The ocean “conveyor belt” may be showing signs of slowing down, according
to provocative research published in December. New measurements of temperature
and salinity in the North Atlantic indicate that changes are occurring in this
segment of the ocean’s circulation that could eventually affect Earth’s
climate.
One way to throw the planet into a cooler state is to flip a switch on the ocean
conveyor belt, which carries warm Atlantic water to northern latitudes, where
it loses its heat, sinks and eventually sends cold deepwater south toward the
equator, keeping Europe’s climate milder than it would be otherwise (see
Geotimes, November 2005). Changes in global climate
and salinity have the potential to affect this transport system of heat in the
oceans, with subsequent consequences for the planet’s climate system, particularly
in the northern latitudes.
| The current
situation “is not The Day After Tomorrow.” Andreas Schmittner, Oregon State University |
One example that illustrates some of the potential feedbacks are so-called
Heinrich events that occurred during the last ice age, when the rapid addition
of thousands of icebergs from the North American ice sheet changed salinity
and slowed ocean circulation. As recorded in the Greenland ice cores, local
temperatures dropped as much as 10 degrees Celsius over timescales of a century.
The most recent such event occurred 16,000 years ago (though the Younger Dryas
event 12,000 years ago may also have been a Heinrich event). Any signs of slowing
today could indicate that some relatively abrupt changes could be in store for
the planet’s climate system.
Recent measurements and analyses of the North Atlantic made by Harry Bryden
and co-workers at the National Oceanography Center in Southampton, United Kingdom,
show some distinct changes over the past five decades that could indicate such
a slowdown could be taking place right now. The researchers collected temperature
and salinity data on cruises following transects across the North Atlantic in
2004, compared with transects made in 1957, 1981, 1992 and 1998 (two of which
were made along a different sightline, from Africa to the Bahamas).
From their data, they calculated that several distinct segments of cooler water
at varying depths showed dramatic decreases and increases in their movement
south, while the volume of warm water moving north remained about the same.
Taken altogether and compared to the past cross-Atlantic transects, circulation
weakened by 30 percent over the half-century covered by the data, according
to Bryden’s team’s new modeling results, published Dec. 1 in Nature.
“The general buzz is that they did a very good job given what they had,”
says Gavin Schmidt, a climate researcher at NASA Goddard Institute for Space
Studies in New York. But with only five transects, separated across decades
and also short in duration, these “snapshots” of ocean slices may
not have covered seasonal and annual variability, “which is, theoretically
speaking, very large,” Schmidt says. Still, “lots of people are excited
because [the result] is quite dramatic.”
The week after its publication, the paper had an excited yet mixed response
at the American Geophysical Union meeting in San Francisco, where researchers
presented new models and data on ocean circulation, as well as on freshening
of North Atlantic waters. The Greenland ice cap and sea ice in the Arctic are
both melting rapidly, with mixed temperature and salinity changes in both the
Atlantic and Pacific, according to data presented by several speakers. The potential
impacts of releasing this freshwater into the North Atlantic remain to be seen.
Andreas Schmittner, an ocean modeler at Oregon State University in Corvallis,
said that the various circulation models do not yet fit the data accumulated
from a variety of sources. The current situation “is not The Day After
Tomorrow,” Schmittner said in a press conference, referring to the
movie in which an abrupt short circuit of the North Atlantic circulation led
to a frozen northern hemisphere in a matter of days.
Researchers hope that long-term monitoring stations deployed on the same cruise
where Bryden and colleagues got their latest measurements eventually will provide
enough information to confirm whether or not the real ocean conveyor belt is
indeed changing.
Naomi Lubick
Link:
"Salting a
stagnant ocean," Geotimes, November 2005
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