A new study published by SRI Consulting (SRIC) finds that cultivating crops to make biofuels – including ethanol and biodiesel – could actually increase global warming in some places in the world compared to using petroleum-based fuels, while reducing them in others.

The group’s report, entitled Carbon Footprint of Biofuels & Petrofuels, calculates the specific impact of land use on specific fuel choices. It compares the total carbon footprints of biodiesel and bioethanol against those of “petrofuels” diesel and gasoline from the production process through its final use in vehicles.

“Carbon footprints of biofuels depend to some extent on crop yields and cultivation emissions, but by far the most significant factor is land use,” said Russell Heinen, vp at Menlo Park, CA-based SRIC. “What could the land be used for and how much carbon would it store if it were not used to grow biofuel crops?”

SRIC’s research indicates that land use is so critical that – at least from a global warming viewpoint – northern European farmers should plant trees and burn diesel rather than plant rapeseed for biodiesel. However, greenhouse gas emissions are reduced more by converting a Malaysian rain forest into a palm oil plantation for biodiesel than by filling tanks with petrodiesel. Again, though, the study finds it is better to fuel with gasoline and to preserve the Brazilian rain forest than to knock it down to grow sugarcane for ethanol.

“Generally speaking, where a crop is grown plays a more important role in the biofuel/petrofuel footprint than what type of crop is grown,” explained Michael Arné, SRIC’s assistant director for greenhouse gas research. “On northern European land, growing a forest and burning petrodiesel is clearly the better choice. But if rapeseed were grown on former prairie land in the U.S. Midwest normally devoted to corn and soybeans, the ‘clear choice’ becomes a tie. The footprint is about the same whether rapeseed biodiesel or petrodiesel is chosen for this area.”

This huge difference for the same crop with two alternative land uses is due to the difference in land carbon capacity, as a forest can store tremendously more carbon than can a prairie, he said.

SRIC’s study compares the carbon footprints – measured in kilograms of carbon dioxide [CO2] equivalent per 100 kilometers driven – of petrodiesel versus biodiesel made from rapeseed, soybeans, tallow and used cooking oil. It also compares gasoline against bioethanol made from sugarcane, corn (maize), and biomass stover (stalks, leaves and cobs) left over from corn harvesting, as well as “hydrotreater” biodiesel and natural gas−derived Fischer-Tropsch diesel.