— The global ocean "conveyor belt" draws cold, subarctic waters south and brings warm waters up to Europe
The surface waters of the North Atlantic are getting saltier, suggests a new study of records spanning over 50 years. And this might actually be good news for the effects of climate change on global ocean currents in the short-term, say the study's researchers.
This is because saltier waters in the upper levels of the North Atlantic ocean may mean that the global ocean conveyor belt the vital piece of planetary plumbing which some scientists fear may slow down because of global warming will remain stable.
The global ocean conveyor belt is the crucial circulation of ocean waters around the Earth. It helps drive the Gulf Stream and keeps Europe warm. The density of waters which drives the flow of ocean currents is dependent on temperature and salinity, so any change in saltiness may have an impact.
Tim Boyer of the US National Oceanographic Data Center and colleagues compiled salinity data gathered by fisheries, navy and research ships travelling across the North Atlantic between 1955 and 2006. They found that during this time, the layer of water that makes up the top 400 metres has gradually become saltier.
The seawater is probably becoming saltier due to global warming, Boyer says. "We know that upper ocean is warming in the North Atlantic, so it stands to reason that there should be more evaporation, making waters more salty," he says.
The global ocean conveyor belt is in part driven by salty and relatively dense subpolar waters sinking and flowing south to the equator.
So when a huge "pulse" of less dense freshwater was found to have been dumped into the sub-polar waters of the North Atlantic in the mid-1960s, researchers speculated the sub-polar waters might just stay floating where they were and cause circulation to stagnate.
The freshwater pulse probably came from a combination of increased rainfall and melting ice, as well as big chunks of ice suddenly pushing through the Fram Straight into the Atlantic.
When in their recent study Boyer and his colleagues zoomed in on the subarctic Atlantic, they found that the waters there became much less salty in the 1960s, as expected. But since the 1990s, they have been getting saltier again, and are now about as salty as they were in the 1970s.
Backing up this finding, when the team looked at the salinity of deeper waters, those flowing more than 1300 metres beneath the surface, they found that these have been getting less salty since the late 1980s. They see this as a sign that the pulse of freshwater has been slowly making its way south.
It takes roughly 10 to 15 years for subpolar water to move away from the Arctic and down to the equator.
While Boyer admits that it looks from his study like there is currently less danger of the ocean conveyor belt shutting down, this could be short-lived relief only.
"Things change rapidly," he told New Scientist. "Just a few years ago, when we had only seen the increase in freshwater, there was speculation that it might shut down. Now we've seen that things have stabilised. But in five years things could change again."
Another team of researchers recently deployed a network of sensors that will be keeping a close eye on the progress of the North Atlantic Drift, the segment of the global ocean conveyor belt which pushes warm water northward from the Equator (see Fickle ocean current foils climate modellers).
Stuart Cunningham of Southampton University, UK, who led that study, says it is too early to say for sure but he believes there is a chance the North Atlantic Drift could still be slowing down in spite of Boyer's findings.
He points out that the conveyor belt depends on temperature as well as salinity. A reassuring signal from one of those factors does not necessarily mean problems with the other will be overcome.
Journal reference: Geophysical Research Letters (DOI: 10.1029/2007GL030126)
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