Climate Change’s Threat to Bay Wetlands

May 1, 2010
Deb Self, Executive Director
by Deb Self, Executive Director

Wetlands are among the most important ecosystems on Earth and provide a number of valuable functions that may be compromised due to climate change and the associated sea level rise. Not only are they highly productive, providing habitat for many specialized plants and animals, but wetlands also control the flow of water in adjacent water bodies and filter out pollutants that would otherwise harm aquatic ecosystems.

Because of their critical functions, wetlands are often thought of the kidneys of the earth, and their role in the San Francisco Bay is no exception. Wetlands along the margin of San Francisco Bay capture and remove metals, nutrients and other contaminants from runoff. Because wetlands can absorb and hold water like a sponge, they also protect people and property from flooding and reduce the forces associated with storm surges and wave impact. In the Bay Area, wetlands provide critical habitat to many rare and endangered species and offer migratory birds on the Pacific Flyway the opportunity to rest and refuel along their route.

The valuable ecological services that wetlands provide to local communities and the Bay ecosystem are threatened by global climate change. As temperatures rise, an increasing percentage of our precipitation in our watershed (which includes the Sierra Nevada Mountains) will fall as rain rather than snow. The snow in the Sierra Nevada also will melt earlier in the spring, so there will be even less freshwater flowing into the Bay during the dry season. If the flow of incoming freshwater is reduced, salty ocean water will push further upstream into the Delta, making naturally brackish water more saline. Warming air temperatures are also causing polar ice to melt and sea levels to rise. If sea levels rise to the predicted height of up to a meter by the end of this century, the shoreline of the San Francisco Bay will be dramatically altered. Even the estimated 16 inches of sea level rise by 2050 (see cover photo) would have a devastating effect on the Bay Area.

The first areas to be affected will be those along the margins of the Bay—the tidal marshes. Even though ecosystems around the world are at risk, Bay Area tidal marshes may be especially vulnerable to climate change because of expected shifts in salinity and inundation. The plants that give wetlands their structure and provide habitat for animals are adapted to the specific conditions at their location in the estuary. Plants in freshwater and brackish marshes may be unable to tolerate higher levels of salinity during the dry season. As a result, saltwater-adapted plants will move further into the estuary, and freshwater plants, like bulrush and cattails, will be displaced.

The disappearance of fresh and brackish water marshes, which tend to be more productive than saltwater marshes, will result in an overall loss of biodiversity and profound impacts to the estuary’s food web. Pacific cordgrass and pickleweed, two common wetland plants, can tolerate salt water but not extended periods of flooding. As rising sea levels overtake tidal marshes, they will become mudflats—areas that are completely submerged at high tide and cannot support plants. While mudflats are biologically necessary ecosystems on which shore birds depend, many other species depend on the tidal marshes that the mudflats would replace.

To escape rising sea levels, wetlands will either have to achieve gradual elevation gains by accumulating sediment, or they will have to move gradually landward. If the wetlands were to shift landward, tidal marshes would become mudflats, and the upland transition areas that are now adjacent to marshes would become the tidal marshes. In most areas around the Bay, however, this will be impossible. As the population of the Bay Area grew, buildings and roads were constructed over these critical upland areas.

Even in newly restored wetlands, upland transition areas are designed to be steep and narrow, allowing little to no space for marshes to migrate. With Bay waters encroaching on the fixed boundary of adjacent development, the loss of Bay Area wetlands is almost inevitable. The ability of wetlands to keep pace with sea level rise by accumulating sediment is highly uncertain.

Also uncertain is the impact of climate change on the biological communities that depend on wetlands. Although climate change is a gradual process, impacts may occur faster than species can adapt. Wetland plants and animals depend on fairly stable conditions yet the shifting and flooding of wetlands will cause significant disturbance. Invasive species, like smooth cordgrass, tend to thrive in disturbed ecosystems. Smooth cordgrass is also more tolerant of salt water and inundation than its native counterpart. Unfortunately, smooth cordgrass is too dense for the foraging habits of many shorebirds, like the endangered California clapper rail, and other endangered species such as the salt marsh harvest mouse. If smooth cordgrass replaces Pacific cordgrass under the stress of climate change, such species may be excluded from the marsh. Rising sea levels, and the associated extreme high tide events and storm surges, may also flood the nests of birds that breed within the marshes. As tidal marshes are squeezed against urban development, sea level rise is likely to reduce both the amount and the quality of tidal marsh habitat. The marshes that do remain will be unable to support the number of animals they once did.

The Bay Area will face many challenges this century as the effects of climate change take hold of the region. The loss of wetlands, which protect water resources and shorelines, will only exacerbate the many impacts of climate change on the San Francisco Bay. The benefits of water filtration, flood control, and ecosystem maintenance will all be lost as rising seas consume wetlands. Because healthy wetlands can sequester carbon, their destruction may even intensify climate change by releasing greenhouse gases into the atmosphere. 

Aggressive protection of existing wetlands and the remaining upland transition areas around the Bay may be the only way to preserve the natural services of wetlands that will protect the water quality of the Bay when rising sea levels encroach on developed lands. Efforts are now underway to expand the extent of wetlands in the South and North Bay and research is addressing how we can manage sediment to build up levels of existing wetlands in the hope of keeping pace with sea level rise. Baykeeper supports efforts to protect critical upland transitional wetlands around the Bay margins, including several critical areas that are in imminent threat of development: Newark Areas 3 and 4, the Redwood City Salt Ponds and Point Molate.

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