A recent study by scientists from the University of Wisconsin, University of Arizona, and McGill University, re-examined the proposed “carbon debt models” for biofuel development, and highlighted the short-term risks in the expansion of tropical biofuels, by growing them in cleared carbon-absorbing forests. An earlier article by Joseph Fargione and colleagues (which appeared in the journal, Science, http://www.sciencemag.org/cgi/content/abstract/1152747) suggested that cultivating land for producing biofuels can result in a “carbon debt”, which must be repaid (“carbon payback period”). For example, the clearing of carbon-absorbing tropical forests to give way for soybean (biodiesel feedstock) production would decrease CO₂-absorbing-capacity, resulting in a so-called carbon debt of 319 years “which must be repaid”. The paper re-examined the carbon debt model of Fargione and colleagues, taking into account some factors which were not considered in the original model. The study largely confirms the original findings that “biofuel expansion into natural tropical ecosystems will lead to net carbon emissions for decades to centuries in most cases”. However, the study also found that certain biofuel expansion pathways can also result in net carbon savings within a decade. Some of these pathways include: (1) expansion of high-yielding crops (sugarcane and oil palm) into largely degraded lands, (2) replacing other crops with agrofuels in croplands that displace tropical ecosystems. The complete results of the study, is published in the journal, Environmental Research Letters (URL above)..