LOS ANGELES, Feb. 7 (Xinhua) -- Researchers have found that biofuel crops may increase carbon emissions rather than reducing them, thus exacerbating global warming, according to studies published on Thursday.

    The conversion of forests and grasslands into fields to grow biofuel crops may offset the benefit of using the fuel itself, according to two studies published in the journal Science.

    One study found that clearing forests and grasslands to grow the crops releases vast amounts of carbon into the air -- far more than the carbon spared from the atmosphere by burning biofuels instead of gasoline.

    "We're rushing into biofuels, and we need to be very careful," said Jason Hill, an economist and ecologist at the University of Minnesota who co-authored the study. "It's a little frightening to think that something this well-intentioned might be very damaging."

    Even converting existing farmland from food to biofuel crops increases greenhouse gas emissions as food production is shifted to other parts of the world, resulting in the destruction of more forests and grasslands to make way for farmland, according to the second study conducted by researchers at the Woodrow Wilson School of Public and International Affairs at Princeton University.

    "Any biofuel that uses productive land is going to create more greenhouse gas emissions than it saves," said Timothy Searchinger, a researcher who led the second study.

    Hill's analysis looked at the amount of carbon in forests and grasslands that is released into the air when soil is overturned and existing vegetation rots or is burned away.

    The study found that clearing an Indonesian peat land rain forest to make way for a biofuel plantation -- a conversion that is occurring rapidly to satisfy Europe's rising demand for biodiesel -- releases so much carbon that a net reduction in emissions would not begin for 423 years.

    Cutting down a tropical rain forest in Brazil to grow soybeans for biodiesel increases emissions for 319 years, the researchers found.

    Dedicating existing fields to production of crops for biofuel has the same effect, indirectly.

    The studies prompted 10 prominent ecologists and environmental biologists to write to President George W. Bush and congressional leaders Thursday, urging new policy "that ensures biofuels are not produced on productive forests, grassland or cropland."

    Since 2000, annual U.S. production of corn-based ethanol has jumped from 1.6 billion gallons to 6.5 billion gallons -- supplying about five percent of the nation's fuel for transportation, according to the Renewable Fuels Association, an industry lobbying group.

    Food crops such as corn, palm oil, sugar cane and soybeans have so far been the main source of biofuels because they are already grown in abundance and are relatively easy to convert.