Monday, June 21, 2021

Satellite Survey Shows California’s Sinking Coastal Hot Spots

A majority of the world’s 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. The most relevant quantity for assessing the impacts of sea-level change on these communities is the relative sea-level rise — the elevation change between the Earth’s surface height and sea surface height. For an observer standing on the coastland, relative sea-level rise is the net change in the sea level, which also includes the rise and fall of the land beneath the observer’s feet.

Areas of land uplift included north of the San Francisco Bay Area (3 to 5 mm/year) and central California (same rate). In the decades ahead, the coastal population is expected to grow to over 1 billion people by 2050, due to coastward migration. (Image: via pixabay / CC0 1.0)
Areas of land uplift included north of the San Francisco Bay Area (3 to 5 mm/year) and central California (same rate). In the decades ahead, the coastal population is expected to grow to over 1 billion people by 2050, due to coastward migration. (Image: via pixabay / CC0 1.0)

Now, using precise measurements from state-of-the-art satellite-based interferometric synthetic aperture radar (InSAR) that can detect the land surface rise and fall with millimeter accuracy, an Arizona State University research team has, for the first time, tracked the entire California coast’s vertical land motion. They’ve identified local hot spots of the sinking coast, in the cities of San Diego, Los Angeles, Santa Cruz, and San Francisco, with a combined population of 4 to 8 million people exposed to rapid land subsidence, who will be at a higher flooding risk during the decades ahead of projected sea-level rise. Manoochehr Shirzaei, who is the principal investigator of the NASA-funded project, said:

The results were published in the journal Science Advances. The research team included graduate student and lead author Em Blackwell, and faculty Shirzaei, Chandrakanta Ojha, and Susanna Werth, all from the ASU School of Earth and Space Exploration (Werth has a dual appointment in the School of Geographical Sciences and Urban Planning). Blackwell, who prefers they/them pronouns, had a keen early interest in geology, and as they began graduate school, the applications of InSAR drew them to pursue this project.

Coastal elevation in California. Coastal zones, which are defined to be those with elevations less than 10 meters, are shown in red. Segments of the coast with elevations higher than 10 meters are colored by a yellow gradient. (Image: USGS NED)
Coastal elevation in California. Coastal zones, which are defined to be those with elevations less than 10 meters, are shown in red. Segments of the coast with elevations higher than 10 meters are colored by a yellow gradient. (Image: USGS NED)

InSAR uses radar to measure the change in distance between the satellite and ground surface, producing highly accurate deformation maps of the Earth’s surface at 10s m resolution over 100s km spatial extent. Land subsidence can occur due to natural and anthropogenic processes or a combination of them. The natural processes comprise tectonics, glacial isostatic adjustment, sediment loading, and soil compaction. The anthropogenic causes include groundwater extraction and oil and gas production. 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 threefold.

The value of property exposed to flooding will increase to about 9 percent of the projected global gross domestic product, with the U.S., Japan, and the Netherlands being the countries with the most exposure. These exposure estimates often rely only on projections of global average sea level rise and do not account for vertical land motion. The study measured the entire 1,350-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 pixels at ~80 m resolution, comprising a wide range of coastal uplift and subsidence rates. Coastal communities’ policymakers and the general public can freely download the data.

The four metropolitan areas majorly affected in these areas included San Francisco, Monterey Bay, Los Angeles, and San Diego, Blackwell said:

Areas of land uplift included north of the San Francisco Bay Area (3 to 5 mm/year) and central California (same rate). In the decades ahead, the coastal population is expected to grow to over 1 billion people by 2050, due to coastward migration. The future flood risk that these 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.

Provided by: Arizona State University [Note: Materials may be edited for content and length.]

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Troy Oakes
Troy was born and raised in Australia and has always wanted to know why and how things work, which led him to his love for science. He is a professional photographer and enjoys taking pictures of Australia's beautiful landscapes. He is also a professional storm chaser where he currently lives in Hervey Bay, Australia.
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