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magazine / mj04

May/June 2004 issue


À LA CARTE
 

Salt of the Earth
Will the rapidly shifting composition of the Atlantic Ocean lead to more serious climate changes?
By Steven Fick and Jessa Sinclair

Salinity Ocean Conveyor
Cartography:Steven Fick /Canadian Geographic; Source: R. Curry, J. Cook. Woods Hole Oceanic Institution

Oceanographers monitoring the planet’s hydrological cycle believe the delicate equilibrium that keeps the global water system in check is shifting. Armed with 40 years of data, they have recently discovered troubling changes in salt levels in the Atlantic Ocean.

The hydrological system constantly recycles water molecules, transporting them from the surface to the atmosphere and back again. Ocean water in the Atlantic evaporates into the warm air near the equator, leaving behind a salty surface. That water vapour is then blown to colder climes by rising air currents, where it falls as rain or snow, freshening polar waters. Powerful ocean currents mix it all, keeping the tropical ocean from getting too salty and the poles from getting too fresh.


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A recent study of salinity by Canadian, British and American scientists shows that water near the equator is becoming saltier and warmer than normal, while the sub-polar North Atlantic is becoming cooler and fresher (below). With evidence of similar changes in the Mediterranean Sea and the Pacific and Indian oceans, the findings suggest that the hydrological cycle is speeding up, moving faster than the ocean’s compensatory mixing process.

The trend appears to have accelerated since 1990. Researchers suggest recent climate changes may be encouraging evaporation of low-latitude waters, which means more water vapour is moving to the poles.

A fresher North Atlantic could eventually mean large-scale disruption of the water-circulation system, which is known as the "ocean conveyor." The North Atlantic is one of the only places on Earth where large masses of cold, salty, and thus dense, water plunge to the ocean floor (above). The sinking action helps drive the ocean conveyor by drawing warm Gulf Stream waters north, which moderates European winters. Historically, the ocean conveyor has slowed, stopped and started again, causing dramatic climatic changes. One such shift occurred some 8,200 years ago, for example, resulting in widespread droughts in western North America, Asia and Africa.

Salinity globes

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