Marcelo Dias de Oliveira and colleagues assess the environmental impacts of ethanol production in Brazil and in the United States (BioScience 55: 593–602), and conclude that “the use of ethanol as a substitute for gasoline proved to be neither a sustainable nor an environmentally friendly option.”

Their paper offers interesting insights into ethanol production, but we are nevertheless in disagreement with their conclusion for Brazilian sugarcane-based ethanol. The reason for this is that their conclusion follows from a common but problematic use of the ecological footprint concept.

Dias de Oliveira and colleagues estimate the ecological footprint of burning gasoline as the forest area required for capturing the associated carbon dioxide (CO₂) emissions. For ethanol, the foot-print is estimated as the amount of land used for the sugarcane production plus the forest area required for capturing the CO₂ emissions from energy input in the production (which is assumed to be fossil fuel–based). In total, the footprint for the ethanol car is approximately 0.56 hectare and that of a gasohol car (76 percent gasoline, 24 percent ethanol) about 0.63 hectare. Thus, according to their calculation of the ecological footprint, an ethanol car seems to be essentially equivalent to a gasohol car.

The major problem with their approach is that it fails to consider that the sequestration of CO₂ in a forest cannot continue forever. This means that when the established forest matures, the carbon sink ceases, a new area has to be planted, and the old area has to be managed so that the carbon stock remains intact.

Taking this dynamic feature into account yields a different picture. The footprint for ethanol remains constant, whereas the footprint for gasohol grows over time. This is illustrated in figure 1. Here we have assumed that bioenergy (instead of fossil fuels) is used in ethanol production, which gives a somewhat lower area requirement in this case.

Finally, although our analysis suggests that ethanol is preferable to gasoline from a dynamic footprint perspective, this does not necessarily mean that it would be unproblematic if sugarcane ethanol became a dominant fuel in the transportation sector. If the 200 million cars in Europe ran on ethanol, they would require some 40 to 100 Mha of land (7 to 15 times the area used today for sugarcane in Brazil). Clearly, the scale of the potential demand is a cause for concern about potential negative environmental impacts. Other, complementary ways to solve the energy problem for transport are thus warranted.