<P> The hydrosphere (from Greek ὕδωρ hydōr, "water" and σφαῖρα sphaira, "sphere") is the combined mass of water found on, under, and above the surface of a planet, minor planet or natural satellite . </P> <P> It has been estimated that there are 1386 million cubic kilometers of water on Earth . This includes water in liquid and frozen forms in groundwater, oceans, lakes and streams . Saltwater accounts for 97.5% of this amount . Fresh water accounts for only 2.5% . Of this fresh water, 68.9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic, and mountain glaciers . 30.8% is in the form of fresh groundwater . Only 0.3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems . The total mass of the Earth's hydrosphere is about 1.4 × 10 tonnes, which is about 0.023% of Earth's total mass . About 20 × 10 tonnes of this is in Earth's atmosphere (for practical purposes, 1 cubic meter of water weighs one tonne). Approximately 75% of Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean . The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%). </P> <P> The water cycle refers to the transfer of water from one state or reservoir to another . Reservoirs include atmospheric moisture (snow, rain and clouds), streams, oceans, rivers, lakes, groundwater, subterranean aquifers, polar ice caps and saturated soil . Solar energy, in the form of heat and light (insolation), and gravity cause the transfer from one state to another over periods from hours to thousands of years . Most evaporation comes from the oceans and is returned to the earth as snow or rain . Sublimation refers to evaporation from snow and ice . Transpiration refers to the expiration of water through the minute pores or stomata of trees . Evapotranspiration is the term used by hydrologists in reference to the three processes together, transpiration, sublimation and evaporation . </P>

What kind of ecosystem covers most of earth's surface