Climate change: Sea rise could kill vital marshes

By Peter Fimrite, Chronicle Staff Writer
Thursday, November 17, 2011

The critical tidal marshes of San Francisco Bay – habitat for tens of thousands of birds and other animals – will virtually disappear within a century if the sea rises as high as some scientists predict it will as a result of global warming.

The sea would inundate the coastline and eliminate 93 percent of the bay’s tidal wetlands if carbon emissions continue unchecked and the ocean rises 5.4 feet, as predicted by scientists under a worst-case scenario, according to a new study by PRBO Conservation Science.

The tidal areas closest to the Golden Gate, including Richardson Bay in Marin County and much of the East Bay coastline, were identified as most vulnerable to sea level rise.

“Marshes cannot keep up with the high-end sea level rise predictions,” said Diana Stralberg, a research associate with PRBO, also known as the Point Reyes Bird Observatory, and the lead author of the study, which was published Wednesday in the online science journal PLoS One.

“If we can’t slow down sea level rise,” said Stralberg, who is working on a doctorate degree at the University of Alberta, “we will need to identify and protect areas where marshes can migrate to.”

The researchers measured the depth of mud, sediment and plant material in the existing marshes along the San Francisco Bay coastline and analyzed the impact on the wetlands under a variety of different scenarios.

Charting sea level rise

The report, the first comprehensive look at the impact of climate change on bay wetlands, started with a 1.6-foot sea level rise this century, a level that scientists consider very optimistic, and then moved up in increments to 5.4 feet.

The 93 percent reduction in tidal marshland would occur over the next 50 to 100 years only if the worst projections come true and assuming the bay does not suddenly become awash in new sediment, according to the report.

Previous studies have predicted that future storms would increase runoff from the state’s rivers and cause huge swaths of land to erode into the ocean over the next several decades, conceivably increasing the amount of mud available for marsh expansion.

Increased runoff

The PRBO report said that under the best-case scenario – that is, with only a 1.6-foot rise in sea level – the marshes would actually increase in size over the next century.

“Some of the marshes will move upslope in areas where there are no barriers,” said Julian Wood, the San Francisco Bay program manager for PRBO and a co-author of the study. “In that case, we will increase marsh extent.”

But that is a very unlikely scenario, he said. Climate scientists say the sea level at the mouth of San Francisco Bay has already risen almost 8 inches over the past century. About 45 billion tons of carbon dioxide is spewing into the atmosphere worldwide every year. As a result, recent estimates of the impact of global warming on the ocean off the California coast show a rise in sea level of between 6 and 16 feet by the end of the century.

California is implementing a greenhouse gas reduction law, which Stralberg said could be helpful in reducing sea level rise, especially if other states and nations follow suit.

Bay tidal marshes are vital to migratory birds, various rodents, fish and invertebrates, according to conservationists. The marshlands filter out pollutants, sequester carbon and act like giant sponges, protecting communities, roadways and businesses from flooding. The abundant food and habitat in wetland areas also helps sustain commercial fisheries.

It is not too late to protect bay wetlands, researchers said, especially considering that the major effects of global warming are not expected to hit until at least 2050 and the mass inundation of coastal wetlands probably won’t happen until the end of the century.

IDing expansion areas

Stralberg said communities around the bay must begin identifying areas where tidal marshes can expand and where new wetlands can be created. The best locations, she said, are undeveloped lowland areas, including diked sloughs, where coastal expansion is not blocked by levees, developments, roads, parking lots or other barriers.”The priority should be in sediment-rich areas,” Stralberg said. “At this point any low-lying land is potential marsh, and it would be wise to consider them for marsh restoration, considering we have so little land available. Those areas, in my opinion, should not be considered for development.”About 8,000 acres have been identified by researchers, much of it in the Napa and Suisun areas, along the Petaluma River and in the South Bay.

“We must start thinking now about where tidal marshes could move up to – the future potential wetlands,” she said.

Marsh maps

View maps of where the marshes would be under various scenarios over the next 100 years

E-mail Peter Fimrite at

This article appeared on page A – 1 of the San Francisco Chronicle

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