Land Clearing and the Biofuel Carbon Debt

Executive summary

Carbon dioxide (CO2) emissions from fossil fuels make switching to lowcarbon fuels a high priority. Biofuels are a potential lowcarbon energy source, but whether biofuels offer carbon savings depends on how they are produced. Converting rainforests, peatlands, savannas, or grasslands to produce food-based biofuels in Brazil, Southeast Asia, and the United States creates a ‘biofuel carbon debt’ by releasing 17 to 420 times more CO2 than the annual greenhouse gas (GHG) reductions these biofuels provide by displacing fossil fuels. In contrast, biofuels made from waste biomass or from biomass grown on abandoned agricultural lands planted with perennials incur little or no carbon debt and offer immediate and sustained GHG advantages.

Excerpts

Demand for alternatives to petroleum is increasing the production of biofuels from food crops such as corn, sugarcane, soybeans and palms. As a result, land in undisturbed ecosystems, especially in the Americas and Southeast Asia, is being converted to biofuel production and to crop production when agricultural land is diverted to biofuel production. Such land clearing may be further accelerated by lignocellulosic biofuels, which will add to the agricultural land base needed for biofuels unless biofuels are produced from crops grown on abandoned agricultural lands or from waste biomass.

Soils and plant biomass are the two largest biologically active stores of terrestrial carbon, together containing ~2.7 times more carbon than the atmosphere. Converting native habitats to cropland releases CO2 due to burning or microbial decomposition of organic carbon stored in plant biomass and soils. After a rapid release from fire used to clear land or from decomposition of leaves and fine roots, there is a prolonged period of GHG release as coarse roots and branches decay and as wood products decay or burn.

We call the amount of CO2 released during the first 50 years of this process the ‘carbon debt’ of land conversion. Over time, biofuels from converted land can repay this carbon debt if their production and combustion has net GHG emissions that are less than the life-cycle emissions of the fossil fuels they displace. Until the carbon debt is repaid, biofuels from converted lands have greater GHG impacts than the fossil fuels they displace.

Our analyses suggest that biofuels, if produced on converted land, could, for long periods of time, be much greater net emitters of greenhouse gases than the fossil fuels that they typically displace. All but two, sugarcane ethanol and soybean biodiesel on Cerrado, would generate greater GHG emissions for at least half a century, with several forms of biofuel production from land conversion doing so for centuries:

• The biofuel carbon debt from biofuels produced on converted Brazilian Cerrado is repaid in the least amount of time of the scenarios we examined. Sugarcane ethanol produced on Cerrado would take ~17 years to repay the biofuel carbon debt.
• Converting lowland tropical rainforest in Indonesia and Malaysia to palm biodiesel would result in a biofuel carbon debt of of CO2 that would take ~86 years to repay.
• Ethanol from corn produced on newly converted US Central grasslands results in a biofuel carbon debt repayment time of ~93 years.
• Converting lowland tropical rainforest in Indonesia and Malaysia to palm biodiesel would result in a biofuel carbon debt that would take ~86 years to repay.
• Converting tropical peatland rainforest to palm production incurs a similar biofuel carbon debt from vegetation, but the required drainage of peatland causes an additional sustained emission which would require ~420 years to repay.
  
• Soybean biodiesel produced on converted Amazonian rainforest would require ~320 years to repay compared with GHG emissions from petroleum diesel.

At least for current or developing biofuel technologies, any strategy to reduce GHG emissions that causes land conversion from native ecosystems to cropland is likely to be counterproductive.

Additional factors may influence biofuel impacts on GHG emissions, including the following:

• biofuel production can displace crops or pasture from current agricultural lands, indirectly causing GHG release via conversion of native habitat to cropland elsewhere.
• greater biofuel production might decrease overall energy prices, which could increase energy consumption and GHG release.

If biofuels are to help mitigate global climate change, our results suggest that they need to be produced with little reduction of the storehouses of organic carbon in the soils and vegetation of natural and managed ecosystems. Degraded and abandoned agricultural lands could be used to grow native perennials for biofuel production, which could spare the destruction of native ecosystems and reduce GHG emissions.

Biofuel production that causes land clearing and GHG release may be favored by landowners who receive payments for biofuels but not for carbon management. To accurately incorporate the costs of carbon emissions in market signals, emerging policy approaches to GHG emissions must be extended to the full life-cycle of biofuels including their net GHG emission or sequestration from land-use change. Moreover, it is important that international policy negotiations to extend the Kyoto Protocol beyond 2012 address emissions from land use change due to increased demand for biofuels.

Joseph Fargione¹ Jason Hill^2,3 David Tilman² Stephen Polasky^2,3 Peter Hawthorne^2
1The Nature Conservancy

²Department of Ecology, Evolution, and Behavior, University of Minnesota

³Department of Applied Economics, University of Minnesota

Published: Science, 07/02/08