From the dawn of time, people have suspected powerful forces lurking
deep in the oceans from the Greeks' fearsome sea-god Neptune
to John Wyndham's submarine aliens in his 1950s novel The Kraken
Wakes. But science is once again going one better than science
fiction. Researchers are discovering that hidden 'rivers' run through
the oceans, and these powerful currents hold the destiny of our
The beneficial aspects of ocean currents have long been known.
For countries on the east side of the Atlantic, winters are a balmy
holiday compared with the same latitudes on the west: the frigid
coasts of Newfoundland and Labrador. It's a reminder that "weather"
is not just a matter of the Sun's heat affecting the Earth's atmosphere.
The world's interconnected oceans can store up solar heat in one
part of the globe in one season, and invisible rivers in the ocean
can transport the warmth thousands of kilometres to another part
of the globe and deliver it in another season.
In the case of the North Atlantic, heat is carried northward and
eastward by the Gulf Stream. This current warms the coast evenly
through the year, in winter as well as summer. Averaged over a year,
the Gulf Stream provides Western Europe with a third as much warmth
as the Sun does.
This ocean warmth is so important to Europe that climatologists
are seriously concerned about the stability of the Gulf Stream.
If it switched off, Europe would be plunged into a mini-Ice Age.
And current studies suggest that the unseen river in the North Atlantic
is dangerously fickle.
The focus of today's worries is the problem of global warming
- the way that human activities are changing the climate, as the
world gets warmer through the build-up of so-called greenhouse gases,
such as carbon dioxide. Climatologists think that global warming
may put the brakes on the Gulf Stream. While the rest of the world
comes to swelter in greenhouse conditions, Europe would freeze!
This concern is based on a new understanding of how the great
ocean currents are all interconnected. The Gulf Stream is part of
a giant pattern of moving water that stretches right around the
The Ocean Conveyor Belt
The warm water that is so important to Europe actually comes from
the other side of the world, in the Pacific Ocean. This tepid stream,
flowing unseen through the oceans, is the longest river in the world.
It flows along the surface, both because it is warm (and warm water,
like warm air, rises) and because it is less salty and so
less dense - than the deeper water. The warm waters travel westward
from the central Pacific, past the north coast of Australia and
round the southern tip of Africa before moving up into the Atlantic.
state of the 'ocean conveyor belt' that transfers warm, less
salty water from the Pacific to the Atlantic as a shallow current,
and returns cold, more salty water from the Atlantic to the
Pacific as a deep current flowing further south. This flow is
threatened by melting ice in the Arctic, and disruptions off
the Antarctic coast.
- Pioneer Online Ltd.
This great ocean river becomes the Gulf Stream by the time it
heads up through the North Atlantic. But as the current surges past
Europe, its nature begins to change. It cools down significantly
as it gives up its precious heat to the European seaboard. And,
all along its long journey, the warmth of this invisible river has
encouraged water to evaporate from its surface layer, so it becomes
increasingly salty. Around the latitude of Iceland, the moving stream
becomes so dense that it sinks into the depths.
This stream now becomes a cold river, flowing back along the ocean
floor. Rounding the south of Africa and Australia, it returns to
the Pacific, where it is pushed to the surface and warms to complete
the cycle. The whole effect is like a conveyor belt bringing Pacific
warmth to the North Atlantic.
The ocean conveyor belt has run more or less smoothly since the
end of the last Ice Age. But global warming may now throw a spanner
into its workings. The planet is undoubtedly warming up, even if
people still argue about how much of this is due to human activities,
and the extra heat is melting ice in the Arctic Ocean. The ice turns
into fresh water, which flows into the salty North Atlantic.
The danger is that this fresh water might dilute the salty current
of the Gulf Stream so much that it stops sinking down into the ocean
depths near Iceland. If the Gulf Stream does stop, there will be
nothing pushing the deep cold river at the bottom of the North Atlantic.
As the Atlantic portion of the ocean conveyor belt grinds to a halt,
then Europe could indeed freeze ironically, as a direct result
of global warming.
Global Warming Kicks In
If that weren't enough, the ocean current specialists have just
found something else to worry about. Global warming may interfere
with not just the North Atlantic currents, but may disrupt the entire
system of ocean currents affecting the entire world's weather.
Leading the new investigation is Wallace Broecker and his colleagues
at Columbia University, New York. They start with evidence that
the great ocean currents tend to make the climate in the Earth's
two hemispheres change in opposite directions: when the north is
hot, the south is colder and vice versa. In Europe, most
of the past millennium (from about 1300 to 1800) was so cold that
it has been dubbed 'The Little Ice Age'. Some researchers have blamed
the Little Ice Age on a slowdown in the ocean current system that
includes the Gulf Stream. But Broecker takes the analysis a stage
further. At the same time, he says, the southern currents were stronger,
and - in a mirror image of the Little Ice Age - the Antarctic region
warmed up, perhaps by as much as 3 oC.
The link involves the ocean rivers that flow along the coast of
Antarctica the deep cold currents flowing back from the South
Atlantic, south of Africa and Australia. Cold, salty water off the
Antarctic coast sinks down into the depths, adding its push to the
interlinked system of ocean currents.
In his latest research, Broecker and his colleagues at Columbia University,
New York, have just announced that the surface water near Antarctica
is sinking at only a third of the rate it was a century ago.
The perpetual chill of Antarctica helps to drive the ocean conveyor
belt, as cold salty water sinks to join the deep current flowing
from the Atlantic to the Pacific. But latest measurements show
a slow-down in the sinking Antarctic waters.
- Sean Leslie
This has really set the cat among the climatological pigeons.
If Broecker is right, the slowdown in the Antarctic deep current
starting about a century ago ought to be doing two
things. It should make the Antarctic colder, and also encourage
the Gulf Stream to make Europe warmer.
The latter result can explain one baffling aspect of global warming.
Temperature records - mainly from the northern hemisphere
show that the present phase of global warming actually began in
the 1880s, before the man-made greenhouse effect got going. It accelerated
in the 1970s, when greenhouse gases started to fill the atmosphere.
So what we are seeing now in the northern hemisphere, anyway
- could be a combination of the recent man-made greenhouse effect
with a longer-term natural warming trend, powered by changes in
the ocean currents.
In Europe, that would mean the climate getting hotter more quickly
than the forecasters have predicted. But in the region around Antarctica,
the situation is by no means as clear-cut. Here, Broecker's analysis
means that man-made warming is fighting against a natural cooling
trend - and nobody knows what the consequences of that might be.
But one thing is for certain. As we enter the 21st century, the
links between ocean currents and weather are moving from academic
analysis into a matter of life and death around the globe.
Copyright © FirstScience.com
Dr John Gribbin is a best-selling author of popular
books on science, ranging from climatology to cosmology and quantum
physics. Formerly a researcher at Cambridge, he is now Visiting Fellow
in Astronomy at the University of Sussex. Gribbin is Physics Consultant
to New Scientist magazine. His books include In Search of
Schrodinger's Cat, The Hole in the Sky, Stephen Hawking: A Life in
Science (with Michael White), and The Little Book of Science