Climate change is one of the biggest challenges of this century, and greenhouse gas emissions from fossil fuel combustion are the main reason for it.
Unlike fossil fuels, bioenergy can be a carbon-neutral or even carbon-negative energy option, but only if the emissions of greenhouse gases produced during the use of biofuels are re-absorbed from the atmosphere during the growth of feedstocks. This is not always the case, however, and some bioenergy options can even lead to higher greenhouse gas emissions over the entire life-cycle from production, conversion, transport and end use.
Relative net life cycle GHG emission improvement of selected biofuel pathways as compared to
gasoline and diesel fuels.
(Source: UNEP/IEA 2008)
Net GHG emissions will depend on the type of land used, choice of crops in different geo-climatic conditions, agricultural practices (including whether carbon-intensive chemical fertilizers are used and the level of mechanization requiring fossil fuel input), and end-use efficiency. Particularly, land use changes can lead to significant GHG emissions if new land is brought into production by converting forests and wetlands into cropland. This creates a 'carbon debt' that may not be 'paid back', even over a long period of time. If land use changes are direct, the impact can be calculated and traced back. In the case of 'indirect land use changes', i.e. where Bioenergy feedstocks do not directly replace forests, wetlands or other areas with high carbon storage capacity, but push into other usages which in turn replace carbon storing areas, both within a country or even across boarders, the effect is more difficult to assess and to trace.
Assessment of the net GHG effect of a bioenergy pathway or project is currently done under several methodologies. To eable choices for the most GHG efficient option in a given context, a common methodology is urgently needed for different pathways over their entire life cycle, including direct and indirect changes in land use.
This is particularly important for an evolving carbon market that can promote Bioenergy pathways with substantial net greenhouse gas reductions. While bioenergy development should generally lead to significant GHG emission reductions, some bioenergy pathways in developing countries, however, may be pursued for their development and environmental benefits, even if they do not produce significant greenhouse gas reductions.
Finally, with expected impacts of climate change on agricultural productivity (according to the IPCC) crop choices need to be made in light of the need for adaptation measures.