Over the centuries leading up to the mid 20th century , the human population has fluctuated. These fluctuations were due to both natural causes and human causes. Famines, genocides, epidemics, such as plagues in Europe, Asia, and conflicts. Natural causes were probably due to an unprecedented number of volcanic eruptions.
The middle ages experienced a global warming period, causing changes in the ocean circulation and resulting in the Little Ice Age, between 1550 and 1850. This event was not a true ice age, but global temperatures decreased.
The most recent study found that an especially massive tropical volcanic eruption in 1258, followed by three smaller eruptions in 1268, 1275, and 1284 that did not allow the climate to recover, caused the initial cooling. The 1452 to 1453 eruption of Kuwae in Vanuatu triggered a second phase of cooling.
The beginning of the 20th Century, saw two major conflicts, resulting in population fluctuations: World Wars 1 and 2 between 1914 and 1918 and from 1939 to 1945, resulting in over 90 million casualties between both conflicts.
Since the beginning of the 21st Century, the Earths population has exceeded 7 billion humans. The 20th Century saw the strongest growth figures from the 1950s, witnessing its highest rate in the 1980s of 138 million births per year. Earths population has seen a steady increase from 370 million at the end of the great famine and Black Death in 1350.