Looking back at how the future was seen half a century ago, it is amazing to see how things have changed. When the conquest of space seemed to be the obvious way forward, nobody would have imagined that, today, we would be discussing the probability of survival of humankind, and that many of us would judge it as low.
Yet, even though the future remains obscure, it still follows the laws of the universe. And one of these laws is that civilizations exist because they have a supply of energy. No energy, no civilization. So, the key element of the future is energy; the idea that it would be cheap and abundant gave rise to the dream of the conquest of space in the 1950s. Today, the idea that it will be neither gives rise to the prospects of doom.
So, let me try a simple "scenario analysis" of what may happen in the future in the next century or so in terms of choices that will determine the energy infrastructure that could support a complex civilization (if any will survive). We are in a moment of transition, and the choices that will be made in the next few years (not decades) will determine the future of humankind.
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Scenario #0: collapse. I call this a "non-scenario" in the sense that it assumes that nothing is done or, anyway, too little and too late. In this case, people remain stuck in their old paradigms, the resources that kept society alive are not replaced, and it becomes impossible to maintain a degree of complexity comparable to the current one. Within some decades, humans return to an economy that we might describe as "medieval," if we are lucky. But we might also go back to hunting and gathering or even, simply, go extinct. Personally, I see this scenario as the most likely one, but not an obligate outcome of the current situation.
Scenario #1: Sticking to Fossil Fuels. Here, we see a repetition of the events that led to stemming the decline of oil production during the first two decades of the 20th century. It was done by pouring large amounts of resources into the "fracking" of tight oil deposits. It produced a temporary resurrection of the oil industry in the US, bringing production to levels never seen before, albeit at enormous economic and environmental costs. The same policy could be continued with renewed efforts, for instance, at exploiting tight oil deposits outside the United States, tar sands, or maybe making synthetic fuels out of coal. That could maintain the production of fossil fuels to levels similar to the current ones. It would make it possible to keep alive the military apparatuses of the main states, and at least some of the current organizations and social structures. But the cost would be enormous, and it would imply beggaring most of the world's population, as well as unimaginable damage to the ecosystem. This strategy could keep a semblance of the current civilization going on for a few decades, hardly more than the end of the century. Then, it will be Scenario #0, but the crash will be even worse than if it had arrived by doing nothing.
Scenario #2: Going Nuclear. Supporting a complex society on nuclear energy may be possible, but it is complicated by several factors. Among these are the limited uranium resources, the need for rare mineral resources for the plants, and the strategic problems involved in disseminating nuclear technologies and uranium processing knowledge all over the world. Because of the limited amounts of mineral uranium, it is well known that the existing technology of light water reactors would not be able to supply the current global energy demand for more than a few decades, at best for a century or so. Then the outcome would be again scenario #0. The fuel supply could be greatly increased by moving to the challenging task of "breeding" new fuels from thorium or non-fissile uranium. If that were possible, a complex civilization could continue to exist for several centuries, or even more. In all cases, a major war that would target the nuclear plants would rapidly send a nuclear civilization to scenario #0.
Scenario #3: The Solar Era. In this case, we see the continuation of the current trend that sees renewable energy technologies, mainly solar photovoltaic and wind, rapidly expanding. If this expansion continues, it can make both fossil fuels and nuclear energy obsolete. Renewable technologies have a good energy return on energy investment (EROI) and little need for rare minerals. Renewables are not a strategic problem, have no direct military interest, and can be used everywhere. The plants can be recycled, and they are expected to be able to support a complex society; even though in a form that, today, we can only barely imagine. A solar-based infrastructure is also naturally forced to reach a certain degree of stability because of the limited flux of solar energy available. So, a solar-based civilization could reach a stable state that could last at least as long as agricultural societies did in the past, thousands of years, or even longer.
Combined Scenarios #1, #2, #3: Feudalization. The three scenarios above are based on the idea that human civilization remains reasonably "global." In this case, the competition between different technologies would play out at a global scale and determine a winner that would take over the whole energy market. But that's not necessarily the case if the world's economic systems separate into independent sections, as it appears to be happening right now. In this case, some regions might adopt different strategies, fossils, nuclear, or renewables, while some would simply be shut off from the energy supply system and go directly to "Scenario #0." With lower demand, the problems of depletion of nuclear and fossils would be greatly eased, although, of course, only for a limited population. Note also that these near-independent regions can be described as "feudal," but need to be much larger and more structured than anything seen during the historical Middle Ages. Keeping alive complex technologies, nuclear in particular, requires maintaining a functioning industrial society, and that may not be obvious in a time of diminishing returns for everything.
The next few decades will decide which direction humankind will take. No one has the hands on the wheel that moves the giant thing we call "civilization," and we are seeing efforts to push it in one of the three scenarios above (some people even seem to be actively pushing for scenario #0, a civilization-level expression of what Sigmund Freud called the "death instinct").
The problem, here, is that the Western governance system has evolved in such a way that no decision can be taken unless some groups or sectors of society are demonized, and then a narrative is created that implies fighting a common enemy. In other words, no decision can be taken on the basis of data and planning for the common good, but only as the result of the confrontation of the lobbies involved in supporting different options. (*)(*) Simon Sheridan provides an interesting discussion of the inner decisional mechanisms of modern society, defined as "esoteric" in the sense of being hidden, unlike the "exoteric," e.g. public decisional mechanism, which is only a reflection of the esoteric process.
(**) For much longer-term scenarios, see my post: "The Next Ten Billion Years"