The second millennium was a period of time spanning the years AD 1001 to 2000 (11th to 20th centuries).[note 1] It encompassed the High and Late Middle Ages of the Old World, followed by the Early Modern period, characterized by the Wars of Religion in Europe, the Age of Enlightenment, the Age of Discovery and the colonial period. Its final two centuries coincide with Modern history, characterized by industrialization, the rise of nation states, the rapid development of science, widespread education, and universal health care and vaccinations in the Western world. The 20th century saw increasing globalization, most notably the two World Wars and the subsequent formation of the United Nations. 20th-century technology includes powered flight, television and semiconductor technology, including integrated circuits. The term "Great Divergence" was coined to refer the unprecedented cultural and political ascent of the Western world in the second half of the millennium, emerging by the 18th century as the most powerful and wealthy world civilization, having eclipsed Qing China and the Islamic World.
World population has grown without precedent over the millennium, from 310 million in AD 1000 to about 6,000 million in AD 2000. Doubling time was at first seven centuries (reaching 600 million in 1700), and during the final three centuries population growth accelerated extremely, growth rate peaking at 1.8% p.a. in the second half of the 20th century. Unchecked globalization and population growth also caused considerable social and environmental consequences, giving rise to extreme poverty, climate change and biotic crisis.
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The Julian calendar was used in Europe at the beginning of the millennium, and all countries that once used the Julian calendar had adopted the Gregorian calendar by the end of it. For this reason, the end date of the 2nd millennium is usually calculated based on the Gregorian calendar, while the beginning date is based on the Julian calendar (or occasionally the proleptic Gregorian calendar).
In 1999, there was some public debate as to whether the millennium should be taken to end on December 31, 1999, or December 31, 2000. Stephen Jay Gould at the time argued there is no objective way of deciding this question. Associated Press reported that the third millennium began on 1 January 2001, but also reported that celebrations in the US were generally more subdued at the beginning of 2001, compared to the beginning of 2000. Many public celebrations for the end of the second millennium were held on December 31, 1999 – January 1, 2000—with a few people marking the end of the millennium a year later.
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