Many of the world’s sinking deltas due to human activities

The study, which is published today in Nature Geoscience, says that human use of the world’s low-lying deltas, many of which are densely populated and heavily farmed, are causing them to sink more rapidly than by rising sea levels alone.

Using globally consistent and high-resolution satellite data, the study assessed 33 deltas in countries including China, India and the US for their tendency to flooding and to see why they are sinking more rapidly than the global sea level is rising. It found that in the past 10 years, 85 per cent of the deltas experienced severe flooding, resulting in the temporary submergence of 260,000 square kilometres of land and that 24 out of 33 major deltas are sinking.

However, according to researchers this vulnerability to flooding, either from their feeding rivers or from ocean storms, is a result of a host of human-induced factors. These are the creation of river dams and reservoirs, where sediment is trapped upstream rather than adding to a delta; the removal of oil, gas and water from a delta’s underlying sediments; and the use of floodplain engineering, all in combination with rising global sea levels.

Professor Robert Nicholls, from the University of Southampton, who co-authored the report says: “We conservatively estimate that the delta surface area vulnerable to flooding could increase by 50 per cent under currently projected values for sea level rise in the twenty-first century. This figure could increase if the capture of sediment upstream persists and continues to prevent the growth and buffering of the deltas.

“These human-induced factors greatly exacerbate the risks faced by inhabitants of deltas, which number nearly 500 million people, under scenarios of global warming. To effectively deal with these problems is challenging and will take a fundamental shift in how we manage these systems, including recognising the multiple drivers of increasing risks.”

These inhabitants and all infrastructure in deltaic lowlands is at risk of flooding from rivers or ocean storms. Already every year, it is estimated that about 10 million people are being affected by storm surges, with most of these people in the big Asian deltas. Hurricane Katrina may be the example that stands out in the US, but in the Asian deltas of the Irrawaddy and Ganges-brahmaputra floods have recently claimed thousand of lives as well. The study predicts that similar disasters could potentially happen in other deltas, such as the Pearl and Mekong deltas, which have tens of thousands of kilometres of area below sea level and are exposed to typhoons.