California’s coast is vanishing, sinking hotspots found in LA, San Diego Santa Cruz, San Francisco: Report
These cities have a combined population of 4 to 8 million, who are being exposed to rapid land subsidence, which is a sudden sinking of the Earth's surface
Local hotspots of California’s sinking coast have been identified in the cities of San Francisco, Los Angeles, San Diego, and Santa Cruz, where people will be at a greater risk of flooding during the decades ahead of projected sea-level rise, according to scientists. A combined population of 4 to 8 million people is being exposed in these cities to rapid land subsidence, which is a gradual settling or sudden sinking of the Earth's surface.
“We estimate between 4.3 million and 8.7 million people in California’s coastal communities, including 460,000 to 805,000 in San Francisco, 8000 to 2,300,00 in Los Angeles, and 2,000,000 to 2,300,000 in San Diego, are exposed to subsidence,” says the research team from Arizona State University, US, in the report published in Science Advances.
A majority of the world population lives on low lying lands near the sea, some of which are predicted to submerge by the end of the 21st century due to rising sea levels. Coastal populations are expected to grow to over 1 billion people by 2050 due to coastward migration. Moreover, these coastal lands are subject to subsidence due to natural processes and anthropogenic or human activities, say experts. Subsidence is most often caused by the removal of water, oil, natural gas, or mineral resources out of the ground by pumping, fracking, or mining activities. The natural processes comprise earthquakes, glacial isostatic adjustment, erosion, sediment loading, and soil compaction. Land subsidence can increase flooding risk, wetland loss, and damage to infrastructure by increasing a region’s relative sea-level rise.
The researchers say that as of 2005, approximately 40 million people were exposed to a 1 in 100-year coastal flooding hazard, and by 2070 this number will grow more than three-fold. The value of property exposed to flooding will increase to about 9% of the projected global Gross Domestic Product, with the US, Japan, and the Netherlands being the countries with the most exposure. But these exposure estimates often rely only on projections of global average sea-level rise and does not account for vertical land motion, the team emphasizes. “The future flood risk that communities will face is mainly controlled by the rate of relative sea-level rise, namely, the combination of sea-level rise and vertical land motion. It is vital to include land subsidence into regional projections that are used to identify areas of potential flooding for the urbanized coast,” the authors explain.
Accordingly, for the analysis, the research team tracked the entire California coast's vertical land motion. They used satellite-based interferometric synthetic aperture radar (InSAR), which can detect the land surface rise and fall with millimeter accuracy, to identify places on the California coast that are sinking. They measured the 1350-kilometer long coast of California from 2007-2018, compiling thousands of satellite images over time, used for making a “vertical land motion map with 35-million-pixel at ~80 m resolution, comprising a wide range of coastal uplift and subsidence rates.”
The analysis shows that the four metropolitan areas majorly affected in these areas included San Francisco, Monterey Bay, Los Angeles, and San Diego. This implies that these areas on the California coast are the most vulnerable to land subsidence. “We highlight four low-lying metropolitan regions that are affected by land subsidence: San Francisco Bay Area, Monterey Bay, Los Angeles, and San Diego,” write authors.
The vast majority of the San Francisco Bay perimeter is undergoing subsidence with rates reaching 5.9 mm/year, shows analysis. Notably, the San Francisco International Airport is subsiding with rates faster than 2.0 mm/year. The Monterey Bay Area, including the city of Santa Cruz, is rapidly sinking without any zones of uplift. The rates of subsidence for this area reach 8.7 mm/year. The Los Angeles area shows subsidence along small coastal zones, but most of the subsidence is occurring inland. San Diego and the surrounding area are characterized entirely by subsidence, with rates reaching 2.7 mm/year. The analysis also reveals that areas of land uplift include north of the San Francisco Bay Area (3 to 5 mm/year) and Central California (same rate).
“This study emphasizes the urgency with which the flood resiliency plans must adapt to scenarios in which coastal land elevation drops rapidly. Understanding the key processes driving relative sea-level change enables policymakers to prioritize the risk reduction and adaptation interventions and to better identify the communities and ecosystems most vulnerable to flooding,” the team concludes.