The risk from sea level rise to California’s Sacramento-San Joaquin Delta system is frequently discussed. As sea level rises, the argument goes, the risk of failing levees increases. With a substantial share of California’s farmland and population depending on water supplies that flow through the delta, failed levees is an enormous risk. Levee failures, as the delta’s low-lying islands fill with water, would draw in sea water, saltying up the farm and municipal water systems that draw from the delta.

But a new paper from Benjamin Brooks of the University of Hawaii and colleagues adds a wrinkle. The land is sinking, Brooks argues, faster than sea level is rising. The team used space-based synthetic aperture radar (which goes by the name “InSAR”). This is not the well-known phenomenon of delta islands sinking as their peat soils is exposed to air and oxidizes:

Because the measurements are insensitive to subsidence associated with peat thickness variations over Delta-island length scales, it is most likely that InSAR rates reflect underlying Quaternary sedimentary column compaction.

This is the vast deposits of sediment below the peat layer, which built up since the end of the last ice age. Brooks et al. put the subsidence rate at 3 to 20 mm per year. The latest satellite estimates put global sea level rise at around 3 mm per year.