||May/June 2004 issue||
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
|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.
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.