There is also an option which may be called virtual biofuels. The pyrolytic produc­tion of ‘black carbon’ (also charcoal, biochar) from biomass has been advocated as an alternative to biofuels (Fowles 2007). Such black carbon would be added to soils, where it said to be ‘very stable’ and able to fulfil useful functions. This is argued to offset CO2 emissions (Lehmann et al. 2006; Fowles 2007; Mathews 2008a). Saun­ders (2008) has proposed to landfill purpose-grown biomass as a ‘virtual biofuel’, which he considers ‘more practical, economic and immediate’ than the use of actual biofuels from lignocellulosics. There are also ‘climate compensation schemes’ of­fered to users of transport (especially car and air transport) to offset their emissions of CO2. Planting trees tends to be major contributor to such schemes. The idea be­hind this is that the C emitted as CO2 into the atmosphere due to the burning of fossil fuels will be sequestered in biomass. For instance, it has been estimated that refor­estation of abandoned tropical land may lead to an aboveground C sequestration of approximately 1.4Mgha-1year-1 and a sequestration in soils of approximately

0. 4Mg C ha^year-1 over an 80-100-year period (Silver et al. 2000). Offsetting by forest conservation or reforestation leads to much lower costs for the alleged reduction of CO2 emissions from transport than the production of biofuels.

The obvious question about these proposals is: are virtual biofuels indeed a so­lution to the impact of fossil transport fuels on climate? This depends on the dura­tion of carbon sequestration in virtual biofuels. Before being used as transport fuel, mineral oil was destined to remain for many millions of years outside the carbon cycle in which biomass participates, and one may argue that to really offset the use thereof, C in black carbon, purpose-grown landfilled biomass and forests should also remain outside the biogeochemical carbon cycle for many millions of years or ‘in perpetuity’. Also, one may focus on CO2 emitted due to the consumption of fossil fuels. Full elimination of the effect of the CO2 emission from fossil transport fuels on the climate is expected to take a very long time. One quarter of fossil-fuel — derived CO2 remains airborne for several centuries, and complete removal may take 30,000-35,000years (Archer 2005; Hansen etal. 2008). So it may be argued that the sequestration in biochar, landfills or forests should at least be for many thousands of years.

Whether C sequestration for at least many thousands of years applies to biochar has been studied in a limited way (Eckmeier et al. 2007; Wardle et al. 2008). There have been reports about the decomposition of biochar and oxidation of the aromatic backbone of biochar, partly depending on the production procedure (Lehmann et al. 2006; Steiner et al. 2007). However, there is also evidence that carbonblack particles may persist in soils over thousands of years (e. g. Carcaillet and Talon 2001; Long et al. 2007), allowing for the possibility that part of buried biochar sequesters C for a very long time indeed (Lehmann et al. 2006). Secondly, there is evidence that biochar may have an effect on soil biological processes: experimental data suggest that this effect may result in loss of native soil carbon (Wardle et al. 2008). As it stands, it would seem likely that the net carbon sequestration by biochar is partial and may show a decrease overtime.

Landfilled purpose-grown biomass will not for thousands of years or in perpetu­ity remain outside the biogeochemical carbon cycle. Landfilled biomass will largely be converted into CH4 by anaerobic processes. This has the added disadvantage that CH4 is, over a 100-year period, 21 times a more potent greenhouse gas than CO2, the main carbonaceous product of biofuel combustion (Barlaz 2006). The rate of con­version of biomass into methane is dependent on a variety of factors, including tem­perature, lignin content, moisture and pH, and such conversion is often a matter of decades (Barlaz 2006, Themelis and Ulloa 2007). Capture of CH4 emitted by land­fills is an option but has in practice limited efficiency (Themelis and Ulloa 2007).

There is also a major snag with planting forests as virtual biofuels. Storage of carbon in trees should be guaranteed for many thousands of years. At the level of individual trees, this is impossible, as storage in perpetuity or for many thousands of years is well beyond the maximum lifespan of tree species. And guarantees at the level of forests are also a problem. Current ‘climate compensation schemes’ have guarantees for forests that do not exceed a hundred years. Even this guaranteed timeframe is questionable in view of (increasing) risks that forests may be destroyed by wildfires and extreme weather events such as storms and droughts (Kirilenko and Sedjo 2007; Gough et al. 2008; Nepstad et al. 2008). The social arrangements safeguarding forests are also unlikely to persist for many thousands of years or in perpetuity. So it would seem that forests as virtual biofuels rather delay than fully offset the emission of CO2 from fossil transport fuels. There is also another problem with planting forests to limit the increase in atmospheric CO2. This has been called ‘leakage’ (Sathaye and Andrasko 2007; Ewers and Rodrigues 2008). When forestry projects are established, people dependent on that area may move elsewhere, where they may reduce C stocks. There has been a series of case studies regarding this phenomenon. In some cases, high levels of leakage have been demonstrated. For instance, Boer et al. (2007) studied forestation projects in the Jambi province of Indonesia and found that reductions in C stock due to leakage exceeded gains in C stock linked to forestation over a 10-year period. Other forestation projects showed lower leakage, and worldwide, an average percentage of about 50% leakage seems to be associated with forestation projects (Sathaye and Andrasko 2007). So forestry as a virtual biofuel is subject to major problems when it comes to full compensation of fossil fuel consumption.

Apart from problems with C sequestration over at least thousands of years, a main problem is that virtual biofuels clearly would not solve the problem of dependence on mineral oil, as one cannot drive or fly on virtual fuels. For this reason, the ‘real’ biofuels that are used or have been proposed for use as transport biofuels will be the main topic of this book.