Sea level rise is increasing fastest in populous coastal areas, study says

Coastal communities are experiencing sea level rise four times worse than global water rise, according to a new study released Monday.

Groundwater pumping, extraction of materials from the ground and sediment production are all happening near the coasts and that is causing the land to actually sink -- compounding the effects of a rising sea level.

It is no coincidence that these are the same locations where people live, worsening the impacts and increasing the vulnerability.

Many of the largest, most populated cities in the world are built along the deltas of major rivers, where there is the added exposure of rivers connecting to the ocean.

• Sign up for The Climate Barometer, delivered to your inbox every week

Much of the coast is uninhabited by people, but where there is civilization, there tends to be a greater rise in water levels.

According to the study, it quantifies "global-mean relative sea-level rise to be 2.5 mm per year over the past two decades. However, as coastal inhabitants are preferentially located in subsiding locations, they experience an average relative sea-level rise up to four times faster at 7.8 to 9.9 mm per year."

COASTAL LANDS ARE SINKING

This is the first ever study that factors in land subsidence into current sea level rise observations globally.

"We've actually quantified (sea level rise) and are able to get the relative magnitude. And it's surprising -- it's surprisingly large. We're making the point that climate change is bad and climate induced sea level rise is bad," Robert Nicholls, lead author of this research and director of the U.K.'s Tyndall Centre for Climate Change Research, told CNN.

"But we have this additional process that is making things even worse. And of course, these things add up. It doesn't really matter whether the sea rises or the land sinks, the people living on the coast still have the same impacts."

Sea level rise is happening in many parts of the world. Where the land is rising, sea level rise is not as significant. Not as many people live where the land is rising, however.

But where the land sinks, the relative rise of the sea is higher -- and unfortunately that is where people tend to live. In fact, more than one in five people live along the coastline where the sea level is increasing at 10 mm (or 0.4 inches) or more per year, despite the fact that it encompasses less than 1% of the world's coastline.

In other parts of the world, like parts of the southeastern U.S., geological changes are not big contributors.

"There's places where the land isn't really moving much at all," said Brian McNoldy, a senior research associate at the University of Miami who was not a part of the study. "And you really are just seeing the effects of ocean levels increasing."

One of the biggest contributors to this subsidence is river deltas.

"Deltas are where rivers bring sediment to the sea," Nicholls said. "And the weight of the sediment plus the compression with the sediment causes consolidation ... So you don't get (rising land) with the deltas, you just get sinking and that can be exacerbated by groundwater withdrawal and drainage."

"Rapid rates of subsidence in deltas and especially cities on deltas are also human-caused, mostly due to groundwater pumping, also oil and gas extraction, and sediment resupply prevented by upstream dams, flood defenses, sand extraction or mining."

Scientists have already been aware of the implications of human-linked climate change to sea level rise, but now there is research that investigates rising and lowering land also caused by humans.

"The process that we're really talking about here is fundamentally down to where people choose to live. And then the fact that they actually made the subsidence worse."

There is a natural cause of the rising and sinking lands, however. According to the study, the melting ice sheets during the ice age thousands of years ago lead to and is still causing rising land near Hudson Bay in Canada.

ASIA EXPERIENCING HIGHEST RATE OF SEA LEVEL RISE

Coastal sections of Asia have been the most impacted by sea level rise in relation to land subsidence. That's because there is a prevalence of deltas and very populous cities.

"South, Southeast and East Asia is noteworthy, as these regions collectively contain 71% of the global coastal population below 10 m in elevation," according to the research.

"In Jakarta, subsidences of over 10 centimetres per year -- It may be even locally faster than that. You can get very, very large changes, but in very small areas," Nicholls said. "But they're important because lots of people live there."

The map below highlights just a few of the many rivers and canals within Jakarta. The combination of so many people and the multitude of rivers has made this city especially prone to relative sea level rise.

In the U.S., cities like New Orleans, which is near the Mississippi River delta, are also sinking. This correlates with one of the highest rises in relative sea level in the country.

According to NOAA, the greatest relative sea level rise has been measured near coastal Louisiana and southeastern Texas.

"A place where the combined effects of sinking land and sea level rise is in the northern Gulf Coast areas, like coastal Louisiana," said McNoldy, of the University of Miami.

In Galveston, Texas, sea levels have risen 6.62 mm per year or about one-quarter of an inch per year during the time period of 1957 to 2011. NOAA said this is "equivalent to a change of 2.17 feet in 100 years."

When asked what can be done about this issue, Nicholls said mitigating the threats of climate change is most crucial.

"I think the important thing is we have a great effort, and rightly so, to actually mitigating climate change and the Paris Agreement," Nicholls said.

The report also says reducing groundwater withdrawal and managing deltas can reduce land subsidence.