But the climate impact of using the grass to make cellulosic ethanol depends on how and where it's grown, processed and transported. With that in mind, researchers from the University of California, Berkeley and Lawrence Berkeley National Laboratory, US, have assessed the optimum conditions for producing the fuel, using six different scenarios.

"Life-cycle assessment is essential to understanding the environmental implications of biofuel production and use," Corinne Scown of the Energy Biosciences Institute told environmentalresearchweb. "It simply doesn't make sense to focus only on tailpipe emissions, as some previous studies of petroleum-based fuels have done."

The team found that, provided indirect land-use change was successfully minimized or mitigated, the major factors affecting the greenhouse-gas emissions of cellulosic ethanol production were the amount of soil carbon emitted or stored during growth of the grass, and greenhouse-gas offset credits for electricity exported to the grid by biorefineries.

"If Miscanthus is planted on land formerly used for tilled agriculture that would otherwise have remained in production, the short-term potential for net carbon sequestration is very large, although this effect is not indefinite as soil will reach carbon saturation after 20–50 years," said Scown. "However, if Miscanthus is planted on Conservation Reserve Program (CRP) land, the net sequestration would likely be insignificant because CRP land has already been taken out of production and would begin sequestering carbon regardless."

Cellulosic biorefineries can generate electricity by burning lignin, which makes up about 22% of the perennial grass but cannot be easily transformed into bioethanol. It's expected the refineries will be able to export up to 0.09 MJ of electricity for every 1 MJ of ethanol produced.

"If these power exports cause carbon-intensive power plants such as coal-fired plants to ramp down in the long term, the carbon credit assigned to ethanol production is large," said Scown. "Conversely, it is possible that biorefinery-power exports will qualify for Renewable Portfolio Standards, in which case they would likely be replacing other renewable, relative carbon-neutral power sources. In that case, the carbon credit would be essentially zero."

Without soil carbon and offset credit factors, the greenhouse-gas intensity of bioethanol from Miscanthus was calculated as 11–13 g of carbon dioxide-equivalent per Megajoule of fuel. The team says this is 80–90% lower than gasoline. Including soil carbon sequestration and the power-offset credit resulted in a net greenhouse-gas sequestration of up to 26 g of carbon dioxide-equivalent per Megajoule of fuel.

"What also became increasingly clear to us is the importance of location; where the biomass is grown, where the biorefineries are located, and by what mode and how far both the biomass and ethanol product must be transported are all key to assessing the environmental impacts," said Scown. "These are all unknowns for an industry such as cellulosic ethanol production that has yet to develop on a commercial scale."

Scown hopes that the findings will encourage other researchers who assess alternative energy systems of all types to "acknowledge the importance of location, and develop geospatially disaggregated scenarios to better understand what these industries will look like and how they will interact with existing infrastructure systems and the environment".

Next the researchers plan to look at the impact of cellulosic ethanol production from Miscanthus on water use, air pollution and soil and water contamination. "Additionally, we hope to build more robust, complex scenarios that better account for uncertainty and cost-benefit trade-offs that will impact choices that farmers and biorefinery operators make as the industry develops," said Scown.

Scown and colleagues reported their work in Environmental Research Letters (ERL).