08/29/2006 10:15 AMOcean currents flip out
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All change: the ocean currents have
flipped directions in the past, and could
do so again.
© Getty Originals
If the ocean flips on
us we may be living
with that change for a
long, long time.
Richard Norris,
oceanographer
University of
California, San Diego
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Published online: 4 January 2006; | doi:10.1038/news060102- 5
Ocean currents flip out
Global warming millions of years ago put seas in a spin.
Mark Peplow
The circulation of the deep oceans reversed abruptly some 55 million years ago, according to a study
of fossilized sea creatures. This rings alarm bells about today's climate change, because the reversal
coincided with a period of global warming driven by greenhouse gases.
Deep ocean currents are intimately linked to our climate, and are driven by colder, saltier water at
the poles plummeting to the bottom of the ocean. Recent studies have suggested that the circulation
of the North Atlantic could be weakened by the influx of fresh water from melting polar ice. As this
watery conveyor belt also brings warm water northwards from the tropics, some scientists fear that a
major shift in the circulation could plunge western Europe into a deep freeze.
Although computer models can predict how ocean circulation reacts to global warming, many
researchers prefer to look for similar situations in the past to see how Earth responded. One useful
period to look at is the Palaeocene/Eocene Thermal Maximum (PETM) about 55 million years ago,
when the average temperature of the oceans shot up by 7 or 8 °C.
Flavia Nunes and Richard Norris, oceanographers at the University of California, San Diego, have now
worked out how ocean currents changed during the PETM, they report in Nature1. They have read
the chemical records held in the shells of tiny fossilized deep- sea creatures called foraminifera.
Carbon capture
Using samples from the Integrated Ocean Drilling Program, Norris and Nunes studied the relative amounts of two isotopes of carbon in the
shells. This acts as a snapshot of the carbon isotope ratios in the deep water that surrounded the creatures.
Scientists already knew that as a water mass travels along the deep section of the ocean's conveyor
belt, it becomes enriched in carbon- 12 compared with carbon-13. This is because it has more time to
accumulate organic matter falling from dead creatures in surface waters, and these creatures tend to use
more carbon- 12 than carbon- 13 in building their tissue.
Comparing the composition of foraminifera shells at different locations throughout the Pacific and Atlantic
Oceans, the scientists found that a gradient in the ratio of carbon isotopes revealed which way the
oceans' conveyor belts were turning during the PETM. They found that although the Atlantic's deep -water
flow normally travels from south to north, it ran in the opposite direction during the period of global
warming. Similar flips were seen in other oceans around the world.
Perhaps most remarkable is how abrupt the changes were. "The switch in circulation took just a few
thousand years," says Norris. "But then it took 100,000 years to revert."
"So if the ocean flips on us we may be living with that change for a long, long time," he adds. "It's sort of frightening."
Measurements have recently shown that the Atlantic current is lessening (see 'Atlantic currents show signs of weakening'), but researchers
are divided about whether modern climate change will be enough to force the current to stop or flip.
Old waters run deep
The research confirms computer models of 55-million -year -old ocean currents made by Karen Bice, an oceanographer at Woods Hole
Oceanographic Institution, Massachusetts and her colleagues2.
Although scientists have found evidence of similar links between ocean current and climate in the last 200,000 years, Bice says that the
PETM is a much better analogue for the climate change we see today, because it occurred in a world that was warming gradually in
