Last month, Gov. Arnold Schwarzenegger announced a plan to borrow $4.5 billion to build two massive new reservoirs. He pitched them as a vital response to climate change.

"With the impact that global warming will cause to our snowpacks," he said, "we need more infrastructure ... so the next generation of Californians is not faced with a shortage of this precious resource."

But new research suggests the governor's water plan may instead aggravate climate change. In recent years, scientists have documented that dams and hydropower -- long considered a "clean" energy source -- may actually pump greenhouse gases into the atmosphere in surprising ways.

Emissions occur, first, during cement-making and construction for a dam. More happens when land behind it is flooded, causing vegetation to rot, releasing carbon dioxide and methane.

Emissions continue throughout the dam's life as more organic matter washes in from upstream, and when water is released to make electricity, causing a pressure drop that frees gases locked within the stored water.

"If these are going to be built as a response to climate change, you at least need to convene some people to study the effect it will have," said Danny Cullenward, a research associate at the Program on Energy and Sustainable Development at Stanford University. "The facts are in that it's not a zero-impact source from an emissions standpoint."

Natural lakes may produce emissions in the same way. But the effects could be greater in man-made reservoirs because water levels change more dramatically behind dams. And, like any other man-made energy source, reservoirs would be counted as an addition of greenhouse gases beyond natural levels.

"Obviously, there's some irony if measures supposed to help us adapt to climate change are themselves contributing to the problem," said Patrick McCully, executive director of the International Rivers Network in Oakland.

Research shows that some reservoirs have a positive effect, absorbing more carbon dioxide than they emit. In either case, the effects vary according to geology, climate, reservoir operations and other factors.

A deeper look at the problem may prove that the governor's main interest -- water storage -- outweighs any negative effect on climate. McCully argues, however, that water conservation would be cleaner and cheaper.

Jerry Johns, deputy director of water planning at the state Department of Water Resources, said he hadn't previously considered reservoir emissions. But he said it should be studied in regard to the proposed dams.

"I don't think it's going to be a big deal, but we probably should look at it," Johns said. "Particularly with where the governor is on greenhouse gases, we probably ought to be looking at all kinds of inputs."

Most of the research has focused on South American dams. There, reservoir emissions actually may be worse than coal-fired power plants, considered the worst offender among energy producers. But those results cannot be applied elsewhere, because rain forests pump far more organic matter into reservoirs than almost anywhere else on Earth. Temperate regions, such as California, likely produce much less reservoir emissions.

Only one study has been done so far in California, led by researchers at the University of Quebec in Montreal. Published in 2004 in the journal Global Biogeochemical Cycles, it looked at Shasta Lake, Lake Oroville and New Melones Reservoir.

It estimated that Shasta Lake released 224 tons per day of carbon dioxide, both through diffusion from the surface and during power generation. That's equal to about 14,500 average automobiles, each driven 40 miles a day. Lake Oroville emissions equaled about 3,400 cars.

New Melones Reservoir -- perhaps, researchers said, because of a difference in water acidity -- actually absorbed carbon dioxide equal to about 975 cars.

Reservoirs also emit methane. These emissions were small in the California reservoirs -- less than a ton per day in each case -- but methane is more potent than carbon dioxide.

In 2008, Schwarzenegger plans to ask voters to approve $4.5 billion in bonds to pay for two new reservoirs. Both are proposed as hydroelectric projects.

The U.S. National Hydropower Association believes more research is needed to understand emissions, said Executive Director Linda Church Ciocci.

"I think there is some validity to the research that's been done. It's an emerging concern," she said. "But we also believe ... hydropower will still be viewed as a very important, clean technology."

Reservoir emissions have not been examined on a broad scale. The Intergovernmental Panel on Climate Change, the leading scientific authority on the issue, has not figured these emissions into its predictions.

California has yet to count reservoir emissions in its own greenhouse gas inventory, said Adam Gottlieb, spokesman for the state Energy Commission.

Cullenward, who published a review of the science in the journal Climatic Change, said methane from reservoirs could boost global inventories 20 percent.

"When you look at this on a global level, this actually is a really, really massive impact," he said. "I would say, right now, this is totally under the radar."

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