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Rising rivers set to wreck Bangladesh

Arguments over the causes of global warming will bring little succour to the people of Bangladesh. Flooding in the country is set to increase by up to 40 per cent this century as global temperatures rise, the latest climate models suggest.

Each year, roughly a fifth of Bangladesh is flooded, and climate change is forecast to exacerbate the problem as sea levels rise, monsoons become wetter and more intense cyclones lead to higher tidal surges.

To make things worse, heavier rainfall triggered by global warming will swamp Bangladesh’s riverbanks, a previously unforeseen effect, flooding between 20 and 40 per cent more land than today, says Monirul Qader Mirza, a Bangladeshi water resources expert now at the Adaptation and Impacts Research Group at the University of Toronto.

Bangladesh is flood-prone because it lies in the delta of three great rivers, the Ganges, Brahmaputra and Meghna, which together drain 175 million hectares. People can grow crops on land regularly fertilised by nutrient-laden silt from the rivers. But extreme floods cause considerable hardship and loss of life: in 1988 and 1998 over two-thirds of the country was under water at some point.

Most climate models predict up to 20 per cent more precipitation in South-East Asia if temperatures rise by 5 °C. But no one had investigated how Bangladesh’s three major rivers would cope, says Mirza.
Scary scenarios
His team collected data on the relationship between current precipitation levels and the resulting discharge of water by the three rivers. They then fed this data into a software program developed by the Danish Hydraulic Institute, which simulates how factors such as sediment and water quality affect the flow of water within river basins. Researchers at the Surface Water Modelling Centre in Dhaka helped calibrate the model to Bangladesh’s particular geography.

Mirza’s team then ran the program for four climate change scenarios, known as global circulation models. In each, the peak mean discharge for all three rivers increased as global temperatures rose by 2, 4 or 6 °C. If temperatures rose by just 2 °C, two of the models showed that the mean flow of the Meghna and Brahmaputra rivers would increase by 20 per cent.

If there is an increase in temperature of 6 °C, the maximum predicted by the International Panel on Climate Change, then the greater flow of water through Bangladesh’s three great rivers will inevitably lead to between 20 and 40 per cent more flooding.

There will also be a steep increase in deeply flooded land – that covered by more than 1.8 metres of water for nine months of the year. Of the 3.1 million hectares that floods each year, 42 per cent is already deeply flooded. That will climb to 55 per cent if temperatures rise by 6 °C.

The land available to grow rice, vegetables, lintel, onion and mustard crops will be significantly reduced, placing an intolerable pressure on farmers. Policy planners should begin working on adaptation measures now, Mirza says.

Journal reference: Climatic Change (vol 57, p 287)

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