Is the sea rising?

Published: Monday 15 June 1992

RISING sea levels and drowning areas is one of the much publicised consequences of global waarming - in fact, sea levels do rise, the effect on island countries like the Maldives and those with long coast lines like India and Bangladesh could be catastrophic.

However, according to P K Daformer director-general of to Indian Meteorological Department who has been analysing sea level data, particularly along the Indian coastline, the relationship between global warming and rise in sea level is still tenuous.

On examining the tide gauge records from Bombay, Madras, Cochin and Visakhapatnam since 1881, Das did not find a uniform rising trend, except at Bombbay between 1940 and 1986 (0.8 mm per year) and at Madras from 1910 to 1933 (0.4 rare per year). These results, he maintains, are not necessarily due to global warming.

Das points out that it is not possible to differentiate the changes sea levels brought about by the thermal expansion of water and the melting of glaciers, both caused by global warming, from those brought about by the tectonic movements of the earth. Tectonic movements in the coastal and deep oceanic regions affect the sea level. In fact, recent modelling of the earth's upper crust has shown that the Australian and Indian plates are slowly moving towards one another and this could be responsible for the rise in sea level. He adds that most of the evidence for sea level rise comes largely from the mid latitudes. Little evidence is found in the tropics.

The understanding of other factors that affect sea level rise has also been changing. The dynamics of melting glaciers and polar ice caps, groundwater withdrawal and its subsequent discharge into rivers, and the atmospheric water vapour content are other influnential factors.

According to Das,, the melting of the floating Arctic ice cap will have a minimal effect on the volume of sea water, as compared to the land-based Antarctic ice mass.

However, certain western scientists hold that with global warming, the western Antarctic ice sheet would grow in weight by too bit Jon tonnes to 400 billion tonnes per year because warm air holds moisture which would freeze, form snow and fall on the polar regions. Thus, it would lower sea levels by 0.3 to 1 men. In the process of his research, Das may have found a lead to explain the Chandler wobble or pole tide, discovered by a Massachusetts merchant, S C Chandler, in 1891, which is as yet an unexplained phenomenon. The Chandler wobble denotes the changes that take place periodically in the inclination of the earth's axis and in the rate of its rotation.

Indian-tide gauge records exhibit certain periodicities, which, on examination, appear to consist of long and short period cye)fs. The long period cycles have a periodic ity of 5-0 years and the shorter cycles four to six years.

These short cycles, according to Das, could be the result of the interadion between an unknown annual cycle and the tidal motion caused by fluctuation of the earth's rate of rotation round its axis - the Chandler wobble - the periodicity of which is' 14.7 months. The difference between the frequencies of the two periodicities - once every twelve months and once every 14.7 months - is equivalent to a periodicity of once every 5.2 years, which is what is observed in the Indian tide gauge records.

Despite the fact that this wobble was discovered over a hundred years ago what causes it is still not understood. Das believes that the movements of vast amounts of air associated with the monsoon may be disturbing the rotation of the earth and hence this wobble. He is now studying coastal rainfall data for supporting evidence.

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