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  • Indian Ocean sea levels 'rising at different rates'


Mapping variations in regional sea level changes of different parts of the Indian Ocean could help developing countries better adapt to the effects of climate change, according to a study published in Nature Geoscience last week (11 July).

Researchers from the University of Colorado, United States, identified distinct patterns of sea-level rises using observational and satellite data combined with climatic and ocean circulation models, including results from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report.

They found that if human effects on the climate continued at the current rate, mid-ocean islands such as the Mascarenhas archipelago, the Indonesian and Sumatran coasts, and parts of India and Bangladesh, could see much higher sea level rises compared to mean sea level increases predicted for the planet as a whole.

Conversely, Zanzibar could experience falls, whilst the Seychelles and the east coasts of Kenya and Tanzania may see little or no rise. The Maldives may only experience substantial sea-rises during the winter monsoon.

The variations are due to oceanic and atmospheric circulation systems, which have been studied together for the first time, lead author Weiqing Han, associate professor at Colorado's Department of Atmospheric and Oceanic Sciences told SciDev.Net.

"This regional sea-level change information will be more important for effective risk assessments in future," said Han.

Global atmospheric circulation systems responsible for south- and north-easterly trade winds, as well as west-to-east surface winds along the equator, are strengthening as the tropical waters of the Indian Ocean heat up. Those winds are combining to drive water movements that lead to sea rises in some areas and falls elsewhere. To compound the problem, the winds blow surface water away, leaving colder sub-surface water behind, which is denser and lowers the sea level further. 

Until now, the extent of the variations has been unknown. Although the research helps build a more accurate picture, it is still only a partial one. For example, the Colorado researchers did not factor in melting ice sheets because of scarce data.

A. Atiq Rahman, executive director of Bangladesh's Centre for Advanced Studies and a lead author of the IPCC report, said tides and sedimentation along coastlines also cause regional variations in sea levels.

Saleemul Huq, senior fellow at the UK-based International Institute for Environment and Development said research should also include more land-based data such as rates of erosion along coasts. "Unless we know what the coast is doing at any one place, we can't say what the net effect is going to be," he said.

Link to full paper in Nature Geoscience


Nature Geoscience doi:10.1038/NGE0901 (2010)

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