Tuesday, August 28, 2007

More confusion over rising sea levels

New Scientist reports: [edited]

The movement of a colossal "mounds" of water in the North Atlantic and Pacific oceans may have caused sea levels to suddenly begin rising more quickly in the 1920s, researchers say.

Their analysis presents a more complex picture of sea-level change and suggests that the rate of change has been more dramatic than previously thought.

Data collected using tidal gauges dotted along the Atlantic and Pacific coastlines during the late 19th and 20th centuries suggest that sea levels suddenly began rising more quickly around the 1920s, from about 1 millimetre per year to about 1.8 mm per year.

This change has previously been attributed to climate change. However, precisely why oceans suddenly began rising more quickly in the 1920s has remained a mystery.

Now, evidence gathered by Laury Miller of the US National Oceanic and Atmospheric Administration and Bruce Douglas, of the Laboratory for Coastal Research in Florida, US, suggests that this change may be partly explained by the pressure-related movement of gigantic amounts of water.

The researchers studied atmospheric pressure records for the late 19th and 20th centuries and used these records to try and work out how rising sea levels may be been affected by shifting ocean peaks, known as "gyres", in the North Atlantic and Pacific Oceans.

These ocean peaks are produced by water swirling in a circular direction around the ocean as a result of atmospheric pressure, wind and heating. The movement, combined with the turning of the Earth, causes water at the centre of the circle to rise upwards, forming a peak, or gyre. The North Atlantic and Pacific gyres are each about 1 metre taller at the centre than at the edge.

But atmospheric records suggest that the gyres in both oceans sank during the 1920s, releasing water held in the centre and allowing it to flow towards the coasts. This would explain the sudden change in the rate at which sea levels changed at this time, measured by coastal instruments. Since tidal gauges only measure sea-levels along the coasts, they could not have detected the drop in levels towards the oceans' centres.

If the researchers are correct, this means the overall sea levels were in fact rising more slowly at the time. This, in turn, implies that the rise of sea levels accelerated faster over the 20th century than previously thought.

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