In the history of the mankind, 14th century was probably the hardest on world population due Black Death, Mongol/Timur wars, and 100 Years War. This whipped out half of the humanity. Chinese population dropped by 1/2, European by 2/3 by the end of the century. For example population of France did not recover from the crisis until in the 1700's. Bohemia had 3 million people around 1350 but only 800,000 by 1500s. It did not reach the population peak again until 1620 and yet later until 1750. Middle East and India had similar drop of population, somewhere between 50% and 80%. Iran, Iraq, Syria, and Central Asia were depopulated for 400 years.
Prior 1350 crisis, similar depopulation happened in 500-550 coinciding with the collapse of the Roman civilization and culture of the Antiquity. Plague of Justinian killed between 30 to 50 million of people. Population of Italy dropped from 12 millions to two and entire western Europe was emptied out. Cities disappeared under time all over the known world in the matter of 50-75 years. Some places, the population shrunk so much that entire city moved to local arena in order to survive. Similar process of depopulation happened in Middle East and China which was fragmented into many smaller kingdoms.
Other scholars will say that collapse of the Bronze Age civilization after 1200BC was also as bad as the collapse of the Roman civilization. Entire civilizations from Mycenae Greece to Hittites to China collapsed as well. This is often attributed by volcanic explosion of Hekla in Iceland (around 1130 BC) which caused widespread cooling of the northern hemisphere for several decades, and destroying food supplies during the Bronze Age.