Underwater currents circle the globe, moving heat from the equator toward the poles.

The longer version

Density differences drive a slow global circulation. Water grows heavier when cooled and when salt is left behind by evaporation or by ice formation, so the densest water sinks in polar seas. That sinking limb drags replacement water from lower latitudes, forming a connected loop through the major basins.

Surface currents driven by wind ride on top, but the deep return takes hundreds to thousands of years. Because the loop moves enormous amounts of heat and dissolved carbon, high-latitude cooling shapes climate everywhere, not just where it happens. Melting ice adds fresh water at the surface, which can slow the sinking that drives everything.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.