Rising seas to displace millions on coasts, demographers warn

A man walks on a ledge of the old Sutro Baths in front of roiling surf in San Francisco. A study reports that 700,000 coastal residents in the Bay Area could be forced to move from their low-lying neighborhoods because of rising sea levels. less A man walks on a ledge of the old Sutro Baths in front of roiling surf in San Francisco. A study reports that 700,000 coastal residents in the Bay Area could be forced to move from their low-lying ... more Photo: Paul Chinn, The Chronicle Buy photo Photo: Paul Chinn, The Chronicle Image 1 of / 24 Caption Close Rising seas to displace millions on coasts, demographers warn 1 / 24 Back to Gallery

Millions of Americans living along the nation’s coasts will be forced to move inland by the end of the century as sea levels rise and storm surges become more frequent in this era of global climate change, a team of demographers foresees.

And nearly 700,000 coastal residents in the Bay Area would be forced to move from their low-lying neighborhoods under the most extreme conditions, the forecasters say.

According to their study, published Monday in the journal Nature Climate Change, America’s coastal areas, where as many as 13.1 million people will be living, could be prone to flooding by 2100. In the Bay Area, 250,000 people living in low-lying parts of San Mateo County would be forced to move, and in San Francisco some 33,000 would be affected, the researchers calculate.

The county-by-county analysis of America’s coastal states was put together by demographers at the University of Georgia who have tracked future population trends facing the predicted rise of oceans as the world grows warmer.

Their forecasts, they say, indicate that that if sea levels rise 3 feet by the end of this century, many sections of America’s coastal lands will be underwater and a total of 4.2 million people will have been forced to move toward higher ground.

And if the oceans rise by as much as 6 feet, the drowning lands will have sent 13.1 million Americans moving to higher ground, the demographers say.

Climate records show that sea levels around the world have been rising steadily since at least 1900, and NASA satellites have determined that the rate has doubled in the past 20 years.

Population trends

Glaciers melting on Greenland and throughout the world, Antarctica’s vast ice sheets falling into the sea, and the ocean water itself expanding as temperatures warm are among the causes, climate scientists say.

Mathew E. Hauer, a demographer at the University of Georgia’s Carl Vinson Institute of Government, said he and his colleagues made their detailed forecasts by studying population trends in every American census block in every county of the 23 coastal states and the District of Columbia that could be affected by rising sea levels.

In the Bay Area’s nine counties, for example, Hauer’s group estimates that more than 350,000 people will be forced to move to higher elevations before a sea level rise of 3 feet drowns their homes by the end of this century. If sea level rise reaches 6 feet, the total forced to move will reach more than 680,000, the group estimates.

One of those people could well be Katharine Mach, a Stanford climate specialist and senior research associate at the Carnegie Institution for Science at Stanford, who lives in San Mateo County.

“The big surprise from the analysis is that the risks are so heavily concentrated in just a few areas, including the county where I live,” she said. “The new work is a sobering example of how the impacts of climate change can really be everywhere, including close to home.”

In fact, Hauer’s analysis indicates that 190,000 people in San Mateo County could be moving in the face of a 3-foot sea level rise, and almost 250,000 would move with a 6-foot rise.

Widespread effects

San Francisco’s Marina neighborhood, Financial District and waterfront would all be imperiled by sea level rise.

Ken Caldeira, an atmospheric scientist at the Carnegie Institution Center for Global Ecology, said the problem will affect coastal cities everywhere.

“If we continue emitting greenhouse gases, cities like New Orleans and Miami will eventually need to be abandoned to the rising seas,” he said. “However, there will be strong incentives to build levees and sea walls and try to protect this infrastructure as long as possible. The question about when to give up on protection and move to abandonment will be politically very difficult, but will be an issue we will increasingly face in the future.”

Hauer’s colleagues include Deepak R. Mishra of the University of Georgia’s Center for Geospatial Research and Jason Evans of Stetson University in DeLand, Fla.

David Perlman is The San Francisco Chronicle’s science editor. Email: dperlman@sfchronicle.com Twitter: @daveperlman

Rising seas

By 2100, researchers from the University of Georgia estimate, a 6-foot rise in global sea levels would force more than 13.1 million people living near U.S. coasts to move from their low-lying homes to higher ground. In the Bay Area this would mean a remarkable migration between now and the end of this century. Here is a county-by-county breakdown of who would be forced to move in the region:

Marin: 90,000

San Mateo: 249,000

Alameda: 176,000

Solano: 37,000

San Francisco: 33,000

Santa Clara: 64,000

Napa: 3,900

Contra Costa: 24,000

Sonoma: 2,500

Source: Nature Climate Change and Mathew E. Hauer of the University of Georgia