Biofuels can reduce emissions, but not when grown in place of rainforests
July 22, 2008
The new research looks at the "carbon payback time" or "carbon debt" of various biofuel feedstocks including oil palm, sugar cane, and soy. Carbon payback time refers to the number of years it takes for the emissions saved by the replacement of fossil fuels with biofuel to offset the carbon emissions generated when the land is converted for growing the biofuel feedstock.
Analyzing the carbon debt for biofuel crops grown in ecosystems around the world, Holly Gibbs and colleagues report that "while expansion of biofuels into productive tropical ecosystems will always lead to net carbon emissions for decades to centuries... [expansion] into degraded or already cultivated land will provide almost immediate carbon savings." The results suggest that under the right conditions, biofuels could be part of the effort to reduce humanity's carbon footprint.
Forest giving way to oil palm plantations in Malaysia
"We're not saying that biofuels are a bad idea," added Jonathan Foley, a co-author on the study and a professor at the University of Wisconsin-Madison's Nelson Institute for Environmental Studies. "This study just indicates that biofuel production should not be occurring at the expense of tropical forests. Other ways of producing biofuels in the tropics — on degraded lands, former agricultural areas and so on — are clearly possible, and could have tremendous environmental, economic and social benefits."
Region-specific look at biofuel crop expansion and carbon payback times
Building on earlier research showing the carbon payback time of biofuel crops grown on certain types of land, the new study uses "a new, geographically detailed database of crop locations and yields, along with updated vegetation and soil biomass estimates, to provide carbon payback estimates that are more regionally specific than those in previous studies," write the authors. "We also estimate carbon payback times under different scenarios of future crop yields, biofuel technologies, and petroleum sources."
Soy and corn score as the worst offenders in terms of carbon payback time when cultivated on forest and savanna lands. Soil palm and sugarcane have the shortest carbon payback times, but the benefits are most significant when these feedstocks are grown on degraded and abandoned agricultural lands.
The researchers find that annual biofuel crops, including soybeans, maize (corn) and sorghum, have the worst carbon balance and the longest carbon payback times. Woody plantations crops like oil palm and coconut rate much better, although their advantages are reduced when they are grown in place of carbon-rich tropical rainforests and peat lands. Sugarcane scores well when grown on degraded and abandoned croplands due to its high efficiency as a feedstock for ethanol, but again, as in all cases, its carbon payback time surges when its cultivation replaces tropical forest or savanna. Overall the researchers note that "converting tropical rainforests requires ~30-300 years for carbon payback for all feedstock crops, even when accounting for these major changes in energy and agricultural technology."
"We argue that the carbon payback times for clearing tropical forests are unacceptably large in the context of any reasonable carbon mitigation efforts," they write. "It is hard to imagine any plausible scenarios where clearing tropical forests for agricultural biofuels could be carbon beneficial."
"No foreseeable changes in agricultural or energy technology will be able to achieve meaningful carbon benefits if crop-based biofuels are produced at the expense of tropical forests," concluded Gibbs.
H K Gibbs et al. Carbon payback times for crop-based biofuel expansion in the tropics: the effects of changing yield and technology. Environ. Res. Lett. 3 (2008) 034001
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