A worldbuilding project for a story about the long run.
In this scenario, humanity continues to progress over the next decades, for so to stagnate in a brief century or two in relative harmony and abundance. Fuelled by cheap carbon and short memories, climate change is too slow for any generation to really care. Demand leads to decline. No single cataclysm happens, but the depletion of all non-renewables starts an era of conflicts. Slow changes snowball, and with the temperature at 8-10 degrees above 20th century, large parts of the globe become completely uninhabitable. Essential resources all but completely gone, modern civilization begins a long fall, dismantles and goes offline. The ice sheets eventually collapse and the last institutions fail. A dark age takes place as survivors scrape by in isolated pockets. The world floods and burns.
After uncountable centuries at the brink of extinction, ancient instincts kick in and we crawl back; rebuilding, retrying and rediscovering. We are no longer masters of nature, and on the way the same instincts lead us to killing, raping and pillaging away. Man eat Man, whole histories destroyed and forgotten, leaving only vague memories about a bountiful Ancient World.
The year is likely somewhere between 4000 and 10000 AD, no-one knows (or cares much) how long the dark age lasted. Life is harsh, but several distinct cultures talk, trade and war across both polar regions. In between are furnaced wastelands of jungles and deserts, oceans overgrown by algae and kelp. Weather is crap. Scattered relics and artefacts still draw the adventurous into the unknown, some returning with tales of monsters. Life below 60° lat. is lethal. Few try the years long journey from north to south, but one trail exists.
