"What we are trying to do by this is to set high standards to show carbon offset programs that work," Gavin Newsom told Reuters, adding he was wary of offsets with little accountability that promise action in distant lands.

For official travel, that means a round-trip flight from San Francisco to New York would cost an additional US$80 to $90, officials say, to be paid into the city's offset programs such as converting restaurant grease into fuel, installing solar energy devices or investing in energy conservation.

The program will not cost additional funds, which means city departments would cut out some travel to pay for other trips with carbon offsets, Newsom said. A second phase of the program would also allow residents to buy offsets.

With growing worldwide focus on the climate change impact of carbon emissions, entities from companies such as Google and Yahoo to organizations including the United Nations and countries such as Costa Rica, Norway and New Zealand are implementing carbon offset programs with the ultimate goal of becoming carbon neutral.

Such plans seek to plant trees (which soak up carbon dioxide, the most common man-made greenhouse gas) or encourage a switch from high carbon-emitting fossil fuels to renewable sources of energy such as wind and water to cut emissions.

WHERE DO THE OFFSETS GO?

The problem with many such programs is that those who pay carbon offset fees often do know know what, if anything, becomes of contributions, the San Francisco mayor said.

"Right now, my offsets, I don't know where the hell they go. They might be going to the Amazon," Newsom, a Democrat, told Reuters. "There are some scam artists doing nothing other than banking on this goodwill."

He said he personally favored paying carbon offset fees for all his travel, whether he is stumping for Democratic presidential candidate Hillary Clinton as he did last weekend in Iowa, or jetting off to the exclusive wedding of Google co-founder Larry Page in the Caribbean earlier this month.

In 2004, Newsom set a goal of cutting the city's annual carbon dioxide emissions by 2.5 million tonnes by 2012, a 20-percent reduction below 1990 emissions.

Newsom said officials were studying proposals to reduce emissions from transportation -- which account for more than half of the city's carbon emissions -- that could include new taxes or restrictions. "The whole movement in the United States is going to be towards congestion management," he said.