The Sea are connected as the world ocean is the body of salty water that covers approximately of a percent of the earth surface. The word sea is also used to denotes the order sections of the sea.
The sea moderates climate of earth and has important roles in the water cycle, carbon cycle, and nitrogen cycle. Humans harnessing and studying the sea have been recorded since ancient times, and evidenced well into prehistory, while its modern scientific study is called oceanography. The most abundant solid dissolved in seawater is sodium chloride. The water also contains salts of magnesium, calcium, potassium, and mercury, amongst many other elements, some in minute concentrations.
Salinity varies widely, being lower near the surface and the mouths of large rivers and higher in the depths of the ocean how ever, the relative proportions of dissolved salts vary little across the oceans. Winds blowing over the surface of the sea produce waves, which break when they enter the shallow water. Winds also create surface currents through friction, setting up slow but stable circulations of water throughout the oceans.
The directions of the circulation are governed by factors, including the shapes of the continents and Earth's rotation the Coriolis effect. Deep sea currents, known as the global conveyor belt, carry cold water from near the poles to every ocean. Tides, the generally twice-daily rise and fall of sea levels, are caused by Earth's rotation and the gravitational effects of the orbiting Moon and, to a lesser extent, of the Sun. Tides may have a very high range in bays or estuaries. Submarine earthquakes arising from tectonic plate movements under the oceans can lead to destructive tsunamis, as can volcanoes, huge landslides, or the impact of large meteorites.
A wide variety of organisms, including bacteria, protists, algae, plants, fungi, and animals, live in the sea, which offers a wide range of marine habitats and ecosystems, ranging vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the cold waters under polar ice caps to the colorful diversity of coral reefs in tropical regions. Many of the major groups of organisms evolved in the sea and life may have started there.
The sea provides substantial supplies of food for humans, mainly fish, but also shellfish, mammals and seaweed, whether caught by fishermen or farmed underwater. The people included the trade, travel, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving in the sea.
