Over the last two decades, as San Antonio and surrounding Bexar County, Texas, grew by more than 600,000 people, some 17% of the city’s blocks experienced a decrease in population.
That delta is largely due to flood risk that climate change exacerbates, according to a new report by the First Street Foundation, a data nonprofit with the mission of communicating climate hazards.
Bexar — sitting in a swath of Texas known as Flash Flood Alley — is part of a national trend of hyper-local migration to avoid flooding, which is hollowing out blocks within cities, the report finds. The research is based on a model, published Monday in the journal Nature Communications, that looks at population changes using granular US Census Bureau data and controls for factors besides flooding, such as nearby job opportunities and school quality.
In all, First Street finds, 3.2 million Americans moved away from high-flood-risk areas between 2000 and 2020. The full extent of the migration has been hidden, however, since most people didn’t move far.
“There appear to be clear winners and losers in regard to the impact of flood risk on neighborhood-level population change,” Jeremy Porter, head of climate implications research at First Street, said in a statement. “The downstream implications of this are massive and impact property values, neighborhood composition and commercial viability, both positively and negatively.”
The analysis also extrapolates these trends 30 years into the future, predicting that vulnerable areas will continue to lose population.
In the US, the frequency of disasters causing at least $1 billion in damages has gone from roughly three a year during the 1980s to an annual average of 17.8 over the period 2018 to 2022, according to the National Oceanic and Atmospheric Administration. Global warming has knock-on effects that exacerbate flooding in particular, including sea level rise, more ferocious hurricanes and more frequent and extended downpours.
Below, a Detroit area local news report about recent flooding in Southeast Michigan. No mention of the word “climate”, but neighbors talk about floods becoming more frequent, area disaster manager mentions “significant rainfall that we’re not used to seeing.”
It’s no secret that some climate migration is underway, whether along the Gulf Coast, which is losing a football field’s worth of land every 100 minutes, or in California, where whole towns have been sent into diaspora by wildfires. However, quantifying the impact of climate change on migration down to the neighborhood level has never been done on a national scale. First Street focused solely on floods, since flooding is the most common weather-related disaster.
The researchers created a model that looks at population change down to the smallest geographical unit used by the US Census, the census block. They overlaid that with historical flood data. They then tried to isolate the influence of flooding on migration as compared to other social and economic factors commonly associated with moving, like excellent or poor schools.
They found that when between 5% to 10% of properties in a census block are at risk of flooding, there is a tipping point, and people begin moving out even if there are other attractive amenities, such as a view of the coast.
In some cases this movement is enough to leave previously thriving areas in decline. In many other cases it is just enough to slow otherwise red-hot growth. Looking deeply at Bexar County, First Street found that neighborhoods with lower flooding risk grew much faster than those with higher risk.
What’s a climate abandonment area? It’s a census block where flood risk has grown high enough to start pushing people to leave. Many of these areas lie along the Texas Gulf Coast, coastal Florida, and the mid-Atlantic.
But it’s by no means confined to these regions, which can get hit repeatedly by storms during the Atlantic hurricane season. Climate abandonment areas are spread throughout the US in places where heavy rainfall, tropical cyclones, and coastal and river flooding are becoming bigger problems.
“People understand which parts of their community to avoid and which parts of their community are more safe, and they’re acting on that,” says Jeremy Porter, demographer and head of climate implications research at the First Street Foundation that led the study. “People, if you look within housing markets, are being much more thoughtful about where to live.”
The phenomenon is more pronounced when you zoom in to see how people are moving from neighborhood to neighborhood. When people think about climate change affecting migration, they might picture someone moving far from home to another part of the country. But that’s just a small slice of overall migration trends. The majority of people move within the same city, county, or metro area, Porter points out.
Obviously, global warming was invented to create migrants.