Data on Atlantic flow are undercut
A massive ocean circulation pattern that plays a crucial role in shaping the world’s climate may not have been slowing down over the last few decades as scientists previously believed, according to a study released Thursday.
The perceived slowdown had been considered alarming support for computer predictions that global warming would disrupt the planet’s heat regulation.
In a single year of measurements, published in today’s issue of the journal Science, the scientists found enough normal variation in the pattern to suggest that previous studies were premature in asserting a long-term trend.
“We can’t strictly say they are wrong, but we can have an alternative explanation,” said coauthor Torsten O. Kanzow, an oceanographer at the National Oceanography Centre in Southampton, England.
The ocean flow, known as the meridional overturning circulation, has been a focus of global warming debates. The Gulf Stream is perhaps its best-known such current.
Often called the “ocean conveyor belt,” the pattern carries tropical Atlantic water north. Along the way, the water emits heat, warming North America and Europe.
The water, laden with carbon dioxide absorbed from the atmosphere, sinks as it cools, sequestering the greenhouse gas in the depths of the ocean. Eventually, the cool water heads south to start the cycle again.
With global warming melting glaciers and increasing precipitation, the surface water would not only be warmer but less salty.
Warmer, less salty water sinks more slowly.
Current theories hold that slower circulation would impair the ocean’s uptake of carbon dioxide and its transport of heat from the tropics.
A previous study on the meridional overturning circulation was based on data collected by ships in 1957, 1981, 1992, 1998 and 2004 that showed a gradually declining flow. It was the first evidence of such a slowdown, and though computer models had predicted one -- projecting a flow reduction of about 25% during this century -- scientists had not expected it so soon.
But because the data points were snapshots in time and not part of a complete record, oceanographers tended to view the analysis with skepticism.
“You could convince yourself that this indicated a trend,” said William E. Johns, an oceanographer at the University of Miami and a coauthor of the new study. “But it could also be a lot of wiggles that happened on shorter time scales.”
Johns and his colleagues installed a dozen moorings across the Atlantic Ocean, from the tip of Florida to the coast of Africa, to measure temperature and salinity daily at various depths.
That information was used with satellite data and a transatlantic cable to help calculate the strength of the circulation.
Over 12 months starting in March 2004, the researchers found that the flow of water varied from 4.4 to 35.3 megatons per second. The average was 18.7 megatons per second.
Their data spanned the range of values previously used to identify a downward trend.
“This really hammers home the importance of making continuous observations,” Johns said.
The researchers had no definite explanation for the wide variation in flow, but Kanzow surmised that the dynamics of wave movements, eddies and currents that interfere with the circulation pattern could be more influential than previously thought.
Despite their own findings, many of the researchers believe that a slowdown is occurring. But they lack proof.
The instruments have been left in place and will continue to collect data.
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