4.3.1 Introduction

Transport biofuels made from plants are often called ‘climate neutral’ or ‘carbon neutral’. These terms can be traced back to the participation of plants in the bio­geochemical C cycle. Plants take up CO2 from the atmosphere and convert this into biomass, and when biomass is burned, the CO2 is ‘given back’ to the atmo­sphere. There is said to be C neutrality: over a short time span, sequestration equals emission of CO2, which is a greenhouse gas. Greenhouse gases are transparent for relatively high energy solar radiation, such as visible light, but absorb infrared radi­ation and thereby influence atmospheric temperature. And thus, carbon neutrality is in this case said to equal climate neutrality. However, there is more to the relation between biofuels and climate. In part, this is linked to the direct effect of plants on local climate and in part to the emission of non-CO2 greenhouse gases, such as N2O and CH4. Also, biofuel production can be accompanied by changes in C sequestra­tion by ecosystems.

If compared with the original ‘natural’ vegetation, cropping biofuels may differ in determinants of local climate, such as surface roughness (Notaro et al. 2006), evapo(tanspi)ration (Gustafsson et al. 2004; McPherson 2007), precipitation (Liu et al. 2006) and albedo (Gustafsson et al. 2004; Schneider et al. 2004; McPher­son 2007). Albedo is a measure of the reflection of solar radiation by the earth’s surface (including vegetation), which in turn is a determinant of net radiation. Net effects of vegetation change may be different dependent on region. In cold regions, replacement of forest by annual biofuel crops tends to have a cooling effect, due to the importance of change in albedo, and in tropical regions, this replacement may cause warming, mainly due to a decrease in evaporation and cloud cover (Betts et al. 2007). When changes in vegetation are widespread, there may be knock-on effects on climate on a wider scale (Delire et al. 2001; Liu et al. 2006; Betts 2007; Betts et al. 2007; McPherson 2007). These will be further discussed in Chap. 5.

Here, of the factors that may impact climate, we will only further consider net greenhouse gas emissions. These may be positive or negative. The latter case corre­sponds with net C sequestration. First, we will consider the major determinants of these net emissions. Thereafter, the actual net greenhouse gas emissions of a number of transport biofuels will be considered.

Potentially important determinants of the net greenhouse gas emissions linked to the transport biofuel life cycle are:

• Carbonaceous greenhouse gas emissions linked to the cumulative demand for fossil fuels.

• N2O emissions linked to N inputs in, and non-product outputs (e. g. NOx emis­sions) of, biofuel production.

• Changes in atmospheric CO2 concentrations following from changes in carbon sequestration. The latter may relate to changes in soil carbon level and/or changes in aboveground biomass.

• Emissions of biogenic, non-CO2 carbonaceous greenhouse gases linked to the biofuel life cycle. These include CH4 emissions linked to anaerobic conversion of biomass and non-CO2 carbonaceous greenhouse gas emissions due to biomass burning.

These determinants will be considered in turn.