More than 1.8 billion people worldwide are at risk of severe floods, new research shows. Most reside in low- and middle-income countries in Asia, and four out of 10 live in poverty.

The figures are substantially larger than previous estimates. They show that the risk is concentrated among those least able to withstand and recover from flooding.

“I thought it was a valuable paper, indeed. Because this link between poverty and flood risk is kind of overlooked,” said hydrologist Bruno Merz, of the German Research Center for Geosciences, who was not involved in the study.

Flood risk assessments typically consider risk in monetary terms, which is highest in rich countries where more wealth is at stake. The new study focused on how flood exposure and poverty overlap.

Published in the journal Nature Communications, the study combined a global flood risk database with information on population density and poverty. The research focused on places where floods 15 centimeters deep or deeper happen at least once every 100 years on average.

The study found that nearly 90% of people at risk of severe flooding live in poor countries, not rich ones. More than 780 million flood-exposed people live on less than $5.50 per day.

The substantial overlap between high flood risk and poverty feeds into a vicious cycle that further concentrates flood protections in rich countries that have more resources to deal with floods in the first place, said flood risk researcher Jeroen Aerts of the Vrije Universiteit Amsterdam. Aerts was not involved in the study.

“It’s doing a cost-benefit analysis,” Aerts said. “Less money is going to poorer countries, because, of course, if the country is poorer, there are less dollars exposed.” Aerts said that this also happens within countries, which tend to invest in pricey flood protections for wealthy urban centers rather than for poorer rural areas.

The new estimate for global flood exposure is higher than some earlier ones. For instance, one previous study predicted that 1.3 billion people would be exposed to severe floods by 2050 — 500 million fewer than are exposed today, according to the new estimate. The authors attribute their higher number to their use of better data covering more regions at higher resolution and combining the risks from coastal, river and surface water floods.

The study did not consider protections, such as levees or dikes, in its assessment of flood exposure. This “distorts the picture,” Merz said, since some flood-prone populations are well-protected, such as those in the Netherlands.

Rather than undermining the study’s findings, Merz thought that this could mean that an even greater proportion of the people threatened by floods lives in poor regions.

“In many low-income countries, there is no flood protection, so people will be flooded by a small flood … that occurs on average every five years. On the other hand, in Europe, in North America, many of the areas are protected (from floods that happen once every) 100 years, 200 years or even higher. And so, this is not included,” he said.

Unprotected, poorer regions could thus shoulder an even greater share of the actual risks from flood exposure than the paper suggests.

The new result offers a snapshot of flood risk around the world as it is today, not a projection of how it will develop in the future. Climate change is projected to increase the frequency and intensity of floods in much of the world. And although early warning systems have decreased flood fatalities, including in resource-poor regions, population growth in flood-prone areas will also put more people at risk in the future, Aerts said.

“The exposure to natural hazards, exposure to flooding — it’s larger than previously investigated. And the majority of those exposed people live in a vulnerable, poor region,” Aerts said. “I think that’s the takeaway, I think, and maybe one sentence more: This means that investments in … flood adaptation should be targeted at those areas.”

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