Sea ice is frozen ocean water that forms, grows, and melts in the polar oceans surrounding Earth's Arctic and Antarctic regions. Unlike icebergs, which are chunks of glacial ice that break off from land-based ice sheets, sea ice forms di…
When polar temperatures plunge below the freezing point of seawater, tiny ice crystals begin forming at the ocean's surface. These crystals, called frazil ice, look like slush or a thick soup floating on the water. As the temperature remains cold, these crystals stick together to form larger structures that eventually consolidate into a continuous ice sheet.
The presence of salt complicates this freezing process in a crucial way. As seawater freezes, most of the salt is expelled from the forming ice crystals and pushed back into the ocean below. This creates pockets of extremely salty, dense water called brine that either drains downward through channels in the ice or gets temporarily trapped. The result is that sea ice contains much less salt than the ocean water from which it formed.
Sea ice grows thicker as freezing continues throughout the winter months. In the Arctic, first-year ice typically reaches 1-2 meters thick, while multi-year ice that survives summer melt can grow to 3-4 meters or more. The Antarctic generally produces thinner ice, averaging around 1 meter, because stronger winds and waves break up forming ice before it can grow as thick.
Sea ice acts as Earth's largest natural mirror, bouncing solar radiation back into space before it can warm the planet. Fresh, snow-covered sea ice reflects about 85 percent of the sun's energy, compared to the dark ocean water beneath, which absorbs roughly 90 percent. This dramatic difference in reflectivity, called albedo, makes sea ice a powerful temperature regulator for the entire planet.
The reflective power of sea ice creates a self-reinforcing cycle known as the ice-albedo feedback. When ice melts and exposes dark ocean water, that water absorbs more heat, which melts more ice, which exposes more dark water. This feedback loop works in reverse during freezing: more ice means more reflection, which means cooler temperatures, which encourages more ice formation.
The condition of the ice surface dramatically affects how much sunlight it reflects. Pristine white snow on top of sea ice reflects the most energy, while older, weathered ice that has accumulated dirt or melt ponds reflects less. Even melt ponds—pools of water sitting on top of the ice—absorb more energy than the surrounding ice, accelerating local melting and demonstrating how small changes in ice surface conditions can have outsized effects.
A layer of sea ice acts like a thermal blanket between the ocean and atmosphere, drastically reducing the transfer of heat and water vapor. The relatively warm ocean water beneath the ice—even at minus 1.8 degrees Celsius—contains far more heat than the frigid polar air above, which can reach minus 40 degrees Celsius or colder. Without ice cover, the ocean would release enormous amounts of heat into the atmosphere through direct contact.
This insulating effect works in both directions depending on the season. During winter, sea ice prevents the ocean from losing heat to the cold atmosphere, keeping the water below warmer than it would otherwise be. During summer, the ice prevents the atmosphere from warming the ocean water too quickly. The thickness of the ice determines how effectively it insulates—thicker ice blocks more heat transfer than thin ice.
Sea ice also dramatically reduces evaporation from the ocean surface. Open polar ocean water in winter releases massive plumes of water vapor into the dry, cold air above, which can be seen as "sea smoke" or Arctic steam fog. When ice covers the surface, this moisture exchange essentially stops. This affects local weather patterns, cloud formation, and even the amount of snowfall in polar regions.
When sea ice forms, the salt expelled from freezing water creates pockets of extremely cold, salty brine that is denser than the surrounding seawater. This dense brine sinks toward the ocean floor, initiating a critical process called thermohaline circulation. In certain key regions—particularly the North Atlantic near Greenland and around Antarctica—this sinking of dense, cold water drives a global conveyor belt of ocean currents that distributes heat around the entire planet.
This ice-driven circulation connects the polar oceans to the rest of the world's seas. The cold, salty water that sinks at the poles flows along the ocean bottom toward the equator, while warmer surface waters flow poleward to replace it. This global circulation pattern, powered in part by sea ice formation, transports heat from tropical regions toward the poles and helps regulate climate patterns worldwide. The Gulf Stream carrying warmth to Northern Europe is part of this ice-influenced system.
The amount and location of sea ice formation directly affects the strength of these currents. When sea ice production decreases, less dense brine sinks, potentially slowing the circulation. Scientists monitor sea ice extent partly because changes in where and how much ice forms can indicate shifts in these fundamental ocean circulation patterns that influence climate far beyond the polar regions.
As spring arrives in polar regions and air temperatures climb above the freezing point of seawater, sea ice begins its seasonal melt. The process starts at the ice surface, where sunlight and warmer air create melt ponds—pools of fresh water sitting atop the ice. These dark pools absorb more solar energy than the surrounding white ice, accelerating melting beneath them. Simultaneously, warmer ocean water below begins eroding the ice from underneath, particularly along the edges of the ice pack.
The melt season determines whether ice survives to become thicker, stronger multi-year ice or disappears completely. In the Arctic, ice that melts entirely during summer must reform from scratch the following winter, creating thinner first-year ice. Ice that survives the summer melt season grows thicker and stronger over successive years. Antarctic sea ice, by contrast, almost entirely melts each summer due to its location at lower latitudes and exposure to more wave action that breaks up the ice.
The timing and extent of sea ice melt has accelerated in recent decades, particularly in the Arctic, where summer sea ice extent has declined by roughly 13 percent per decade since satellite records began in 1979. This melting ice doesn't directly raise sea levels—since the ice was already floating in the ocean—but it does trigger the ice-albedo feedback, exposing dark ocean water that absorbs heat and promotes further warming. The fresh water released from melting sea ice also affects ocean salinity and can influence the formation of new ice the following winter.