River deltas are created where large rivers reach the ocean and deposit their natural sediment load. Although they make up less than 0.5 percent of the Earth’s land area, deltas are highly productive, contributing over four percent of the global GDP and three percent of the global crop production, while being home to 5.5 percent of the world’s population.
Unfortunately, according to a new study led by Stanford University, river deltas are currently highly vulnerable to imminent environmental change, and the livelihoods of the millions of people living there may be at risk.
“It is often not rising seas, but sinking land due to human activities that puts coastal populations most at risk,” said study lead author Rafael Schmitt, a postdoctoral fellow in Environmental Management at Stanford. “Our research highlights that this relevant global risk is grossly understudied for all but very few coastal regions.”
Deltas are usually subject to various factors creating dynamic yet stable systems. For example, the sediment supplied from upstream river basins creates new land even if sea levels are rising, while also offsetting the effect that recent, unconsolidated delta land compacts continuously under its own weight.
However, since today river deltas are cut off from their natural sediment supply by human-made dams and reservoirs – and the small amount of sediment that still manages to reach deltas cannot spread due to artificial dikes and levees – all the processes that occur naturally in undisturbed deltas are out of balance.
Moreover, coastal vegetation is increasingly lost to make space for tourism and farmlands, while groundwater pumping and extraction of hydrocarbons creates subsidence (sinking of the ground caused by underground material movement).
Along with global sea level rise, these local factors lead to relative sea level rise, in which sinking lands amplify the effects of rising seas, possibly causing major parts of the world’s largest deltas to fall below sea level by the end of the century.
According to the experts, it is not sea level rise, but sinking land which puts deltas most at risk. Thus, while climate change mitigation remains important to curb global sea level rise, combating overuse of local natural resources in river deltas and their basins would likely provide greater and more immediate protection to deltas and their inhabitants.
The study is published in the journal One Earth.
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