Putting switchgrass in your tank

CARS AND trucks are responsible in the United States for about one-third of the greenhouse gases that are causing global warming. Greater use of ethanol is a promising way to lower vehicles' carbon dioxide emissions and reduce reliance on oil from the Mideast, where Saudi Arabia recently arrested 172 terrorism suspects accused of plotting to attack its oil facilities.

Congress is weighing bills to boost ethanol production in several ways, but the industry worries that on-again, off-again subsidies will subject it to the fate of the solar- and wind-power industries, which have seen federal support come and go. To ensure a long-term transition to ethanol and away from gasoline, Congress should mandate limits on carbon emissions, either through a tax or a cap and trade system.

Ethanol needs this kind of a backup because, while corn-based ethanol can already compete with gasoline in the market, research must yet be done on cellulosic ethanol, which can be derived from agricultural wastes, sugar cane residue, timbering slash, and plants like switchgrass. Without ethanol derived from these abundant sources, the fuel is not an answer for transportation. Analysts believe that ethanol based on corn, which is also a staple in everything from soft drinks to livestock feed, could never replace more than about 10 percent of US demand for gasoline.

Moreover, corn requires fertilizer and, often, irrigation while the sources of cellulosic ethanol do not. All this gives cellulosic ethanol -- the subject of several presentations at this week's 2007 BIO convention in Boston -- a distinct advantage over the corn-based product when energy input is weighed against energy output.

The hurdle in the path toward a cellulosic ethanol future is finding economic ways to break down cellulose fibers and then to ferment the sugars produced in that breakdown, a more complex process than processing corn starch. A mandated lid on carbon emissions would ensure investors in this technology that there will be a reliable market demand for cellulosic ethanol.

A carbon-reduction mandate would also push automakers to build more flex-fuel cars and signal to the petroleum industry that it must stop dragging its feet on making ethanol available at service stations as an alternative to gasoline and diesel. In most of the country it is still impossible to get ethanol as more than a fractional additive in gasoline.

A mandated carbon reduction will face stiff resistance in Washington. But the modest increases in cars' fuel efficiency that many officials support as a way to curb greenhouse gases and promote energy independence do not, by themselves, go far enough. Drivers need both more efficient vehicles and the alternative of ethanol to reduce their climate change impact. . A mandate on carbon emissions would make sure they can put greener gas in their tank.