Even those who regard ethanol as the holy grail of energy policy concede that there is a right way and a wrong way to produce it. Done right, ethanol could help wean the country from its dependence on foreign oil while reducing the emissions that contribute to climate change. Done wrong, ethanol could wreak havoc on the environment while increasing greenhouse gases.

Congress was sufficiently excited about ethanol's promise to order a fivefold increase in production by 2022, as part of the 2007 energy bill. Less than half would come from corn ethanol, which has been on the market for years. The rest would come from other sources, like small trees and plants. These advanced biofuels are nowhere near commercial production.

The final bill correctly included environmental safeguards. The most important is a requirement that ethanol, regardless of its source, achieve at least a 20 percent reduction in greenhouse gases compared with conventional gasoline. Congress gave the necessary job of calculating emissions from various forms of ethanol to the Environmental Protection Agency.

The calculations would have to account for both direct emissions -- those associated with growing, harvesting and refining corn or other feedstocks -- and indirect emissions, including those caused by changes in land use as acres devoted to producing food were converted to producing fuel.

These safeguards now look very smart. Science magazine recently published two studies arguing that earlier calculations had underestimated or largely ignored the potential emissions from land use changes.

For example, the studies said, a proper accounting would include not only the carbon absorbed by the corn grown to produce ethanol but the carbon released into the atmosphere when the soil was prepared for planting.

Such an accounting would also have to weigh the prospect that an acre of corn diverted to fuel production in this country could cause farmers elsewhere in the world to clear an equivalent acre of forest or grassland to replace that food, thus adding to carbon emissions.

The studies' authors say that some ethanol sources -- wood wastes, or grasses planted on previously degraded land -- could yield net benefits in terms of greenhouse gas emissions. Still, they are one more reminder that regulators will have to design the tightest possible standards for ethanol production. And Congress, which is responsible for this huge mandate, will have to ensure that they do.