Huge population growth in major river deltas puts tens of millions at increased risk from climate change floods

The world’s 48 major deltas, where huge rivers like the Mekong in Vietnam empty into the sea, are 2.6 times more densely populated and 1.7 times more built up than other low lying coastal zones, according to new research due to be published in Environment & Urbanization, a journal edited by IIED.

With an estimated 240 million people living in river delta areas, they are highly vulnerable to the rising sea levels and extreme weather events that climate change will bring. 

Although river deltas have been popular places to settle throughout human history because of their rich soil and potential for trade, populations living around the largest ones have exploded in recent decades, particularly in Asia at the mouths of major rivers like the Irrawaddy, Yangtze and Mekong. The size and dynamism of cities in these areas continue to attract further development, regardless of long-term risks.

In 'Is rapid urbanisation of low-elevation deltas undermining adaptation to climate change? A global review', researchers calculate that although river deltas account for only 0.35% of the planet’s land mass they are home to 3.8% of its people and 3% of its built-up area.

The paper combines new data on population, the extent of built-up land and urbanisation levels to create a global assessment of environmental risk for delta dwellers.

It reveals that population growth has been much faster in the world’s major deltas even than in other low-lying coastal regions. The populations of deltaic settlements in areas 10 metres or less above sea level increased by 1.8% per year between 1990 and 2015, with built-up land expanding by 2.1% annually. That compares to 1.3% and 1.1% respectively for areas below 10 metres not sited on deltas. Estimates suggest populations in land five metres or less above sea level are growing faster still.

Gordon McGranahan, the paper’s lead author, said: “People are often drawn to cities by the promise of jobs and prosperity, and the deltas we studied are no exception.

“But what might seem like a good idea in the short term is going to have long-term consequences. It will become increasingly difficult for residents and governments in these densely populated deltas to adapt to the rising seas, flooding and storms we know are coming.

“If nothing is done, many tens of millions of people face seeing their homes destroyed or neighbourhoods made unliveable by flooding in the coming decades.”

River deltas, like other low-lying areas, will be increasingly threatened by sea-level rise as large parts of the Antarctic and Greenland ice sheets melt over the coming decades and centuries. Deltas are further jeopardised by subsidence, both due to groundwater extraction by people and a lack of new sediment being deposited. Sediment deposition is how deltas are formed.

Planners, politicians, private companies and individual residents making decisions today about the expansion of cities on large deltas could be locking in an inability for their descendants to adapt to climate change in future. 

While the study examined major deltas across the globe, it is the Asian deltas that are the most populated. The issue of population density is particularly acute in China. Since 1980, Beijing has opened a number of special economic zones in coastal cities, relaxing regulations to attract investment to areas geographically primed to feed an export market. As a result, China has seen much higher rates of urban growth in low-level delta areas, despite the growing risks.

Deborah Balk, director of the CUNY Institute for Demographic Research at the City University of New York, and co-author of the paper, added: “With hundreds of millions of people living in hazard-prone delta regions, and both hazards and urban populations increasing, planning for existing and future cities will need to become far more effective and equitable than it is now to avoid a serious increase in major disasters.

“Evidence from the past 25 years suggests that people are not reluctant to settle in the riskiest areas despite the known risks.”