ice fringed the sea, and the only ubiquitous sign that life still existed was a green slime that coated the rocks of the shore.
"All the sounds of man, the bleating of sheep, the cries of birds, the hum of insects, the stir that makes the background of our lives – all that was over."
An alien, cold, and pessimistic view of the future? Well—it can hardly be classified as pessimistic; pessimism is an emotion based on the seeming unchangeability of things we can in fact change. But the Earth's future is commanded by forces scarcely within our power to affect.
For one, the Earth's rotation is slowing down. It used to spin much faster—maybe three times as much—but tidal effects of the Moon and Sun have been slowing it down for four and a half billion years. Imagine an eight-hour day, with the Sun crossing from horizon to horizon in about four. Wake up, it's only a couple of hours until lunchtime, and another two ‘til dinner. I got a whole three hours of sleep last night! Ahh!
Where is Earth's spin going? Shakespeare had the answer: "The Moon's an arrant thief…." The momentum of Earth's spin is being slowly siphoned off by the Moon through tidal interaction, which is simultaneously causing the Moon to move farther from the Earth. Once much closer to Earth, even today the Moon continues to inch away into space–quite literally, at less than two inches per year.
So in the very distant future, we can project that the Moon will have moved much farther from the Earth, and the Earth's rotation will have slowed down even more. At some point the Earth's rotation would match the Moon's orbital period and the Earth will become tidally locked with the Moon, always keeping the same face to it, just as the Moon is currently tidal-locked to the Earth.
In H.G. Wells' vision, the far distant future Earth is tidally locked to the Sun, and the Moon is apparently gone. Would this happen? Will there ever be an Earth with an unending day and unending moonless night (depending on your address)? That could happen, but the Moon would have to leave the picture first, perhaps wandering far enough out that a chance gravitational disturbance by another planet would knock it off the edge of its orbit.
The Sun is changing too—has changed, and will continue to change—as the dynamics of its nuclear fuel supply mix shifts. As atomic fusion converts hydrogen into helium, helium to carbon, and so forth, the availability of easily released energy will diminish, causing the core to shrink and heat up, in turn causing the outer layers to inflate, becoming more expansive but also cooler and redder. In the very long run, the outer layers will expand beyond Earth's present orbit.
So there is a future out there that we can be more certain of than the future shaped by human affairs. It's further out in time than the decades or centuries ahead—and frankly further out than H. G. Wells penned in at 30 million years (little will have changed with the length of a day and the mile markers to the Moon in that time, and I believe the Sun won't make much of a fuss for at least a billion, or more).
In the meantime, it's captivating to think what the scenery may be like around the place I stand today, a million or a billion years hence.