DELTA CITIES
A woman works in a paddy field in a village in Sunderbans delta region in India. Given the rapid extraction of groundwater in deltas, there are higher risks of ground subsidence.REUTERS

Excessive groundwater removal is causing the ground to sink, adding to concerns of rising ocean levels facing coastal cities.

A study on sea-level rise from groundwater extraction notes that while Bangkok in Thailand has sunk more than three feet since the mid-1970s, many coastal regions with large populations are also sinking in parts of Bangladesh, India and China.

It has been documented that Jakarta in Indonesia is the world's fastest sinking megacity, with some areas of it having subsided more than 12 feet in the past 35 years.

John Moore, a glaciologist at Beijing Normal University in China, and colleagues determined an upper limit of 5.9 feet for global sea level rise by the year 2100, reports NBC News.

This was based on studies of the various dynamics that contribute to rising seas, including the melting of ice sheets in Greenland and Antarctica, the expansion of ocean waters as they warm, and changes in water stored on land.

The worst-case sea-level rise could be reached if countries around the world continue to grow their economies predominantly with fossil fuels, says the study while placing a low probability of 5% for sea-level rise above six feet.

The findings have been published in Environment Research Letters.

Two years ago, a study had already suggested that extracting "fossil" water could be pushing up seas as much as melting glaciers outside of Greenland and Antarctica, which is on the order 0.6 millimetres a year.

In the process of extracting groundwater and applying it for surface irrigation, additional water is being dumped into the hydrological cycle which normally ends up in the ocean. How much of the water applied to the surface infiltrates back into the ground, how much runs off, how much evaporates has not been studied.

Cities built on deltas housing large populations face sinking possibilities as the deltas store a lot of groundwater which when pumped out leads to ground subsidence.