False-color composite image of the Mississippi River delta, as acquired by the ASTER instrument on NASA’s Terra satellite on May 24, 2001. Image provided by the USGS EROS Data Center Satellite Systems Branch as part of the Earth as Art II image series (Courtesy of USGS EROS Data Center Satellite Systems Branch)

A river delta is, by definition, a place in flux — coastal land naturally sinks, and is naturally rebuilt by the flow of a vast river that carries in new sediment. Across the globe, from the Amazon to the Nile to the Yangtze, we humans rely on such deltas for the many benefits they bring — access to fisheries, good locations for shipping, and much else.

But we don’t just rely on them — we change them. We dam rivers upstream and channelize them downstream – actions that reduce the flow of sediment and, thus, the growth of land. Meanwhile, we cut channels through wetlands and cause land to sink further by pulling lots of oil and gas and water out of it.

Such changes have an insidious consequence: Even as they entice larger populations to live on deltas by providing jobs and the semblance of environmental stability, they also increase long-term vulnerability to storms and floods by exacerbating the land’s sinking. Meanwhile, global warming makes the whole dynamic worse because it introduces a huge arrow pointing in only one direction — sea level is going up, and up, and up.

Now, a sweeping new study released in Science Thursday takes a comprehensive look at how much such factors are increasing the vulnerability of 48 major global river deltas, the home to 340 million people — including the Mississippi River delta, which lies between the city of New Orleans and the sea.

“We characterized the rate of change of risk in delta systems due to combined land subsidence, sea level rise, geophysical setting, and socio-economic capacity to protect themselves,” explains lead study author Zachary Tessler of the City University, who conducted the research with scholars from three additional universities.

Here’s an image Tessler provided with the research, showing the loss of delta wetlands in Louisiana over the past four decades:


Four decades of coastal land and wetland loss in the Mississippi River Delta, as observed by Landsat (USGS/NASA Landsat Program). Credit: Zachary Tessler (Data: USGS/NASA Landsat Program)

The study used multiple datasets and indices to examine changing risks to river deltas based upon 1) their inherent risk of storms and floods; and 2) how much humans are exacerbating those risks by changing their landscapes in a way that causes land to sink, even as seas rise.

Along the first axis, deltas range from the Orinoco in South America, which faces relatively few flooding and storm threats, to the Yangtzee and Han, where these risks are very high.

When it comes to human perturbations of the system, meanwhile, river deltas range from the pristine Yukon in Alaska, which is relatively lowly populated and has been little changed by humans in a way that would make it more vulnerable (much of it is a wildlife refuge), to the Ganges-Brahmaputra of Bangladesh and India — home to over 100 million people, where human activities and sea level rise are pushing the risks forward dramatically.

Based on these two factors — how much human activities and sea level rise are changing a delta and worsening its plight, and how much it is exposed to storm and flood risks — the study found a complex tapestry of changing risks, depending on the delta. On the one hand, the Yukon wasn’t at much additional risk at all. “It’s definitely clear that some of these deltas are almost completely untouched, that tends to be at the high latitudes,” says Tessler.

However, most of the deltas showed at least some increased risk, and some showed quite a lot of it. In particular, the heavily populated Krishna and Ganges-Brahmaputra deltas had the most rapidly growing risk of flooding related disasters. “They have…lots of dams, lots of development on the coastline, wetland loss — we expect the relative sea level rise that’s happening there to have a large impact on the risk outcomes of the communities,” Tessler says.

Indeed, Bangladesh has historically been the home to the deadliest tropical cyclone (or hurricane) disasters known to history, including the 1970 Bhola cyclone, which struck this very delta, the Ganges-Brahmaputra, and caused 300,000 to 500,000 deaths.

But there’s another key factor involved in determining a given delta’s risks — a society’s wealth, which translates into its ability to protect itself. Thus, while the Mississippi River is heavily managed and has been changed nearly beyond recognition by humans, the delta also has a high level of artificially imposed resilience — with vast levees along much of the river to prevent flooding, not to mention New Orleans’ huge new hurricane protections.

A similar story can be told about the Rhine, heavily altered by humans but also the home to arguably the world’s most impressive flood protections, constructed by the Dutch.

“A high capacity for investment in risk-reducing technologies…is the primary reason that several wealthy, developed deltas today have relatively low risks,” the study found.

But that may not always be the case. The research found that if the cost of building flood-defense infrastructure increases steadily over 100 years — as might be brought on, for instance, by rising energy prices — then the rich but heavily exploited deltas like the Mississippi would see a spike in vulnerability. In that scenario, a delta like the Mississippi “reverted back toward expectations based on geophysical hazards and anthropogenic change alone.”

That’s a sobering message this year, as most observers of New Orleans 10 years after Hurricane Katrina are likely to look at its gigantic and costly hurricane protections and see them as a sign of reassurance. And they are — but Tessler is suggesting that it’s a little more complicated than that.

“If you want to maintain your [level of] risk, in light of increasing cost, you need to spend increasing amounts of money,” he says. “And while you’re doing that, you’re not addressing the fact that land subsidence is increasing. So your costs are rising.”

Deltas, after all, have been doing their thing for thousands upon thousands of years — flooding, washing in sediment, rebuilding land. But we changed the system — meaning that simply building higher, on sinking land, may not be enough of a sustainable solution.