"During this year a most dread portent took place. For the sun gave forth its light without brightness and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear."
~ Procopius of Caesarea, 536 A.D.
A crater the size of Rhode Island sprawled across Wyoming where Yellowstone National Park once attracted tourists from around the world. Two hundred and eighty cubic MILES of magma and volcanic ash heaved into the air while a cloud of burning wind and debris ripped outwards to destroy towns and level mountains for twenty miles. Fault-lines flexed and shuddered. Earthquakes rippled like fallen dominoes around the globe. Other volcanoes joined in the apocalypse and the Ring of Fire could clearly be seen by horrified astronauts from the Earth’s orbit.
It wasn’t the first time humanity was thusly fucked in such an epic scale.
But modern humans aren’t the hearty breed they once were. Technology and advanced medicines trumped natural selection, allowing the sick and meek to survive far longer than nature would allow. Entire genetic lines, families with ailments passed from one generation to the next, couldn’t make it a year after the world ended.
The basics of life had been handed to much of mankind for over a hundred years. Food in cans and delivered through fast food windows took the place of hunting, gathering and harvesting. Cities grew to barely sustainable levels, kept viable only through the use of petroleum-based delivery methods and plumbing systems that required an army of workers to maintain.
Without their lifelines, the cities became death-traps.
And none of that takes the zombies into consideration.
Yellowstone exploded on April 15, 2010. Heavy ash particles fell like snow around the world, ferrying microscopic undeath wherever it landed. There were twenty days of night before the denser matter finally settled itself and the skies cleared. Kinda. What remained could not be seen by the naked eye, though its results were clear.
So much sulfur dioxide gas had been blasted into the stratosphere that the atmosphere could not compensate and oxidize all the gas to sulfuric acid aerosol. The gas reflected and filtered sunlight, casting the skies in bloody shades of red. Acid rain fell, destroying life already starving for the light of day. Holes in the ozone allowed deadly levels of radiation to pass freely through the atmosphere, further stressing survivors of all species to the breaking point.
The average temperature, globally, dropped by 18°F (9.6°C) within the first year. Blizzards struck New England in June. Lakes and rivers remained frozen well into July. Entire ecosystems crashed and burned.
The zombies didn’t mind the colder weather at all. Decay slows down, after all.
Unlike their ancestors, modern humans had different means of survival. Canned food. Electricity, in some places, to stay warm and grow plants indoors. Gasoline to power vehicles and make the world a smaller place. Bleach and filters to make water pure.
Eventually that shit runs out and people die.
By the summer of 2012, temperatures averaged 15°F cooler than normal around the world. Gasoline became unusable unless treated properly. Diesel fuel still had a bit longer to last. But not everyone knew that. Cars stopped working. Little gas generators stopped working. Heaters stopped working. Microwaves stopped working. More humans died.
Naturally, some communities fared better than others. Southern states were bearable with the lower temperatures, even though native plants couldn’t manage. Humans migrated south, expecting sunny skies and verdant fields of grain. Many didn’t survive the trek. Those that did were sorely disappointed.
Things settled down by the fall of 2013. Those that still lived did so because they adapted and learned to survive. Or were very lucky. Human migrations came to an end as the snowstorms began. Dead trees in their abundance provided heat. Food was their main problem.
Canned foods. That’s what saved humanity. At least in the United States.
Livestock died as the last stored grains were devoured. Medicines gradually lost their potency. Gasoline couldn’t sustain combustion engines. The world got a lot smaller.
Humans are a strange bunch. They do strange things. Hold strange beliefs. Among the beings that populate the earth, only humans have religion. Like liquor, humans turned to that faith in times of dire need.
Their prayers didn’t get answered though. Most figured Hell was full.
So new religions and beliefs filled the hole left by those unanswered prayers. New cultures began to grow where crops and trees could not. Old faiths were resurrected. New religions and customs took root.
Slowly, each cluster of survivors became a tribe. Isolated from their peers by distance and eternally hungry hordes of the undead, small communities huddled together and formed their new cultures. Most were influenced by the dominant ethnic or religious majority. Others by strong leaders. Still more by their geography and environment.
The global civilization of man devolved into tribes and villages long before the volcanic winter ended in 2020. Ten years. A new generation born into a world filled with ruins where new beliefs and cultures became the norm.
Mankind, changed though it was, survived.