Life cycle assessment (LCA) is the study of impacts that occur from "cradle to grave," or in the case of transportation fuels, from "farm to wheel" or "wheel to well." LCA is currently being used to inform and set policy in the area of low-carbon fuel use. In two research papers appearing in the Jan. 1 issue of Environmental Science and Technology, the researchers found that life cycle analyses of greenhouse gas emissions from petroleum fuels and biofuels do not acknowledge the uncertainty in emission estimates. This omission leads to policies based on average values that may promote reductions that may not be able to meet their goals. Estimates based on single point value hide the underlying uncertainty that can be appropriately represented by a range of possible values.
"For instance, LCA is being used to qualify fuels for renewable fuel standards, however, there is not a full appreciation for the considerable uncertainty associated with the fuel production chain," said Griffin, executive director of CMU's Green Design Institute. "We have shown that even in a mature industry like petroleum refining, the uncertainty range for the life cycle greenhouse gas emissions is 13 percent greater than anticipated reductions in some legislation. For example, this could mean failing to meet the goal of a 10 percent reduction from the current baseline."
The researchers' second paper explored the uncertainty in greenhouse gas emissions for some major biofuels like corn ethanol and how this uncertainty should influence decision-making.
"Essentially, what we are looking at is how much uncertainty the public and industry are willing to accept," said Matthews, a professor of civil and environmental engineering and engineering and public policy at Carnegie Mellon.
Biofuels, for example, received legislative support from the Low Carbon-Fuel Standard issued in California and the Federal Energy Independence and Security Act (EISA). But Carnegie Mellon researchers report that neither act provides any methodological guidelines for dealing with the inherent uncertainty in evaluating potential life cycle greenhouse gas emissions.
"Incorporating the right uncertainty guidelines in the decision making process can highlight the risks of policy failure resulting in increased emissions. And a calculated risk of this failure due to uncertainty can then be used to inform more appropriate target reductions in future biofuels policies," Matthews said.
The Carnegie Mellon researchers report that corn ethanol has only a 25 percent chance of having emissions lower than gasoline while cellulosic ethanol has a 90 percent chance. However, the probability of cellulosic ethanol meeting the 60 percent emissions reduction targets based on the EISA is only 40 percent.
"We suggest that policymakers decide what level of confidence they want in achieving reductions and then set the percent reduction required to meet that specific level," Griffin said.
Source: Carnegie Mellon University