Ocean

The ocean is a massive, interconnected body of salt water. It covers approximately 70.8% of the Earth's surface. This vast system contains 97% of all the water on our planet. Because of this, the ocean is the primary component of the hydrosphere. It serves as a giant reservoir for heat and energy. This role helps regulate the Earth's energy budget. The ocean is also vital for the carbon cycle and the water cycle. These processes form the basis for global climate and weather patterns.

Scientists study the ocean by dividing it into vertical and horizontal zones. Horizontally, the ocean covers the oceanic crust. Where the water meets land, it covers shallow continental shelves. These shelves are part of the Earth's continental crust. Vertically, the water column is called the pelagic zone. This zone is divided into layers based on light and depth. The photic zone is the top layer, starting at the surface. It extends to about 200 meters deep in the open ocean. This depth is where light intensity drops to only 1% of surface levels.

In the photic zone, a process called photosynthesis occurs. This process is essential for life. Microscopic algae known as phytoplankton use sunlight, water, and carbon dioxide. They also use nutrients to produce organic matter. This makes the photic zone the most biodiverse part of the ocean. It provides the food supply that sustains most marine ecosystems. Below this, the water becomes cold and dark. These deeper areas are known as the mesopelagic and aphotic zones.

Ocean water is constantly in motion through various currents. These currents are caused by several different forces. Temperature and salinity differences play a major role. Atmospheric circulation, such as wind, also drives water movement. The Coriolis effect is another force that affects these patterns. Tides create tidal currents, while wind and waves cause surface currents. Major examples include the Gulf Stream and the Antarctic Circumpolar Current. These currents transport heat, gases, and nutrients across the globe.

Geologists believe the ocean has existed in some form for billions of years. Some evidence suggests water was present during Earth's formation. Rocks in the Nuvvuagittuq Greenstone Belt show water existed 3.8 to 4.28 billion years ago. The shape of the ocean changes due to plate tectonics. In mid-ocean, magma rises through the seabed at mid-oceanic ridges. This process can create underwater mountains or volcanic islands. Sometimes, one oceanic plate slides beneath another in a process called subduction. This movement can cause deep trenches and earthquakes.

Ocean temperatures vary greatly depending on location. In the tropics, surface temperatures can exceed 30 °C (86 °F). Near the poles, temperatures can drop to about −2 °C (28 °F). In the deep ocean, temperatures stay between −2 °C and 5 °C. The ocean also absorbs dissolved gases like oxygen and nitrogen. It also absorbs carbon dioxide from the atmosphere. However, rising CO2 levels can cause ocean acidification. This happens when the pH value of the water drops.

The ocean is home to a massive variety of life. Scientists have identified over 230,000 species. Some researchers believe there may be over two million species. Humans rely on the ocean for seafood, transport, and ecosystem services. However, human activity creates many environmental threats. Marine pollution and overfishing are major concerns. Climate change also causes ocean warming and sea level rise. Coastal waters and continental shelves are often the most affected by these changes.