El Niño-Southern Oscillation.

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of -10,911.4 m.[note 11][108]
The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.[109] If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[note 12] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.[110]
The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).[111] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[112] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[113] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[114] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[115]
Atmosphere
Main article: Atmosphere of Earth


This is a picture of Earth in ultraviolet light, taken from the surface of the Moon. The day-side (right) reflects a lot of UV light from the Sun, but the night-side (left) shows bands of UV emission from the aurora caused by charged particles.[116]
The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[3] It is 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.[117]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 bya, forming the primarily nitrogen–oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[118] 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 and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface would be -18 °C, in contrast to the current +15 °C, and life would likely not exist.[119]

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