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Flooding set to increase in Ganges-Brahmaputra-Meghna river basin

25 Oct 2019
Flooding in Bangladesh
(Image courtesy: iStock/Supratim-Bhattacharjee)

Warming of 1.5 °C is often portrayed as a “safe” limit but the consequences of climate change don’t distribute themselves evenly around the world. For some regions even 1.5 °C of warming will have serious consequences. A new study shows that 1.5 °C of warming will bring significant increases in extreme precipitation events and flooding to the highly populated Ganges-Brahmaputra-Meghna Basin.

The Ganges-Brahmaputra-Meghna river system covers a wide area through Bangladesh, Bhutan, China, India and Nepal. Monsoon rains between June and September dominate the system. In 2017 monsoon flooding produced record river levels, resulting in around 1200 deaths and severe loss of crops and infrastructure. Will events like the 2017 floods become more common with 1.5 °C of warming?

Previous studies used high-emission scenarios to look at the impact of warming on this region. But no-one had investigated the impact of a low-emission scenario aimed at stabilising the climate at 1.5 °C or 2 °C of warming. Some climate drivers, such as aerosol emissions, are likely to be different under a low emissions scenario and could produce a differing precipitation response to similar levels of warming under a high-emissions scenario. To investigate this further, Peter Uhe from the University of Bristol, UK, and colleagues combined a high-resolution flood hazard model for the Ganges-Brahmaputra-Meghna region with climate model simulations for 1.5 °C and 2 °C of warming.

All but one of the models analysed show a significant trend in increasing rainfall in the region, with the trend being stronger for extreme rainfall. Flood hazard also increases. For floods with a 1 in 5 year probability the flood area under 1.5 °C of warming is estimated to increase between 7 and 25% compared to present day.

“Bangladesh will be particularly vulnerable to the more frequent floods because it is flat and very low lying and a large portion of the county is a flood plain,” explains Uhe, whose findings are published in Environmental Research Letters (ERL).

More severe floods (1 in 20 year and 1 in 100 year probability) also increased in frequency and extent under the 1.5 °C and 2 °C warming scenarios. The percentage increase in area was not as great as for the 1 in 5 year floods but the waters were deeper. This is partly because of the topography of river valleys, with the more frequent floods spilling out onto the flatter valley bottom, whilst more severe floods tend to create deeper waters because they are spatially constrained by the steeper valley sides.

“These floods could become more dangerous by impacting areas that are not often flooded, which may be less prepared,” says Uhe.

Even under low emission scenarios and just 1.5 °C of warming, the research shows that there will be an increase in extreme precipitation events for this region, and a greater likelihood of flood events like the one seen in 2017.

“People living there may have to change how and where they build, and their farming practices,” says Uhe. “Bangladesh, for example, has a large amount of flood defences along the Brahmaputra river and these could be modified to adapt to future flood risk. However, these measures will not remove the flood risk completely.” Preparing and adapting will be vital.

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