PARIS – The California Air Resources Board expects the Environmental Protection Agency to decide in May whether it will allow California to establish de facto limits on vehicle fuel economy, and CARB clearly anticipates a favorable decision.

CARB Chairman Mary Nichols, attending a recent conference on climate change here, tells Ward’s she has suggested the federal government adopt the California proposal nationally if it wants to promulgate a single fuel-economy standard for the 50 states.

“That would be a very good idea,” she says. “That would be great.”

The EPA is proposing a declaration that greenhouse gases are a health threat, which would allow the agency to regulate carbon-dioxide emissions in the same way it already regulates carbon monoxide, oxides of nitrogen, particulate matter and other traditional pollutants.

The proposal will undergo a period of public comment, but Nichols is sure it will be accepted, saying “the Obama administration will find that CO2 endangers public health and the environment and take steps to reduce CO2.”

If CO2 is ruled a pollutant, it would strongly suggest the agency under President Obama’s appointee Lisa Jackson is favorable to the California petition. Nichols wrote Jackson on Jan. 21, the day after Obama’s inauguration, asking the EPA reconsider a 2007 ruling against California.

The prospect of two fuel-economy rules in the American market upsets the auto industry, as California, which represents about 10% of the U.S. market, could be joined by up to 15 other states that want to follow its rules.

However, the U.S. Congress likely will act on greenhouse gases before the EPA can establish any limits of its own.

“Everyone is saying that tailor-made Congressional legislation would be preferable,” David Bookbinder, chief climate counsel for the Sierra Club, tells Bloomberg.

If Congress were to develop such rules, they presumably would reflect input from all stakeholders and be less subject to legal challenge than EPA rules. Also, they could be enacted much faster.

Meanwhile, an international conference in Copenhagen next December will tackle global-warming questions.

Nichols says if Obama can attend the conference with a sufficient mandate from Congress, he will be able to negotiate effectively. A mandate need not be a law, she says. It could be a Congressional resolution or a law in progress. Obama wants to roll back U.S. CO2 emissions to 1990s levels.