Timeline: 800-1100 AD
Inuit boat (North America)
About 1000 AD, the world's climate got a few degrees warmer, and this climate change brought many other changes with it. In North America, Tuniit moved south from the Arctic into Greenland, and then they were wiped out by the Inuit, who were also on the move. Vikings from northern Europe also took advantage of the warmer weather to settle in Greenland and Nova Scotia.
Further south in North America, the Mississippian culture replaced the Hopewell, building walled towns and earth mounds from Wisconsin to Florida. In Central America, the Moche culture collapsed, and the Maya seem to have begun a decline after 900 AD, building fewer buildings and carving hardly any inscriptions, but the Zapotec continued their civilization.
Across the Pacific in China, the T'ang Dynasty collapsed about 800 AD, but China stayed more or less unified under the Sung Dynasty. In India, the Chola kings ruled the south, while Rajput kings in the north spent a lot of time fighting off Arab invasions from West Asia.
In West Asia and Africa, the Arab empire also collapsed into smaller kingdoms ruled by different dynasties. The Turks took over Baghdad, the old capital, and the Fatimid dynasty took over Egypt. But many scientific discoveries happened in these small kingdoms: about 850, Arabic scholars invented algebra, and about 1000 AD, the first glass mirrors. There were small Islamic kingdoms in West Africa, and along the East African coast Indian and Arab traders brought the Islamic faith.
When Charlemagne died in the 800s, his sons split up his European empire, and soon it fell apart into (roughly) the modern states of France and Germany. The Lombards ruled Italy, and the Slavs ruled Eastern Europe.
Just as the warmer climate encouraged some Vikings to sail to Greenland and Nova Scotia, it also encouraged other Vikings to head south and east. Some Vikings settled in Russia. Some Vikings settled in northern France about 1000 AD, and from there invaded England in 1066 and Sicily a little later.
In the name of Christianity, these same Vikings and their friends launched the First Crusade in 1096, where the French, English, and Germans united to try to capture Jerusalem and the Mediterranean coast from the Fatimids. The Christians succeeded in taking Jerusalem, and the Crusaders established a kingdom along the coast.