Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, Asia to the east, and the Mediterranean Sea to the south. It comprises the westernmost part of Eurasia. It is the 6th largest continent in the world. Europe is commonly considered to be separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas and the waterways of the Turkish Straits. Although the term "continent" implies physical geography, the land border is somewhat arbitrary and has been redefined several times since its first conception in classical antiquity. The division of Eurasia into two continents reflects East-West cultural, linguistic and ethnic differences which vary on a spectrum rather than with a sharp dividing line. The geographic border between Europe and Asia does not follow any state boundaries: Turkey, Russia, Azerbaijan, Georgia and Kazakhstan are transcontinental countries. France, Portugal, the Netherlands, Spain and the United Kingdom are also transcontinental in that the main portion is in Europe while pockets of their territory are located in other continents. Europe covers about 10,180,000 square kilometres (3,930,000 sq mi), or 2% of the Earth's surface (6.8% of land area). Politically, Europe is divided into about fifty sovereign states of which Russia is the largest and most populous, spanning 39% of the continent and comprising 15% of its population. Europe had a total population of about 741 million (about 11% of the world population) as of 2018[update]. The European climate is largely affected by warm Atlantic currents that temper winters and summers on much of the continent, even at latitudes along which the climate in Asia and North America is severe. Further from the sea, seasonal differences are more noticeable than close to the coast. Europe, in particular ancient Greece and ancient Rome, was the birthplace of Western civilisation. The fall of the Western Roman Empire in 476 AD and the subsequent Migration Period marked the end of ancient history and the beginning of the Middle Ages. Renaissance humanism, exploration, art and science led to the modern era. Since the Age of Discovery started by Portugal and Spain, Europe played a predominant role in global affairs. Between the 16th and 20th centuries, European powers controlled at various times the Americas, almost all of Africa and Oceania and the majority of Asia. The Age of Enlightenment, the subsequent French Revolution and the Napoleonic Wars shaped the continent culturally, politically and economically from the end of the 17th century until the first half of the 19th century. The Industrial Revolution, which began in Great Britain at the end of the 18th century, gave rise to radical economic, cultural and social change in Western Europe and eventually the wider world. Both world wars took place for the most part in Europe, contributing to a decline in Western European dominance in world affairs by the mid-20th century as the Soviet Union and the United States took prominence. During the Cold War, Europe was divided along the Iron Curtain between NATO in the West and the Warsaw Pact in the East, until the revolutions of 1989 and fall of the Berlin Wall. In 1949 the Council of Europe was founded, following a speech by Sir Winston Churchill, with the idea of unifying Europe to achieve common goals. It includes all European states except for Belarus, Kazakhstan and Vatican City. Further European integration by some states led to the formation of the European Union (EU), a separate political entity that lies between a confederation and a federation. The EU originated in Western Europe but has been expanding eastward since the fall of the Soviet Union in 1991. The currency of most countries of the European Union, the euro, is the most commonly used among Europeans; and the EU's Schengen Area abolishes border and immigration controls between most of its member states.