The ocean is the enormous body of salt water that covers more than half of our planet. It is the source of most of the world’s oxygen, and it also contains vast stores of energy and minerals. The sea connects the continents and their people, providing important shipping routes for trade and tourism. It is a living, changing environment that supports life, sustains our economies, and influences weather and climate around the globe.
Most people use the words sea and ocean interchangeably, but there is a difference between them. The sea is a division of the ocean, “of considerable extent and more or less definitely marked off by land boundaries.” The Atlantic, Indian, and Pacific oceans all fit this definition. Seas are smaller than oceans, but larger than lakes or rivers. They may be bounded by one or more large landmasses, or they may contain numerous islands. For example, the Caribbean Sea is bounded by Jamaica and Cuba.
The water in a sea is saltier than the water in a river or lake. This saltiness is due to evaporation, or the loss of freshwater from the surface. The water in a sea also has more salt than the water in freshwater bodies because it absorbs more sodium and chloride from the surrounding environment.
Ocean currents are caused by the Earth’s rotation and wind, which move heat from the tropics toward the poles. They also transport oxygen to organisms throughout the ocean from the deep, cold waters. Ocean plants and algae, known as phytoplankton, convert sunlight into food for themselves through photosynthesis in the epipelagic zone. This makes the phytoplankton visible as green or blue splotches in the water.
A major part of a sea’s surface is covered by sediments. These sediments are formed from the decomposition of organic matter and from the erosion of other material, such as rock or ice. Some seas are very shallow, while others can be thousands of meters deep. In addition, a sea’s temperature and salinity vary with the seasons.
In the 19th century, scientific societies began to process a huge flood of new terrestrial botanical and zoological information, and oceanography became a scientific discipline. Scientists divided the ocean into a number of major oceanic divisions, including (in ascending order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Southern Ocean. These are often referred to as the seven seas, but many scientific publications do not include the Southern Ocean. In the future, the ocean may be divided into further subdivisions. Each of these new subdivisions will have its own name and specific boundary. This will allow us to better understand how these changes affect the oceans’ biodiversity, and how they interact with each other.