With ferocious lobbying by the corn belt, America has been planting more and more corn for the purpose of making ethanol, a gasoline additive that supposedly reduces tailpipe emissions of carbon dioxide.

Both Senators Clinton and Obama, the Democratic presidential contenders, have published plans to expand ethanol use, going beyond the mandates enacted in the energy bill in December in response to the lobbying.

Now it begins to appear that ethanol may not be a panacea. In fact, more corn may mean more greenhouse gases.

Even for people who don’t drive or worry about global warming, this is a vital subject. Ethanol has been driving up food prices by attracting agricultural resources to corn, and diverting corn from the food supply to gasoline.

Earlier this month, two new papers in the journal Science questioned the efficacy of ethanol in reducing greenhouse gases. They argued that the production of ethanol causes more harmful emissions of CO2 than it prevents. Other research has shown that even a moderate increase in our use of ethanol is not cost-effective, because America does not have the infrastructure to use it efficiently.

Is it possible to reconfigure the production and delivery of ethanol so that it assists in reducing greenhouse gases, or has our use of ethanol reached a natural ceiling, above which we get a net gain in greenhouse gases and more ethanol than the economy can absorb?

Senator McCain, the presumptive Republican presidential candidate, doesn’t want to spend taxpayers’ money on ethanol subsidies. However, he has not indicated that he would repeal the new congressional mandates for renewable fuel use that start this year and reach 36 billion gallons in 2022.

Mrs. Clinton wants to spend $50 billion, funded mostly by taxes on oil companies, on a Strategic Energy Fund, to pay for investments in alternative energy. She wants “aggressive action to transition our economy toward renewable energy sources, with renewables generating 25 percent of electricity by 2025 and with 60 billion gallons of home-grown biofuels available for cars and trucks by 2030.”

Mr. Obama proposes to fund research so that America can use 2 billion gallons of cellulosic ethanol — ethanol from plant matter instead of corn — by 2013. In addition, he would require 60 billion gallons of advanced biofuels like cellulosic ethanol by 2030.

The problem is that the more ethanol we produce, the more greenhouse gases we generate. Princeton University professor Timothy Searchinger, along with other researchers, wrote in Science magazine that corn-based alcohol doubles greenhouse emissions over 30 years and causes increases that continue for 167 years.

That happens because higher corn prices encourage farmers all over the world to transform their land from forests and unplanted fields to corn. Opening new land to agriculture volatilizes the CO2 stored in dead trees, dirt, and other biomatter, thereby increasing the atmospheric CO2. The paper found this release to be massive, dwarfing any potential benefits.

A companion paper in Science, by the Nature Conservancy’s Joseph Fargione, showed that converting undeveloped land to biofuel production releases 17 to 420 times more carbon dioxide than annual greenhouse gas reductions due to ethanol’s substitution for gasoline. Only if biofuels are made from waste products or grown on idle farm acres are greenhouse gases reduced.

This might suggest, then, that there are ways of producing biofuels that do not lead to more greenhouse gases. Just produce them out of waste material, without cutting down forests or plowing over fields, and Mr. Obama and Mrs. Clinton can proceed with their plans.

Not so fast, according to Energy Policy Research Foundation vice president Larry Kumins. His paper, published in the Oil and Gas Journal last November, shows that the present supply of ethanol in the American economy, about 8 billion gallons a year, or 5% of the gasoline pool, is optimal. Congressional mandates of even a small increase to 9 billion gallons this year will have harmful effects, and a level of 35 billion gallons, as Congress has mandated in 2022, is currently impossible.

Here’s why: because ethanol separates from gasoline in the presence of water, blends of ethanol and gasoline that many of us put in our cars cannot be transported by pipeline. Instead, ethanol is shipped by rail or tank truck, at greater cost than the transport of gasoline, and mixed with gasoline near the point of distribution. That’s why 10% ethanol-gasoline blends are not available all over the country, only in major metropolitan areas.

Mr. Kumins writes that “even if cellulosic sources became commercially viable on the most optimistic schedule, consumption of ethanol still won’t exceed 10% of gasoline supply without substantial changes in the stock of capital. ” Auto manufacturers won’t guarantee vehicles’ engines if the gasoline contains more than 10% ethanol, and higher quantities of ethanol violate Environmental Protection Agency air quality standards.

But a better, simpler approach may be unfolding: in response to high gasoline prices, Americans are reducing their vehicle emissions by turning to more fuel efficient cars. Congress and presidential candidates, take note.

Ms. Furchtgott-Roth, former chief economist at the U.S. Department of Labor, is a senior fellow at the Hudson Institute. She can be reached at dfr@hudson.org.