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; intergovernmental organizations and cultural homogenization through developments in emerging transportation and communications technology; poverty reduction and world population growth, awareness of environmental degradation, ecological extinction; and the birth of the Digital Revolution, enabled by the wide adoption of MOS transistors and integrated circuits. It saw great advances in communication and medical technology that by the late 1980s allowed for near-instantaneous worldwide computer communication and genetic modification of life.
The 20th century saw the largest transformation of the world order since the Fall of Rome: 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. The average global temperature on Earth has increased by about 0.8° Celsius (1.4° Fahrenheit) since 1880; Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20 °C per decade.
The repercussions of the World Wars, Cold War and Globalization crafted a world where people are more united than any previous time in human history, as exemplified by the establishment of international law, international aid, and the United Nations. The Marshall Plan—which spent $13 billion ($100 billion in 2018 US dollars) to rebuild the economies of post-war nations—launched "Pax Americana". Throughout the latter half of the 20th century, the rivalry between the United States and the Soviet Union created enormous tensions around the world which manifested in various armed conflicts and the omnipresent danger of nuclear proliferation. The dissolution of the Soviet Union in 1991 after the collapse of its European alliance was heralded by the West as the end of communism, though by the century's end roughly one in six people on Earth lived under communist rule, mostly in China which was rapidly rising as an economic and geopolitical power.
It took over two-hundred thousand years of human history up to 1804 for the world's population to reach 1 billion; world population reached an estimated 2 billion in 1927; by late 1999, the global population reached 6 billion. Global literacy averaged 80%. Up until a couple hundred years ago, life expectancy was about thirty in most populations; global lifespan-averages exceeded 40+ years for the first time in history, with over half achieving 70+ years (three decades longer than it was a century ago).