Oceans resist freezing because dissolved salts push their freezing point well below 0 °C, their enormous volume stores and circulates vast amounts of heat, and energy from the sun, ocean currents, and even the seafloor keeps temperatures above that lowered threshold across most of the planet. Typical seawater, with a salinity around 35 parts per thousand, does not begin to freeze until it drops to roughly −1.8 °C. That might seem like a small margin, but combined with the sheer thermal mass of the world ocean and its constant motion, it is enough to keep the overwhelming majority of ocean water liquid year-round.
How Salt Keeps the Freezing Point Down
Pure freshwater freezes at 0 °C. Add salt, and you need to pull more energy out of the water before ice crystals can form. The dissolved ions get in the way of water molecules trying to lock into the orderly lattice structure that makes ice. The more salt, the lower the temperature has to drop. At the average ocean salinity of about 35 grams per kilogram, the freezing point sits near −1.8 °C. In especially salty bodies like the Red Sea or parts of the Mediterranean, where salinity can exceed 40 parts per thousand, the freezing point drops even further.
The relationship between salt content and freezing point has been measured with precision. Researchers studying Western Pacific Ocean water derived empirical formulas linking chlorinity to freezing temperature, confirming that even modest changes in salinity shift the freezing point measurably.1Bulletin of the Chemical Society of Japan. Chemical Studies of the Western Pacific Ocean. III. Freezing Point, Osmotic Pressure, Boiling Point, and Vapour Pressure of Sea Water This is why river mouths and polar seas, where meltwater dilutes the salt, freeze more readily than the open ocean at the same temperature. Dilute the salt, and the freezing point creeps back toward zero.
The Ocean as a Heat Reservoir
Water has an extraordinarily high heat capacity. It can absorb and store large amounts of thermal energy without its temperature rising much. The global ocean holds roughly a thousand times more heat than the entire atmosphere. Sunlight warms the surface, particularly in tropical and subtropical latitudes, and the ocean hangs onto that warmth for months or even years.
That stored heat does not just sit in the tropics. Ocean currents act like a planetary heating system, moving warm water from lower latitudes toward the poles. The Gulf Stream, for instance, carries warm Caribbean water northeastward across the Atlantic, which is a big reason why western Europe stays milder than you would expect for its latitude. Modeling work on ancient ocean configurations has shown that the arrangement of continents and ocean passages profoundly shapes how efficiently heat moves poleward, influencing global temperature gradients over geologic time.2Paleoceanography. Impact of a Tethyan circumglobal passage on ocean heat transport and “equable” climates The modern Atlantic overturning circulation is one version of this system, and it keeps enormous stretches of high-latitude ocean well above their freezing point.
Glacial meltwater from sources outside Greenland is now altering parts of this system, introducing fresh, low-salinity water into high-latitude seas. Research shows that this extra freshwater can change local temperatures and salinity patterns, adjusting the very conditions that determine whether surface water stays liquid or begins to ice over.3Earth System Dynamics. Freshwater input from glacier melt outside Greenland alters modeled northern high-latitude ocean circulation Where warmer water gets pushed northward and mixes with cold, diluted surface layers, the interplay between temperature and salinity becomes the key factor deciding ice formation.
Convective Mixing Stirs Heat Upward
Even without horizontal currents, the ocean has a built-in mechanism for resisting a frozen surface: vertical mixing. When surface water cools and becomes denser than the water below it, it sinks, and warmer water from below rises to replace it. This overturning process constantly brings heat up from deeper layers, making it harder for the surface to stay cold long enough to freeze.
This convection can be dramatic. During the winter of 2014–2015, intense atmospheric cooling over the Irminger Sea south of Iceland drove mixing so deep that the stratified upper layer was completely overturned down to about 1,400 meters.4Geophysical Research Letters. Strong winter cooling over the Irminger Sea in winter 2014–2015, exceptional deep convection, and the emergence of anomalously low SST That kind of deep convection pulls up water that has been accumulating heat for years, effectively resetting the surface with warmer water from below. Even though sea surface temperatures in the subpolar North Atlantic were unusually low that winter, the ocean did not freeze over; convective mixing kept redistributing heat fast enough to prevent it.
Freshwater behaves differently. In a freshwater lake, water reaches its maximum density at about 4 °C and then becomes lighter as it continues cooling toward 0 °C. That means the coldest water sits right at the surface, insulated from the warmer water below, which is why lakes freeze from the top down so efficiently. In saltwater, density keeps increasing all the way down to the freezing point, so cold surface water keeps sinking and getting replaced by warmer water from depth. The convective loop is harder to shut off.
How Pressure Plays a Role at Depth
Down in the deep ocean, hydrostatic pressure adds another layer of protection against freezing. Increasing pressure lowers the freezing point of seawater slightly, so water at the bottom of a four-kilometer-deep basin needs to be even colder than −1.8 °C before it could freeze. The effect is modest on a per-meter basis, but it accumulates over thousands of meters of water column. Research on this pressure-dependent freezing-point depression has described a convection mechanism in which the difference between the freezing point at the surface and the freezing point at depth contributes to vertical water movement.5Deep Sea Research and Oceanographic Abstracts. Conditional instability of sea water at the freezing point
In practical terms, no part of the deep ocean is anywhere close to freezing. Even the coldest abyssal water, like Antarctic Bottom Water, sits just above 0 °C at depth, still well above its pressure-adjusted freezing point. The combination of pressure, salinity, and continual mixing means that deep-ocean freezing is essentially impossible under current conditions.
Heat From the Seafloor
One factor people rarely think about is the heat seeping up through the ocean floor. The Earth’s interior is hot, and geothermal energy steadily radiates through the crust into the water above. On its own, this heat flux is small compared to solar energy at the surface, but it matters where it accumulates: in the deep ocean, far from direct solar warming.
Modeling studies have shown that geothermal heating warms the bottom waters of the ocean by roughly 0.3 to 0.4 °C on average. If that heat were simply trapped in place and diffused upward, the deep ocean would warm by over a degree, but because geothermal heating triggers its own circulation changes, the actual warming is about a quarter of that diffusive estimate.6Geophysical Research Letters. Impact of geothermal heating on the global ocean circulation Instead, the added warmth destabilizes the water column and accelerates the formation of deep-water masses. One study found that geothermal heat enhances the production of Antarctic Bottom Water by roughly a third and North Atlantic Deep Water by about 10%.7Geophysical Research Letters. Geothermal heat flux and its influence on the oceanic abyssal circulation and radiocarbon distribution Another estimated that the Antarctic Bottom Water cell strengthens by about 15% when geothermal input is accounted for, with deep-ocean temperatures in the North Pacific rising by as much as 0.3 °C above what they would be without it.8Ocean Science. Geothermal heating, diapycnal mixing and the abyssal circulation
None of these temperature bumps sound large in absolute terms, but for abyssal water that already hovers just a fraction of a degree above freezing-adjusted thresholds, the extra warmth is a meaningful buffer. And by driving circulation, geothermal heat also speeds up the mixing that carries warmth from other sources to the deep.
When the Ocean Does Freeze
Of course, parts of the ocean do freeze. The Arctic Ocean develops a seasonal ice cover every winter, and sea ice surrounds Antarctica for much of the year. But even in polar regions, the ocean does not freeze solid. It forms a relatively thin skin of ice on the surface while the vast bulk of water below remains liquid. Understanding how that ice forms shows why it stays limited.
In calm conditions with light winds, seawater that has cooled to its freezing point forms a smooth, continuous sheet of columnar ice. But the polar ocean is rarely calm. At moderate wind speeds, the growing ice gets rafted and ridged, with a mix of crystal textures. In strong winds above roughly 10 meters per second, the ice cover is heavily deformed, and a distinctive type of granular ice dominates.9Journal of Geophysical Research: Oceans. Development and properties of sea ice in the coastal regime of the southeastern Weddell Sea Waves and turbulence prevent a neat frozen lid from forming. Instead, the first ice to appear in rough open water is frazil ice: tiny suspended crystals that swirl through the upper water column like an icy slurry.
Those frazil crystals gradually collect at the surface to form a soupy layer called grease ice. Laboratory experiments have measured the solid fraction of this layer, finding that it starts out extremely dilute, around 3 to 5% ice by volume, and thickens to about 23 to 31% before it consolidates into the next stage.10The Cryosphere. Laboratory study of frazil ice accumulation under wave conditions That next stage is pancake ice: rounded discs that bump and jostle against each other in the swell, developing raised rims from the collisions. Eventually the pancakes freeze together into a continuous pack ice cover. Studies tracking this process in the Weddell Sea have used drifting buoys to follow the full sequence from frazil crystals to consolidated pack, modeling how thermodynamics, wave action, and mechanical scavenging of frazil into the growing pancakes partition ice volume at each stage.11Journal of Geophysical Research: Oceans. Simulating pancake and frazil ice growth in the Weddell Sea: A process model from freezing to consolidation
The whole process is messy and gradual. It is nothing like a pond freezing on a still night. And even once a solid ice cover forms, it typically reaches only one to three meters thick in the Arctic and one to two meters around Antarctica in a single season, leaving the other 99.9% of the water column liquid.
Brine Rejection and the Self-Limiting Nature of Sea Ice
Sea ice has a built-in brake. When seawater freezes, the ice crystals exclude most of the dissolved salt. The rejected brine drains downward, making the water just below the ice saltier and denser. This dense, salty water sinks, and warmer water from below moves up to take its place, delivering heat to the underside of the ice and slowing further growth.
This brine rejection is most intense during the early stages of ice formation, when the ice is young and porous. The salty drainage from new sea ice contributes to the large-scale overturning circulation, feeding into the deep-water masses that drive global ocean currents.12Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. Sea-ice thermodynamics and brine drainage So the act of freezing itself triggers a response that resists more freezing. It is a negative feedback loop: ice forms, ejects salt, the saltier water sinks, warmer water rises, and ice growth slows.
This is fundamentally different from freshwater ice. When a lake freezes, the ice is nearly pure water, and the lake water below stays roughly the same composition. There is no brine rejection, no enhanced convection from the freezing process itself. The ice just keeps growing downward until equilibrium is reached with whatever heat is left in the lake. In the ocean, the freezing process actively fights itself.
Why Arctic Sea Ice Is Shrinking
If the ocean has all these defenses against freezing, you might wonder why the Arctic has any ice at all. The answer is that polar regions receive far less solar energy than lower latitudes, especially during the months of polar darkness. At those extremes, heat loss to the atmosphere can outpace the heat supplied by currents and mixing, particularly in semi-enclosed basins like the Arctic Ocean where water exchange with warmer oceans is restricted.
But the balance is shifting. Warming of both land and ocean surfaces, combined with increased heat advection into the Arctic, has driven substantial losses of sea ice extent and thickness. Satellite observations and model analyses document the recession of Arctic sea ice, including thinning and a sharp reduction in older, multiyear ice.13PubMed Central. Global warming leading to alarming recession of the Arctic sea-ice cover: Insights from remote sensing observations and model reanalysis The ocean beneath is absorbing more solar energy as reflective ice gives way to darker open water, creating a feedback that accelerates further melting.
The disappearance of Arctic sea ice does not mean the ocean is warming to the point where it cannot freeze at all. Winter temperatures in the central Arctic still drop far below the seawater freezing point, and new ice forms every cold season. What is changing is how much ice survives the summer, how thick it gets, and how far south it extends. The ocean’s resistance to freezing has not fundamentally changed; the atmosphere is simply delivering more warmth into the system, tipping the balance further away from ice and toward open water.
Subglacial Oceans on Earth
Some of the most striking examples of liquid water persisting against the odds sit under the Antarctic ice sheet. More than 400 subglacial lakes have been identified beneath kilometers of glacial ice, kept liquid not by salinity or ocean currents but primarily by geothermal heat from below and the insulating pressure of the ice above. Research has shown that Earth’s geothermal flux drives vigorous convective mixing within these buried lakes, stirring the water enough to suspend fine particles and distribute nutrients and oxygen from the overlying ice melt throughout the water column.14Science Advances. Dynamic flows create potentially habitable conditions in Antarctic subglacial lakes
These subglacial lakes demonstrate that liquid water can persist in extreme cold as long as some energy source keeps feeding it. They also serve as analogs for thinking about oceans on other worlds, where solar energy is negligible but other heat sources might do the job.
Oceans Beyond Earth
If you extend the question to the rest of the solar system, the same principles apply in unfamiliar settings. Jupiter’s moon Europa almost certainly has a global liquid ocean beneath an ice shell perhaps 10 to 30 kilometers thick. Without meaningful sunlight reaching beneath the surface, that ocean stays liquid thanks to tidal heating from Jupiter’s gravity stretching and compressing the moon’s interior, along with radiogenic heat from decay of elements in its rocky core.15PubMed Central. Dynamic controls on subsurface water chemistry and habitability on icy moons
Thermal modeling of Europa’s ocean suggests an interesting twist on the freshwater-versus-saltwater story. Most of the ocean is likely at the temperature of maximum density rather than at the freezing point. A thin layer of stably stratified water at the freezing point sits just beneath the ice shell, while the bulk of the ocean below convects vigorously at a slightly warmer temperature.16Icarus. The temperature of Europa’s subsurface water ocean The same density-driven convection that keeps Earth’s oceans from freezing solid appears to operate inside a moon hundreds of millions of kilometers from the sun.
Saturn’s moon Enceladus tells a similar story on a smaller scale. Geysers at its south pole spray water vapor and ice particles into space, direct evidence of a subsurface liquid reservoir maintained by tidal forces. The common thread across all these worlds is the same one that keeps Earth’s oceans from freezing: energy input, whether from the sun, from internal heat, or from gravitational interactions, combined with the physical properties of water that make it remarkably resistant to solidifying.
Everyday Misconceptions About Ocean Freezing
One common misunderstanding is that the ocean does not freeze because it is “too big.” Size matters, but only because a larger body of water stores more total heat and has more opportunity for circulation. A vast, completely still basin of seawater with no energy input would eventually freeze from the surface down, regardless of its size. What saves the ocean is not its volume alone but the constant replenishment of heat through solar radiation, current transport, geothermal input, and convective mixing.
Another misconception is that salt merely “prevents” freezing, as if ocean water simply cannot freeze. Salt lowers the freezing point; it does not eliminate it. If you cool seawater to −1.8 °C and keep extracting heat, it will freeze. The reason it does not happen across most of the ocean is that heat arrives faster than it leaves. In the polar regions where heat loss does win out, the ocean freezes right on schedule, just at a lower temperature and in a more complicated, self-limiting way than freshwater does.
People also sometimes assume the deep ocean must be close to freezing because it is cold and dark. Deep water is indeed cold, typically 1 to 4 °C, but its freezing point under the immense pressure at depth is lower still. And geothermal heat, however modest it seems in absolute terms, acts directly on the deepest water, keeping it comfortably above the threshold. The deep ocean is cold, but it is nowhere near frozen.