Vulnerability to Sea Level Rise

How will rising tides affect Bay Area communities?

Vulnerability to Sea Level Rise

Definition: 
Vulnerability to sea level rise refers to the share of the historical and current Bay Area population located in areas at risk from forecasted sea level rise over the coming decades. Given that there are varying forecasts for the heightened high tides (i.e., mean highest high water mark), projected sea level impacts are presented for six scenarios ranging from a one foot rise to six feet. A neighborhood is considered vulnerable to sea level rise when at least 10 percent of its land area is forecasted to be inundated by peak high tides in the coming years.

As a result of global climate change caused by greenhouse gas emissions, sea levels are expected to rise over the coming decades. Despite our region’s commitment to reducing these harmful emissions, the Bay Area is not immune to this global trend that may seriously affect low-lying coastal areas across the world. Understanding which neighborhoods are most at risk from rising tides should help the region set key priorities when it comes to regional resilience, adaptation and future growth patterns.

Regional Performance
Bay Area development patterns are neither increasing nor decreasing the regional risk from sea level rise.

Depending on the magnitude of sea level rise over the coming decades, between three to seven percent of Bay Area residents may be vulnerable to rising tides. While this is just a small share of the region’s population, the impacts to these communities may be dramatic, with broader consequences felt beyond directly impacted communities. Even with the most modest forecast for sea level rise – just 12 inches from today’s levels – 220,000 residents will be affected. Looking at historical growth patterns since 1990, the share of our region’s population living in low-lying zones has remained steady. This indicates that new housing developments are occurring at the same rate in areas at risk from sea level rise as in the region overall.

Local Focus
While no Bay Area county is immune from the threat of rising tides, the coastal counties of San Mateo and Marin are perhaps the most at risk.

The unique geographical features of each Bay Area county – combined with the location of existing residential developments – have a strong influence on the number of residents who may be vulnerable to future sea level rise. Marin County, with much of its population clustered in low-lying areas close to the Bay, is the most at risk, with approximately 27 percent of its population expected to be affected by a six-foot rise in tides. Even under a much more conservative forecast of just 12 inches of sea level rise, more than one in ten Marin residents would be affected. The situation is only slightly better in San Mateo County, with its similarly low-lying communities such as Foster City and Redwood City.

Notably, the region’s most populous county – Santa Clara – is the least at risk from sea level rise, thanks to smaller residential populations in communities close to the Bay. While numerous salt ponds inevitably will be flooded in the event of sea level rise – thus expanding the size of the Bay significantly – few residents live in or near these zones. Only 14,500 Santa Clara residents would be affected by a six-foot sea level rise scenario – approximately one percent of the county’s current population.

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Sea Level Rise

Select sea level rise forecast:

Impacted Land Area

Population at Risk of Impacts

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Sources: 

National Oceanic and Atmospheric Administration: Sea Level Rise Maps (2015)

San Francisco Bay Conservation and Development Commission: Sea Level Rise Maps (2015)

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U.S. Census Bureau: Decennial Census

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U.S. Census Bureau: American Community Survey

5-Year Population Estimates (2008-2012)

Image: Flickr (Creative Commons license), Photographer: Vicki Moore, https://www.flickr.com/photos/vickimoore/2757400175

Methodology Notes: 

Projected areas of inundation were developed by BCDC at one-foot intervals ranging from one foot to six feet of sea level rise. To determine the impacts on historical and current populations, inundation areas were overlaid on a U.S. Census shapefile of 2010 Census tracts using Census Bureau population data. Because census tracts can extend beyond the coastline, the baseline scenario of zero feet was used to determine existing sea level coverage of census tracts. Sea level rise refers to the change from this level, and the area of the tract was determined by measuring the component of the tract area not currently under water. This area, rather than the total tract area, was used as the denominator to determine the percentage of the census tract that is inundated under future sea level rise projection scenarios. When at least 10 percent of tract land area is inundated with a given sea level, its residents are considered to be affected by sea level rise. Note that SLR scenarios therefore do not reflect periodic inundation due to extreme weather events, which may lead to an even greater share of residents affected on a less frequent basis.