What caused the decrease in the global human population in 1400?

Pandemic plague

Explanation:

The Black death or the Great Plague occured during 1347 to 1351 in Eurasia causing death of almost 75 million to 200 million people. The cause of this plague was identified a bacterium known as Yersinia pestis . This plague outbreak was first major European plague pandemic and it killed almost 30% to 60% population of the Europe.