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Following up on the reader’s earlier question about what we need to do to stabilize greenhouse gas emissions and the role alternative fuels play in that.

I talked to Richard Somerville, a theoretical meteorologist at Scripps Institution of Oceanography at University of California, San Diego.

The world will have cut carbon dioxide emissions by about 70 percent to stabilize levels in the atmosphere, said Somerville, a contributing lead author to the recent report from the United Nations’ Intergovernmental Panel on Climate Change.

“You basically have to cut emissions down to the level at which you’re not putting gases into the atmosphere faster than nature can remove them,” Somerville said. “It’s a fuzzy number because you don’t know how the carbon cycle will change as the CO2 levels go down.”

He added: “It’s a massive number.”

For context, Somerville pointed to the Kyoto Protocol. If every country signed it (the United States and Australia, for example) and no one cheated, carbon dioxide levels would be cut by 7 percent. What’s needed is 10 times larger than that, Somerville said.

So how do we do it?

“You wean the world from fossil fuels, so the only carbon sources are things like cement manufacturing,” he said. “Or you sequester the carbon.”

The Los Angeles Times examined that issue in a February story. Reporter Alan Zarembo wrote:

“All trucks, all trains, all airplanes, cars, motorcycles and boats in the United States — that’s 7.3 percent of global emissions,” said Gregg Marland, a fossil fuel pollution expert at the Oak Ridge National Laboratory in Tennessee.

Scrapping all fossil fuel-powered electricity plants worldwide and replacing them with windmills, solar panels and nuclear power plants would make a serious dent in the problem — “a 39 percent reduction globally,” Marland said. …

For some perspective on that number, consider an icon of the green movement: a 2007 Toyota Prius. Driving it 12,000 miles over the course of a year releases an average of 4,200 pounds of carbon dioxide.

If all 245 million cars in the United States — more than one-third of all the cars in the world — were replaced with hybrid cars, the carbon savings would be less than 3 percent of the needed reduction.

I’ll continue answering this question as the afternoon progresses.

ROB DAVIS

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