The ocean is important to life on Earth because of the role it plays in both the hydrologic cycle and the carbon cycle. The hydrologic cycle, also known as the water cycle or H2O cycle, refers to the relationship that exists between the circulation, evaporation and precipitation of water on the planet. The carbon cycle determines the amount of carbon dioxide in the atmosphere.
The evaporation of water from the ocean is the primary interphase transport system of the hydrologic cycle. In addition to moving and exchanging water across the Earth's land masses through weather systems, the atmospheric transport of evaporated water helps to move thermal energy from the equatorial regions to the subtropical regions above and below.
The carbon cycle enables the ocean to absorb almost 50 percent of the atmospheric carbon dioxide produced by human activity. The ocean is capable of storing 50 times as much carbon dioxide as the atmosphere, and more carbon can be found in the ocean than the amounts produced by the use of fossil fuels. Cold ocean water dissolves carbon dioxide at high latitudes. The dissolved gas is then carried by sinking currents to lower depths where it can remain for hundreds of years.