Parts of San Francisco are sinking faster than the sea is rising

By Michael Le Page

Rising seas aren’t the only problem facing low-lying coastal areas. Many of these areas are also sinking, vastly increasing the risk of flooding.

In the San Francisco Bay area, sea level rise alone could inundate an area of between 50 and 410 square kilometres by 2100, depending both on how much action is taken to limit further global warming and how fast the polar ice sheets melt. But when land subsidence is also taken into account, the area vulnerable to flooding during high tides and storm surges rises to between 130 and 430 square kilometres.

That’s the conclusion of Manoochehr Shirzaei at Arizona State University and Roland Bürgmann at the University of California, Berkeley. They used satellite data from 2007 to 2010 to work out how land heights changed in the Bay area at this time.

A few areas such as Santa Clara Valley were rising slightly, likely because of increased groundwater storage. But most areas were sinking slightly, by 1 or 2 millimetres per year.

Slowly sinking

Some places, including parts of the city itself, plus San Francisco International Airport and Foster City, were sinking by up to 10mm per year. That’s because these areas are built on natural mud deposits, or landfill sites that are still compacting.

The subsidence means these areas are sinking even faster than sea level is rising because of global warming: currently 3mm per year and accelerating. The results show the importance of taking land subsidence into account when calculating the risk from coastal flooding, the researchers say.

“From 1990 to 2010, most people viewed sea level rise as pouring water into a bath tub,” says James Syvitski at the University of Colorado at Boulder. “But most people are now including land subsidence in their calculations.”

In 2009 Syvitski and colleagues showed that subsidence of up to 150mm a year is a major problem in most of the world’s river deltas, affecting cities from New Orleans to Bangkok and Shanghai. A major cause is the extraction of groundwater, gas and oil. The blocking of flooding is another: without annual deposits of fresh material, deltas subside fast due to compaction.

Journal reference: Science Advances, DOI: 10.1126/sciadv.aap9234

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• climate change
• floods
• oceans