Flatter oceans caused sea levels to rise in the 1920s

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

London, Aug 26 : The movement of colossal mounds of water in the North Atlantic and the Pacific oceans may have caused sea levels to suddenly begin rising more quickly in the 1920s, according to a new study by US researchers.

The analysis by Laury Miller of the US National Oceanic and Atmospheric Administration and Bruce Douglas of the Laboratory for Coastal Research in Florida, US, presents a more complex picture of sea-level change and suggests that the rate of change has been more dramatic than previously thought.

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

Oceanologists have previously attributed this rise to climate change, though the precise reason why oceans suddenly began rising more quickly in the 1920s has remained a mystery.

Warming temperatures boost sea levels in two ways: melting glaciers release more water into the ocean and seawater expands as it warms.

Over the past few decades, sea level measurements taken using satellites have shown that this trend has continued, with the current rate of increase standing at about 3.36 mm per year.

As part of their study, the scientists analysed atmospheric pressure records for the late 19th and 20th centuries and used these records to try and work out how rising sea levels may have 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 revealed that the gyres in both oceans sank during the 1920s, releasing water held in the centre and allowing it to flow towards the coasts.

The scientists believe this could probably explain the sudden change in the rate at which sea levels changed at this time, measured by coastal instruments.

The researchers say the findings implied that 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, they said.

"However as measurements have only recently become sufficiently accurate, it may take some time before the full picture will be known. My guess is that it will be 20 or 30 years before we are able to identify how fast sea-level rises are truly accelerating," said Miller.

The research appears in the Geophysical Research Letters, reports New Scientist.

ANI