<P> 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 . </P> <P> The atmospheric pressure on Earth's surface averages 101.325 kPa, with a scale height of about 8.5 km . It has a composition of 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide, and other gaseous molecules . The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors . </P> <P> Earth's biosphere has significantly altered its atmosphere . Oxygenic photosynthesis evolved 7000270000000000000 ♠ 2.7 Gya, forming the primarily nitrogen--oxygen atmosphere of today . This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O into O . The ozone layer blocks ultraviolet solar radiation, permitting life on land . Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature . This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature . Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere . Without this heat - retention effect, the average surface temperature would be − 18 ° C, in contrast to the current + 15 ° C, and life on Earth probably would not exist in its current form . In May 2017, glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere . </P> <P> Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space . Three - quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface . This lowest layer is called the troposphere . Energy from the Sun heats this layer, and the surface below, causing expansion of the air . This lower - density air then rises and is replaced by cooler, higher - density air . The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy . </P>

Principle constituent of the atmosphere of the earth