River deltas face risk from straying cyclones #ClimateChange #RiverDeltas

The growing tendency for cyclones to change their paths means river deltas risk being starved of the vital sediment the storms deliver.

Some of the world’s great river deltas could be at risk, not because of increased cyclone hazard but paradoxically because, as climates change, the track of the cyclones has begun to shift away from them.

And although typhoons, hurricanes and tropical cyclones – three names for one storm phenomenon – bring with them destruction, they also deliver prodigious quantities of water to wash billions of tonnes of silt and sediment downstream to create that marshy, meandering and immensely fertile pattern of rich soil and waterways known as a delta.

Without regular deliveries of sediment, deltas could drown as sea levels rise because of global warming.

British scientists report in the journal Nature that they analysed two decades of water and sediment concentrations discharged into the Mekong River delta in Vietnam.

Planetary ricebowl 

The Mekong is the world’s third largest delta: it covers 39,000 square kilometres, it is home to 20 million people, and its moist fertile soils help make Vietnam one of the planet’s great rice producers.

But a gradual change in the path of tropical cyclones – reported to be moving northwards at 50kms a decade – means that the burden of suspended sediment in the river declined by more than 50 million tonnes between 1981 and 2005. Of this decline, 33 million tonnes was due to change in cyclone climatology.

“Our study is the first to show the significant role tropical storms play in delivering sediment to large river deltas. We show that although human impacts affect the amount of sediment in a river, cyclonic activity is also a very important contributing factor,” said Stephen Darby, a physical geographer at the University of Southampton, who led the study.

“These results are very significant because the Mekong’s sediment load is already declining as a result of upstream damming and other human impacts such as sand mining.

“Understanding the role played by changes in tropical cyclone climatology gives us a broader knowledge of the threats facing this delta and others like it around the world.”

“ . . . Although human impacts affect the amount of sediment in a river, cyclonic activity is also a very important contributing factor”

What is true for the Mekong is likely to hold for the other great river deltas of the planet: the Ganges-Brahmaputra, the Yangtze, the Mississippi and other river systems that benefit from the 19 billion or so tonnes of sediment washed down the world’s rivers every year.

An estimated 500 million people make their homes on river deltas, but most are under threat from a cocktail of effects: rising sea levels, local ground subsidence and human interference in the flow upstream. And climate change in one way or another delivers a challenge to them all.

So, ironically, as global atmospheric and sea surface temperatures rise because of the combustion of fossil fuels and the consequent release of atmospheric greenhouse gases, the intensity of tropical windstorms threatens to increase, but at the same time the track of the storms seems likely to shift and deliver new threats to cities not normally at hazard.

The study is, once again, a reminder of the intricate dependence of human habitats and natural ecosystems on stable climates: hurricanes and typhoons are a source of danger for people in the great river deltas, but they also deliver the water that keeps the deltas fertile.