Issue 487 to

Sea Level Rise and the Future of the Bay Area

How will we adapt to rising tides?

Urbanist Article

See also SPUR's analysis of seven methods for managing sea level rise.

[Image: flickr user Mr. Read]

The Fight of Our Lives

Global warming is one of the biggest challenges the world has ever faced. At long last, it is widely accepted to be a threat and has become the subject of great debate over how to reduce emissions and stabilize the atmosphere that has nurtured humanity for so long. The sooner we act, the better chance we may have to ward off the most devastating impacts of rapid climate change, including stronger storms and hotter days, as well as disease, famine and drought.

In spite of the urgency and clarity of the threat, however, even if we stop producing greenhouse gases tomorrow we still will be unable to stop certain climate impacts, including the rise of worldwide sea levels. Such is the legacy of industrialization and our past century of carbon-fueled growth. Stabilizing concentrations of carbon dioxide will begin the process of stabilizing global temperatures, slowing the rate at which oceans absorb atmospheric heat and slowing down the rise of sea levels. But even with swift and effective climate action today, it will take at least the next 100 years for our efforts to produce significant impacts. Unlike other disasters (such as earthquakes) that require us to plan ahead, rising seas will not present a one-time event from which we can recover. As the world's population increasingly lives — and exerts development pressure — in coastal cities, we must figure out how to adapt. And if we don't make radical progress on reducing emissions in the next few years, our adaptation efforts will be even more necessary, and more severe.

At SPUR, we've convened a blue-ribbon task force to explore how climate change will affect the Bay Area and how we should plan to adapt. In this article, we share with you what we know about sea level rise, one of global warming's most confounding impacts for our region. We also summarize some of the best thinking about what we can do to minimize its worst effects.

Figure 1

The Science of Sea Level Rise

Sea level rise occurs because of two natural processes that have been ocurring since the last ice age ended approximately 10,000 years ago. One process, thermal expansion of the oceans, describes the oceans increasing in mass as they absorb atmospheric and land-generated heat, pushing them higher up onto shore. The second process is the melting of land-based ice, such as glaciers and ice sheets that occupy vast areas of Greenland and Antarctica.

Scientists working with ice cores, coral reefs and computer models have pieced together estimates of historical sea levels, showing wide variations over time — time, that is, on the scale of millennia. In the modern world, tidal gauges have measured a slow and steady sea level rise over the past 150 years. The observed rate varies somewhat, depending on what part of the ocean is being measured. For example, the Indian and the Pacific Oceans near Asia have experienced more rapid rise in the last 15 years than the Pacific Ocean off of the Americas. The oldest tidal gauge still in operation in the United States was installed near the Golden Gate in 1854. Over that time, it has measured a rise of about 2 millimeters per year, or about 8 inches per century. (see Fig. 1)

In the last 10 to 15 years, the rate of global sea level rise has increased by about 50 percent, and is now averaging 3 millimeters per year. The scientific consensus is that human-induced global warming is a major contributor to this accelerated rise. Although this rate of increase may have been matched or even exceeded in past interglacial periods — the intervals of warmer temperatures between ice ages that occur every 20,000 to 40,000 years — different global conditions existed then, as did different patterns of human settlement. It is now widely accepted that the world's coastlines and coastal cities will be faced with seas that are rising faster than ever experienced. In California, we are very likely to experience a sea level rise of 16 inches by 2050 and 55 inches (1.4 meters) by 2100, and much more after that.

Isle of Palms, South Carolina
A hotel in Isle of Palms, South Carolina, attempts to keep the encroaching seas out with mounds of sea bags. [Image: flickr user kmoliver]

Of course, there is uncertainty surrounding these estimates, uncertainty that grows the farther into the future we look. Some of the uncertainty surrounds the feedback loops that could be set off by the rapid melting of large ice sheets. But the latest measurements of global warming emissions and atmospheric carbon dioxide concentrations indicate that the only uncertainty is on the upper boundary. That is, things may be worse than anticipated but are unlikely to be better, so an estimate of 1.4 meters by 2100 may be quite conservative. In 2007, the Intergovernmental Panel on Climate Change released a range of future emissions scenarios, each of which predicted atmospheric changes and global temperatures based on a set of assumptions about global energy use and population growth.1 In 2009, measured production of greenhouse gases exceeded even the IPCC's worst-case scenario. Leading scientists studying sea level rise now think that we are on an unstoppable trajectory that could raise the seas 5 meters (16 feet) in 300 years.2

What Will Sea Level Rise Actually Look Like?

Both the IPCC's projections and the Golden Gate tide gauge only tell us how the world baseline, or "eustatic," sea level is rising. In any one place on the globe, sea level rise can be felt quite differently. Two factors contribute to this. The first is in some northern latitudes, continents are on the rise, still rebounding from 20,000 years ago when they were covered by glaciers three miles thick. These places will experience eustatic sea level rise at a much lower rate than equatorial areas, which tend to be slowly sinking. Land subsidence due to groundwater extraction or the loss of marshes contributes to relative sea level rise being much greater than eustatic sea level rise in some places. For example, some communities of the South Bay, which heavily mined groundwater up through the 1960s, have sunk below today's sea level by as much as 13 feet. Parts of the Sacramento-San Joaquin Delta that have been heavily channelized and diked are now 25 feet below sea level. Areas that are sinking or losing land area or wetlands to erosion will experience the impacts of sea level rise sooner and perhaps with greater intensity.

Figure 2

The second factor is weather. Most of the near-term damage that sea level rise is expected to inflict on developed areas is from surges that occur during storms at the same time as high tides. Storms cause extreme lows in air pressure, allowing the sea level to instantaneously rise above predicted tides. Storms also increase winds, especially onshore winds, that cause bigger, erosive waves. Finally, they bring rain, which increases water volume in creeks and rivers. Approximately 40 percent of California's land drains to San Francisco Bay, which means that storm floods will last longer here than in higher-elevation regions. Under existing conditions, the combination of high tides, storm surges and river flooding can raise water levels in the Delta by 51 inches for as long as a day.3 As sea levels rise, low-lying areas protected by already fragile levees will be even more at risk.

How is sea level rise expected to work in the Bay itself? A recent study conducted by the United States Geological Survey and Deltares, a Dutch research institute, modeled hydrodynamic changes in the South Bay that could result from sea level rises ranging from 0.5 meters to 5 meters.4 The study found that sea level rise in the Bay will be linear with the ocean; that is, the Bay will not amplify or change tidal characteristics. It found that flow speeds, particularly south of the Dumbarton Bridge, will be higher, which could increase erosion there and affect safe shipping at our Central Bay ports. Finally, it found that wave heights will generally increase as a result of higher (deeper) water and higher winds. This will also increase erosion, and pose a potential hazard to recreational uses.

What Can We Do?

There is a continuum of strategies we can use to manage changing sea levels, ranging from armoring the shoreline — keeping the sea out — to abandoning low-lying development altogether. Some of these strategies are familiar and reliable.For example, the Netherlands has used dikes and levees to keep the ocean at bay for hundreds of years, and most of that country's population lives below sea level. Other strategies, such as large-scale wetland restoration projects in the Chesapeake Bay and the southern San Francisco Bay, are ongoing experiments of wetlands' ability to absorb waves, attenuate flooding and protect developed areas while improving the environment. Not all strategies are financially, politically, culturally or environmentally appropriate for all areas. The essence of planning for sea level rise is matching the right strategies to the right locations and conditions.

Today, nobody knows exactly which strategies are best for our region, or where we should apply them.

In the meantime, we are proposing that the many ways of managing sea level rise can be organized into a typology of seven core strategies. (See pages 9-15.) Our summary draws on the work of BCDC, the Pacific Institute, the Netherlands Office of Science and Technology, the Public Policy Institute of California, and entries from BCDC's "Rising Tides" design competition. All of this thinking informs our ongoing work to figure out where and how we should apply these strategies in the Bay Area.

We do know that appropriate and resilient strategy selection may require thinking about a much longer planning horizon than we are accustomed to. We may also have to work more in concert with, rather than in opposition to, natural processes. Today, BCDC and the California Coastal Commission assume for permitting purposes that new development will have a 50- to 90-year life. But we don't really know if that time frame is appropriate, given the permanent changes that sea level rise will bring. Is it a good idea to build new development in the water's path, knowing that flooding won't be likely for 100 or 300 years, but will very likely happen eventually? Or is there a way to keep water from encroaching on some places forever? Is that cost-effective? If we look far enough into the future, it is possible that San Francisco itself, along with every other coastal city, will have to be abandoned. How long should we be planning for? Uncertainty about when rising seas will make their impact felt raises numerous questions about what the appropriate planning horizon should be.

We also know that the Bay of the future is going to need to be designed around more than one of these strategies. We will not be able to recreate the ecological intactness of San Francisco Bay before it was developed, even if we fully retreat from its shore, just as we will not we be able to hold the shoreline at 2009 levels to protect all existing and planned development. The shape of the Bay will become the product of our choice of strategies.

First, stop climate change: SPUR’s recommendations for reducing emissions in SF
SPUR has previously recommended economical and effective ways San Francisco can reduce global warming emissions. Our May 2009 paper “Critical Cooling” analyzed 42 options to reduce emissions in the transportation, energy and waste sectors, and considered but rejected dozens of other ideas. We found that compact land-use planning is the most cost-effective tool that our region can wield to reduce future emissions, for the fundamental reason that it is the most effective way to reduce driving. San Francisco could contribute more to this effort by increasing the amount of housing and jobs to be built within the city, which will reduce regional emissions due to the relatively low carbon footprint of city residents and workers. We found that other powerful and cost-effective ways to reduce emissions include local policies to price parking and roads, increase waste diversion, increase bicycling and improve the energy efficiency of existing buildings.

While San Francisco can take important steps toward curbing greenhouse gas emissions, SPUR found that the State of California can achieve reductions an order of magnitude beyond what our city can do. The state can make a real impact by employing mechanisms such as allowing “pay as you drive” insurance, imposing better fuel economy standards on vehicles and raising the renewable energy standards utilities must meet. Cities have a role to play, but counties and states can make a greater impact because they control more territory and more activity.

Nevertheless, we in San Francisco have perhaps a unique opportunity within America to demonstrate what a truly sustainable city and region would look like. If we can’t find a way to create a carbon-neutral way of life here, with all of our wealth, innovation and political support for tackling climate change, then we are lost.

Read the report at

On the ocean coastline, of course, we have less control, due to the shoreline being much more rugged, and less developed – so fewer strategies are realistic. The coastlines of Marin and San Mateo counties, while home to towns and cities such as Stinson Beach and Pacifica, are uninhabited in many places. San Francisco, of course, has medium-density residential areas — the Sunset and Richmond districts — that were built right over sand dunes to the very edge of the beach. Although most of our region's population and urbanized area is concentrated along the Bay shoreline, we will still need to figure out what to do about development on the ocean coast, especially where critical infrastructure is concerned.

Managing Sea Level Rise

Many public agencies have responsibilities for managing the challenges of climate change. Water supply and wastewater agencies will have to deal with changes in flow, facilities at risk and saltwater intrusion into intake systems. Airports and ports will have to deal with shoreline infrastructure that is not at the right height. Transit and transportation agencies will have to deal with roads, railways and subways vulnerable to flooding. Parks, planning, and redevelopment agencies will have to figure out how to deal with floodwater in residential neighborhoods, especially in those neighborhoods that are least prepared to cope with new risks.

In the Bay Area, there are two special purpose government agencies with jurisdiction over the water that surrounds us: the San Francisco Bay Conservation and Development Commission, and the California Coastal Commission. These agencies have severely limited authority to implement strategic decisions about adapting to sea level rise. BCDC issues permits for filling, dredging, and changes in use in the San Francisco Bay, salt ponds, managed wetlands, and on the shoreline. BCDC makes these permitting decisions in concert with the policies in its long-term guidance document, the San Francisco Bay Plan, which, among other things, specifies which areas along the shoreline should be used for ports, recreation, wildlife refuges and other purposes. However, BCDC's shoreline jurisdiction to regulate development only extends to 100 feet upland from the Bay. In many places, 100 feet inland is well within the elevation that will be flooded by a sea level rise of 1.4 meters. Many people have recently suggested that state law be amended to broaden BCDC's scope of authority to allow better management of rising seas.

Along the ocean coastline, the California Coastal Commission (CCC) shares responsibility for developing coastal plans with 60 cities and 15 counties. Local coastal plans set ground rules for the location and type of land uses that can take place in the coastal zone, as described by law. Typically, LCPs are developed by local governments and certified by the CCC, at which time the commission transfers permitting authority for most new development to the local government. The CCC retains appellate authority over development within 300 feet of the high tide line or the first public road, whichever is landward. About 90 percent of the state's coastal zone falls into an LCP. However, most of these plans were developed in the 1980s, before sea level rise became a well known concern, and there is no legal requirement for them to be updated.

One of the big gaps in our waterway governance system is an entity to manage the Delta, where two major climate adaptation issues have come to a head: water supply and sea level rise. The Delta is the largest estuary on the West Coast, and supplies drinking water to two-thirds of California. It is home to half a million people, and hundreds of species of plants and wildlife — including numerous endemic species. Over the years, a lack of cohesive planning and a lack of a clear manager have left the Delta in crisis. Agricultural and developed areas are severely threatened by sea level rise and by old, unmaintained levees. Delta water is not being managed or supplied well enough to meet growing demand from human uses and ecological restoration. Today's challenges in the Delta give us a glimpse of the extreme vulnerability the Bay Area could face in the future if we don't give our institutions the powers they need to manage the changes coming our way, with the whole region's best interests in mind.

Finally, there is no identified funding source to help local governments develop vulnerability assessments and plan for sea level rise, let alone conduct on-the-ground implementation. Local governments generally are not doing enough on their own to prepare for rising waters.

If we don't make changes in how our shorelines are governed, we will end up with an unplanned and ineffectual solution. Most communities will want shoreline armoring to secure property tax bases and to protect coastal infrastructure in the short term. But these structures diminish ecological and recreational values. If every project or jurisdiction, particularly on the Bay side, hardens its shoreline, we will not build enough flood-attenuating wetlands. We do not want to set ourselves up for a New Orleans-like tragedy in 20, 50 or 100 years because we have too heavily invested in constructing levees and not enough on maintaining them. We need the right combination of approaches to produce an answer that is sustainable, cost-effective and resilient to whatever change in sea level the future brings.

SPUR's task force endeavors to determine what changes are needed to allow our regional institutions to be best equipped to manage rising sea levels.

SPUR’s blue-ribbon task force on climate change adaptation
In the summer of 2009, SPUR formed a panel of experts in infrastructure, planning, sustainable development and environmental protection to try to understand the impacts of climate change on the Bay Area and to make recommendations to manage it. The task force, headed by BCDC executive director and SPUR Board member Will Travis, aims to develop a regional strategy for managing the many threats of climate change, including sea level rise, higher temperatures, extreme weather and drought. Our task force builds on the enormous amount of research being done, particularly by state agencies, to model sea level changes, snowpack and air quality changes, and more. We will apply the best available science to the task of figuring out how people must change the ways and places we live and work to minimize these threats.

So far, the group has developed an at-a-glance threat assessment, studied best practices in climate planning from other cities, and reviewed proposed strategies for addressing rising seas in shoreline development projects such as Treasure Island. Its work will wrap up in 2010 with recommendations for planning, and a map of appropriate sea level rise strategies for various parts of the Bay. SPUR’s climate planning work is generously supported by an Urban Land Institute Community Action Grant and the San Francisco Foundation.

Resources on Sea Level Rise

In the past year, four major reports have assessed how climate change will affect California and what we should do. For further reading:

  • The Public Policy Institute of California completed a suite of studies assessing the state’s preparedness for climate change, including analyses of water management, electricity, ecosystem management, coastal management, air quality and public health. (November 2008) (
  • The Pacific Institute issued “The Impacts of Sea-level Rise on the California Coast,” funded by the Energy Commission, CalTrans, the Metropolitan Transportation Commission, the California Environmental Protection Agency, and more, finding 480,000 people and $100 billion at risk. (March 2009) (
  • In “Living with a Rising Bay,” BCDC described sea level rise in the Bay Area, including maps showing inundation areas for rises projected at 16 inches and 55 inches. (April 2009) (
  • The California Resources Agency published a draft “California Climate Adaptation Strategy,” summarizing the best-known science in seven sectors, with management recommendations. (August 2009) (


Sea level rise is not a new problem, but its historic rate is rapidly increasing, and won't slow down for as long as we can project. Today we have some sense of what it may look like, and what the flooding risks are, for parts of San Francisco Bay and the ocean in 50 and 100 years, and beyond. We also have a broad array of management tools. We can try to stop floodwaters from encroaching on developed areas. Retrofit our developed areas to accommodate floodwaters. Build things that float. Make room for sponge-like wetlands to try to solve our flooding problem and restore the Bay at the same time. Or retreat from shorelines altogether.

Today's challenge is that we don't know where to apply these strategies, and in what combination — or how they will change the Bay. We also don't know how we will pay for them. We do know that we need a regional strategy to develop a cost-effective and equitable solution. Stay tuned as SPUR continues to study this issue, and makes recommendations for how to move forward.


1 Intergovernmental Panel on Climate Change, Climate Change 2007 Synthesis Report, November 2007. (
2 According to the United Nations Conference of the Parties web page, reporting on an Oxford University conference ( 2238).
3 Bay Conservation and Development Commission, "Living with a Rising Bay: Vulnerability and Adaptation in San Francisco Bay and on the Shoreline," April 7, 2009.
4 Arcadis, Deltares, United States Geological Survey and Bay Conservation and Development Commission, "San Francisco Bay: Preparing for the Next Level," Sept. 21, 2009.

Laura Tam is SPUR’s sustainable development policy director.

We would like to thank the Bay Conservation and Development Commission for their thought leadership on this issue.