Melting of Antarctic Ice Sheets Is Making Southern Ocean Less Salty
Melting Antarctic ice sheets are measurably reducing the salinity of the Southern Ocean. The research, published Thursday in Nature, notes that more freshwater entering the ocean “is more important for changes in global climate than has been appreciated so far.”
Scientists have seen a reduction in salinity of the waters around Antarctica over the past decades. They haven’t understood why until now. The study—by researchers from ETH Zurich, the GEOMAR Helmholtz Centre for Ocean Research Kiel and the University of Hamburg—describes a process whereby sea ice forms each winter, releasing salt in the seawater, then drifts northward and melts in the summer, spreading fresh water over a wide area.
On average, seawater is about 3.5 percent salts, mainly sodium chloride. Salt water is denser than fresh water, and the freezing point of water goes down as the amount of dissolved salts goes up. If all the salt in the world’s oceans were removed and spread over the Earth, it would cover the planet in a 500-foot thick salt lick.
But salinity is not uniform across the world’s waters. The highest salt content is generally found in the center of ocean basins, away from continents and the fresh water flowing from their rivers every day. Equatorial waters, where evaporation is highest, also boast above-average salinity. But these are not constant throughout the year.
Ocean circulation is driven by surface winds, temperature and salinity. Cooler, high-salinity water sinks to great depths, while lower-salinity water, which is less dense, rises. Over a long period—about 1,000 years—the Global Conveyor Belt moves water around the oceans in response to temperate and salinity.
The maximum seasonal ice cover in the Southern Ocean now reaches further north than it did 30 years ago. The German and Swiss study explains, “This expansion is mainly due to a stronger transport of sea ice that has pushed the sea-ice edge further to the north.” The northernmost edge of the ice at maximum is at about 60 degrees south.
That may seem paradoxical in the face of a warming planet, but NASA explains: “Just as the temperatures in some regions of the planet are colder than average, even in our warming world, Antarctic sea ice has been increasing and bucking the overall trend of ice loss.”
The significance of the greater mass of Antarctic sea ice is that, as it melts each summer, the fresh water sinks to a depth of about 600 to 1,500 meters, a region known as the Antarctic Intermediate Water. It then spreads north to the equator, into the eastern Atlantic and as far as the Iberian Peninsula.
As the water in the upper layers of the ocean become fresher and less dense, they may block the heavier, saltier waters below from rising to the surface. This is a process known as stratification.
James Hansen, the former NASA scientist, pointed to stratification as a potential wild card in the climate disruption process. He thinks that it could accelerate the melting of the Greenland and Antarctic glaciers, and perhaps change current patterns in the Atlantic. Research published Aug. 23 in Nature found evidence that stratification is occurring in Prydz Bay in East Antarctica.
In announcing the German and Swiss study, the researchers noted, “So far, the Southern Ocean has acted as a climate regulator and carbon sink: climate models show that this ocean has absorbed around three quarters of the excess heat. The Southern Ocean has also taken up around half the total amount of anthropogenic carbon dioxide absorbed by the world’s oceans.”
Further studies will test these theories, and new satellites will help researchers better observe the ocean and sea ice. The Southern Ocean will get more attention in the years to come.