Tide-Gauge Data Archaeology in Coastal Louisiana Reveals Relative Sea-Level Trends Up to Twenty Times the Global Average since 1950

Coastal Louisiana, particularly the Mississippi River Delta region, faces some of the largest rates of relative sea-level rise worldwide (>9.3 mm/yr since 1947 at Grand Isle). These large rates are dominantly driven by (nonlinear) land subsidence, but larger-scale oceanic processes in the Gulf of Mexico and the adjacent North Atlantic have also contributed to these rates, particularly over the past ~15 years. In the past, relative sea-level rise in the region has usually been approximated by means of the Grand Isle tide gauge record even though it is well known that local processes, such as subsidence and hydrologically driven processes, can vary significantly locally. Here we introduce a set of thirty-one daily tide-gauge records maintained by the U.S. Army Corps of Engineers and located throughout southern Louisiana. However, there exist several inhomogeneities within these records, including undocumented datum shifts, which necessitated the development of a homogenization framework to properly analyze trends. Given the unique issues with the dataset, particularly the spatial isolation of some tide gauges, we develop a new approach using probabilistic principal component analysis to homogenize these records in place of the traditional buddy-checking approach. Significant spatial and temporal variability in long-term sea-level trends is found in these newly homogenized records. The Lower Mississippi Delta region (also known as the Birdsfoot) stands out with the largest long-term trends on the order of 35 mm/yr, more than three times the value obtained at Grand Isle and more than twenty times the value obtained from the global average. We identify subsidence as the main driver of these changes and provide new evidence that oil and gas withdrawals have significantly contributed to them.