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 the environment including loss of forests shortages of energy and water the decline in 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 1988 and 6 billion in 2000. For the 21st century the continued exponential growth in science and technology raises both hopes (e.g. advances in medicine) and fears (e.g. development of even more lethal weapons of war).
The surface of the earth is approximately 70.9% water and 29.1% land. The former portion is divided into large water bodies termed oceans. Gheos World Guide recognizes and describes five oceans which are in decreasing order of size: the Pacific Ocean Atlantic Ocean Indian Ocean Southern Ocean and Arctic Ocean.
The land portion is generally divided into several large discrete landmasses termed continents. Depending on the convention used the number of continents can vary from five to seven. The most common classification recognizes seven which are (from largest to smallest): Asia Africa North America South America Antarctica Europe and Australia. Asia and Europe are sometimes lumped together into a Eurasian continent resulting in six continents. Alternatively North and South America are sometimes grouped as simply the Americas resulting in a continent total of six (or five if the Eurasia designation is used).
North America is commonly understood to include the island of Greenland the isles of the Caribbean and to extend south all the way to the Isthmus of Panama. The easternmost extent of Europe is generally defined as being the Ural Mountains and the Ural River; on the southeast the Caspian Sea; and on the south the Caucasus Mountains the Black Sea and the Mediterranean. Africa's northeast extremity is frequently delimited at the Isthmus of Suez but for geopolitical purposes the Egyptian Sinai Peninsula is often included as part of Africa. Asia usually incorporates all the islands of the Philippines Malaysia and Indonesia. The islands of the Pacific are often lumped with Australia into a 'land mass' termed Oceania or Australasia.
Although the above groupings are the most common different continental dispositions are recognized or taught in certain parts of the world with some arrangements more heavily based on cultural spheres rather than physical geographic considerations.

$70.14 trillion (2008 est.)
$68.08 trillion (2007 est.)
$64.77 trillion (2006 est.)

2.9% (2008 est.)
5% (2007 est.)
5% (2006 est.)

$10,500 (2008 est.)
$10,300 (2007 est.)
$9,900 (2006 est.)

$NA (31 December 2008 est.)
$64.99 trillion (31 December 2007)
$53.38 trillion (31 December 2006 est.)

$16.04 trillion (2008 est.)
$13.89 trillion (2007 est.)

$15.97 trillion (2008 est.)
$13.74 trillion (2007 est.)

$60.96 trillion (31 December 2008 est.)
$60.26 trillion (31 December 2007 est.)

$16.65 trillion (31 December 2008 est.)
$14.77 trillion (31 December 2007 est.)

$16.22 trillion (31 December 2008 est.)
$15.43 trillion (31 December 2007 est.)