If adopted by the state's influential air quality regulators, the low-carbon fuel standard would become the first measure in the nation to impose on motor fuels limits on the amount of greenhouse gases that contribute to global warming.

Similar rules are under consideration in 11 other states that are waiting for California to act. President Barack Obama also has called for a nationwide low-carbon fuel standard to help meet his goal of cutting greenhouse gas emissions more than 80 percent by mid-century.

California's proposal takes a sweeping "cradle-to-grave" approach that aims to reduce the carbon footprint of fuels from production to combustion. It also seeks to spur development of cleaner-burning alternatives to gasoline and diesel fuels that will help abate climate change and reduce oil imports.

Transportation alone accounts for 40 percent of greenhouse gas emissions in California, which ranks as the nation's leading automobile market.

"It's a model for the country and other nations in that it provides a mechanism to move away from today's carbon-intensive fuels towards tomorrow's cleaner, more sustainable fuels," said Patricia Monahan, state director for the Union of Concerned Scientists.

The new standard would require refineries, producers and importers of motor fuels sold in California to reduce the "carbon intensity" of their products 10 percent by 2020, with greater cuts thereafter.

As a result, carbon emissions in California would decline by 16 metric tons over the next decade, while the measure would lead to 20% of the state's fossil fuels being replaced by cleaner alternatives, such as electricity, hydrogen, natural gas and biofuels.

But the proposal has drawn heavy opposition from the beleaguered ethanol industry, which has argued that the regulation, as drafted last month, would unfairly penalize grain-based biofuels.

Ethanol advocates say the measure's "indirect land-use" provisions erroneously calculate the carbon impact of such biofuels by factoring in the clearing of forests, which store carbon, when corn is grown on a large scale.

Critics say the land-use models are flawed and are selectively applied to grain ethanol, putting a cleaner-burning fuel that already is widely available at a disadvantage compared to other alternatives still under development.

"Ethanol is not the reason for deforestation," said Doug Berven, an executive for Poet, the largest U.S. ethanol producer. He said deforestation in the Amazon had declined 50% while ethanol production grew five-fold.

California Air Resources Board spokesman Stanley Young defended the proposed rule, denying a built-in bias against grain fuels.

"We're not gunning for ethanol. We're gunning for carbon," he said. "We actually looked at 11 different ways of making corn ethanol, and seven of those came out lower than the standard, so we believe that corn ethanol has an important role to play."

Supporters of the measure say the new rule also would give a boost to low-carbon fuels made from nonfood sources, such as switchgrass and even raw trash.