Climate-driven depopulation of a low-elevation coastal zone

The latest IPCC report projects that regardless of the climate scenario, the highest rates of future sea-level rise will occur along the west-central US Gulf Coast. Meanwhile, recent research has shown that coastal wetlands in the Mississippi Delta are unable to survive rates of sea-level rise higher than 3 mm/yr, a number that was exceeded two decades ago. Thus, the 1M+ inhabitants surrounded by marshland will have to adapt to rapidly changing environmental conditions. Here we adopt an interdisciplinary approach to assess this problem.

The archeologic record shows that indigenous people adapted quickly to changing conditions in the rapidly evolving Mississippi Delta, abandoning areas subject to transgression and settling on prograding delta lobes. Present-day populations are much less nimble, yet rapid coastal degradation (notably wetland loss) has been related to the population decline that has already commenced in this region. While catastrophic events (i.e., major hurricane strikes) are commonly thought of as driving population loss, we argue that socio-economic factors (notably a dwindling home insurance industry) may become equally important. One key question is how much continued sea-level rise this region will see.

The last interglacial (LIG, ~125,000 years ago) featured a global average temperature that reached about 0.5-1.5 °C above pre-industrial values. Remnants of a LIG shoreline have been identified in SE Louisiana more than 100 km landward of the present shoreline, with a reconstructed sea level of 3.1 ± 0.8 m higher than present (7.5 ± 1.1 m after correction for fault motion). Since anthropogenic climate change (~1.5 °C in 2024) has already brought us near the upper end of LIG warming, it is plausible that future sea-level rise to such an elevation is already locked in, although the timescale for this to play out remains uncertain. With respect to the LIG shoreline, the New Orleans metropolitan area is located on the “wrong” side.

If future warming is kept well below Paris Agreement levels (2 °C) the shoreline may eventually stabilize at a position comparable to that from the LIG. Conversely, if Paris goals are exceeded, sea level can be expected to rise to an extent that puts other metropolitan areas, farther inland and at slightly higher elevation, in jeopardy as well. The next few decades will be decisive as to whether Paris climate goals are met. As a consequence, the ultimate fate of several million inhabitants, along with trillions in economic and ecologic capital, will likely be determined by mid-century.