Carbon payback times for crop-based biofuel expansion in the tropics: the effects of changing yield and technology

Abstract

Biofuels from land-rich tropical countries may help displace foreign petroleum imports for many industrialized nations, providing a possible solution to the twin challenges of energy security and climate change. But concern is mounting that crop-based biofuels will increase net greenhouse gas emissions if feedstocks are produced by expanding agricultural lands. Here we quantify the 'carbon payback time' for a range of biofuel crop expansion pathways in the tropics. We use 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. Using this cropland database, we also estimate carbon payback times under different scenarios of future crop yields, biofuel technologies, and petroleum sources. Under current conditions, the expansion of biofuels into productive tropical ecosystems will always lead to net carbon emissions for decades to centuries, while expanding into degraded or already cultivated land will provide almost immediate carbon savings. Future crop yield improvements and technology advances, coupled with unconventional petroleum supplies, will increase biofuel carbon offsets, but clearing carbon-rich land still requires several decades or more for carbon payback. 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.S Supplementary data are available from stacks.iop.org/ERL/3/034001

Solutions for a cultivated planet

“…This is particularly true when tropical forests are cleared for agriculture 27,49,50 , estimated to cause 5-10 million hectares of forest loss annually 18,51 . Slowing (and, ultimately, ceasing) the expansion of agriculture, particularly into tropical forests, will be an important first step in shifting agriculture onto a more sustainable path.…”

Tropical forests were the primary sources of new agricultural land in the 1980s and 1990s

“…Agroindustrialists and some scientists suggest that expansion is occurring largely on degraded or previously cleared land (23)(24)(25), but others posit that agriculture is expanding into rainforests (22,(26)(27)(28). This debate became even more pressing with the surge in the demand for biofuel, because recent studies have argued that expansion of biofuel crops into rainforests may substantially increase rather than decrease net carbon emissions (29)(30)(31).…”

The life cycle emission of greenhouse gases associated with plant oils used as biofuel

“…In the case of palm oil, the time to pay back the carbon debt is probably in the order of 60e100 years when oil palms are cultivated on mineral soils after recent deforestation [2,4] and in the order more than one century to over nine centuries, when the oil palms are cultivated on peat [4e6]. When soybeans are cultivated for oil on recently deforested land, the carbon pay back time is in excess of 300 years [4]. These studies of carbon pay back times [2,4e6] are consistent with the outcomes of the life cycle studies amortizing large changes in carbon stocks over a 20 year period, as the pay back times estimated for palm oil and soybean oil are much in excess of 20 years.…”