There is really one connected world ocean. The boundaries between its named parts have changed over time for historical, cultural, geographical and scientific reasons. The named parts are the Pacific, Atlantic, Indian, Arctic and Southern oceans.
Water that is cold and very salty is heavier, so it sinks. This sinking water starts slow, deep currents that move around the whole ocean. Wind-driven surface currents and density-driven deep currents are linked in one global system, the ocean conveyor. It carries heat, salt, nutrients and carbon around the world and helps regulate climate.
Coastal upwelling happens when winds blowing along the shore push surface water away, and the Earth's rotation (the Coriolis effect) turns the moving water. Cold deep water then rises to take its place.
El Niño begins when the trade winds weaken. In normal years the winds push warm surface water west across the Pacific and cold water rises along South America. In El Niño, warm water flows back east and the weather changes around the world. It comes back every two to seven years.
Warm water takes up more space than cold water. As the ocean warms, its water expands, and this thermal expansion raises sea level even without any added ice melt.
Carbon dioxide that dissolves in sea water reacts to make more hydrogen ions, which leaves fewer carbonate ions. The water becomes slightly more acidic. This makes it harder for corals, oysters, clams and other shell-builders to make shells and skeletons.
A marine protected area is any part of the ocean that has legal protection. They include sanctuaries, monuments, parks and wildlife refuges, and some allow no fishing at all.
As of June 2025, overfishing has reduced some fish populations, but many fisheries are recovering where management is strong and based on science.
Words to know
- upwelling: water rising
- Coriolis: spin-turn effect
- conveyor: moving belt
- expands: takes more space
- refuges: protected places
