Ocean

The ocean is a vast expanse of salt water that covers a large portion of Earth's surface. Ocean water makes up more than the largest share of all water on Earth, while fresh water (rivers, lakes, and ice) comprises only a small fraction.

Oceans are conventionally divided into several major regions: the Pacific, Atlantic, Indian, Arctic (northern), and Southern (Antarctic) oceans. This division is a human convention based on the continents; the water itself forms one continuous, interconnected system.

Properties of seawater

Seawater contains salinity composed mainly of sodium chloride, derived from rock minerals that rivers carry to the sea over long periods. Surface temperature varies: warm near the equator, cold near the poles. Density depends on temperature and salinity, and this variation drives the movement of deep water.

Water movement and climate

The ocean has currents driven by wind and density differences. Surface currents, such as the Gulf Stream, carry heat to distant regions in the higher latitudes. Deep currents create a global circulation pattern that distributes heat and carbon systematically. For this reason, the ocean profoundly influences Earth's climate and plays a role in absorbing heat and carbon dioxide that has accumulated in the atmosphere.

Life and depth zones

Ocean life is organized by light zones. The upper zone, where light penetrates, is inhabited by phytoplankton, which forms the base of all marine food chains and produces part of the atmosphere's oxygen. The deeper zones are dark, cold, and under extreme pressure, yet life exists there, including organisms clustered around hydrothermal vents on the seafloor.

Uses and threats

Humans use the ocean for commercial shipping, fishing, energy, and mining. Major threats include overfishing, plastic pollution and oil spills, warming seawater, and acidification resulting from carbon dioxide absorption. Sea levels are also rising, affecting low-lying coasts and islands.