Relative sea level rise, caused by local drivers, together with global sea level rise, could cause significant parts of the world's largest deltas to fall below the rising sea before the turn of the century, according to a new study.
Relative sea level rise also takes into account sinking lands which amplify the effect of rising seas.
Local drivers such as groundwater pumping and extraction of hydrocarbons creates subsidence, and coastal vegetation, which can provide some protection, is lost to make space for farmland and tourism.
Scientists of the Stanford University-led study have found overwhelming evidence that it is not sea level rise, rather sinking land, that puts global deltas most at risk.
While climate change is increasingly recognized as a risk to coastal livelihoods and global wealth and security, this is only one part of the story.
This is of great importance for managing river deltas, according to Rafael Schmitt, lead author of the study, a lead scientist with the Stanford Natural Capital Project.
"It is often not rising seas, but sinking land due to human activities that puts coastal populations most at risk," said Schmitt.
"Our research highlights that this relevant global risk is grossly understudied for all but very few coastal regions," said Schmitt.
Little is known about local and regional drivers of relative sea level rise.
So, Schmitt's study set out to identify key drivers of land loss and vulnerability across the world's major deltas, and the knowledge gaps impeding more sustainable delta management, for specific deltas and on a global scale.
Under natural conditions, deltas are subject to a number of factors that together create dynamic but stable systems.
For instance, sediment supplied from upstream river basins builds new land even when sea levels are rising. Sediment supply is also critical to offset the effect that the recent, unconsolidated delta land compacts continuously under its own weight.
Today, all of these processes are out of balance.
River deltas are cut off from their natural sediment supply by dams and reservoirs, and the little sediment still reaching deltas cannot spread because of artificial levees and dikes.
The livelihoods of millions of people who live in river deltas, among the world’s most productive lands, are at risk.
Created where large rivers meet the ocean and deposit their natural sediment load, river deltas are often just a few meters above sea level.
While they make up less than 0.5 per cent of the world's land area, river deltas contribute more than 4 per cent of the global GDP, 3 per cent of global crop production, and are home to 5.5 per cent of the world's population.
All of these values are highly vulnerable to imminent global environmental change, the study said.
Of course, climate mitigation is important to curb global sea level rise.
However, fighting overuse of local natural resources in river deltas and their contributing basins would have much greater and more immediate effects, posing both an opportunity and a responsibility for coastal nations, the study said.