California and Bay Area cities must start planning for new and costly systems to control increasing runoff from urban storms, spring floods from swollen rivers and rising sea levels as they invade lowlands, all as a result of global warming, climate scientists and water experts warn.
Climate change will thin winter snowpacks in the Sierra and other western mountains. As the snowpacks melt earlier each spring, the meltwater will increase river flows. Meanwhile, even a small increase in sea levels would threaten cities and farmlands in low-lying areas, like the Delta and Silicon Valley.
These forecasts are underscored in two reports to be published Friday and Saturday in the journal Science. The long-term problems call for a new urgency in planning for the future, according to the analysts.
The two reports -- an online version Friday and the other in print Saturday -- highlight the issues:
The first, by a group of climate researchers headed by Tim P. Barnett of the Scripps Institution of Oceanography in La Jolla, warns of "a coming crisis in water supply for the western U.S." that is largely due to what the scientists call "human-caused climate changes." Those changes, according to Barnett's team, have already affected the west's river flows, water temperatures and snowpacks for the past 50 years, according to records that Barnett's team has analyzed.
The other report, from an international team headed by Paul C. Milly of the U.S. Geological Survey, focuses on policy and warns that because of climate change, the planners who design dams and flood control projects can no longer rely on records of orderly variations in recent past climates, but must look ahead to an era when variations in the pace of warming will be rapid and unpredictable.
And that will mean rapidly changing seasonal averages and extremes of snow, rainfall, river flow and floods year-by-year, they say.
New urban systems to handle winter storm runoff, new designs for dams and flood control systems, and higher dikes and levees around lands that even now lie below sea level will be needed, the scientists argue.
"The challenge is daunting," Milly and his team of experts from Sweden, Poland, Germany and the University of Washington said in their report. "Patterns of change are complex, uncertainties are large; and the knowledge base changes rapidly."
And what about California?
In a phone interview this week, Milly, who is based at Princeton University with scientists from the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory, said the California Department of Water Resources and many of the state's local water districts have done a good job of planning for the future, but it won't be enough.
"You're in an area with substantial vulnerability to the impact of global warming," he said. "So with more rain in coming years, a diminishing snowpack, and more danger of floods, you'll have your work cut out to start planning now."
Peter H. Gleick, a leading water resources expert and president of the Oakland-based Pacific Institute, was not part of either research group, but said both teams are correct in warning that planners of dams, canals and systems for controlling floods and stormwater runoff must consider "the undeniable prospect of global warming as they design new facilities."
Jonathan Loiacono, an engineer at San Francisco's Public Utilities Commission and project manager of the city's master plan for upgrading its ancient and crumbling sewer system, agreed.
"Our sewers are 70 years old on average, and we have 60 to 70 miles of brick sewers that were built in the 1890s and are leaking badly," he said in an interview. "The system is falling apart. To meet the problems that are bound to come with more frequent and intense storms the scientists predict from global warming -- that calls for imaginative solutions, like capturing storm waters for irrigation, and other 'green' uses."
Stephen H. McDonald, a partner with Carollo Engineers, a national firm that consults on water projects for San Francisco and many other California cities, said his company is already helping Silicon Valley towns design new dikes and levees that will be needed to protect against sea level increases.
A $5.4 billion bond issue for dams and water conservation issues passed by California voters in 1986 includes $1 billion for "integrated water management infrastructure," to cope with future problems posed by a warming climate, said Jenine Jones, an engineer and interstate resources manager at the State Department of Water Resources. "One of the elements we'll have to deal with in planning future work on California dams will be the best models available of ongoing climate change," she said.
Milly, lead author of the Science policy report, said the best global warming models indicate, first, that annual runoff everywhere will come earlier from diminishing snowpacks in the mountains, and second, that runoff totals will diminish sharply.
"A third would be sea-level rise and all that means for the Delta, but let's focus ourselves here," he said in an e-mail message.
"Planners must develop methods to grapple with increased hydrologic uncertainty in the design of systems to balance water supply and demand and to protect lives and property."
In the Science report on global warming and water, Barnett and his colleagues said that two-thirds of the measured climate change in the past 50 years has been "human-induced" -- a conclusion Milly argued is understated.
But Barnett maintained that models of warming in the future mean "a coming crisis in water supply for the Western United States." And in an interview he put it more dramatically: "We're headed for a train wreck," he said.