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The Once and Future Circulation of the OceanClues in seafloor sediments link ocean shifts and climate changes |
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Enlarge Image Today (top), the oceans’ overturning circulation carries a tremendous amount of heat northward, warming the North Atlantic region. It also generates a huge volume of cold, salty water called North Atlantic Deep Water—a great mass of water that flows southward, filling up the deep Atlantic Ocean basin and eventually spreading into the deep Indian and Pacific Oceans.
Paleoceanographers have found evidence for very different patterns of ocean circulation in the past. About 20,000 years ago (bottom), waters in the North Atlantic sank only to intermediate depths and spread to a far lesser extent. When that occurred, the climate in the North Atlantic region was generally cold and more variable. (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution)
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Enlarge Image A schematic diagram of the global ocean circulation pathways, sometimes referred to as the "Ocean Conveyor." (W. Broecker, modified by E. Maier-Reimer)
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| By Jerry McManus, Associate Scientist
and Delia Oppo, Senior Scientist
Geology and Geophysics Department
Woods Hole Oceanographic InstitutionSource: Oceanus Magazine
The short history of modern oceanographic observations—less than a
century’s worth, really—doesn’t give us a long track record to evaluate
how the ocean’s circulation has operated and changed in the past. Nor
does it give us enough data to assess how changes in the ocean shifted
Earth’s climate in the past, or how they could cause climate changes in
the future.
To examine the ocean’s role in climate change, scientists use computer
models that simulate the workings of Earth’s ocean and climate. The
models point to a possible “switch” that speeds up or slows down a
global system of ocean currents that transports heat and warms the
North Atlantic region and surrounding continents. It seems likely that
any changes in the rate or strength of this circulation would have
substantial impacts on climate.
Did changes in ocean circulation play a role in melting the vast ice
sheets that covered North America and Europe during the last ice age?
Could global warming cause ocean circulation changes that lead to
dramatic climate changes in the future?
The overturning ocean
In the North Atlantic today, cold surface waters sink to the abyss, and
salty, warm surface and near-surface currents, including the Gulf
Stream, flow northward from the tropics to replace them. When the warm
waters reach high latitudes, they release heat to the atmosphere and
warm the region. The waters become colder and less buoyant. They sink
to continue this grand ocean overturning, which is approximately equal
to 20 times the combined flow of all the world’s rivers.
This overturning circulation carries a tremendous amount of heat
northward, while also generating a huge volume of cold, salty
water—which we call North Atlantic Deep Water. After descending, this
great mass of water flows southward, filling up the deep Atlantic Ocean
basin and eventually spreading into the deep Indian and Pacific Oceans.
Computer climate models show that if fresh water is added to sensitive
locales in the North Atlantic Ocean, it would increase surface water
buoyancy enough to brake the overturning circulation, and less warm
water and heat would flow northward. Nothing like the changes predicted
by computer models has ever been seen during the brief interval of
modern studies of the sea. So to evaluate the possibilities, scientists
must extend oceanography back in time, long before recorded human
history, to reveal the full range of possible ocean circulation changes
and their impacts on climate.
Sticky isotopes
We and other paleoceanographers have found evidence for very different
patterns of ocean circulation in the past. This evidence come from
clues that are preserved in sediments deposited on the seafloor over
tens of thousands of years. The sediments contain fossilized shells of
foraminifera—ocean-bottom-dwelling, single-celled organisms the size of
sand grains. The shells contain differences in trace elements and
carbon isotopes, which reflect different seawater conditions at the
times when the foraminifera were alive and growing.
The foraminifera analyses showed us where and when different types of
water masses formed in the past. Water masses similar to today’s North
Atlantic Deep Water seemed to have intensified and diminished in the
past—sometimes sinking deeply and spreading to fill the North Atlantic
basin and beyond, and sometimes sinking only to intermediate depths and
spreading to a far lesser extent.
The carbon isotopes and trace elements, however, don’t provide
information on how fast or how vigorously these different water masses
circulated. To investigate that, we used a different set of clues
preserved in deep-sea mud, based on the “clock” inherent in the
radioactive decay of naturally occurring uranium in seawater to its
daughter isotopes, protactinium and thorium.
Both chemically adhere to particles in the ocean that sink to the
seafloor. Thorium is inherently “stickier;” however, so it is removed from
seawater within decades, while protactinium remains in seawater for
centuries.
As a result, about half of the protactinium produced in North Atlantic
water today lasts long enough in the water column to be exported into
the Southern Ocean by the ocean’s overturning system. At times when the
rate of overturning circulation slows, the proportion of protactinium
buried in North Atlantic sediments increases. Thus, the ratio of
protactinium-to-thorium levels in the sediments tells the story of past
changes in how fast North Atlantic Deep Water was produced and exported
by the overturning circulation.
Disrupting circulation with fresher water
When we compared ocean circulation records to records of climate since
the peak of the last ice age 20,000 years ago, we confirmed that the
rate of ocean overturning, with its northward heat transport, has a
critical influence on climate. When North Atlantic Deep Water filled
the deep ocean and spread southward vigorously, the climate of the
North Atlantic region was warm and generally stable. When North
Atlantic Deep Water filled less of the Atlantic and did not spread
southward extensively, the climate was generally cold and more variable.
Roughly 14,500 years ago, Earth’s surface was warming, and Northern
Hemisphere ice sheets had melted considerably. Paralleling these
changes on land were changes in the ocean: North Atlantic Deep Water
had already begun filling more of the deep Atlantic basin and was
spreading vigorously southward, balanced by a greater flow of warm
water northward.
About 12,700 years ago, however, this warming trend was abruptly
interrupted. Average air temperatures dropped by at least 5°C (9°F)
within a few decades and stayed that way for a 1,000-year cold snap.
Simultaneously, the presence and vigor of North Atlantic Deep Water
dwindled.
Some researchers suspect that ice dams melted in front of vast lakes of
glacial meltwater, releasing a flood of fresh water to the ocean.
During this period, more icebergs drifted and melted into the North
Atlantic, perhaps providing an alternative or additional source of
fresh water to curtail the overturning circulation.
What does the future hold?
Now we are investigating if there is a similar link between overturning
circulation and more modest climate fluctuations, such as the Medieval
Warm Period and the Little Ice Age, during the last 10,000 years or so
when the Northern Hemisphere was free of large ice sheets. Maybe the
lack of ice sheets has reduced the likelihood of glacial outbursts or
iceberg discharges, but ample potential freshwater sources still exist,
especially in a greenhouse world.
Global warming raises the potential of unlocking large amounts of fresh
water now frozen in the vast Greenland ice sheet and in Arctic Ocean
sea ice. Warming air temperatures could also increase evaporation in
low latitudes and transport freshwater vapor toward high latitudes,
where it falls as rain or snow into the oceans.
Could these factors tip the freshwater balance in the North Atlantic in the future? The answers may lie buried on the seafloor.
Funding for this research came from
the National Science Foundation, the WHOI Ocean and Climate Institute,
and the Comer Science and Education Foundation.
Originally published: November 16, 2006
Last updated: September 3, 2009 |
