The Ice Age
Around A.D. 2400, six thousand years of civilization were about to be swallowed by a blanket of ice descending toward the equator. After a frantic (and fruitless) race to counter the coming Ice Age through technological solutions, and after several near-miss ecological disasters, everyone on the planet realized the fight was pointless and moved toward the warmer climates of the equatorial countries to wait out the new Ice Age.
Cultures were born and died during the rush toward the equator and old racial differences were largely broken down by global mixing. Millions died in the Emigration Wars, as overburdened equatorial countries did their best to keep refugee convoys from entering their borders, and still the ice continued to creep forward. Brutal warlords sprung up with frightening regularity, but this period also saw some great social experiments. The socialist utopia of Greater Samarkand stood as the pearl of Earth until it collapsed under its own weight.
Great technological advances were made, especially in the field of genetic engineering, resulting in the design of the fabled Prime Knight warriors in Kir Arya, a small country nested between warring giants. That success, however, was short lived and Prime Knights were killed to the last during the 24th Crusade that also destroyed the fledging Kir Aryan society.
Ultimately the population of Earth could only wait, one generation after the other, for the ice to retreat. It finally did in the 44th century, and brought a Colonization Era with it.