Water freezes at 32°F (0°C), yet the world’s oceans remain largely unfrozen even in frigid regions. The oceans represent the planet’s single largest reservoir of water. This resistance to freezing is due to the sheer volume of water, chemical properties, and constant movement.
The Role of Salt
The primary chemical reason oceans resist freezing is the presence of dissolved salt, mainly sodium chloride. The average salinity is approximately 35 parts per thousand. This dissolved material causes freezing point depression, lowering the freezing temperature of the liquid. Salt ions disrupt the crystalline lattice structure water molecules must form to become solid ice. Consequently, the average freezing point of seawater is lowered to about 28.4°F (-1.9°C).
Constant Motion and Deep Currents
Ocean water is in a state of continuous motion, which physically prevents molecules from settling into a stable ice structure. Wind-driven surface waves and tides provide constant agitation, mechanically disrupting ice formation and mixing near-freezing molecules with warmer water. Below the surface, massive deep currents known as thermohaline circulation act as a global conveyor belt. This density-driven flow constantly mixes and redistributes water masses, preventing localized cold spots from stabilizing and freezing.
Thermal Mass
The enormous volume of the oceans gives them a vast thermal mass, allowing them to absorb and store tremendous amounts of heat. This high heat capacity makes the ocean temperature stable, requiring a prolonged loss of energy for the entire body of water to cool significantly.
Density
The density behavior of water also prevents freezing from the bottom up. Unlike fresh water, the salt content in seawater shifts the maximum density point below the freezing point. Surface water continues to become denser as it cools until it reaches its freezing point, causing the coldest water to continuously sink. This sinking pulls warmer water from the depths up to the surface, cycling heat away from the freezing zone.
Where Ocean Water Does Freeze
Ocean water does freeze in the polar regions, forming sea ice. This occurs when temperatures drop significantly below the seawater freezing point of -1.9°C for extended periods, especially where the water is relatively still. When seawater freezes, the resulting ice rejects most of the salt. This salt is expelled into the remaining liquid water, making the water immediately beneath the ice saltier and denser. This super-saline water sinks, contributing to deep ocean circulation, while the resulting sea ice becomes significantly less salty over time.