Disappearing cities on US coasts

Abstract

The sea level along the US coastlines is projected to rise by 0.25–0.3 m by 2050, increasing the probability of more destructive flooding and inundation in major cities1–3. However, these impacts may be exacerbated by coastal subsidence—the sinking of coastal land areas4—a factor that is often underrepresented in coastal-management policies and long-term urban planning2,5. In this study, we combine high-resolution vertical land motion (that is, raising or lowering of land) and elevation datasets with projections of sea-level rise to quantify the potential inundated areas in 32 major US coastal cities. Here we show that, even when considering the current coastal-defence structures, further land area of between 1,006 and 1,389 km2 is threatened by relative sea-level rise by 2050, posing a threat to a population of 55,000–273,000 people and 31,000–171,000 properties. Our analysis shows that not accounting for spatially variable land subsidence within the cities may lead to inaccurate projections of expected exposure. These potential consequences show the scale of the adaptation challenge, which is not appreciated in most US coastal cities.

Variable vertical land motion for sea level rise projections

“…High temporal variability with respect to its surroundings indicates locations with significant trend fluctuations, while low temporal variability suggests constant trends throughout the observation period. This analysis advances the state of high-resolution VLM observations for the purpose of improving relative sea level projections, as previous studies [13,[17][18][19] neglected internal variability and based their analysis on the data modeled solely as linear processes, despite the evidence to the contrary [12,20]. On a regional scale, our InSAR-VLM confirms a known 1-3 mm/yr widespread tectonic-induced land uplift in northern CA (associated with the Cascadia subduction zone) [21,22], and subsidence around San Francisco and San Diego (associated with the San Andreas Fault System) [22,23](Fig.…”

“…The IPCC AR6 framework [3,7] relies on historical data (geological and model rates) interpolated on tide-gauge locations for VLM contributions to relative sea level projections (IPCC-VLM, [6,7]). The results can be at odds with estimates of contemporary (1993 to present) VLM derived either from InSAR [13,19], nearby GNSS sites [9,20], or indirectly by subtracting gridded altimetry from tide-gauge data [40,41]. In Fig.…”

“…[10][11][12]). Interferometric Synthetic Aperture Radar (InSAR), on the other hand, provides high spatial resolution estimates of VLM over large areas [13] that can fill these gaps, but requires GNSS as tie-in points for its placement in a common reference frame [14]. Geodetic observations from both GNSS and InSAR are limited by record length extending back 2-3 decades [15,16].…”

“…As such, simple affine transformation (e.g. [12,13]) is not adequate for a translation of relative large-scale InSAR displacement into a GNSS reference frame. We develop a GNSS displacement rate model as a calibration plane to accommodate for this spatially variable alignment between GNSS and InSAR.…”