A better way to grow biofuel crops is to re-use abandoned agricultural lands. Or farmlands that are less productive. Both are better than current practises: clearing wilderness and converting food farms to energy farms.

There are 1.5 million square miles of abandoned cropland and pastureland available around the world. Energy crops raised on these could yield up to 27 exajoules of energy a year—equal to 172 million barrels of oil. Yet even this would still satisfy only about 5% of global primary energy consumption—483 exajoules in 2005, and rising.

Better than nothing, you say. But only if it doesn't further aggravate climate change. The study by Carnegie Institution and Stanford University scientists used historical data, satellite imagery, and productivity models to estimate how to maximize the benefits from biofuels while also mitigating global warming. Recycling old farms yields the best atmospheric returns.

Julia Whitty is Mother Jones' environmental correspondent, lecturer, and 2008 winner of the Kiriyama Prize and the John Burroughs Medal Award.

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