The sea level rise that will occur around the world as a result of climate change has been predicted to submerge many coastal regions around the San Francisco Bay by 2100. However, a recent study published in the online journal Science Advances has found that – due to sinking land – the flooding will be even worse than previously thought.
Researchers from the University of California, Berkeley and Arizona State University used precise mapping and measurements of subsidence around the Bay Area from 2007 to 2011 in order to map out the waterfront areas that will be impacted by various estimates of sea level rise in the next 100 years. The results showed that, depending on how fast sea level rises, the regions at risk for flooding may be as much as twice what had been previously estimated just from sea level rise alone.
Past studies of this area did not take subsidence into account, and estimated that between 20 and 160 square miles of San Francisco Bay shoreline are at risk of flooding by the next century. When the effects of sinking ground along the shoreline were added, the researchers found that the area at risk of inundation rose to between 48 and 166 square miles.
“We are only looking at a scenario where we raise the bathtub water a little bit higher and look where the water level would stand,” says Roland Bürgmann, a UC Berkeley professor of earth and planetary science and senior author of the study. “But what if we have a 100-year storm, or king tides or other scenarios of peak water-level change? We are providing an average; the actual area that would be flooded by peak rainfall and runoff and storm surges is much larger.”
Low-end estimates of flooding are based on conservative predictions of sea level rise by 2100, which stand at about one and a half feet. However, these estimates are now being questioned, as ice sheets in Greenland and West Antarctica are melting faster than expected. Some extreme estimates are now as high as five and a half feet. With this in mind, the subsidence that geologists found makes less of a difference in extreme cases. Most of the Bay Area is subsiding at less than 2 millimeters per year, but subsidence was found to be as high as 10 millimeters per year in some areas.
“Flooding from sea level rise is clearly an issue in many coastal urban areas,” says Bürgmann. “This kind of analysis is probably going to be relevant around the world, and could be expanded to a much, much larger scale.”
Additionally, flooding is not the only problem that comes with rising seas and sinking land. When previously dry land becomes flooded, it causes saltwater contamination of surface and underground water, which increases coastal erosion and wetland losses. The consequences of these findings have wide-ranging effects.
“Accurately measuring vertical land motion is an essential component for developing robust projections of flooding exposure for coastal communities worldwide,” explains Patrick Barnard, a research geologist with the U.S. Geological Survey in Menlo Park. “This work is an important step forward in providing coastal managers with increasingly more detailed information on the impacts of climate change, and therefore directly supports informed decision-making that can mitigate future impacts.”