Major global cities are now sinking faster than sea levels are rising

Land subsidence might now be the biggest climate threat in places like New Orleans and Shanghai

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For decades, the story of coastal risk has been dominated by climate change and rising seas. But a major new global study suggests that, for hundreds of millions of people living on river deltas - including those in cities such as New Orleans and Bangkok - an even more immediate threat is unfolding beneath their feet.

Across much of the world, the land is sinking - and in many places, it's sinking faster than the ocean is rising. Using satellite radar to track tiny changes in Earth's surface, scientists have discovered that more than half of the world's river-delta regions - the low-lying land where major rivers meet the sea - are now sinking. In many of the most densely populated deltas on the planet, this gradual subsidence, rather than rising seas alone, is now the dominant driver of flood risk.

"It's a real call to arms," Prof Robert Nicholls, a coastal scientist at the University of Southampton and coauthor of the study, told BBC Science Focus. "Before this, nobody had a global view of delta subsidence. This study shows how widespread the problem is and really brings home the need to respond to it.”

GROUND TRUTHS

River deltas occupy just one per cent of Earth’s land area, but they support between 350 and 500 million people, in some of the world’s largest cities and on its most productive farmland. They are economic powerhouses, ecological hotspots and vital food baskets. They're also fragile.

Deltas are built from loose, waterlogged sediments deposited over thousands of years. Even without human interference, these sediments slowly compact under their own weight, causing gradual sinking.

Historically, that natural sinking was balanced by floods that replenished the land with fresh sediment. But modern development has changed that balance.