Climate change will put tens of millions more people around the world at risk of exposure to flooding rivers over the next 25 years, an alarming new study reports—unless policymakers invest in significant adaptation measures.

While rising sea levels can increase the risk of coastal flooding, the study focuses instead on fluvial floods, which happen when rivers overflow their banks. As global temperatures rise, storms and heavy rainfall events are expected to become more severe in many parts of the world, increasing the risk of these flooding events.

According to the study, published yesterday in Science Advances, more than half the United States alone will need to double existing flood protections—through the building of new dikes, levees and other infrastructure updates—to prevent additional people from being affected in the future. Other regions around the world, including central Europe and parts of Africa, Central and South America, as well as large areas of India, Pakistan, Indonesia and China, will also need to take similar measures to protect their populations.

Otherwise, millions more people in each region may be hit by future floods. In the United States and parts of Canada, the affected population could rise from 100,000 people to a million, a tenfold increase. In China, high-end flood risks could rise from 24 million affected people to 55 million.

And that's only if human populations remain at their current levels, the researchers note. If communities in affected areas continue to grow over the next few decades, even more people could be at risk.

"We ask the question: How much do you have to improve your local protection levels now in order to keep the flood risk in the next 25 years the same as you had it in the last 25 years?" said study co-author Anders Levermann, a climate scientist at the Potsdam Institute for Climate Impact Research.

Populations in different parts of the world experience differing levels of flood protections. For instance, infrastructure in many U.S. states is designed to protect citizens from floods severe enough that they should only be expected to occur once every 100, or even 500, years. In other parts of the world, protections are designed for much less severe events, meaning greater proportions of the population would be affected by more damaging floods.

But the new research investigates the amount of action that must be taken in any region simply to keep conditions at their current levels—in other words, to prevent any more of the population from being affected than is already at risk. That means parts of the world with relatively low flood risks, such as the United States, may need to work similarly hard just to keep those dangers from rising as regions with much higher flood risks.

The research shows that "the adaptation need is similarly high in highly developed countries in North America, Europe, and East Asia, as well as for developing countries in Africa, Asia, and South America," the study authors write.

The researchers focused only on the next few decades of climate change mainly because "the next 25 years of climate change are already determined by the emissions that we have carried out so far of greenhouse gases," Levermann noted. Even if nations around the world halted their carbon emissions tomorrow, the climate system would still experience additional warming for years afterward. It's a phenomenon known as "climate inertia"—the idea that some climate effects don't occur immediately after emissions are put into the atmosphere, but take some time to unfold.

This means the change in flood risk over the next few decades is likely to be the same regardless of global climate mitigation efforts. If immediate measures are taken to reduce greenhouse gases over the next few decades, these risks may not rise much more throughout the rest of the century, Levermann pointed out. But if not, flood risks beyond the next 25 years will likely grow even more severe.

The researchers conducted their study using the help of hydrological and climate models, which helped them simulate future climate change and its effects on precipitation and global water systems. They were able to calculate the risks to populations in different regions using a special database of flood protection standards around the world.

The paper comes on the heels of a new report, just issued by NOAA, that found 2017 was the most expensive year on record for damage caused by natural disasters. The cumulative cost of all the events, including storms, wildfires, floods and droughts, was more than $300 billion. Included on the list were the February flooding in California and the April floods in Missouri and Arkansas. Both events were spurred by heavy rainfall.

The new research reinforces the need for immediate adaptation efforts to prepare for the next few decades, if even more damage is to be prevented. And it underscores the idea that even greater flood risks can only be avoided through strict climate mitigation, starting now.

"Everything beyond the next 25 years will be determined by what we are doing now," Levermann said.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.