In the 1950s, we knew what the future would be: an age of prosperity and unprecedented wonders. Energy too cheap to meter, flying cars, vacations on the moon, and the conquest of space. Then, space heroes would return to Earth to relax on the edge of their swimming pool while the robot-butler would bring them their margaritas. To be sure, the future had a dark side: that of the nuclear holocaust. But it was still a future where human ingenuity would trump everything else.
The future today is completely different. The way we see the destiny of humankind is inextricably linked to the great "pulse" of carbon burning that has been ongoing for a couple of centuries and which is now reaching its peak. Fossil carbon has taken us to where we are now, creating the prosperity of our industrial civilization. But fossil fuels are rapidly running out and that creates a number of consequences; one is the impossibility of running an industrial society without abundant and cheap energy, the other is global warming which is transforming the earth into a completely new planet. These effects will shape the future of humankind in ways that can't be exactly predicted, but that we can imagine in the form of "scenarios" - futures that could happen. So, here are some possible futures of humankind, arranged from the least exciting one (near term extinction) to highly exciting ones, involving expansion over the whole galaxy.
Extinction is a simple scenario to describe: humankind goes extinct and that's it. The time scale of extinction may be millennia, centuries or, perhaps, just decades (in the last case, it may go under the name of "Near Term Extinction," a term popularized by Guy McPherson). In any case, extinction would be very rapid in comparison to the time span of existence of homo sapiens, at least two hundred thousand years.
Extinction is a perfectly possible scenario if we assume the playing out of some of the most dire effects of the human impact on the ecosphere, in particular the emissions of greenhouse gases. The great "methane burp" that could result from the thawing of the Earth's permafrost could raise temperatures up to 6-8 degrees C and even more in times of the order of a few centuries or even much faster. In its extreme version, global warming could evolve into the "Venus catastrophe", where the whole biosphere could be sterilized by extremely high temperatures. To be sure, this scenario seems to be ruled out by the results of the current climate models, but we don't need the Venus catastrophe to unbalance the ecosystem to such a degree that the resources humans need in order to survive would be destroyed. At that point, the outcome could be only one: extinction.
This is a scenario that leaves little to discuss about the destiny of humankind. But, assuming that the biosphere is not completely destroyed, could the planet recover afterward? Perhaps it could, but not necessarily. Nowadays, the Earth is perilously close to the inner edge of the habitable zone in the Solar system and it is being pushed out of it by the gradual increase of solar radiation. It is a very slow process by human standards, but it is estimated that vertebrates have no more than some 100-150 million years to go before the Earth becomes too hot for them to survive. A major disaster such as the one we are contemplating in this scenario could kick the Earth out of the vertebrate habitable zone. In this case, the Earth's biosphere might revert to a world of unicellular creatures such as it was during the Archean or the Proterozoic eons. In such case, it is possible, and perhaps likely, that vertebrates would never re-evolve and that the planet would remain dominated by unicellular life forms until it gets sterilized by further increases in solar radiation, about one billion years from now.
But let's assume that the ecosystem can recover without major losses of phyla. In times of the order of hundreds of thousands of years, the excess CO2 in the atmosphere would be removed and transformed into solid carbonates. That would slowly cool down the planet and the ecosystem would gradually recover its former productivity. At that point, vertebrates could become again abundant and the Earth would look very much like it looked millions of years ago, when the ancestors of human beings didn't seem to be destined to the great explosion of numbers that was to take place with the Anthropocene.
Is there a chance that the Earth would evolve again a species of sentient beings? It is not impossible. If some species of primates could survive the great carbon pulse, they might re-develop tool making abilities and, in time, human-like intelligence. That would take time, considering that it took some 50 million years to arrive to homo sapiens from the earliest primates, but it would still be possible within the remaining lifetime of the biosphere for vertebrates. If all primates go extinct, then the task becomes more difficult considering that it took more than 400 million years for primates to appear after the evolution of vertebrates. But, again, it would not be impossible and, anyway, perhaps sentient beings don't need to be primates. So, there might be a second (and probably last) chance for intelligent creatures to do better than we did. Good luck to them!
2. The Olduvai Scenario.
The "Return to Olduvai" was proposed by Richard Duncan in 1996 to describe the effect of the gradual depletion of fossil fuels; taking the name "Olduvai" from the name of a region in Tanzania, Africa, where our remote ancestors lived. The idea is that, without fossil fuels, humans would lose their principal source of energy and would be forced to return to their oldest survival lifestyle: hunting and gathering.
The Olduvai scenario could play out as the result of a combination of factors. First of all, fossil fuels would gradually become so expensive to make an industrial economy impossible. In parallel, global warming would raise temperatures so much that tropical and temperate latitudes would become impossible to inhabit year round for human beings. At this point, humans would be forced to retreat to extreme northern and southern regions, where it is not obvious that agriculture is possible. As we move away from the equator, a strong limiting factor is the low level of solar irradiation. Crops can grow nicely at high latitudes, but the problem is the slow rate of the reforming of fertile soil and the consequent erosion. It is a problem already evident today in regions such as in Iceland and Greenland and which might make agriculture impossible to maintain for long times.
So, humans living in high latitude regions could find that the best survival strategy for them is to adopt a lifestyle similar to that of modern Inuit, even though at much higher temperatures. They would live mainly by fishing and hunting marine mammals in the warm season - retreating in their shelters during the long polar night. In the Northern Hemisphere, this lifestyle would be possible in the ring of land around the North Pole, part of Eurasia and of the American Continent. In the Southern Hemisphere, it would mean the tip of the South American continent, Tierra Del Fuego, and perhaps an ice-free Antarctica, where humans could live for the first time in their history.
Modern humans have been hunters and gatherers for at least two hundred thousand years. Their hominid ancestors have been using this strategy for a couple of million years, at least. So, hunting and gathering is a stable and successful way of living that humans could adopt for a long time, at least as long as the planetary ecosystem would be able to maintain a sufficient biological productivity. In time, the ecosystem could stabilize and return the planet to the conditions of the past ten million years or so. In this case, the high latitude regions would probably freeze again and become covered by ice. Humans could then move back to lower latitudes. At this point, they would probably rediscover agriculture and restart with agricultural civilizations, as they had done tens or hundreds of thousands of years before. And so, we move to the next scenario; the return to agriculture.
3. The return to agriculture.
Suppose that we run out of cheap fossil fuels, that is, fuels as cheap enough to sustain an industrial society. And suppose that we haven't used the energy we had - while we had it - to build up an alternative. Then, we will be forced to return to the world as it was before we started burning fossil fuels: an economy wholly based on biological resources; that is on agriculture.
This is a straightforward scenario that doesn't imply special events other than assuming that the effects of climate change would not be so drastic and ruinous as some scenarios describe them. Not that the transition won't be traumatic for humans. The world without fossil fuels and without alternatives to them won't be able to support, not even remotely, the same population that the fossil-powered agriculture had supported. And it is not just the lack of fossil fuels that will reduce agricultural productivity, it is the fact that centuries of intensive agriculture have destroyed a large fraction of the fertile soil that had created the human civilization. That would necessarily bring a drastic reduction in human population. In such a scenario, "traumatic" is surely an understatement. But humankind would survive.
In this farming future, there would hardly be a chance for a new industrial revolution. The fossil fuels that created the present one will be gone and will need millions of years to reform, if they ever will. Metal ores would also be scarce, although our farming descendants would do well by scavenging the ruins of our cities for metals. They would have plenty of iron and copper and they could even use aluminum for their cooking pans by melting down the zillions of beverage cans that we left behind. But their technological level would be severely limited by the lack of fuels: they would have only wood charcoal for their metallurgy. So, our descendants could still work iron and they could still kill each other with swords and spears (and, maybe, even with occasional muskets and cannons). But we know of no society in the past that could develop an industrial revolution without a cheap and abundant source of energy.
Curiously, however, there is a possibility for a new burst of industrialization in this remote future. It would be the result of mining Antarctica and, in minor measure, Greenland and other high latitude northern regions. Because of the ice cover, so far these regions have been scarcely exploited for minerals (or not at all, in the case of Antarctica). But the great carbon pulse could heat the planet enough that the world's glaciers would melt completely and open up these lands to mining. In this case, our ancestors could have a second (and likely last) chance to develop a new coal based industrial revolution. That would bring back everything to square one: with the new industrial society threatened by the deadly combination of depletion and climate change. Would our descendants be able to do better than us? Considering that they are - indeed - our descendants, probably not. Hence, this second cycle of industrialization might truly be the last one on the planet.
Apart from Antarctic coal, our descendants could remain farmers for a long, long time. It is said that agricultural societies of the past could be described as "peasants ruled by brigands", but this is an over-simplification for an integrated social structure where different layers perform highly specialized tasks: peasants, warriors, priests, artisans, and more. In time, agricultural societies could evolve converging to the social structure typical of other species which practice agriculture: mainly ants and termites. These species are "eusocial" (or "ultrasocial", according to some definitions) and practice extreme specialization, for instance with "queens" taking care of reproduction, while the other members of society are sterile female workers and warriors. Could future human agricultural society become something similar? Why not? At least one other species of mammals has developed full eusociality (the naked mole rat).
Eusocial species are highly resilient and tend to dominate the ecosystem, as ants and termites do and have been successfully doing for at least 50 million years. In principle, eusocial humans could also maintain their dominance of the ecosystem and continue in this role for tens or hundreds of millions of years, until they gradually disappear in a remote future as the earth becomes too hot for vertebrates to survive. If that happens, they would have been the most successful vertebrate species of earth's history; a species that even briefly dreamed of conquering space.
4. The great metabolic revolution
In more than four billion years of existence, the Earth never stood still. Powerful forces have shaped it in a continuous series of revolutions which have seen the development of more and more complex life forms, increasingly able to exploit the thermodynamic gradient created by sunlight. During this long time span, we have seen several metabolic revolutions; of which two have been the most important ones. The first was photosynthesis, some 4 billion years ago. The second is the aerobic metabolism, about 2.5 billion years ago. It is the latter revolution which, eventually, generated vertebrates and us.
Today, we seem to have reached an impasse in this ever increasing growth of biological complexity. Actually, we may be heading for an inversion of tendency created by long term changes of the ecosphere. The planetary thermostat which stabilizes the Earth's temperature works by regulating the concentration of CO2 in the atmosphere. But with the gradually increasing solar radiation, these concentrations are already near the lower limits necessary for photosynthesis. So, the present ecosystem is in a no-win situation: in the long run, either it will be destroyed by the lack of CO2 or by high temperatures. So, in order for a complex ecosystem to survive, we need a truly drastic metabolic revolution. Organic photosynthesis has reached its limits: we need to move to a completely different kind of substrates.
What is in photosynthesis, after all? It is a way to transform solar energy into excited electrons and use them to create chemical compounds which can give back this energy on demand. The efficiency of photosynthesis in this process is reported to arrive to about 13% in ideal conditions - in practice it is of the order of 8%. Note also that plants can't function as photosynthetic machines outside a narrow range of temperatures and without of nutrients and chemicals which are not always available.
So, if we want another metabolic revolution, we need something that can be both more efficient and less demanding in terms of environmental conditions. A possibility is the photovoltaic (PV) cell. The efficiency of a modern silicon PV cell can be higher than 20% in creating excited electrons. By themselves, the cells do not store energy, but can be coupled to energy storage devices and used to power a variety of processes and reactions for an overall efficiency that is comparable (and arguably higher) to that of photosynthesis. Silicon PV cells function using abundant elements: mainly silicon and aluminum, plus traces of nitrogen, boron, an phosphorous. The present generation uses also silver, but that's not a crucial. But the great advantage of "silicon photosynthesis" is that solid state PV cells do not need water or gaseous oxygen, and can operate in freezing temperatures or at high temperatures, up to a few hundred degrees centigrade. The "habitable zone" for PV cells is not a narrow shell around the sun: it spans a huge volume that includes all the major planets and probably extends even closer and farther from the sun. The quantity of solar energy that can be gathered in this volume is incredibly larger than the tiny amount intercepted by the Earth.
Of course, solid state PV devices are not normally considered the photosynthetic part of an ecosystem. They enjoy the name of "cells"; but unlike biological cells they don't reproduce themselves. But PV cells delegate their reproduction to specialized entities; cell factories, just like worker ants delegate their reproduction to specialized entities: queen ants. So, it is all part of a new ecosystem that is emerging; one which starts from the beginning as eusocial.
We know that complex systems become more complex the more energy flows through them. If the solid state ecosystem turns out to be more effective than the biological one, then the perspectives are mind boggling even if we limit our horizon to the surface of the Earth. Of course, it is hard for us to imagine the consequences of such a revolution (think of how difficult it would be for a protist of the Proterozoic age to imagine the advent of vertebrates). What we can see is that such a system is born connected at the planetary scale. The rapid development of the internet is giving us a taste of this new situation of extended interconnectedness. From our viewpoint of human beings, it is an unpleasant loss of privacy. On the other hand, ants in an anthill don't enjoy much privacy. It is, again, one of the characteristics of eusociality: you pay the advantages of efficiency with a loss of individuality. But we can hardly say more than that: if the new system is to be born, it will. What it will do, it is impossible to say, but it can - theoretically - expand to the whole solar system and survive for the whole remaining lifespan of the Sun, about 5 billion years - and even more.
In a way, it would be the ultimate triumph for human beings who would have engineered the birth of a new ecosystem encompassing the whole solar system and perhaps over the whole Galaxy. Would they still exist in this new ecosystem? If so, which role could they play? And, if not, will they be remembered with gratitude? (Note, however, that we don't feel particularly indebted to our one-celled ancestors).
5. Where are we going, anyway?
All civilizations of the past have declined and collapsed. But collapse is nothing more than rapid change and, as long as the sun shines, the ecosystem has at least a chance to move to higher levels of complexity. The future that we can dimly see today is rich in possibilities. Billions of years ago, Mars - and possibly also Venus - had a chance to develop an organic ecosphere. But in both cases the time available was too short and soon both planets left the habitable zone and were sterilized. The Earth has had a much longer time, billions of years more to develop the ecosystem we know today. But the Earth never stood still and it is not standing still: change is accelerating to speeds never seen before in history. We may go down to a sterile planet or move on to a new system of unbelievable complexity. It is the ultimate challenge for humankind; one that we cannot avoid to face.