 The average salinity of Earth 's oceans is about 35 grams of salt per kilogram of sea water ( 3 @.@ 5 % salt ) . Most of this salt was released from volcanic activity or extracted from cool igneous rocks . The oceans are also a reservoir of dissolved atmospheric gases , which are essential for the survival of many aquatic life forms . Sea water has an important influence on the world 's climate , with the oceans acting as a large heat reservoir . Shifts in the oceanic temperature distribution can cause significant weather shifts , such as the El Niño @-@ Southern Oscillation . 
