The Australian flag features symbols of its British colonial history and geography. The Union Jack acknowledges ties to Great Britain, while the Southern Cross constellation and Commonwealth Star represent navigational landmarks and Australia's original states. Australia has diverse landscapes including Ayers Rock, the Great Barrier Reef, and the Great Sandy Desert. The climate varies from over 50 degrees Celsius in summer to below freezing in winter. Natural resources include forests, marsupials, and water. As of 2009, Australia's population was over 21 million people.