Biomass energy—energy generated from agricultural waste or specially grown energy crops—has been widely touted as a clean, renewable alternative to fossil fuels. Research is booming to improve energy crops and methods of converting crops to fuel. Already, Brazil gets 30% of its automotive fuel from ethanol distilled from sugar cane. But critics warn that “energy farming” will gobble up land needed to grow food or will impinge on natural ecosystems, possibly even worsening the climate crisis.

In the February Trends in Ecology and Evolution, Global Ecology director Chris Field, with postdoctoral fellow Elliott Campbell and a colleague, took a sober look at the prospects for biomass energy. They found that while biomass has many benefits—in principle it can be carbon neutral—there are limits to the extent that it can sustainably contribute to global energy needs. For example, the total mass of carbon fixed by all croplands worldwide each year (about 7 billion tons) is still less than that released by fossil fuel emissions (7.7 billion tons). This fact, the authors write, “highlights the challenge of replacing a substantial part of the fossil fuel system with a system based on biomass.”

The researchers used a combination of historical data, satellite imagery, and productivity models to determine best-case estimates of potential yields and of how much biomass could sustainably contribute to the world’s energy needs while also mitigating global warming.

“The area with the greatest potential for yielding biomass energy that reduces net warming and avoids competition with food production is land that was previously used for pasture but that has been abandoned and not converted to forest or urban areas,” they write.

Globally, suitable abandoned cropland and pastureland amounts to approximately 1.5 million square miles. Realistically, energy crops raised on this land could be expected to yield about 27 exajoules of energy each year. This is a huge amount of energy—an exajoule is a billion billion joules, equivalent to 172 million barrels of oil. Yet the biomass yield could still satisfy only about 5% of global primary energy consumption by humans, which in 2005 was 483 exajoules.

The study concludes that at a proper, sustainable scale, biomass energy presents exciting opportunities for increasing energy independence, sustaining farm economies, and decreasing the forcing of climate change. But deployed at a larger scale, it could threaten food security and exacerbate climate change.