Sea ice changes through two sets of processes:

• thermodynamics: freezing, melting, and heat conduction determine how much ice can grow;
• dynamics: wind and currents move, crack, and pile the ice, determining where different thicknesses occur.

A realistic ice cover requires both.

Why seawater freezes differently from a lake

Fresh water is densest near 4 °C. When a lake's surface cools below 4 °C, that colder water becomes less dense and remains at the surface. Only a thin surface layer must cool to 0 °C before ice can form.

Salt water behaves differently. Above a salinity of about 24.7 g/kg, water continues becoming denser as it cools toward its freezing point. Cooled surface water sinks and mixes, so the full upper mixed layer must lose heat before freezing begins.

Typical seawater with 33–35 g/kg of salt freezes near −1.8 °C. A nearby freshwater lake can therefore freeze weeks before the sea under the same weather conditions.

This is why stored summer heat delays sea-ice formation: all of that heat must leave the mixed layer before the first crystals form.

From crystals to an ice sheet

Freezing begins with small plate-like crystals called frazil suspended in the upper water.

• In calm water, frazil collects into a thin elastic sheet called nilas.
• In rough water, frazil becomes grease ice, then circular pancake ice with raised rims.
• Both paths eventually consolidate into a continuous sheet.
• Once a sheet exists, most further growth occurs on its underside as congelation ice.

From open water to growing sea ice

Why ice growth slows

When water freezes on the underside, it releases latent heat. That heat must conduct upward through the existing ice before more water can freeze.

The process creates a negative feedback:

1. water freezes beneath the sheet;
2. the ice becomes thicker;
3. thicker ice conducts heat less efficiently;
4. heat escapes more slowly;
5. further freezing slows.

The lecture's Stefan-law examples hold air temperature constant and show the declining growth rate:

• at −30 °C, thickness reaches roughly 2.5 m after 180 days;
• at −20 °C, thickness reaches about 2 m;
• one example grows about 16 cm on the first day but less than 1 cm on day 180.

Thermodynamic growth therefore does not add the same thickness each day. Old, undeformed Arctic ice approached an equilibrium near 3 m when winter growth balanced summer melt.

Snow increases the insulation

Snow conducts heat roughly seven times less effectively than sea ice. The lecture comparison treats 10 cm of snow as approximately equivalent to 67 cm of ice for insulation.

A 15 cm snow layer therefore provides about the same added insulation as 1 m of ice. Snow depth strongly affects winter ice growth even though the snow layer is thin.

Sea ice is not simply frozen seawater

The crystal lattice of ice excludes salt. As sea ice grows, most salt is rejected into the ocean while some remains as concentrated liquid brine in pockets and channels between ice crystals.

The rejected brine is cold, salty, and dense, so it sinks and can mix the upper ocean. This brine rejection contributes to deep-water ventilation and ocean circulation.

The brine remaining in the ice changes with temperature:

• cooling freezes more water out of each pocket, shrinking it;
• warming melts more ice around each pocket, enlarging it.

First-year ice in spring can contain about 5% brine by volume at −5 °C, with the fraction increasing rapidly as the ice warms.

Why brine matters

Strength

Brine pockets act like holes in the solid structure. Warm ice contains more liquid brine and is therefore weaker. Thickness alone is not a complete safety measure: ice type and temperature also matter.

Radar response

Liquid brine makes ice electrically lossy:

• salty first-year ice tends to produce surface scattering;
• older, fresher multi-year ice allows more penetration and volume scattering.

This difference helps satellites distinguish ice types.

Age and salinity

Summer meltwater flushes brine from the ice. Ice that begins near 8–10 g/kg can decline to about 1–2 g/kg after surviving several summers.

A first-year core often has a C-shaped salinity profile: high salinity near the top from early growth, lower salinity in the middle, and higher salinity near the still-growing bottom. Multi-year ice is fresher throughout, especially near the top.

The loss of old ice

The fraction of the March ice pack older than four years fell from about 33% in 1985 to about 3% in 2023. Over the same broad change, first-year ice grew from just over half of the winter pack to more than three quarters.

This matters because younger ice is generally thinner, saltier, weaker, and darker.

How the pack moves

Wind is the main driver of sea-ice motion. Ice typically drifts at about 2% of wind speed and, in the Northern Hemisphere, about 20–40° to the right of the wind because of Earth's rotation. Ocean currents also contribute.

Two large circulation patterns organize much of the drift:

• the Beaufort Gyre, a slow clockwise circulation on the Canadian side;
• the Transpolar Drift, which carries ice from Siberia across the Arctic toward the Atlantic.

Nansen's ship Fram entered the ice north of Siberia in 1893 and emerged near Svalbard three years later, demonstrating the cross-Arctic drift.

Fram Strait, between Greenland and Svalbard, is the main export route. On the order of one tenth of the ice cover exits there each year and melts in the Atlantic. Export removes ice even when it has not melted inside the Arctic.

Convergence: rafting and ridging

When floes are pushed together, the ice cannot compress smoothly:

• thin ice can slide over itself in rafting;
• thicker ice can break and pile into a pressure ridge.

Most of a pressure ridge lies below the water as a keel. Thermodynamic growth usually produces only a few metres of ice, while ridging can produce thicknesses of 10–30 m. The thickest ice is therefore built mechanically rather than frozen in place.

Divergence: leads and polynyas

When floes pull apart, a crack called a lead exposes ocean water. In winter this can place −1.8 °C water beside air near −30 °C.

Leads may cover only a few percent of the winter pack but can lose heat roughly one hundred times faster than nearby thick ice. They refreeze quickly into thin, salty, weak ice.

A polynya is an area of open water that remains open through winter because wind continually removes new ice or warm water rises from below. Continuous freezing and removal make it an “ice factory.” Brine rejection and mixing can also bring nutrients upward, supporting productive ecosystems.

Thermodynamics and dynamics together

Thermodynamics sets average growth and melt. Dynamics redistributes the ice:

Why the oldest ice is disappearing

Several mechanisms reinforce one another:

1. warmer summers melt more ice, so less survives to become multi-year ice;
2. thinner ice drifts faster, reaching Fram Strait before it can age;
3. fewer old, strong floes means fewer large pressure ridges;
4. younger ice is darker and ponds more readily, strengthening the albedo feedback;
5. younger, weaker ice breaks more easily, creating more leads and rapid heat exchange.

The changing Arctic ice cover is therefore not only an area-loss problem. It is also a change in age, thickness, salinity, strength, motion, and seasonal behaviour.