20th century 20th century The 20th (twentieth) century was a century that began on January 1, 1901 and ended on December 31, 2000. It was the tenth and final century of the 2nd millennium. It is distinct from the century known as the 1900s which began on January 1, 1900 and ended on December 31, 1999. The 20th century was dominated by a chain of events that heralded significant changes in world history as to redefine the era: flu pandemic, World War I and World War II, nuclear power and space exploration, nationalism and decolonization, the Cold War and post-Cold War conflicts;
Death rates in the 20th century the 20th century. A natural population increase occurs when birth rates are higher than death rates. Recently and most notably, the years immediately after World War II saw an explosion in fertility rates called the Baby Boom because the returning soldiers and displaced people started new families. Death rates were significantly lower during the baby boom and thus populations increased substantially. Today these baby boomers are approaching old age and driving up the average age of the overall population. The World Bank predicts a dramatic decrease in population size from the increase in death rates over the next decade or