The carbon emissions category is called indirect or market-mediate effects.

In a letter to California Governor Arnold Schwarzenegger, they acknowledged that all fuels have indirect carbon effects. But they challenged the notion that they are well understood and were critical of the plan to enforce indirect carbon effects on biofuels only.

The letter recommends that California adopt LCFS regulation based on direct carbon effects, or those emissions directly attributable to the production and use of the particular fuel.

The model used to determine these effects is well-grounded and peer-reviewed, and for biofuel includes land conversion needed to produce biofuel feedstock. California would then lead an international effort to investigate the indirect, market-mediated carbon effects of all fuels, including but not limited to biofuels, they recommended.

``A fundamental principle of any comparative carbon lifecycle analysis, and of a performance standard in general, is that all fuels are judged through the same lens,’’ said Dr. Blake A. Simmons of Sandia National Laboratories, who led the effort to submit the letter. ``We have abandoned that principle here.’’

The scientists, who express their personal beliefs in the letter but conduct research at some of the most prestigious laboratories in the country, outlined two fundamental problems with the proposed LCFS.

They challenged the science of indirect land use change, calling it controversial and in its nascent stage with ``clear omissions relative to the real world.’’ They also criticized selective enforcement of indirect effects against biofuels only. ``Enforcing different compliance metrics against different fuels is the equivalent of picking winners and losers, which is in direct conflict with the ambition of the LCFS,’’ they said.

``No level of certainty justifies asymmetrical enforcement of indirect effects,’’ added Simmons. ``What the current proposal basically says is that using more biofuel will have ripple effects in the economic marketplace but using more petroleum, natural gas or electricity to power our cars and trucks will have zero ripple effects in the marketplace’’ he added, ``which is, of course, not the case.’’

Dr. Harvey W. Blanch, who is a member of the National Academy of Engineering, observed: ``This is the rare case in which a very complicated problem has a simple short term solution. An LCFS based on direct carbon effects advantages the optimal kind of biofuel without creating major and unjustifiable asymmetries in the regulation.’’