Under current conservative projections, sea level will rise 1.0 to 1.4 meters by the year 2100, which will inundate 200 square miles of low-lying shoreline areas of San Francisco Bay, impacting wildlife, habitat, and millions of Bay Area residents who rely on at-risk infrastructure. The cost of replacing property at risk of coastal flooding with a 1.4 meter rise in sea levels is $62 billion. We already get a glimpse of what this might look like during extreme high tides, commonly called King Tides, which happen a few times a year. The next ones are next week, on December 21-23, 2014.
Tidal marsh wetlands are both part of the solution, and at risk of disappearing. They capture carbon to lessen global warming, and mitigate the effects of sea level rise by buffering us from floods and storms. But thousands of acres of Bay wetlands are at risk of drowning as sea level rises. Wetlands have evolved with sea level rise, and can resist and adapt to it by building up their elevation or moving upland. However humans have interfered with these natural responses, by changing the climate, obstructing sediment delivery and creating obstacles to migration. It is projected that 95% of San Francisco Bay tidal marsh habitat may be drowned by 2100.
When marshes flood during high tide, plant roots trap mineral sediment from the water, adding new soil to the ground. As sea level rise accelerates and flooding occurs more often, marshes can react by building soil faster. Below ground, the growth and decay of plant roots adds organic matter as well. Given access to sediment rich waters, wetlands can gain elevation, keeping pace with sea level rise. If sediment delivery to a wetland is cut off or reduced due to upstream restrictions, tidal wetlands can no longer build soil to outpace rising seas, and will drown. Sediment flow has been disrupted by development, landfills, and levees around the bay, while upstream dams and reservoirs reduce sediment load by 20%.
Another natural response to sea level rise is migration. As sea levels rise, current wetlands can go underwater, while new mudflats are constructed and new tidal zones grow upland. This is possible if there are no obstacles impeding the wetlands migration. Examples of obstacles are roads, railroads, levees and buildings. Ironically, conventional ways of protecting coastal properties, such as dikes and seawalls, impede wetland migration, and in some cases increasing vulnerability to floods and storms. It is estimated that the San Francisco Bay wetlands require approximately 93 square miles (60,000 acres) of land into which they must migrate to survive a sea level rise of 1.4 m. (See Map of SF Bay county viable accommodation areas for wetland migration.)
Whether wetlands continue to survive rising seas depends largely on us. In order to ensure the long term viability of our wetlands, we need to protect their sediment delivery, as well as preserve the inland areas to ensure their viability as wetland habitat in the future. This means halting development adjacent and directly inland to wetlands, planning our cities with an eye towards sea level rise and wetlands survival, and building smarter flood controls using horizonal levees fronted by wetlands.
Scientists agree that the Bay needs 100,000 acres of tidal marsh to thrive. Less than half that exists. Save The Bay is working to double the amount of Bay tidal marsh over the next ten years, while continuing to battle development that not only threatens current and restorable wetlands, but also the future viability of those wetlands as we and they adapt to sea level rise.