Professor Azar and colleagues raise a number of interesting points. We agree that forest sequestration cannot continue indefinitely—we addressed the issue of uncertainty about forests' potential to sequester CO2 in our article—and we do not advocate it as a solution to the problem. Azar and colleagues note that the land area required to grow sugarcane or corn for 200 million cars fueled with ethanol is unrealistic. Indeed, that was one of the main points we tried to make, and it is true despite the imprecision involved in determining ecological footprint values, which we pointed out was problematic in some aspects. Nonetheless, the ecological footprint approach is still a valuable screening tool.
Regarding Azar and colleagues' argument that the footprint of gasohol-fueled (but not ethanol-fueled) cars will grow over the years, it seems to us that the assimilation area needed per car would increase as well for ethanol. Consider the production of sugarcane: If only labor-intensive agricultural production (as in Brazil) is taken into account, the assimilation component with regard to ethanol would remain smaller. However, with any major scale-up of production, most countries would turn to industrial agriculture (e.g., supplementary nitrogen, pesticide, and fuel inputs) for biomass production, which would mean that the ecological footprint would increase.
Another important observation: After 30 years of the ethanol program, Brazilian production is still far from deserving the label of “clean energy.”