An unusually large solar flare erupted from the surface of the sun, virtually destoying Mercury and Venus and irradiating the side of the earth exposed to the sun. The force of the flare knocked the earth off its axis and pushed it several million miles farther away from the sun.
Three fourths of the worlds population died from radiation poisoning. The polar ice caps grew in size very quickly. The earth's orbit became oblong and the summers often reach 140 degrees while the winters dipped as low as -90 degrees on frequent occasions. The equator was snowed on in the winter and the snow didn't melt until early May most of the time. The surviving humans migrated to the current equator and huge cities, large enough to put Los Angeles to shame, sprang up everywhere along it.
All tropical plants and animals died out except those in conservatories, of which there were few. The humans at first acted barbaric, taking and killing whatever they liked until the most powerful realized that they faced extinction and quickly forced everyone to conform or be imprisoned. Most of those imprisoned died the first year because the prisons were farther north than the cities, and quickly froze.
Humans began to grow more hair all over their bodies. Their hair began to shade towards white as their bodies grew shorter and stouter.
Scientists began talking of populating Venus since it had moved very close to resuming Earth's previous orbit. Fresh water became scarce and power struggles occurred between those who sought to control the major sources of it. Technology advances began to turn toward environmentally controlled cities and devices to to control the weather.
The types of food that are able to be cultivated was drastically reduced. Malnutrition diseases like scurvy soared. Genetically changed foods became common but few people ate them because the nutritionally enhanced food was bland and left a terrible chemical after-taste.
Government systems began to tend towards dictatorships. A caste system developed with the rich controlling the poor, and no middle class.
Search parties began searching the wastelands for weapons. The benevolent groups destroyed those that they found or else used them to protect the little of civilization that there was. The malevolent ones stockpiled the weapons and used them to threaten the cities. Aggression escalated to the weapons being put in use on both sides. Finally nuclear holocaust was achieved, virtually decimating what was left of the human race, except for a few groups of tundra nomads.