Changing storm dynamics have made flooding a much bigger problem in the US, according to a nationwide study published in Nature Climate Change today. By analyzing historical storm data, researchers have found that the likelihood of a coastal city experiencing severe rainfall and storm surge at the same time is far greater nowadays than it was in the middle of the 20th century. And that combination has substantially amplified the risk of flooding in coastal counties over the course of the past century — counties that house about 40 percent of the US population.

Floods are one of the most common natural disasters in the US. They don't claim as many lives as heat and hurricanes, but they cause a ton of damage. Between 2010 and 2014, the average flood claim was almost $42,000. And that doesn't even take into account the number of people that are displaced following a severe flooding event. That's why researchers are trying to figure out which factors contribute to flooding; it's the kind of information that can really come in handy when rebuilding a city, for instance.

In the study, researchers used records of rainfall, sea level, and hurricanes to assess the relationship between heavy precipitation on land and the abnormal rise of water that occurs during a storm, a phenomenon that's referred to as a "storm surge." They found that, for both US coasts, the kind of weather event that blows water toward the coast — thus causing a storm surge — is now more likely to also cause heavy rainfall over land, compared with weather events that took place in the middle of 20th century.

(Theodore Scontras / University of Maine)

The study "clearly identifies" an increasing connection between storm surges and heavy rain, says Carling Hay, a geophysicist at Harvard University who didn't work on the study — and that's pretty novel. "It's well known that the combination of storm surges and heavy precipitation has the potential to increase coastal flooding; however, I'm not aware of other studies that have examined long term trends in the co-occurrence of the two."

The researchers don't know if climate change contributes to this altered relationship, says Thomas Wahl, an oceanographer at the University of South Florida and a co-author of the study. That's a question that they plan to tackle next.

That said, researchers already have evidence that global warming has led to heavier rainfall in some places, and many think that warming will lead to more heavy rain events globally, says Philip Orton, a physical oceanographer at the Stevens Institute of Technology who didn't work on the study. In addition, researchers have shown that sea level rise causes more coastal floods — and this will intensify as sea level rise accelerates. "Whether this pair of troublemakers will become closer allies in the future is harder to predict," Orton says. "But this paper is one of the first ever to look at the problem over such a broad area, so it's a step in the right direction for better understanding the science, and monitoring how things change."

If researchers can figure out which communities are most at risk and why, they might also be able to limit damage — and increase people's safety.