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Climate change

Tractor in distance spreading fertilizer in ploughed field
Studies have found nitrogen fertiliser use to be responsible for 51% of greenhouse gas emissions from biodiesel from oilseed rape

The primary motive behind promoting and supporting the development of bioenergy is climate change mitigation - reducing greenhouse gas emissions. But can biofuels really deliver?

Assessing how effective they are in tackling climate change means looking at greenhouse gas emissions over the life-cycle of each fuel product. A full life cycle analysis calculates the emissions associated with the entire production chain to give a total emissions figure. This can be compared to the emissions associated with the fossil fuel counterpart, such as petrol for biofuels or coal for biomass.

The emissions savings on offer from biofuels have always been smaller and considerably more variable than for heat and power from biomass, particularly when they originate from purpose grown crops. There is, moreover, increasing evidence to show that the production of some biofuels can actually lead to increases in greenhouse gas emissions.

Farmers worldwide are responding by converting forests and grassland to croplands

This variability in greenhouse gas emissions savings is principally due to differences in how the processing stage is powered, what happens to any by-products such as straw, and also the level of fertiliser use. Processing wheat into bioethanol, for example, is an energy intensive process. If coal is used to fuel the processing stage, this could result in an overall increase in greenhouse gas (GHG) emissions.

The biggest impact on the lifecycle is fertiliser use. Studies have found nitrogen fertiliser use to be responsible for 51% of greenhouse gas emissions from biodiesel from oilseed rape, and 40.2% of emissions in bioethanol production.

Emissions associated with nitrogen fertiliser arise from their:

  • Manufacture – nitrogen fertiliser is energy intensive to make, with calculations suggesting that each kilogram of fertiliser is responsible for 6.7 kg CO2 equivalent emissions
  • Use - applying nitrogen fertiliser releases nitrous oxide (N20) into the atmosphere, a potent greenhouse gas 296 times more potent than carbon dioxide. 

Together, these figures add up to considerable overall emissions. But this isn’t the whole story. Recent research into the nitrous oxide emissions from fertilizers indicates that there is a much greater level of uncertainty about what the full emissions are than previously thought, and that current Life Cycle Analyses of temperate biofuel crops in particular, may significantly underestimate the total net greenhouse gas emissions released in their production (1).

Emissions from land use change

Until recently, many of the life cycle analyses of biofuels ignored a critical factor: the carbon impact of land clearance, drainage and cultivation in order to plant biofuel feedstocks. Recent research has shown that converting carbon rich habitats such as rainforests, peatlands, savannas or grasslands to produce biofuel feedstocks creates a ‘biofuel carbon debt’, releasing 17 to 420 times more CO2 than the average annual greenhouse gas reductions these biofuels provide by displacing fossil fuels (see below). This is often referred to as the carbon ‘payback’ time.

Time to repay the biofuel carbon debt for various biofuels grown on different ecosystems around the world

  • Palm biodiesel from peatland rainforest in Indonesia: 423 years
  • Soybean biodiesel from tropical rainforest in Brazil: 319
  • Soybean biodiesel from Cerrado grassland in Brazil: 37
  • Corn ethanol from abandoned cropland in US: 48
  • Prairie biomass ethanol from marginal cropland in US: 0

Source: Fargione et al (2)

But it is the indirect landuse impacts of biofuels production that still pose enormous risks to the planet, and which must be addressed before we proceed any further with the kind of public support and subsidy for biofuels that the UK Government and the EU are implementing.

If biofuel crops take up land previously used for food crops, these food crops will inevitably be displaced elsewhere, potentially leading to loss of important habitats and the release of stored carbon into the atmosphere. The drive for biofuels is already distorting world commodity prices, and farmers worldwide are responding to these higher prices by converting forests and grassland to croplands.

Conclusion

In summary, there remains significant uncertainty surrounding the potential for biofuels to contribute to climate change mitigation. Current evidence suggests that there are some biofuels that can deliver a high level of greenhouse gas emissions savings, without impacting on valuable natural habitats because they are produced from waste materials, eg waste vegetable oils. And some tropical crops, which need little if any fertilizer inputs, could also deliver high greenhouse gas emissions savings, as long as they do not replace valuable carbon stores.

In the future, it is hoped that wood-based feedstocks, such as forestry products and coppiced willow, can be used for commercial biofuel production (often referred to as second generation biofuels). It is suggested that this would be considerably more promising in terms of greenhouse gas emissions. However, these biofuels are not currently commercially available.

1. Crutzen, PJ, Mosier, AR, Smith, KA, Winiwarter, W (2008) N20 Release from agro-biofuel production negates global warming reduction by replacing fossil fuels, in Atmospheric Chemistry and Physics 8, 389-395

2. Fargione et al (2008) Land Clearing and the Biofuel Carbon Debt

Last modified: 25 February 2008