The ocean is a continuous body of salt water that covers more than 70 percent of the Earth's surface. Ocean currents govern the world's weather and churn a kaleidoscope of life. Humans depend on these teeming waters for comfort and survival, but global warming and overfishing threaten Earth's largest habitat. Geographers divide the ocean into five major basins: the Pacific, Atlantic, Indian, Arctic, and Southern. Smaller ocean regions such as the Mediterranean Sea, Gulf of Mexico, and the Bay of Bengal are called seas, gulfs, and bays. Inland bodies of saltwater such as the Caspian Sea and the Great Salt Lake are distinct from the world's oceans. The oceans hold about 321 million cubic miles (1.34 billion cubic kilometers) of water, which is roughly 97 percent of Earth's water supply. Seawater's weight is about 3.5 percent dissolved salt; oceans are also rich in chlorine, magnesium, and calcium. The oceans absorb the sun's heat, transferring it to the atmosphere and distributing it around the world. This conveyor belt of heat drives global weather patterns and helps regulate temperatures on land, acting as a heater in the winter and an air conditioner in the summer.
Oceanography also known as oceanology, is the study of the physical and biological aspects of the ocean. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamics; plate tectonics and the geology of the sea floor; and fluxes of various chemical substances and physical properties within the ocean and across its boundaries. These diverse topics reflect multiple disciplines that oceanographers blend to further knowledge of the world ocean and understanding of processes within: astronomy, biology, chemistry, climatology, geography, geology, hydrology, meteorology and physics. Paleoceanography studies the history of the oceans in the geologic past.
The study of oceanography is divided into these four branches:
- Biological oceanography, or marine biology, investigates the ecology of marine organisms in the context of the physical, chemical and geological characteristics of their ocean environment and the biology of individual marine organisms.
- Chemical oceanography and ocean chemistry, are the study of the chemistry of the ocean. Whereas chemical oceanography is primarily occupied with the study and understanding of seawater properties and its changes, ocean chemistry focuses primarily on the geochemical cycles.
- Geological oceanography, or marine geology, is the study of the geology of the ocean floor including plate tectonics and paleoceanography.
- Physical oceanography, or marine physics, studies the ocean's physical attributes including temperature-salinity structure, mixing, surface waves, internal waves, surface tides, internal tides, and currents.