Where the Sun can stand straight overhead lives forty percent of us
The tropics belt Earth where the Sun can shine directly overhead, unlike temperate or polar zones. Axial tilt sets their width at twice that tilt—about ±23.4° today, or ±23°26′08.9″. They cover 39.8 percent of Earth’s surface and 36 percent of land, and as of 2014 held 40 percent of people, projected toward 50 percent by 2050.
Because the Sun stays high all year, temperature barely shifts between seasons, so people in the tropics tend to mark the calendar by rain rather than by heat or day length. Under the Köppen scheme a wet-season month is one averaging at least 60 millimetres of rain, and some places see a mid-season lull when the Intertropical Convergence Zone or monsoon trough slides poleward.
Plenty of land inside the belt is not climatically tropical at all. The Sahara, the Atacama and the Australian Outback count as dry, and alpine tundra caps Mauna Kea, Kilimanjaro, Puncak Jaya and the Andes. The Tropic of Cancer was named for the constellation behind the northern summer Sun as reckoned in Mesopotamia around 2500 BC, but precession of the equinoxes has since shifted those overhead positions into Taurus and Sagittarius. The moon's pull makes the tilt wobble slightly, so the boundary lines are a convention.
Over the past century the region warmed by 0.7 to 0.8 °C, a little under the global average, with El Niño driving much of the year-to-year swing; 1998 was the hottest year in most areas. Models project another 1 to 2 °C by 2050, and subtropical areas are taking on tropical conditions with harsher heatwaves and storms that could make some places unliveable.
Biogeographers split the belt into the Paleotropics of Africa, Asia and Australia and the Neotropics of the Americas. Rainforest soils are often poor in nutrients, leaving them exposed to slash-and-burn clearing. The French geographer Pierre Gourou revived debate about tropicality, the outsider's image of the region as either Eden or untamed wilderness, with his late-1940s book on the tropical world.
Source: Tropics
