The future of our deltas

What if the past is a key to the future?

Around 11,000 years ago, global warming induced a rapid rise in sea level, which slowed down again around 7,500 to 7,000 years ago. Geological research has revealed that this slowdown coincided with the beginning of the formation of a large part of the world’s deltas. The scientists who carried out this research concluded that, based on their data, the world’s deltas formed when sea level rise slowed down to a range of 5 to 10 mm per year. If this range could be considered representative of the situation of today and in the future, the scientists argue, a tipping point to delta decline is likely reached when sea level rise reaches that annual 5 to 10 mm range.

We may start losing coastal deltas 50 years from now

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Deltas and coastal zones are home to hundreds of millions of people, many of them living in cities. These people are facing the consequences of climate change, including sea level rise and changes in weather patterns. By 2070, 70% of the world’s population is projected to be living in cities.

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Sea level rise, sediments and land subsidence determine the future of our deltas

The future of our deltas will be determined not only by sea level rise, but also by local land subsidence and sediment supplied by the rivers that have formed the deltas. Sea level rise and land subsidence constitute relative sea level rise: the rise of sea level with respect to the level of the delta. Deltas where fluvial sediment supply is insufficient to compensate for relative sea level rise will erode and shrink.

Many deltas are sinking because of land subsidence due to the large-scale, long-term abstraction of groundwater and fossil fuels. Today, land subsidence often exceeds sea level rise. In addition, in most large rivers, dams are trapping substantial quantities of the sediment that flows from upstream, and, in combination with the large volumes of sand mined in downstream reaches, reduce the sediment supply to the deltas. Deforestation, on the other hand, may increase sediment production in the catchment area and the sediment supply to the delta.

A recent study estimates the combined impact of all these drivers for the future, for over 6,000 deltas. The authors conclude that, at the global level, deltas will gain land if the relative sea level rise is less than about 5.5 mm per year. From 1985 to 2015, delta land gain has slowed down mainly because of sea level rise. A shift towards a net land loss may result, once the rate of sea level rise reaches 5.5 mm per year. By the end of the century, sea level rise will be the dominant driver that determines the future of our deltas. This may lead to the disappearance of up to 5% of total global delta area by 2100. In addition, waves and tides increasingly leave their mark on the shape of deltas, at the expense of the influence of the river.

Land area change observed in 1985–2015 and projected for the period between now and 2081–2100 #

Observed

1985 - 2015

Climate change scenario

Low Intermediate High

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Land subsidence is a major contributor to flood risk

For 1.6% of the global land surface, subsidence is very likely. This area includes 19% of the global population and 12% of global GDP. Land subsidence is a major contributor to flood risk. Around 484 million people are living in areas that are flood-prone and potentially subsiding. Subsidence threatens 15 of the 20 major coastal cities ranked as having the highest flood risk, worldwide.

Over the next decades, global population and economic growth will continue to increase groundwater demand. Groundwater abstraction will probably increase land subsidence and its related damage and impacts. By 2040, the part of the global population that is threatened by land subsidence is projected to increase by 30%, affecting 1.6 billion people, with 635 million of whom living in flood-prone areas.

Land subsidence

The ground is subsiding in parts of many deltas and coastal zones, mostly due to human interventions. Globally, groundwater abstraction and – to a lesser extent – oil and natural gas extractions are the main drivers of subsidence. In some areas, the construction of buildings and infrastructure, soil compaction and decomposition of organic matter after drainage may also lead to subsidence. Today, in many deltas and coastal cities, land subsidence is exceeding the rate of sea level rise. This map shows the maximum rate of subsidence in 99 cities around the world.

A strong reduction of groundwater abstraction has already slowed down or stopped land subsidence in the urbanized regions of Tokyo and Shanghai.

Impact of global sea level rise and average land subsidence, for several coastal cities

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Sediment supply from rivers to deltas is declining, in many rivers

According to an assessment of global trends, the sediment fluxes of 63% of the world’s large rivers show no significant change, because of human intervention. Sediment flux has decreased for 32% of the rivers and increased for 4%. Rivers with decreasing trends are located in North America and Eastern Asia.

Reduction in sediment flux is mainly caused by dam construction, reduced river flow because of irrigation, water–soil conservation, and local sand mining for infrastructure and urban development. Increase in sediment flux is mainly due to deforestation, mining activities, and land clearance in the catchment area. Sand mining is a major threat to the sustainability of, particularly, Asian deltas. In the Mekong Delta, for instance, almost 18 million m³ of sand were extracted from the riverbed, in 2018 alone.

The reduction in sediment flux is widespread in rapidly developing Asian countries where numerous dams have been constructed in recent years. Long-term sediment flux has decreased in 71% of the large rivers in Asia because of human interventions.

In the past 5 to 10 years, the global land–ocean sediment flux has reduced by 20.8%, mostly (for 13.2%) contributed by Asian rivers due to accelerated human disturbance of river basin sediment budgets, because of dam construction and sand mining.