Mediterranean Sea Getting Saltier, Hotter
Oceanographic stations were visited four times a year to monitor the temperature, salinity and biochemical variables, and check changes in the Mediterranean.
Credit: Manuel Vargas-Yáñez

The Western Mediterranean Sea is heating up and getting saltier, a new study finds.

Each year the temperature of the deep layer of the Western Mediterranean increases by 0.0036 degrees Fahrenheit (0.002 degrees Celsius), and its salt levels increase by 0.001 units of salinity, researchers monitoring the sea found. The change is consistent with the expected effects of global warming.

These changes may sound like small beans, but they have been building up at a faster pace since the 1990s, the study, detailed in the April 1 edition of the Journal of Geophysical Research, suggests.

The results show a consistent trend, "but to confirm this accelerating trend, we need to monitor it over the years to come," said study author Manuel Vargas-Yáñez of the Spanish Institute of Oceanography.

The researchers analyzed the temperature and salt levels of the three layers of the Mediterranean Sea: the upper layer (from the surface to 656 feet, or 200 meters, deep with water that enters from the Atlantic); the middle layer (from 656 to 1,968 feet, or 200 to 600 meters, deep with water from the eastern Mediterranean that enters the western basin via the Strait of Sicily); and the deep layer (from 1,968 to the sea bed, or 600 meters and deeper, with water from the western Mediterranean).

"These layers, especially the deep one, take up a huge volume, and raising its temperature each year by one-thousandth of a degree requires an enormous amount of heat," Vargas-Yáñez said. 

The team has also observed an increase in the salt level and the temperature of the middle layer of the sea. This has not been clearly observed in the upper layer, "but it can be deduced from the heating of the deep water and from studies done by other teams and our current research projects," Vargas-Yáñez said.

An increase in ocean salinity suggests an increase in the net evaporation of the water — the difference between evaporation and precipitation. When evaporated water leaving the ocean is greater than water entering the ocean as precipitation that means overall less water staying in the sea with the same amount of salt. So the sea gets saltier.

A greater evaporation rate can be due to a warmer ocean, but other factors can come into play, such as the humidity and temperature of the atmosphere, said oceanographer Ruth Curry of Woods Hole Oceanographic Institution in Massachusetts.

Also, the Mediterranean is a closed space, so certain land-use issues such as damming rivers could change how much freshwater is flowing into the ocean and diluting the saltwater. 

Scientists are seeing a change in the evaporation-precipitation patterns of the ocean, which is consistent with what's expected under greenhouse gas-driven warming, Curry told LiveScience.

The top layer of the entire ocean has warmed significantly over the past 16 years, according to another study detailed in the May 20 issue of the journal Nature. From 1993 to 2008, the top 2,300 feet (700 meters) of the world's oceans warmed 0.64 watts per square meter. 

That's equal to adding the energy from 100 million atomic bombs to the ocean each year during the 16-year period, said John Lyman of the University of Hawaii.