<P> The 20th century was dominated by a chain of events that heralded significant changes in world history as to redefine the era: 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 . 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 term "short twentieth century" was coined to represent the events from 1914 to 1991 . </P> <P> 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 . It took all 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%; 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). </P> <P> The century had the first global - scale total wars between world powers across continents and oceans in World War I and World War II . Nationalism became a major political issue in the world in the 20th century, acknowledged in international law along with the right of nations to self - determination, official decolonization in the mid-century, and related regional conflicts . </P> <P> The century saw a major shift in the way that many people lived, with changes in politics, ideology, economics, society, culture, science, technology, and medicine . The 20th century may have seen more technological and scientific progress than all the other centuries combined since the dawn of civilization . Terms like ideology, world war, genocide, and nuclear war entered common usage . Scientific discoveries, such as the theory of relativity and quantum physics, profoundly changed the foundational models of physical science, forcing scientists to realize that the universe was more complex than previously believed, and dashing the hopes (or fears) at the end of the 19th century that the last few details of scientific knowledge were about to be filled in . It was a century that started with horses, simple automobiles, and freighters but ended with high - speed rail, cruise ships, global commercial air travel and the Space Shuttle . Horses, Western society's basic form of personal transportation for thousands of years, were replaced by automobiles and buses within a few decades . These developments were made possible by the exploitation of fossil fuel resources, which offered energy in an easily portable form, but also caused concern about pollution and long - term impact on the environment . Humans explored space for the first time, taking their first footsteps on the Moon . </P>

20th century developments and transformations in the us