When Hurricane Helene, the 420-mile-wide, slow-spinning conveyor belt of wind and water, drowned part of Florida’s coastline and then barged its path northward through North Carolina last week, it destroyed more than homes and bridges. It shook people’s faith in the safety of living in the South, where the tolls of extreme heat, storms and sea level rise are quickly adding up.

Helene was just the latest in a new generation of storms that are intensifying faster, and dumping more rainfall, as the climate warms. It is also precisely the kind of event that is expected to drive more Americans to relocate as climate change gets worse and the costs of disaster recovery increase.

Researchers now estimate tens of millions of Americans may ultimately move away from extreme heat and drought, storms and wildfires. While many Americans are still moving into areas considered high risk, lured by air conditioning and sunny weather, the economic and physical vulnerabilities they face are becoming more apparent.

One study by the First Street Foundation, a research firm that studies climate threats to housing, found that roughly 3.2 million Americans have already migrated, many over short distances, out of flood zones, such as low-lying parts of Staten Island, Miami and Galveston, Texas. Over the next 30 years, 7.5 million more are projected to leave those perennially flooded zones, according to the study.

All of this suggests a possible boom for inland and Northern cities. But it also will leave behind large swaths of coastal and other vulnerable land where seniors and the poor are very likely to disproportionately remain.

The Southern United States stands to be especially transformed. Extreme heat, storms and coastal flooding will weigh heavily on the bottom third of this country, making the environment less comfortable and life within it more expensive and less prosperous.

The young, mobile and middle class will be more likely to leave to chase opportunity and physical and economic safety. That means government — from local to federal — must now recognize its responsibility to support the communities in climate migration’s wake. Even as an aging population left behind will require greater services, medical attention and physical accommodation, the residents who remain will reside in states that may also face diminished representation in Congress — because their communities are shrinking. Local governments could be left to fend alone, but with an evaporating tax base to work with.

In December, the First Street Foundation created one of the first clear pictures of how this demographic change is unfolding. It looked at flood risk and migration patterns down to the census tract, across the country, and identified hundreds of thousands of so-called abandonment zones where the out-migration of residents in response to rising risk had already passed a tipping point, and people were making small, local moves to higher ground.

The research contains plenty of nuance ⎯ cities like Miami may continue to grow overall even as their low-lying sections hollow out. And the abandonment areas it identified were scattered widely, including across large parts of the inland Northeast and the upper Midwest. But many of them also fall in some of the very places most susceptible to storm surges from weather events like Helene: Parts of low-lying coastal Florida and Texas are already seeing population declines, for instance.

In all, the First Street report identified 818,000 U.S. census blocks as having passed tipping points for abandonment ⎯ areas with a combined population of more than 16 million people. A related peer-reviewed component of the organization’s research forecasts that soon, whole counties across Florida and Central Texas could begin to see their total populations decline, suggesting a sharp reversal of the persistent growth that Florida has maintained as climate pressures rise, by the middle of this century.

Such projections could turn out to be wrong ⎯ the more geographically specific such modeling gets, the greater its margin of error. But the mere fact that climate research firms are now identifying American communities that people might have to retreat from is significant. Retreat has not until recently been a part of this country’s climate change vernacular.

Other research is putting a finer point on which Americans will be most affected. Early this year Mathew Hauer, a demographer at Florida State University who has estimated that 13 million Americans will be displaced by rising sea levels, was among the authors of a study that broke out what this climate-driven migration could mean for the demographics of the United States, examining what it might look like by age.

Hauer and his fellow researchers found that as some people migrate away from vulnerable regions, the population that remains grows significantly older. In coastal Florida and along other parts of the Gulf Coast, for example, the median age could increase by 10 years this century — far faster than it would without climate migration.

This aging means that older adults — particularly women, who tend to live longer — are very likely to face the greatest physical danger. In fact, there is notable overlap between the places that Hauer’s research suggests will age and the places that the First Street Foundation has identified as the zones people are abandoning.

The exodus of the young means these towns could enter a population death spiral. Older residents are also more likely to be retired, which means they will contribute less to their local tax base, which will erode funding for schools and infrastructure, and leave less money available to meet the costs of environmental change even as those costs rise. All of that is very likely to perpetuate further out-migration.

The older these communities get, the more new challenges emerge. In many coastal areas, for example, one solution under consideration for rising seas is to raise the height of coastal homes. But, as Hauer told me, “adding steps might not be the best adaptation in places with an elderly population.” In other places older residents will be less able and independent, relying ever more on emergency services. This week many of Helene’s victims have simply been cut off, revealing the dangerous gaps left by broken infrastructure, and a mistaken belief that many people can take care of themselves.

In the future authorities will have to adapt the ways they keep their services online, and the vehicles and boats they use, in order to keep flooded and dangerous places connected. Such implications are worrisome. But so is the larger warning inherent in Hauer’s findings: Many of the effects of climate change on American life will be subtle and unexpected. The future demographics of this country might look entirely unfamiliar. It’s past time to give real thought to who might get left behind.