Climate change and expanding seas could cause the waters beneath the Golden Gate Bridge to rise 3 to 4 feet by century's end, a new study finds.

Warmer temperatures also would transform the environment of the San Francisco Bay and Sacramento-San Joaquin Delta, a system that provides water for 25 million people and irrigates $36 billion in annual crops. Changed conditions would affect native species, including the endangered delta smelt and winter-run chinook salmon, the U.S. Geological Survey reported in research published in the journal PLoS ONE.

"Virtually every aspect of the San Francisco Bay Delta system that we looked at is going to change substantially in the future," said James Cloern, a USGS scientist and the study's lead author. Less snowpack and faster-melting snow could lead to more frequent and intense flooding and droughts, he said.

"We have to really think about anticipated changes in the frequency of extreme events," Cloern said. "As global warming proceeds, we're going to experience combinations of environmental conditions unlike any we've seen in the past."

USGS looked at the likely effects of climate change using two models, one where the temperature rises 2 degrees Fahrenheit by 2099 and the other where it climbs 7 degrees by century's end. Researchers examined the impact on air and water temperatures as well as the frequency of extreme events.

The findings can help government agencies and conservation groups as they plan strategies to adapt to the changes, Cloern said. Those include the city of San Francisco and the state's San Francisco Bay Conservation and Development Commission, which is studying how to protect water supplies and sustain native species. San Francisco, Cloern said, believes rising seas will affect 270,000 people and threaten more than $60 billion in infrastructure, he said.

"The protection of California's Bay-Delta system will continue to be a top priority for maintaining the state's agricultural economy, water security to tens of millions of users, and essential habitat to a valuable ecosystem," said USGS Director Marcia McNutt. "This new USGS research complements ongoing initiatives to conserve the Bay-Delta by providing sound scientific understanding for managing this valuable system such that it continues to provide the services we need in the face of climate uncertainty."

Widening a partisan divide
The findings could also have political punch. There is a partisan divide over water policies and fish protections in the Bay Delta region.

Rep. Devin Nunes (R-Calif.) earlier this year offered H.R. 1837, which would repeal a 2009 law on central California water uses and replace it with 1994 rules from an agreement known as the Bay-Delta Accord. The measure would set compliance with the Endangered Species Act to that year. Nunes and other GOP lawmakers at a House Natural Resources Subcommittee on Water and Power hearing in June argued that the existing law prioritizes fish over people. The law, which stems from a court settlement on water flows, salmon and endangered species, places limits on the amounts of water certain farmers can take for irrigation.

"Protecting endangered species is a worthy goal, and worthy goals need to be pursued with common sense and sound science, not left-wing ideology and junk science," Subcommittee Chairman Tom McClintock (R-Calif.) said at that hearing.

USGS researchers found that climate change would alter conditions in the Bay Delta, affecting the fish that also are part of the current fight over protections. Both the moderate and the faster increases in temperature would lead to water that is warmer, saltier and less muddy.

That's the opposite of the conditions best for the endangered delta smelt.

"As temperatures warm, the habitat becomes less friendly for delta smelt and more friendly to introduced species like blue gills," Cloern said, adding, "what's a new perspective here is that one of the consequences of climate change is progressive favoring of the introduced species and disfavoring of the native species."

The habitat conditions changed under both the 2- and 7-degree temperature increase scenarios, he said.

"The habitat changes are bigger and faster in the [7-degree] warming scenario," Cloern said. "So it's just a question of how fast things will change. Even in the moderate warming scenario, there are predictions of big changes in the delta habitat as this century unfolds."

Uncertainty about rain, little uncertainty about sea level rise
Climate change could also affect precipitation in California, though the two models USGS used in its research produced different results.

If temperatures rise 7 degrees by century's end, the state's climate will become significantly drier, Cloern said. With the 2-degree temperature increase, California would not see a noticeable impact on precipitation patterns. That difference has been seen in other climate change research, he said.

There were longer droughts, however, in the model using a 2-degree temperature increase.

"Even in the moderate warming scenarios, there are going to be changes in California's water supply," Cloern said, but "there's real uncertainty about whether California will become drier or wetter."

"There's little uncertainty about sea level rise," he added. "Both of the scenarios we used project substantial levels of sea level rise, whereas the models did not agree on the direction of change of California's precipitation."

Looking at sea level rise, water would rise 37 inches in the moderate warming scenario and 48 inches in the faster model. The measurements reflect what would happen at the tide gates beneath the Golden Gate Bridge, which is the entrance to the Bay Delta. Water levels will rise less closer to the shore, Cloern said, though the research only looked at the one site.

"It's a powerful conclusion that these two different scenarios give rates of sea level rise that aren't that different from one another," Cloern said.
