Ocean Salinity

Why is the sea salty, and why is some seawater saltier than the rest? The answer is salinity, the amount of dissolved salt in the water. Salinity shapes the ocean's chemistry, its life, and even its great deep currents, making it one of the most important properties of seawater.

Before asking how salty the sea is, fix the bigger picture: where Earth's water actually sits. About 97 per cent of all water on Earth is salt water in the oceans. Fresh water is therefore rare, only about 3 per cent of the total, and most of it is locked away where we cannot easily reach it.

The fresh-water store itself is very unevenly divided:

• Polar ice caps and glaciers: hold roughly 69 per cent of all fresh water, the single largest store.
• Groundwater: holds about 30 per cent of fresh water, the second largest store and far more than all surface water.
• Rivers, lakes and other surface water: together hold only a tiny fraction, well under 1 per cent of fresh water.

Two comparisons are worth memorising. Ice caps and glaciers hold more water than groundwater. Groundwater in turn holds far more water than all rivers and lakes combined. Because so much fresh water is frozen or buried, the fresh water readily available for human use is less than 1 per cent of all the water on Earth. Note the common trap figure: ice caps and glaciers hold about 69 per cent of fresh water, not 95 per cent.

Salinity is the total content of dissolved salts in seawater. It is measured as the amount of salt (in grams) dissolved in 1,000 grams of seawater, and expressed in parts per thousand (ppt or ‰).

• The average salinity of the ocean is about 35 ppt (that is, 35 grams of salt in every kilogram of water).
• The most abundant salt is common salt, sodium chloride (NaCl), which makes up over three-quarters of the dissolved salts.
• Lines joining places of equal salinity on a map are called isohalines.

Water with salinity above about 24.7 ppt is classed as fully saline (brackish water lies below this).

Salinity is not the same everywhere. It is raised where water is lost and salt is left behind, and lowered where fresh water is added. The main factors:

• Evaporation: high evaporation in hot, dry regions concentrates the salt, raising salinity (the highest of the open seas is in the subtropics).
• Precipitation and river inflow: heavy rain and large rivers add fresh water, lowering salinity (low near the equator and river mouths).
• Ice: freezing leaves salt behind, raising salinity; melting adds fresh water, lowering it.
• Ocean currents: mixing redistributes salinity.
• Latitude and depth: surface salinity is highest in the subtropics, lower at the equator and poles.

Enclosed seas show extremes: Lake Van, the Dead Sea and the Great Salt Lake are among the most saline water bodies on Earth.

Salinity is closely tied to density (how tightly water's mass is packed). The rule is simple:

• higher salinity → denser water, and
• lower temperature → denser water.

Dense water sinks below lighter water. So cold, salty surface water (for example near the poles) sinks to the deep ocean and flows along the bottom. This density difference is the engine of the global deep ocean currents (the great "conveyor belt"). Where salinity rises sharply with depth, the zone is called the halocline.