1. Yellowstone finally blows: The North American Supervolcano(s) erupt after thousands of years of dormancy. Life over much of North America is wiped out, pockets still exist on the Eastern and Western coast. The federal government absolutely fails to respond to and deal with the crisis and collapses in the ensuing Chaos. Ash and soot are thrown into the atmosphere blocking out the sun, at first seen as a boon that reverses some of the worst of climate change, however, the ensuing ash cloud eventually leads to a crisis of global cooling. Crop shortages are abundant, and famines are rampant. The hardest-hit territories are those already cold and difficult to inhabit, followed by the very urban very industrial European states. The territories least affected are warmer more rural societies, better known as the "global south". Ironically, the societies that are best able to deal with the crisis are those that are the poorest. Even then the Global South still faces societal collapse and revolution as Neo-Colonial ties and western funding for their regimes dries up.
2. Gradual Systems Collapse: or how I stopped worrying and learned to love the second fall of Rome: Capitalism proved surprisingly capable of facing its challenges, even climate change was eventually halted and reversed. However, the internal contradictions of capitalism continued to grow, until eventually, they grew too great. The numerous complex wheels and cogs to maintain society and deal with minor crises had grown too great, the cost to maintain each one outweighed the value they provided, and yet society was too dependent on each of them to get rid of, leading to a greater cost to eliminate or change the structure. The ruling classes, unable and unwilling to make the drastic changes necessary and give up their power, were content to leave the system chugging along as normal slowly dying. The working classes, lacking class consciousness, being too content, or being too impoverished to do anything were unable to stop the collapse. Eventually, the systems just... fell apart. Following the collapse was great conflict and destruction, new societies and ideologies were born and old ones died. While some still hold onto the glory days of the past and seek to restore them, due to current material conditions, they are no more capable of restoring capitalist civilization's height than Charlemagne was capable of restoring Rome.
3. Age of Strife: For decades humanity has been a member of the galactic community. For decades humanity has sought its place among the stars. New worlds, new species, new challenges, all of which have been overcome by the growing Interstellar Human Government. However, it was not to last. Uprisings, food shortages and interstellar system collapse have taken hold of the galaxy. The relatively young, and weak Human government quickly collapsed. Contact and trade were severed with Earth's colonies and the United Nations of Earth was unable to maintain its control over Humanity's home planet. Local governments collapsed without support from the UN, and eventually, the Earth was nothing but a bunch of bickering warlords seeking to stabilize their economies, and reunite the Earth, and restore Humanity's place among the stars.
BONUS (MEME) - 4. Rapture/The Snap: No one knows or understands what happened. The new societies across the globe all have different names for it. The Rapture, the Apocalypse, the Event, the Great Dying, Armageddon, Ragnarök, and The Snap just to name a few. What we do know, however, is that one day roughly half of all life on Earth just vanished in an instant. Not even bodies were left behind. The fallout was immediate, supply chains collapsed overnight, cars and trucks ran themselves off the road, governments collapsed as leadership vanished and lines of succession were broken, new religions formed that tried to explain what happened, old religions struggled to cope and find an explanation in their holy texts or leaders. Chaos reigned as the survivors tried to pick up the pieces from a broken system, but the damage had been done, and new societies must build upon the dust of the old.