It's easy enough to imagine rising sea levels as a geographically isolated problem. Me, I live inland at a sufficient elevation to not be threatened by even a few meters of sea level rise, at least directly. I'll catch plenty of climate change hell, but probably won't find myself living in an aquarium.
Most research on the effects of rising sea levels looks at immediate, direct outcomes—who is going to be underwater and when. But that's just the beginning of the story. The United States' largest population centers suddenly rent uninhabitable will cause mass migration via several different mechanisms, and this effect has been much less studied. What happens to the rest of the country when its biggest, fastest-growing cities are no longer tenable?
This is what University of Georgia geographer Mathew Hauer set out to explore in new simulations described in a paper published Monday in Nature Climate Change. The results suggest bad things, generally, with large-scale population redistribution putting intense pressure on unprepared inland communities. At the extremes, Hauer identifies Florida and Texas, with the prior losing some 2.5 million residents due to rising sea levels (RSLs) and the latter gaining nearly 1.5 million residents from newly depopulated regions (see below).
"Infrastructure challenges required to protect coastal communities are well documented, but the infrastructure challenges of accommodating millions of SLR migrants in largely unprepared inland municipalities is virtually unexplored," Hauer writes. "For many destinations, such as Riverside California, Phoenix Arizona, Las Vegas Nevada, and Atlanta Georgia, already experiencing water management and growth management challenges, the SLR migrants who wash across the landscape over the coming century could place undue burden in these places if accommodation strategies are left unplanned."
Hauer identifies the likely recipients of SLR migrants by examining current US migration patterns according to IRS datasets (basically what locations people are filing tax returns in as a function of time). Obviously, this is imperfect, as people faced with forced mass-migration can't really be depended upon to follow the "normal" migration patterns of 1990s/2000s United States. Environmental studies professor Jeroen Aerts points this out in a commentary accompanying the new study.
"This research, and the necessity of making and justifying assumptions in migration models, highlights that there is much we still do not know about how climate change will influence migration," Aerts writes. "Although migration is a common strategy for upgrading socio-economic conditions, movement is costly and disruptive and is perceived as last-resort adaptation. It is also unclear whether people will follow IRS migration patterns when faced with longterm sea-level rise."
It's also unclear who will be migrating. Hauer assumes that people that make a lot of money are likely to remain in flooded areas because they have the means to adapt. And yet moving requires money as well, so there will likely remain a population of "trapped" residents unable to migrate. At the same time, research has demonstrated that people tend to behave irrationally in crisis mode, which throws another wrench into the problem of predicting migration patterns.
Nonetheless, it's a crucial task. With regions poised to take on millions of displaced migrants, being unprepared is the worst possible outcome.
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