“All civilisations are mortal. This idea, however widespread, wasn't always accepted as fact. For three centuries, the thermal-industrial civilisation assumed it would go on forever. That the future would only be an exponential recreation of the present. And then it died.”

-- Collapse historian Sybil Masani

Historians still discuss the exact causes of the collapse of the mid-21st century, which marked the end of the civilisation born from the industrial revolution. Ecosystem collapse and resource depletion played an obvious and overwhelming role, however recent examination of previously unknown sources has led collapse studies to re-evaluate the importance of the systemic failure of technological networks, resource wars and political unrest. Regardless of the details, however, one thing is painfully certain: if the 21st century hadn't breached the IPCC baseline of +2 °C by 2050, the collapse would have been either averted or greatly cushioned. Anthropogenic climate change is the Gordian knot of the death of the industrial world. It strangled the Earth, choked humankind in its own mistakes, and was not far away from causing civilizational extinction.

Collapse studies, however, warn against the ever-popular vision of a brutal, cliff-like collapse. The apocalypse was a slow transition from a high-energy, stable state to a low-energy, unstable state. It began in the Third World and took decades to extend to the First World, following the exhaustion of state resilience and the rise of authoritarian governments. A few prominent collapse historians have even argued for the concept of “centennial apocalypse”, positing that the first steps of collapse began as the twentieth century ended. Such a teleological vision of an inevitable collapse, a century in the making, is hotly debated in academic circles. However, the 21st century apocalypse necessarily colours the popular vision of the 20th century and the industrial era as a whole. Historical movies and books concerned with this time period have to deal with a cultural zeitgeist where everyone knows how it ends: badly and into the Low Age.

Though many history books still consider the Low Age as the “second Middle Ages”, it didn't see the complete end of modern civilisation nor the erasure of a thousand years of technical and social progress. It was a complex and chaotic era, first characterised by the “energy cliff”. As the collapse forced polities to brutally transition towards a small-scale, post-carbon energy mix, the overall energy capacity of the human civilisation decreased immensely during the 2050-2150 period. If the Low Age lasted five centuries, most of the physical destruction and loss of knowledge happened during the initial stages of the collapse, also dubbed “early Low Age” or “dark Low Age”. Computer science, manufacturing, agriculture and transportation were the most affected, with several capabilities, such as space access, outright erased. Even in places where knowledge remained, the know-how was lost. A profound population decrease followed, driven less by epidemics or famines and more by plummeting birth rates at a worldwide scale.

The world, however, didn't fracture into warlord kingdoms and post-apocalyptic raider dominions. When the dust settled by the mid-22nd century, the human civilisation had reverted to a stable, lower technology state, comparable to the late 17th century, albeit with significant fragments of modern scientific knowledge and a limited but real capacity for renewable and to a certain extent nuclear energy production. Within that framework, industrial states couldn't work any longer. The disappearance of worldwide markets, combined with the drastic reduction in infrastructure capability led to the rise of local communes and syndicates as the main framework of governance, with old world fragments such as AUSCOM or Eurofront remaining as ghost entities, moving forward on inertia and derelict AI systems. As the Low Age moved transitioned to its middle era, loose continental federations emerged as a way to unite the communes together. Sciences were geared towards immediate survival in a hostile world, ravaged by ecosystem collapse, hypercanes and deadly wet bulb temperatures. This led to a relative decline in fundamental sciences (which hasn't been fully covered up to this day) with domains such as agronomy, biology, geography, urban planning and practical engineering favoured by the scribes and researchers of this new world. Conservation and cross-referencing of knowledge was deemed paramount and handed to semi-religious groups acting as medieval monks, copying and archiving documents on physical storage. Warfare favoured skirmishes, small-scale battles and diplomatic resolutions, with very few groups capable of organising and financing professional militaries. A select few polities kept and maintained nuclear weapons, albeit with very limited means of delivery. They acted as a counter-power to local hegemonies, cementing the rise of the syndicates that would later become Laniakea and the USRE.

“The Low Age -- why did we name it that way? Because we had lost the ability to go beyond our atmosphere? Because we saw in it a repeat of the Middle Ages? I like to think it is because it forced us to slow down. To go lower, to go better.”

-- quote attributed to Oak-class AI Thot.

Three centuries and a half after the collapse, the world entered the late Low Age. The new world order had achieved a stable state, with the largest communes and syndicate relinking together into continental markets. Technological innovations such as radio-webs and re-discovery of ancient capabilities enabled the emergence of worldwide information networks and logistics similar to what would have been available in the mid-19th century, albeit on a renewable-nuclear energy mix. Worldwide population shot back to three billion inhabitants. As many a city had been abandoned or ravaged by climate catastrophes, new metropolises started emerging from the ruins. Africa, Southern America, northern India and small parcels of Europe arose as the new urban centres of the wounded planet. At the end of the late Low Age, the new renewable-nuclear civilisation had reached a level of capability akin to the mid 20th century. Laniakea and the USRE rose to prominence, destroying the last remnants of the industrial world in short world wars. Space was reached again as a strange, colourful 25th century rose upon the Earth. 

History accelerated. A tense, dynamic era swept across the solar system -- the Kinetic Age, the Interplanetary Age, the century of broken shackles and renewed dreams. Cities were built on the Moon and around Saturn; fission drives criss-crossed the solar system; communes and syndicates were established in the void; and somewhere in the eerily familiar wreck of an ancient solar sail, the geometry drive was found.

The Low Age ended when the first geometry drive-equipped probe performed an Earth-Mars translation under six milliseconds. The Great Filter had fallen; in a wake of blueshifted light began the Interstellar Age.

But the Low Age isn't dead yet. The Earth remains wounded, its five billion inhabitants still living under the constant threat of wet bulb temperatures, hypercanes and decaying ecosystems. A million works of art have been destroyed, forever lost to dead cloud storage and unreadable file formats. And, deep down, the fear remains -- for now, we know everything can be lost again.

Illustration from Steven Sander's Symbiosis Creative Commons artbook, licensed under a Creative Commons Attribution Non-Commercial Share-alike 3.0 unported licence.