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Ocean freshens up

June 16, 2005 By Jenny Hogan This article courtesy of Nature News.

Measurement of fresh water entering the Atlantic will help climate predictions.

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About 19,000 cubic kilometres of fresh water have flooded into the North Atlantic in the past 40 years, a new study has found. That's more than 3 times the annual outflow of the Amazon River. The analysis provides vital clues for modellers who are trying to predict how ocean circulation might change in the future, and how that might affect the climate.

"There has been lots of evidence of ocean freshening, but this is the first quantification," says Rowan Sutton of the Centre for Global Atmospheric Modelling at the University of Reading, who studies climate change in the Atlantic. "The results are going to be very useful in putting models to harder tests."

Climate scientists fear that fresh water entering the northern Atlantic from melting ice caps and surging rivers might upset the currents that carry heat from the tropics towards the pole, such as the Gulf Stream in the North Atlantic.

Such currents are like a conveyor belt: dense, cold, salty water sinks in the north, and returns to the tropics along the ocean floor. Dilution with fresh water could prevent the sinking, and cause the current to falter.

There is, as yet, little evidence that the current is weakening. But Ruth Curry of the Woods Hole Oceanographic Insitution in Massachusetts and Cecilie Mauritzen from the Norwegian Meteorological Institute in Oslo, who report their results in Science1, say their measurements of freshwater input suggest that slowing will happen.

"We believe we will measure a change in the overturning circulation in our lifetime. It will not be on the scale of disaster, but I do imagine being able to see it before I am too old and frail to go out and make measurements of the ocean," says Curry.

The century after tomorrow?

The movie The Day After Tomorrow envisaged a sudden shutdown of the overturning current leaving Europe and America blanketed by ice. Scientists dismiss this scenario as extreme and unrealistic, but there is huge uncertainty about how the ocean and the climate will respond to changes in the circulation.

"It's very difficult and complicated plumbing," says Thomas Stocker, a climate modeller from the University of Bern in Switzerland.

Curry and Mauritzen analysed data on water pressure, temperature and salinity collected by research vessels and floating buoys since 1965.

They found that between 14,000 and 24,000 cubic kilometres of fresh water poured into the North Atlantic Ocean between 1965 and 1995, with nearly half arriving in one great pulse of water in the late 1960s. This 'great salinity anomaly', which has been known about for decades, was caused by unusual wind patterns that first built up masses of ice in the north, and then blew it south, where it melted.

The team discovered that this melt water migrated to areas of the ocean just south of Greenland that are not greatly involved in overturning waters. This explains why the great salinity anomaly had little effect on ocean currents.

Since then, an average of 100 cubic kilometres of fresh water per year has accumulated in the more sensitive upper layers of the Nordic seas. At this rate, Curry and Mauritzen estimate that the circulation could start to weaken within a century.

But the amount of fresh water entering the ocean may change. Sutton points out we don't know where all the water is coming from, or exactly how climate change will affect rainfall, run-off and melting ice. So a slowdown in circulation may happen sooner than predicted.

References

  1. Curry R. & Mauritzen C. Science, 308. 1772 - 1774 (2005).

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