Chapter 1 · Try it yourself

Brine Freezer

Seawater does not freeze at 0 °C. The salt gets in the way, so polar seawater (salinity about 34) waits until roughly −1.9 °C. And when it finally freezes, the ice mostly refuses to take the salt with it. Freeze some ocean and follow the salt.

1. Freeze the sea

Side view of polar seawater. Air −20 °C Day 0 Wind pushes new ice away Dense plume sinks Surface water (top 50 m) Deeper shelf water Seafloor, about 500 m down
  • Salt (more dots = saltier)
  • Brine pocket trapped in ice
  • Ice crystals (fresh water)

Not to scale: the ice is drawn about 50 times thicker than it would be next to 500 m of water.

  • Ice made so far0 m
  • Salinity of the ice–
  • Salinity, surface water34.00
  • Freezing point−1.87 °C
  • Density, surface water27.37
  • Density, water below27.78

How close the surface water is to sinking

2. White ice, dark water

Sea ice also changes how much sunlight the polar ocean keeps. The share of sunlight a surface bounces back is its albedo. Pick a surface.

Sunlight hitting a surface. Sun

Albedo ≈ 0.06

  • Incoming sunlight
  • Sunlight bounced back up
  • Sunlight absorbed (warms the surface)

Simplified model. Freezing point and density (σt, kg/m³ minus 1,000) from the UNESCO seawater equations at surface pressure. Ice growth uses Stefan’s law for bare ice with no snow (real ice grows more slowly under snow); ice salinity follows Cox and Weeks (1974), about 12 in thin new ice falling to about 5–6 in ice a meter thick, versus about 34 in the water. The top 50 m of water starts at salinity 34.0 above deeper shelf water at 34.5; polynya ice production (about 2.5 mm of ice per day for each degree of cold) is illustrative. Albedo values from NSIDC: open ocean about 0.06, bare sea ice about 0.5–0.7, snow-covered ice up to about 0.9.