Hydrosphere

More than |70\ \%| of the Earth's surface is covered with water. Seen from space, this abundance has earned Earth the nickname "Blue Planet.” It is estimated that there is approximately |1.34| billion km³ of water on Earth. Water is found in the three states of matter: solid, liquid or gas. It exists in the form of water vapour in the atmosphere, and in the form of ice crystals in glaciers, ice floes, and snow. It is also in liquid form in oceans, seas, lakes, waterways, groundwater bodies, and raindrops.

Water is essential for life. It provides a habitat for many living organisms and it is the foundation for multiple biological processes which sustain life on Earth. Indeed, the presence of water is one of the basic conditions for the existence of life on a planet.

The hydrosphere is responsible for the continuous circulation of elements in the various spheres of the Earth. All of these processes form the biogeochemical cycles.

It is possible for humans to harness the energy produced by the movement of water in the hydrosphere. Examples are hydroelectric power, tidal power, and energy from ocean waves and currents.

More than |70\ \%| of planet Earth is covered with water. Despite this abundance, only |1\ \%| is available for the needs of humanity, because most of this water is salty, therefore not drinkable.

Water on Earth is distributed as follows:

• |97\ \%| salt water (including brackish water);

• |3\ \%| fresh water.

From this percentage, fresh water is distributed as follows:

• |69.6\ \%| of fresh water is found in solid form (as in glaciers);

• |30.0\ \%| of fresh water is found in groundwater;

• |0.4\ \%| of fresh water is available on the Earth's surface in liquid or gaseous form (water vapor).

This fresh water available for consumption is not distributed equitably on Earth. Some countries are therefore faced with problems of supplying fresh and drinking water. The following map illustrates the availability of fresh water in different regions of the world.

Water is responsible for the formation of a large part of the lithosphere’s relief through the phenomenon of erosion. Water is indeed an important agent of erosion since it is continuously in motion. As a matter of fact, the water cycle allows a form of constant circulation of water. In addition, all the currents, tides, and winds that occur on Earth sometimes cause significant water movement. By passing regularly over the rocky surfaces of the lithosphere, the rough and moving waters end up wearing down and shaping the relief.

There are many interactions between the hydrosphere and the atmosphere. The Sun plays a crucial role as it changes water from a liquid to a gas state. This has several effects:

• water heated by the Sun evaporates into the atmosphere and, as it cools, forms clouds;

• water stored in the clouds falls back down in the form of precipitation (rain, snow, etc.);

• the evaporation of water from the oceans by the Sun allows the regulation of the water level on the Earth’s surface;

• water that evaporates into the sky is warmer than water that forms the clouds. This temperature difference is the cause of winds and other meteorological phenomena (tornadoes, hurricanes, etc.);

• heat stored in the water that evaporates from the oceans moves through the atmosphere and tempers the continents by contributing to climate regulation.