Carnegie Mellon University
November 15, 2022

Michalek and NASEM team inform national low-carbon fuel policy

A report co-authored by Professor Jeremy Michalek of Engineering and Public Policy and Mechanical Engineering has been published by the National Academy of Sciences, Engineering, and Medicine to evaluate methods for assessing greenhouse gas (GHG) emissions of transportation fuels. 

Transportation is the largest source of GHG emissions in the U.S., and transitioning to low-carbon fuels is important for decarbonizing the transportation sector. But alternative fuels also produce emissions in their production and use, so understanding emissions over their full life cycle is key to identifying which fuels can best reduce GHG emissions from transportation.

States like California and Oregon have used life cycle analysis (LCA) in regulating transportation fuels via low-carbon fuel standards, and the report notes that both the House of Representatives Select Committee on the Climate Crisis and a bipartisan network of former EPA career employees have recently expressed interest in a national low-carbon fuel standard.

The committee of experts assembled by the National Academies looked at methods for estimating GHG emissions from different transportation fuel sources, including electricity, fossil fuels, and biofuels, over their entire lifecycle, and they produced a consensus report with over 100 conclusions and recommendations.

Among the committee’s key recommendations is the importance of choosing the right type of life cycle assessment for the question at hand. There are two types of life cycle analysis: attributional and consequential. 

In an attributional approach, researchers track emissions throughout a supply chain and determine which emissions to assign to which fuels. This can be challenging for cases like electric vehicles, where many types of power plants—powered by coal, natural gas, nuclear, solar, wind, water, and other sources—all feed into a massively interconnected transmission network that supplies electricity to many demand applications.

In a consequential approach, researchers instead estimate how net emissions may change in response to a decision or action, such as shifting fuel use from gasoline to an alternative fuel. This can also be challenging, because effects can be wide ranging – such as market effects where petroleum displacement lowers global petroleum prices, inducing new demand elsewhere.

The report finds that these two types of life cycle analysis answer fundamentally different questions, and it is important for policymakers to choose the correct type to answer the question at hand. The authors recommend that attributional analysis can help identify potential emission reductions within existing supply chains, but a consequential life cycle analysis is needed to determine whether a proposed policy like a national low carbon fuel standard will reduce net emissions and provide net benefits to society.

Their work will help inform the increasing number of policymakers, researchers, and citizens interested in the decarbonization of our transportation system.