The 20th (twentieth) century began on January 1, 1901 (MCMI), and ended on December 31, 2000 (MM). The 20th century was dominated by significant events that defined the era: Spanish flu pandemic, World War I and World War II, nuclear weapons, nuclear power and space exploration, nationalism and decolonization, technological advances, and the Cold War and post-Cold War conflicts.The 20th century saw a massive transformation of the world order: global total fertility rates, sea level rise and ecological collapses increased; the resulting competition for land and dwindling resources accelerated deforestation, water depletion, and the mass extinction of many of the world's species and decline in the population of others; consequences which are now being dealt with. Man-made global warming resulted in more extreme weather conditions.