section_path: Physical science/Seawater/Salinity
section_name: Salinity
paragraph_text: Salinity
  
  A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand (‰ or per mil), and the open ocean has about 35 grams (1.2 oz) solids per litre, a salinity of 35 ‰. The Mediterranean Sea is slightly higher at 38 ‰, while the salinity of the northern Red Sea can reach 41‰. In contrast, some landlocked hypersaline lakes have a much higher salinity, for example, the Dead Sea has 300 grams (11 oz) dissolved solids per litre (300 ‰).
  
  While the constituents of table salt (sodium and chloride) make up about 85 percent of the solids in solution, there are also other metal ions such as magnesium and calcium, and negative ions including sulphate, carbonate, and bromide. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout the world's oceans. Seawater is too saline for humans to drink safely, as the kidneys cannot excrete urine as salty as seawater.
  
  Although the amount of salt in the ocean remains relatively constant within the scale of millions of years, various factors affect the salinity of a body of water. Evaporation and by-product of ice formation (known as "brine rejection") increase salinity, whereas precipitation, sea ice melt, and runoff from land reduce it. The Baltic Sea, for example, has many rivers flowing into it, and thus the sea could be considered as brackish. Meanwhile, the Red Sea is very salty due to its high evaporation rate.