Some of the world’s expanding coastal cities face a two-pronged threat involving water: Sticking giant straws into the ground to suck up freshwater can cause the ground below to sink at the same time that sea levels are rising.

That interplay between subsiding land and rising seas highlights an underappreciated risk in global climate change, according to scientists.

It’s not known how many people live on coastal lands that are sinking due to excessive groundwater pumping, but about 150 million live within 3.3 feet of today's high-tide mark. And the worst-case scenario for sea level rise by the end of this century is nearly six feet, according to a recent study.

"The thing about the subsidence is that it is localized, but where it happens it is bad," Jay Famiglietti, a hydrologist and groundwater expert affiliated with the University of California, Irvine, and NASA's Jet Propulsion Laboratory in Pasadena, Calif., told NBC News. "It can be a foot or more a year, which is a lot."

Some areas in Bangkok, Thailand, for example, have sunk more than three feet since the mid-1970s. Other coastal regions at the confluence of rapid population growth and sinking land include parts of Bangladesh, India and China, noted John Moore, a glaciologist at Beijing Normal University in China, and a co-author of the study on sea-level rise published Oct. 10 in Environmental Research Letters.

"In China there is a huge amount of new (construction) on reclaimed coastal land that's within perhaps three meters (10 feet) of sea level, so the cities, of course, are under big threat from storm flood events," he told NBC News.

A recent warning from the insurer Lloyd's of London pointed to Jakarta, Indonesia, as the world's fastest sinking megacity, with some areas of it having subsided more than 12 feet in the past 35 years.

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Worst-case sea level rise

Moore and colleagues determined an upper limit for global sea level rise by the year 2100 using a combination of expert opinion and 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 of 5.9 feet could be reached if countries around the world continue to grow their economies predominantly with fossil fuels, as they are now, noted Moore. "There is only, in our estimation, a 5 percent probability of exceeding that" nearly six-foot rise by 2100, he said.

Determining an upper limit for sea level is a tricky, largely because significant scientific uncertainty remains on the processes that drive ice sheets to melt, Moore and colleagues note in their study.

Groundwater gray area

Another gray area comes in calculating the contribution to sea level from groundwater pumping. A 2012 study published in Geophysical Research Letters suggested 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 millimeters a year.

"You are basically extracting water that was stored hundreds or thousands of years ago and you're adding additional water to the hydrological cycle which normally ends up in the ocean," Yoshihide Wada, a global water and food security researcher at Utrecht University in the Netherlands who led the groundwater study, told NBC News.

According to Famiglietti, Wada's finding and similar ones from other research groups likely overstate the significance of the effect of groundwater pumping on sea level rise. He is currently using data from NASA's gravity sensing satellites to study the issue and, in unpublished work, said he sees a zero-sum trend. As some areas lose water, he noted, others get wetter, which tends to balance things out.

In addition, he said, "we just don't have good measurements to say with any confidence when you extract groundwater for pumping, for irrigation, and you apply it to the surface, … how much infiltrates back into the ground, how much runs off, how much evaporates."

Nevertheless, he added, excessive groundwater pumping that leads to subsidence "is a very big issue." Where it is happening, and to what degree, however, is currently poorly studied. A good place to look, he said, is cities built on deltas. "Some big fraction of the population lives on deltas," he noted. "And those deltas actually store a lot of groundwater. When you pump it out, there is a lot of subsidence."

On the rebound

On the flipside, Moore and colleagues note in their paper, some cities such as Stockholm, Sweden, are actually rising on the order of a few millimeters a year due to a phenomenon known as post-glacial rebound, where the land springs up in the absence of the weight of ice sheets from the last ice age.

"That isn't going to be a dramatic effect except for a few places around Scandinavia and maybe Canada," Moore said. "They are not massive at-risk cities for future sea level rise. Most of the population is fairly outside of that region that was depressed by the large ice sheets in the glaciation."

One other effect to consider when planning for sea level rise locally is the amount of gravitational pull from the ice sheets in Greenland and Antarctica. Currently, both are so large they actually attract higher seas. As the ice melts, this pull relaxes, causing regional sea levels to fall. Thus, if Greenland melts faster than Antarctica, for example, sea levels could actually fall in northern Europe.

"You get a bigger rise if Antarctica melts preferentially because we don't get any gravitational bonus, if you like, from Antarctica in Europe and North America," Moore explained. "But our most likely guess is that they won't melt that differently in the sort of 100-year time scale."