Have you ever read the folktale “Why the Sea is Salt?” It’s a cute little story about a magic handmill spirited away from Satan himself by an unfortunate thief. While hiding behind the door to hell, the thief sees the devil order the handmill to produce all sorts of lavish food and objects. Once he gets his hands on the magic handmill, the thief escapes by boarding a boat and setting sail. The devil had directed the handmill by simply saying “handmill grind…,” so the thief proceeds to tell the magic handmill to grind gold. Unfortunately for the thief, only Satan can make the handmill grind luxuries. For everyone else, the magic handmill just grinds salt. Once the handmill starts grinding salt, the thief realizes he’s forgotten the words the devil used to get the handmill to stop. The salt from the handmill overloads the boat and it sinks to the bottom of the ocean where the handmill keeps on grinding out loads of salt even today.
It’s a nice legend, but in reality we all know the main mechanism causing the sea to be salty is the erosion of minerals from rocks and soils. The eroded minerals are flushed down rivers and streams and deposited in the oceans. There’s also some contribution of minerals to the oceans from volcanic eruptions both above and below sea level. (Yes there are volcanic eruptions from the ocean floor.) The minerals in the ocean are concentrated by evaporation of freshwater from the ocean’s surface. Some of these minerals like calcium and carbonate are used by organisms to form shells. Other minerals are used as nutrients. Only the minerals which are not actively used by organic life in vast amounts are left behind. As you might guess the largest component of the dissolved solids left in the oceans are chloride and sodium, the components of salt. Useful for seasoning French fries but not much else.
With all you’ve heard in the media about the melting glaciers and the polar ice sheets, you would think the ocean was getting less salty, right? After all, the freshwater from all this ice is going into the ocean and scientists say that at least half of sea level rise is being caused by melting ice. Surely this addition of freshwater is having an impact. In reality though the contribution of melting glaciers to salinity changes in the oceans is not as great as you might think, because there are a whole lot of other things going on between the oceans and the atmosphere which are far more influential on how salty the water is. The International Panel on Climate Change (IPCC) in their 2013 report “The Physical Science Basis” states that the oceans are getting saltier in some locations and less salty in others. For example the Atlantic Ocean is becoming saltier and the Pacific Ocean is becoming fresher. Interestingly enough, the Atlantic has always been saltier than the Pacific. Why? It is that old hydrologic cycle of evaporation and precipitation. It just rains more in the Pacific. Low salinity is found in areas where precipitation exceeds evaporation and runoff from rivers and streams. High salinity is found where evaporation exceeds precipitation and runoff.
Increases in atmospheric temperature as part of climate change increases this overall hydrologic cycle. Think about it. Warmer air holds more moisture than dry air. The IPCC cites that the atmosphere can actually hold about 4 % more water vapor for each degree Fahrenheit the temperature increases. Scientists have been recording atmospheric vapor since the 1970s and the data show increases in atmospheric water vapor. So essentially, it is raining more in warm areas of the Pacific Ocean and raining less in mid latitude areas of the Atlantic Ocean. The IPCC report notes that surface salinity of the tropical Pacific Ocean has declined by 0.1 – 0.3 parts per thousand over 50 years in the western equatorial regions and by up to 0.6 – 0.75 in the intertropical and southern Pacific. The subtropical Atlantic is saltier by 0.1 – 0.3. Polar regions also get more precipitation in the form of both ice and snow, so the ocean water in the arctic regions is becoming fresher as well, although I didn’t see an exact number in the IPCC report.
Interesting right? But exactly how vital is the role of salinity in the oceans? Colder water is denser than warmer water. Denser water sinks. In the arctic, cold fresh water sinks and spreads toward the equator, supplying the deeper ocean with freshwater and reducing salinity. It is a process that impacts the oceans circulation and ocean circulation impacts our climate. Fresh water is less dense than salty water. Will the freshening of the waters in the arctic impact the movement of polar waters in some way? Will increasingly dense salty water in the subtropics result in changes to the composition of deeper water? Now we are entering the realm of speculation. Other scientists don’t hesitate to speculate on possibilities. No less an august organization as the National Science Foundation speculates that based on research they funded back in the early first decade of the 21st century fresher water at the northern pole could impact ocean circulation in the North Atlantic, in particular the Gulf Stream, and consequently weather in the Northern Hemisphere.
The impact of the Gulf Stream on North Atlantic climate appears to be as much of a myth as the thief’s magic handmill, according to the Lamont Doherty Earth Observatory at Columbia University. Here is a link to an article on why the Observatory says it is a myth:
http://www.ldeo.columbia.edu/res/div/ocp/gs/.
So I will leave speculation about the impacts of changes in ocean salinity to earth’s climate for someone else.