Before reading this part in the series, please review its predecessors: Part 1, Part 2, and Part 3, concerning the history of life on Earth up to the present, the next decade, and the next century, respectively. The concepts outlined in these entries are critical for the longer-term predictions to make sense, because otherwise they will seem entirely arbitrary - which they are not. In part 4, we expand the time horizon to 1000 years.
Allow me to reiterate a few standard caveats, as they keep popping up in comments despite my having addressed them in earlier entries:
First, this prediction sequence does not attempt to account for either radical technological breakthroughs or catastrophic events, because it is my considered understanding of history that these are short-term phenomena whose consequences (positive or negative) are averaged out over time. While a radical advance can make humanity suddenly aware of extreme new potential, its long-term realization nevertheless will tend to proceed along incremental lines.
Likewise, while a catastrophe (e.g., the Black Death) can effectively destroy a civilization, it either recovers in a few generations or is surpassed by another that was less affected. Even the most extreme historical scenario, the collapse of the Roman Empire, did not end progress, but merely moved it Eastward into Byzantium and the first few centuries of the Caliphate - an era that saw significant (albeit slow) progress in mathematics, astronomy, maritime technology, and other fields. We refer to this period as the Dark Ages only because Western Europe became degenerate and chaotic, but progress of a kind continued in the Byzantine Greek and Arabic worlds.
The point of these two arguments is that, although progress may be a process of "punctuated" equilibrium, it is still an equilibrium whose general trajectory can be rationally and credibly assessed over long timescales. In fact, the longer the timescale, the more credible an assessment becomes when derived from underlying principles (in the case of this series, the Root/Spore cycle and energy pathway-climbing).
Secondly, these predictions are conservative - they assume no new physics or radical social progress, and on longer timescales do not speculate about specific technologies beyond the vaguest outlines suggested by the overall pattern. So the purpose is to show a baseline future: One that is likely to occur if humanity discovers nothing fundamentally new about the universe in a million years. Since this is extremely unlikely, the picture I paint is intended to establish a minimum-progress scenario that is inevitable in all cases short of total human extinction. Since I don't rule out intervening Dark Ages at any given point, the shorter timescales are less supportable, but the picture approaches inevitability on longer ones.
Thirdly, human extinction is one of the least likely scenarios imaginable. Even a species-wide Dark Age is incredibly unlikely over century timescales, let alone longer epochs, so the total eradication of a viable human population with at least latent access to scientific principles is almost ridiculous. Not impossible, but also not relevant - if human ingenuity and will to survive is insufficient, then no amount of foresight would make a difference, so positing such a case is beyond moot: It's an intellectual abortion.
Furthermore, planetary-scale disasters will not endanger the species as a whole much beyond the next century due to initial spaceward expansion. We should still be aware of short- and medium-term risks in order to limit damage and maximize potential, but the "WE'RE ALL GONNA DIE!!!" school of dystopic futurism adds nothing to human progress.
Fourth, every technology that exists today or has ever existed will still exist in some form regardless of what progress has occurred. If I fail to mention something - e.g., if I treat wind and geothermal energy dismissively - it is not because it doesn't exist in some capacity, but because it has probably ceased being a major component of human infrastructure. There may be niche applications here or there, but nothing with a lot of long-term (and I do mean long-term) potential.
Table of Contents
(Current part in bold)
I. The Energy History of Life (Part 1)
II. The Energy History of Humanity (Part 1)
III. The Next Decade (Part 2)
IV. The Next Century (Part 3)
V. The Next Millennium (Part 4)
VI. 10,000 Years
VII. 100,000 Years
VIII. Mark: One Million Years
To recap humanity's situation as I left it in Part 3, covering progress over the next century, we have a world with a fully renewable energy infrastructure; fully electric transportation, including ground, sea, and air (but not Earth-to-orbit spacecraft, of course); denser, more localized, more efficient, and more self-sufficient economies; the core of a renewable water infrastructure through desalinizing and piping of seawater inland; and a significant (though not great) level of human settlement on the Moon and Mars, with initial human tendrils extending to asteroids and traffic in the Earth-Moon system dominating. We see the first kernel of global Root in the solarization of the renewable energy infrastructure, and the first iteration of planetary Spore in quixotically-driven settlement activity in LEO, the Moon, the Earth-Moon system, and Mars.
Now, keeping in mind the scenario painted in Part 3, zoom outward in time by an order of magnitude so that the 21st century as described is just 1/10th of the total picture. Naturally, a lot more happens over a millennium than a century, so some of the details blur into insignificance while larger-scale patterns come into sharper focus. The motion of the Root/Spore dynamic becomes much more obvious both in the evolution of society on Earth and in the growth of off-world civilizations, particularly as it regards energy. Without laying out a specific timeline, we can at least establish a sequence of developments likely to occur within the thousand-year horizon.
The first Spore iteration (since this is a new Root/Spore cycle, we will call this the N-Spore) is driven by quixotic individual impulse and Earth-focused business, so it does not expand much beyond the orbital region between the Earth-Moon system and Mars. Since fissile elements are scarce even on Earth - let alone on the Moon, Mars, or asteroids - and fusion will not be initially practical even when the technology has been demonstrated in principle, the N-iteration Spore will depend on photovoltaic development, which further limits the scope of its expansion to the inner solar system where sunlight is powerful.
While there will be outlying N settlements beyond this region, they would grow much more slowly due to energy limitations - either because they rely on marginal solar flux or expensively-imported nuclear fuels. These would overwhelmingly be scientific outposts and tourist destinations for the elite, although there might be some grim isolationist communities founded by cults or the like. The furthest I would expect viable N-Spore to reach would be Jupiter, although the gradient of viability (and thus the population) would fall off sharply beyond Mars for this iteration.
This is interesting because N-Spore will be expanding away from their energy source, since they are either guided by non-rational motives (wonder, adventure, escape, etc.) or what they seek is raw material that is available in vastly larger quantities further from the Sun - an especially relevant motive in asteroid development. As a result, growth beyond Mars during this phase will be relatively slow because its energy source is tenuous or expensive.
But something quite different would be happening along Earth's solar orbit: Energy is abundant, and material products from the Moon easily distributable. This is because the delta-v (i.e., energy) required to leave the lunar surface is small, and that required to move along a given orbit is trivial if you don't mind waiting - an ideal condition for bulk cargo transport. In particular, the stable Earth-Sun Lagrange points (L4 and L5 - areas of gravitational equilibrium in empty space) will achieve substantial growth because of lunar resources, solar energy, and the ability to cheaply remain in one location in free-space.
However, I do not see much activity occurring within Earth's solar orbit on the part of N-Spore: Although solar energy only becomes more powerful the closer to the Sun you get, accessible raw materials practically disappear. Virtually all of the materials within Earth's solar orbit are on Venus - which might as well not be there for all the good it does us at this point - or on Mercury, which will still be too hot and irradiated to deal with economically. So any inward migration at this point will be checked by the high cost of increased shielding, more temperature-resistant solar panels (resistant to both extremes, because even though sunlit areas are hotter, shadow is still frigid), and delta-v required to deliver cargo.
The same inward boundary will persist even at N+1 Spore, which will be kicked off by the development of compact, economical fusion power. What will change radically with this technology is the outward-bound viability gradient: Even though fusion may still be costly compared to passive solar harvesting near Earth, it will be increasingly cost-effective at greater distances from the Sun.
Settlements that had before relied on marginal solar or expensive, imported fissile materials could now generate abundant energy without relying on external inputs. Since these settlements are already abundant in metals and water, the addition of bountiful, locally-creatable energy causes explosive growth in existing settlements; rapid creation of new settlements; and expansion of the viability envelope to encompass the entire ecliptic plane of the solar system out to the Kuiper Belt. Humanity will then diffuse outward at a much quicker pace, seeding the solar system with a mind-boggling array of new civilizations. Although in this time frame they each would have smaller populations than the core worlds - Earth, the Moon, and Mars - their combined population and wealth would come to exceed that of the inner solar system.
This is just a pet conjecture of mine, but I would expect the economic center of gravity in the solar system to shift to Saturn due to its high concentration of easily-accessible raw materials, volatiles, and even organics - not to mention its awesome beauty, lower escape velocity and radiation environment than Jupiter, and central location between the core worlds and regions more distant. The Saturn system would fill the role that China is increasingly filling on Earth today, albeit hopefully without the ominous political and military connotations. The Main Belt asteroids, on the other hand, I can see becoming like a new Aegean, with a multitude of highly diverse, creative, and dynamic civilizations. Again, these are not essential predictions, but I find them likely.
I will not speculate on exactly when N-Spore will transition to N+1 - it could be 200, 300, or 500 years before nuclear fusion becomes that advanced - but I am strongly confident that this is the proper time horizon in which to address it. Progress on fusion will be slow but inexorable, and this transition point - when fusion is simple and economical enough for frontier deployment and local maintenance - will be reached at some point within the next millennium, almost certainly in the first half.
Regardless of its exact epicenter or timing, the N+1 Spore iteration will come to dominate civilization and facilitate an ongoing mass-exodus from the inner solar system. But far from depopulating the core worlds, it will simply relieve potentially destructive pressures and allow them to grow in a healthier way. However, an energy bifurcation will occur: Mars and more distant points will go full fusion due to the tenuousness of solar energy at their orbit, but Earth, the Moon, and settlements along Earth's solar orbit will either have a mixture of fusion and PV-derived1 technologies or else entirely the latter. The ability to passively harvest solar energy this close to the Sun will be so trivial and advanced by this point that fusion will have difficulty penetrating the system, and will evolve much less quickly due to its relative complexity.
This bifurcation will be the result of humanity climbing what is ultimately the same energy pathway - nuclear fusion. One side will find it more economical to generate its own little stars to shine on the passive collectors surrounding them, while the other is close enough to an existing star to let nature do most of the hard work for them and just focus on harvesting its output. The border between these two domains will never be clear-cut - there are plenty of reasons that some fusion power would be utilized on Earth, and some passive solar harvesting occur further out - but it will exist nonetheless, and Earth will fall on the passive-utilization side of the boundary.
The reasons for this are simple: The passive-harvesting infrastructure will by then be so massive, cheap, and adaptable that the marginal cost of increasing it will be trivial compared to building fusion plants, however inexpensive they've become due to outer system development. I won't speculate on the exact nature of the harvesting technology beyond stating that it will be extremely low-mass, cheap, and ubiquitous both on surfaces and in orbit - i.e., space-based solar power will be realized on a grand scale, probably early in this millennium, and come to dominate the system's energy infrastructure. As a result, expanding it by umpteen thousand square kilometers in a decade would not be an economically significant task by the time that fusion becomes potent in the outer solar system.
If you haven't already guessed, the solar/fusion boundary demarcates the line between Root and Spore - between a civilization whose energy pathway leads inward toward the Sun, and disparate civilizations with a mobile energy source who follow the raw materials. This is important because Root will no longer be limited to Earth - the Earth-Moon system and settlements in our solar orbit will gradually cease to follow Sporelike development patterns, and instead fuse into a single Sun-centered Root economy with ever-increasing production of solar harvesting material.
Over time, the greater efficiencies obtainable by being closer to the Sun will lead to successive rounds of innovation to deal with higher temperature gradients and more powerful input per unit area. Ever so slowly, the Root civilization will creep inward toward the source of its energy, but the effect will not be significant over this timescale. Rather, the majority of Root growth will be along Earth's solar orbit.
There will likely be numerous space elevators leading out of Earth's gravity well - this is a practical certainty over this time horizon - so the economic boundary between the surface and the rest of the system will be trivial. But since we have a sentimental desire to think about what the Earth's surface will be like in the future, I'll take a stab at it: As it will have undergone a millennium of Root development, Earth civilization will look like a technological version of a rainforest - unbelievably dense, diverse, complex, and interdependent. My guess is that government will be primarily authoritarian with a strong slant toward theocracy, but this is nonessential.
Human geography becomes the geography of Earth: With the exception of areas preserved for historic reasons, if a road moves uphill, it will not be because it was built on a natural surface that moves uphill - it will be because there's an enormously large building with a vaulted ceiling beneath the road. Skyscrapers will be little more than the pinnacles on top of truly mountainous structures, and yet throughout all the man-made valleys, hills, and mountains, there will be green everywhere - forests and fields, fed with all the sunlight and water they need through easily-imaginable means.
I've tried to find an artistic rendering of a future city that conveys what I'm thinking of, but none of the examples on Google quite do the job. They're all either hyper-techno, imposing Blade Runner-style urban cityscapes, or else touchy-feely low-height "eco" fantasies, and the reality is an amalgam of the two: There will be lots of trees and plants everywhere, but it will be within terraced urban canyons with architecture on a scale that would seem daunting to anyone not accustomed to it. This is the best example I've found, and it's not that great - I'd say it's not even 1000 years from now, but more like 200 (note the tiny Chrysler Building):
The terrestrial water supply will consist of...the terrestrial water supply - all of it. Every square inch we choose to irrigate with desalinized seawater becomes verdant, and despite attempts to preserve desert ecologies, they are ultimately doomed. The world comes alive, and humanity takes control of the climate by managing the ratio of energy input/ouput through manipulation of Sun-exposed surface albedo. Still, it may not be the climate we recognize: The sky could be milk-white and the air thick, but generations of people would see it as Home and not pine after the blue skies we know - after all, they can still see blue skies by going up to the 3,000th floor of a building, or during the part of a space elevator ride before the sky turns black.
In summary, over the next millennium we can expect to see two Spore iterations and the growth of Root to encompass the Earth's solar orbit. The first (N) Spore iteration will settle the Earth-Moon system, Mars, some nearby asteroids, and late in its term, generate a small number of marginal settlements further out. The second (N+1) Spore iteration will utilize cost-effective fusion energy to expand outward to the utmost reaches of the solar system along the plane of the ecliptic, and radically shift the focus of human civilization away from Earth and toward the outer planets. However, the Earth-centered Root economy will continually expand its solar harvesting capacity and grow along its solar orbit.
Footnote:
1: I suspect that energy will not, in the future, correspond to electricity, but to direct photonic manipulation, so I am not referring literally to "photovoltaics" when talking this far into the future - just to passive harvesting of solar energy.