Scientists Expose Devastating False Carbon Accounting for Biofuels

False carbon accounting for biofuels that ignores emissions in landuse change is a major driver of global natural habitat destruction, incurring carbon debts that take decades and centuries to repay; at the same time, the emissions of nitrous oxide from fertilizer use has been greatly underestimated Dr. Mae-Wan Ho

Simultaneously as biofuels are blamed for increasing world hunger and landlessness ([1] Biofuels and World Hunger, SiS 49), scientists have been lifting the lid on biofuels’ massive contribution to global warming.

Fatal flaw in bioenergy accounting

A team of thirteen scientists led by Timothy Searchinger at Princeton University, New Jersey, in the United States, pointed to a “far-reaching” flaw in carbon emissions accounting for biofuels in the Kyoto Protocol and in climate legislation. It leaves out CO₂ emission from tailpipes and smokestacks when bioenergy is used, and most seriously of all, it does not count emissions from land use change when biomass is grown and harvested [2]

They said that replacing fossil fuels with bioenergy does not by itself reduce carbon emissions, because the CO₂ released by tail pipes and smokestacks is roughly the same per unit of energy regardless of the source, while emissions from producing and/or refining biofuels also typically exceed those for petroleum as other critics have highlighted [3] (see Biofuels: Biodevastation, Hunger & False Carbon Credits, SiS 33). The team maintained that bioenergy reduces greenhouse emission only if the growth and harvesting of the biomass for energy captures carbon above and beyond what would be sequestered anyway, and offsets the emissions from energy use. This additional carbon may result from land management changes that increase plant uptake or from the use of biomass that would otherwise decompose rapidly.

For example, if unproductive land supports fast-growing grasses for bioenergy, or if forestry improvements increase tree growth rates, the additional carbon absorbed offsets emissions when burned for energy. Energy use of manure or crop and timber residues may also capture additional carbon. But harvesting existing forests for electricity adds net carbon to the air. That remains true even if limited harvest rates leave the carbon stocks of regrowing forests unchanged, because those stocks would otherwise increase and contribute to the terrestrial carbon sink.

The worst case is when the bioenergy crops displace forest or grassland, the carbon released from soils and vegetation, plus lost future sequestration generate huge carbon debts against the carbon the crops absorb, which could take decades and hundreds of years to repay (see later).

Unfortunately, the Kyoto Protocol exempts landuse emissions from bioenergy accounting. It caps the energy emissions of developed countries; but the protocol applies no limits to land use or any other emissions from developing countries, and special crediting rules for “forest management” allow developed countries to cancel out their own land-use emissions as well. Thus, maintaining the exemption for CO₂ emitted by bioenergy use under the protocol wrongly treats bioenergy from all biomass sources as “carbon neutral”, even if the source involves clearing forests for electricity in Europe, or converting them to biodiesel crops in Asia.

Massive carbon debts from land use changes of practically all biofuels

David Tilman and colleagues have earlier worked out the extent of carbon emissions from landuse changes associated with different biofuels.

They found that converting rainforests, peatlands, savannas, or grasslands to produce food crop-based biofuels in Brazil, Southeast Asia, and the United States creates a “biofuel carbon debt”, through releasing 17 to 420 times more CO₂ than the annual greenhouse gas (GHG) reductions that the biofuels would provide in displacing fossil fuels, which would take decades to hundreds of years to repay [4].

In contrast, biofuels made from waste biomass or from biomass grown on degraded and abandoned agricultural lands planted with perennials incur little or no carbon debt and can offer immediate and sustained GHG advantage.

Soil and plant biomass are the two largest biologically active stores of carbon on land that together contain ~2.7 times more carbon than the atmosphere. Converting native habitats to croplands releases CO₂ as the result of burning or decomposition of organic carbon stored in plant biomass and soils. The amount of CO₂ released during the first 50 years of this process is referred to as the “carbon debt” of land conversion. Over time, biofuels from converted land can repay this carbon debt if their production and combustion have net GHG emissions less than the life-cycle emissions of the fossil fuels they displace. For crops with co-products, the carbon debt can be partitioned into biofuel carbon debt and co-product carbon debt. These carbon debts are calculated for a range of biofuel crops grown on different converted ecosystems - from tropical rainforests, peatland rainforest, wooded cerrado, cerrado and  grasslands to abandoned and marginal lands - in Indonesia, Malaysia, Brazil and the US (see Figure 1).

Figure 1  Carbon debt of biofuels from different land conversions around the world

The biofuel carbon debt varies from 17 years for sugarcane ethanol grown on land converted from wooded cerrado, to 423 years for palm biodiesel on land converted from peatland rainforest. Even corn ethanol grown on abandoned agricultural land incurs a debt of 48 years. The only debt-free options are ethanol made from biomass harvested from natural prairies regenerated with native perennials from abandoned croplands and marginal croplands in the US.

The team commented that their estimates are conservative because they have only counted CO₂ generated from decaying material of former habitats for only 50 years. The results indicate that biofuels remain net carbon emitters long after conversion.

That’s not all that counts against biofuels

Nitrous oxide emissions cancel out carbon savings

Nitrous oxide, N₂O, a greenhouse gas with a 100-yr average global warming potential (GWP) of 296 compared with an equal amount of CO₂, is emitted as a by-product of the nitrogen fertilizers used for agriculture, including agri-biofuels. It turns out that the amounts of N₂O emitted is considerable; in some cases sufficient to more than cancel out the biofuels’ savings in carbon emissions, and contribute to global warming instead [4].

An international team led by Paul Crutzen at Max Planck Institute for Chemistry, Mainz, Germany, showed that when the extra N₂O emissions from biofuels production is calculated in CO₂ equivalent, and compared with the CO₂ emissions “saved” in burning the biomass grown instead of fossil fuels, common agri-biofuel crops such rapeseed (for biodiesel) and corn (for ethanol), offer little or no net saving, thereby contributing as much, or more, to global warming than simply burning fossil fuels.

The researchers took a global approach to work out the amount of N₂O generated by N fertilization in industrial agriculture. By comparing (1) the pre-industrial N₂O concentration, assumed to be the natural source, with (2) the current concentration plus the atmospheric growth rate, the anthropogenic source was obtained by subtracting (1) from (2), correcting for an amount due to 30 percent global deforestation since preindustrial times. This gave an anthropogenic source of 5.6-6.5 Tg N₂O-N per year.

To obtain the agricultural contribution, an estimated industrial source of 0.7 to 1.3 Tg N₂O-N per year was subtracted, giving a range of 4.3-5.8 Tg N₂O-N per year. This is 3.8 to 5.1 percent of the anthropogenic input in N fertilizers of 114 Tg N₂O-N per year for the early 1990s, derived from 100 Tg of N fixed by the Haber-Bosch process plus 24.2 g of N fixed due to fossil fuel combustion, plus 3.5 Tg of excess biological N fixation between current and pre-industrial levels, and minus 14 Tg of the Haber-Bosch N not used as fertilizer.

This global ratio of 3.8 to 5.1 percent N₂O will be the same in agri-biofuel production systems; and is much larger than the 1 percent value typically assumed, which was recommended by the Intergovernmental Panel on Climate Change.

When compared in CO₂ equivalents with the carbon dioxide emissions “saved” simply by counting the carbon contained in the biomass harvested, a relative global warming potential of the biofuel, due to N₂O alone, can be readily calculated (see Table 1).

Table 1   Relative warming of crops and nitrogen content

Crop	gN/kg dry matter	Relative warming*	Type of fuel produced
Rapeseed	39	1.0-1.7	Biodiesel
Maize	15	0.9-1.5	Ethanol
Sugarcane	7.3	0.5-0.9	Ethanol

*N-efficiency, 0.4

In Table 1, the N₂O emissions are calculated taking into account the nitrogen fixed in the plant, thus, rapeseed with the highest concentration of N, will emit proportionately more than maize and sugarcane. The N use efficiency (efficiency with which nitrogen is taken up and converted into grain or biomass) is assumed to be 0.4. The current average N use efficiency of cereals is 0.33 [5].

Thus, simply growing the crop is sufficient to cancel out a substantial part of the potential saving, even before any processing to turn the crop into biofuels; and this does not include fossil fuels used in making and transporting fertilizers and pesticides for the crop. When you add the fossil fuel energy required for the many stages of processing to biofuels, etc., it is clear that not a single biofuel actually saves any carbon emissions, even if we disregard the huge amounts of extra emissions caused by deforestation and destruction of natural ecosystems to make place for biofuel plantations.

Article first published 29/11/10

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References

1. Ho MW. Biofuels & world hunger. Science in Society 49.
2. Searchinger TD, Hamburg SP, Mellilo J, Chameides W, Havlik P, Kammen DM, Likens GE, Lubowski RN, Obersteiner M, Oppenheimer M, Roberson GP, Schlesinger WH and Tilman GD. Fixing a critical climate accounting error. Science 2009, 326, 527-8
3. Ho MW. Biofuels: biodevastation, hunger & false carbon credits, Science in Society 33, 36-39, 2007.
4. Fargione J, Hill J, Tilman D, Polasky S and Hawthorne P. Land clearing and the biofuel carbon debt. Science 2008, 319, 1235-8
5. Crutzen PJ, Mosier AR, Smith KA and Winiwarter W. N₂O release from agro-biofuel production negates global reduction by replacing fossil fuels. Atmos Chem Phy 2008, 8, 389-95.
6. Nitrogen use Efficiency, Oklahoma State University Department of Plant and Soil Sciences, accessed 24 October 2010, http://www.nue.okstate.edu/