Extreme flooding has struck almost every corner of the country over the past year, from rural areas in Tennessee and California to the Michigan suburbs and the streets of Brooklyn, New York. Floods have always been by far the most widespread and costliest weather disaster in the U.S., and they have only gotten worse as climate change has accelerated. Total damages from floods and hurricanes last year eclipsed $100 billion, according to data from the National Oceanic and Atmospheric Administration, or NOAA.

A new study published this week in the journal Nature Climate Change projects that the number of people in the U.S. who are exposed to flooding will almost double over the next 30 years — but not for the reasons you might think. Most new risk will come not from climate change but from population growth in areas that are already vulnerable to flooding. The findings underscore a hard truth with dire implications for climate adaptation policy: The lion’s share of U.S. flood risk does not stem from the changing nature of storms and seas, but instead from our decisions about where to build and where to live.

That’s not to say climate change isn’t playing a major role: The study’s authors found that climate change will render around 700,000 more people vulnerable to flooding by 2050, mostly as a result of rising sea levels and stronger hurricanes. The lion’s share of current flood risk is borne by low-income white communities in places like Appalachia, but the new climate-driven risk that will arrive by 2050 will fall hardest on Black communities. (People of color are more likely to live in flood zones overall.) Many of these are located in coastal cities or hurricane-vulnerable Southern states, which puts them right in the crosshairs of rising seas and whopper storms.

When the authors measured the role of future population growth on flood vulnerability, though, they found an even stronger effect. The report finds that population growth in flood-prone areas will put over 3 million more people at risk of flooding by 2050 — four times the increase that will result from climate change. Unlike the new risk that results from climate change, most of the new risk from population growth will come in places that don’t have very much exposure to flooding right now, from Arkansas to Kansas to Idaho. As cities and suburbs in these areas sprawl out onto untouched land, more people will put themselves in the water’s way.

Read the full article about flood risk by Jake Bittle at Grist.