Treasure Island, SFO sinking as sea levels rise

As sea levels rise, San Francisco gets almost monthly projections from climate researchers about how a creeping water line will affect low-lying areas like the airport, but a new paper published Wednesday in the peer-reviewed publication Science Advances suggests that previous projections downplayed the likely scenario by overlooking certain eccentricities of the land itself.

The paper by Manoochehr Shirzaei (head of Arizona State University’s Radar Remote Sensing and Tectonic Geodesy Lab) and Roland Bürgmann (a planetary scientist at UC Berkeley),”Global climate change and local land subsidence etc,” uses a sensitive satellite network to measure coastal conditions and claims that previous projections underestimated flood risks in certain Bay Area regions by as much as 90.9 percent.

Here’s a few takeaways about the real lie of the land, at least as Bürgmann and Shirzaei have it:

• Not all coasts are created equal. Some places in the Bay Area, particularly those built on landfill, compact and sink as years go by, stooping to meet rising seas: “Satellite system data [shows] subsidence rates of less than two mm/year along most of the coastal areas along San Francisco Bay. However, rates exceed 10 mm/year in some areas underlain by compacting artificial landfill and Holocene mud deposits.”
• When researchers factor in the movement of the land along with predicted sea level rise, the risk of flooding gets worse: “Maps estimating 100-year inundation hazards solely based on the projection of sea level rise from various emission scenarios underestimate the area at risk of flooding by 3.7 to 90.9 percent compared with [our] revised maps.”

Photo by Scott F Smith

• This new statistic, Relative Sea Level Rise [RSLR], may be antagonized by specific phenomena that aren’t a problem for coastal areas in general but can spell trouble for specific hot spots: “The rate of relative SLR can vary significantly due to a number of other processes such as isostatic adjustments, ocean currents, earthquakes, and volcanic episodes, as well as local land subsidence (LLS) associated with sediment and aquifer-system compaction.”
• Which means that, while the entire Bay Area must reckon with climate change, some locales face additional burdens over the next century or so: “Most of the Pacific shorelines and areas adjacent to the San Francisco Bay are subject to subsidence at less than ~2 mm/year. Portions of Treasure Island, San Francisco, San Francisco International Airport, and Foster City are subsiding as fast as 10 mm/year.”
• In fact, the shifting conditions of some of this land would still create flood risks even if sea level rise stopped entirely: “Even if SLR was completely halted, LLS [local land subsidence] alone would put 45 km square at risk. Thus, a much larger area will be affected by inundation once the effect of LLS is taken into account, especially for the more modest SLR scenarios.”
• Even previous, more modest flooding estimates already projected significant hazard in affected areas: “Major consequences of exacerbated inundation risk for coastal areas include saltwater contamination of surface and underground waters, accelerated coastal erosion, wetland losses, and increased flooding. [...] In California by 2100, more than 480,000 people and $100 billion worth of property will be exposed to flood risk caused by SLR.”

To read the full paper, including a detailed explanation of the methodology and calculations, check it out here.

• Science Advances
• Manoochehr Shirzaei
• Roland Bürgmann
• Global climate change and local land subsidence exacerbate inundation risk to the San Francisco Bay Area
• What is glacial isostatic adjustment? [NOAA]