We know that average sea levels have risen over the past century, and that global warming is to blame. But what is climate change doing to the saltiness, or salinity, of our oceans? This is an important question because big shifts in salinity could be a warning that more severe droughts and floods are on their way, or even that global warming is speeding up.
New research coming out of the United Kingdom (U.K.) suggests that the amount of salt in seawater is varying in direct response to man-made climate change. Working with colleagues to sift through data collected over the past 50 years, Peter Stott, head of climate monitoring and attribution at the Met Office in Exeter, England, studied whether or not human-induced climate change could be responsible for rises in salinity that have been recorded in the subtropical regions of the Atlantic Ocean, areas at latitudes immediately north and south of Earthâ€™s tropics.
By comparing the data to climate models that correct for naturally occurring salinity variations in the ocean, Stott has found that man-made global warming -- over and above any possible natural sources of global warming, such as carbon dioxide given off by volcanoes or increases in the heat output of the sun -- may be responsible for making parts of the North Atlantic Ocean more salty.
Salinity levels are important for two reasons. First, along with temperature, they directly affect seawater density (salty water is denser than freshwater) and therefore the circulation of ocean currents from the tropics to the poles. These currents control how heat is carried within the oceans and ultimately regulate the worldâ€™s climate. Second, sea surface salinity is intimately linked to Earthâ€™s overall water cycle and to how much freshwater leaves and enters the oceans through evaporation and precipitation. Measuring salinity is one way to probe the water cycle in greater detail.
Illustration shows the Aquarius/SAC-D spacecraft, scheduled for launch in May 2010. It will be the first NASA instrument to measure sea salinity from space.