Globally, the 20th century was marked by: (a) two devastating world wars;
(b) the Great Depression of the 1930s;
(c) the end of vast colonial empires;
(d) rapid advances in science and technology, from the first airplane flight at Kitty Hawk, North Carolina (US) to the landing on the moon;
(e) the Cold War between the Western alliance and the Warsaw Pact nations;
(f) a sharp rise in living standards in North America, Europe, and Japan;
(g) increased concerns about environmental degradation including deforestation, energy and water shortages, declining biological diversity, and air pollution;
(h) the onset of the AIDS epidemic; and
(i) the ultimate emergence of the US as the only world superpower.
The planet's population continues to explode: from 1 billion in 1820 to 2 billion in 1930, 3 billion in 1960, 4 billion in 1974, 5 billion in 1987, 6 billion in 1999, and 7 billion in 2012. For the 21st century, the continued exponential growth in science and technology raises both hopes (e.g., advances in medicine and agriculture) and fears (e.g., development of even more lethal weapons of war).