Changes in rainfall due to global warming are sure to raise the possibilities of river flood throughout the globe. At present, already fluvial floods are the most usual and destructive natural disasters.

Researchers have now computed the necessary increase in flood protection across the globe till the 2040s, dividing it into single regions and cities. They discovered that the requirement for adaptation is highest in the United States, specific parts of Africa and India, Indonesia and in Central Europe (including Germany). Passivity will leave millions of people unprotected from acute flooding.

“More than half of the United States must at least double their protection level within the next two decades if they want to avoid a dramatic increase in river flood risks,” stated Sven Willner, lead author of the study, who hails from the Potsdam Institute for Climate Impact Research (PIK). In the absence of additional adaptation steps, for example increasing building standards, enhancing dykes, relocating settlements or improved river management—the number of people hit hard by the worst 10% of all river flooding occurrences will increase in a number of places: in Northern America, from 0.1 to 1 million. Although this may not be a large number, it is still an increase of ten times. In the case of Germany, it could increase by seven times, from 0.1 to 0.7 million.

Elsewhere, the absolute values are still bigger—in South America, the number of people hit by flooding risks is anticipated to increase from 6 to 12 million, in Africa from 25 to 34 million and in Asia from 70 to 156 million. In the future, the real figures might be greater than this because further urbanization and population growth have not been considered.

“Even in developed countries with good infrastructure the need for adaptation is big.”

The research is based on comprehensive computer simulations performed by using prevalent data on rivers from various sources. “While this data is not perfect for each and every river in the remotest corners of our planet, it certainly is sufficient for places where a lot of people live, a lot of financial values are accumulated, and where flood risks are substantial—we know enough about the places that matter,” explained Willner. Data related to changes in evaporation, rainfall, and similar occurrences have been acquired from the worldwide largest modeling intercomparison project of climate impacts (ISIMIP), coordinated by Katja Frieler at PIK. The spatial detail of the new research is approximately 10 times more accurate than in usually adopted climate computer simulations.

“We have been surprised to find that even in developed countries with good infrastructure the need for adaptation is big,” stated Anders Levermann, co-author of the study, who is head of global adaptation research at PIK and a researcher at Columbia University’s Lamont-Doherty Earth Observatory in New York. “Our yardstick is that people want to keep the protection level they have today—they don’t want things to become worse. Consequently, in countries with a fairly good level of protection, much has to be done to keep the same level of protection and prevent that people indeed have to leave their homes due to flooding.”

“If we do not limit climate change, risks will surpass our abilities to adapt.”

The elevation in the risks of river flood for the next 20-30 years will be governed by the quantity of greenhouse gases that have been already liberated into the atmosphere; therefore, it is not dependent on whether or not we restrict and contain global warming. “However, it is clear that without limiting human-caused warming to well below 2 degrees Celsius, river flood risks in our century will increase in many regions to a level that we cannot adapt to,” stated Levermann. “To keep people safe climate-change-induced risks must be taken seriously and money must be spent for adaptation. If we act now, we can protect against the risks of the next two decades. But further climate change must be limited by cutting greenhouse gas emissions from fossil fuels to avoid risks that surpass our abilities to adapt.”