WETLANDS ARE WATER-BASED ecosystems that cover approximately 6 percent of the Earth’s land area. an estimated 3.3 million square mile (8.6 million square kilometer). Wetlands occur on every continent except and in every climatic zone from the humid tropics to desert to tundra. Three climatic zones of the world (tropical, subtropical, and boreal) support 70 percent of the world’s wetlands. Together, tropical and subtropical zones have an estimated wetland area of 1.3 million square mile (3.4 million square kilometer); the boreal zone has an estimated 1 million square mile (2.6 million square kilometer) of wetlands. Wetlands include many familiar ecosystems, such as swamps, bogs, fens, mires, and moors, which have different names in various regions of the world. As wetlands often occur at the interface between dry terrestrial ecosystems and permanently saturated aquatic ecosystems (rivers, lakes, estuaries, and oceans) they are sometimes considered to be ecotones, transition areas or interfaces between ecosystems.