There were two main crises of the fourteenth century in Afro-Eurasia—the effects of the Medieval Warming Period and the Black Death.
The Medieval Warming Period was at its height in the 12th and 13th centuries. Due to extra solar flares and activities, the global temperature warmed slightly for the first time in around 1000 years, causing famines, droughts, and other environmental problems. As a result, populations had to adapt. The Mexica, or Aztecs, had to leave their home in the Southwestern United States (called Aztlan; historians still argue on where it is located today) and move South to more fertile lands, eventually settling in the Valley of Mexico on Lake Texcoco. The Mongols banded together under the first Mongol Emperor Chinggis (Genghis) Khan and swept South into China to take advantage of its fertile land and settled agrarian populations. The Mongols were nomadic pastoralists, relying on pastureland to graze their horses, sheet, and goats, but the warming period was killing off grasslands, necessitating their migration South. They eventually conquered a massive empire encompassing much of Asia. In Medieval Europe, the medieval warming period caused crop failures and droughts, leading to famine and starvation.
As a result of all of these changes, people began to rely more on trade. The increase in trade between the East and West led to the spread of the bacteria from an endemic plague reservoir on the Mongolian-Chinese border in 1333 to Europe from 1348-1351. The short term result of this was death; around 33% of Eastern, Middle Eastern, and European populations died. The effects of the plague in Europe were exacerbated by the Medieval Warming Period, as people were already overly dependent on their feudal manors for support and a food supply.
After the plague finished its course and spread throughout Eurasia, and once global temperatures dropped down a bit and agriculture recovered from the warming period, governments started to grow in power, reducing decentralized feudal control and encouraging monarchies and/or empires. In Europe, monarchies started to form, centralizing power and designating borders. In the Middle East, Islamic Empires grew from gunpowder technology. In the East, Chinese Dynasties reclaimed land from the Mongols and centralized power once again as the Ming. This re-centralization of power, establishment of trade, and new technologies that propelled Europeans to start exploring and sailing helped Afro-Eurasia recover from the famines, droughts, and diseases of the 14th century.