A majority of the world’s population lives on lowland near the sea, some of which are projected to be flooded by the end of the 21st century due to rising sea levels.
The most important amount to assess the effects of sea level change in these communities is the relative sea level rise – the difference in altitude between the height of the Earth’s surface and the height of the sea surface. For an observer standing on the shore, the relative rise in sea level is the net change in sea level, which also involves the rising and falling of the earth beneath the feet of the observer.
Now, using accurate measurements from the highest art-based synthetic interferometric synthetic aperture (InSAR) radar, which can detect the rise and fall of the earth’s surface to millimeter accuracy, an Arizona State University research team has followed, for the first time, by the entire vertical ground movement of the California coast.
They have identified local drowning coastal hotspots 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 landslides, who will be at a higher risk flood in the decades before projected sea level rise.
“We have created a new era of coastal maps in detail greater than 1,000 times higher and resolutions than ever before,” said Manoochehr Shirzaei, who is the lead investigator on the NASA-funded project. “The unparalleled detail and accuracy of the sub-millimeter solved in the vertical ground motion database can transform the understanding of natural and anthropogenic changes to the relative level of sea level and the associated risks.”
The results were published in this week’s issue of Advances in science.
The research team included graduate student and lead author Em Blackwell, and Manoochehr Shirzaei faculty, Chandrakanta Ojha, and Susanna Werth, all from ASU School of Earth and Space Research (Werth has a dual appointment to the School of Urban Geography and Planning) .
Em Blackwell had a keen interest in geology, and as Blackwell started graduate school, InSAR applications attracted them to pursue this project. InSAR uses radar to measure the difference in distance between the satellite and the Earth’s surface, producing highly accurate deformation maps of the Earth’s surface at 10s m resolution over 100s km spatial extent.
Land degradation can occur due to natural and anthropogenic processes or a combination of them. Natural processes include tectonics, isostatic regulation of glaciers, sediment loading, and soil compaction. Anthropogenic causes include groundwater extraction and oil and gas production.
Since 2005, nearly 40 million people were exposed to a 1 in 100 risk of coastal flooding, and by 2070 that number will more than triple. The value of assets exposed to floods will increase to about 9% of projected Global Gross Domestic Product, with the US, Japan and the Netherlands, the countries with the largest exposure. These exposure estimates often rely solely on global average sea level rise forecasts and do not take into account vertical land movement.
The study measured the entire 1350-kilometer-long California coastline from 2007-2018, compiling 1,000 satellite images over time, used to make a vertical map of terrestrial motion with a 35-pixel pixel resolution of 80 m, including a wide range of coastal ups and downs. Coastal community policymakers and the general public can download the data (link to additional data).
The four metropolitan areas mainly affected in these areas include San Francisco, Monterey Bay, Los Angeles and San Diego.
“The vast majority of San Francisco Bay perimeter is declining at rates reaching 5.9 mm per year,” Blackwell said. “In particular, San Francisco International Airport is declining at rates faster than 2.0 mm / year. The Monterey Bay area, including the city of Santa Cruz, is sinking rapidly with no take-off area. Reduction rates “For this area they reach 8.7 mm / year. The Los Angeles area shows declines along small coastal areas, but most of the depletion is happening on land.”
Landslide zones include north of the San Francisco Bay Area (3 to 5 mm / year) and Central California (same rate).
Moving forward in the decades ahead, the coastal population is expected to grow to over 1 billion people by 2050, due to coastal migration. The future flood risk that these communities will face is mainly controlled by the relative sea level rise rate, respectively, the combination of sea level rise and vertical land movement. It is essential to include land reduction in the regional forecasts used to identify potential flood areas for urbanized coasts.
Beyond the study, the ASU research team hopes that others in the scientific community can base their findings on measuring and identifying coastal hazards more widely in the US and around the world.
The study says the seas could rise faster than previously thought
“Following California’s coast from space: Implications for relative sea level rise” Advances in science (2020). DOI: 10.1126 / sciadv.aba4551
Provided by Arizona State University
citation: Satellite Study Shows California Coast Hotspots (July 31, 2020) Retrieved July 31, 2020 from https://phys.org/news/2020-07-satellite-survey-california-coastal-hotspots.html
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