Heightened concern about oil dependence is generating growing support for alternative transportation fuels, but some fuels, like liquid coal and gasoline from tar sands would emit significantly more global warming pollution than gasoline or diesel, according to a new report issued today by the Union of Concerned Scientists (UCS). In Biofuels: An Important Part of a Low-Carbon Diet, the UCS offers an overview of the lifecycle emissions of different alternative fuels, and two scenarios for their future role in America’s transportation fuel mix. It also stresses the importance of comprehensive lifecycle analyses (LCAs) to take into account the entire emissions profile of alternative fuels.

Transportation is responsible for two-thirds of US oil consumption and nearly 40 percent of the country’s global warming pollution on a life cycle basis. To dramatically cut emissions from this sector, a comprehensive solution must include improved vehicle fuel efficiency, smart growth policies that reduce vehicle miles traveled, and clean fuel alternatives.

Liquid coal, for example, can release 80 percent more global warming pollution than gasoline, the report found. Corn ethanol, conversely, could be either more polluting or less than gasoline, depending on how the corn is grown and the ethanol is produced. On average, corn ethanol can reduce emissions about 20 percent, though there is uncertainty due to differing land use practices. The cleanest alternative, cellulosic ethanol from grasses or wood chips, could reduce emissions by more than 85 percent (graph, click to enlarge). (Note that the study did not look at first generation biofuels made from tropical crops like sugarcane or sweet sorghum which reduce emissions far more than corn ethanol; for sugarcane ethanol, the reduction is as large as that of cellulosic biofuels, earlier post.)

Biofuels can quickly become a staple of a low-carbon fuel diet because they integrate well with the existing fuel distribution infrastructure and offer potentially abundant domestic supplies with significant opportunities for growth, the report says. But not all biofuels are the same. There is a wide range in the estimated heat-trapping emissions and other environmental impacts from each biofuel over its life cycle (i.e., from farm to finished fuel to use in the vehicle), depending on the feedstock, production process, and model inputs and assumptions. There are also concerns about emissions and impacts from land conversion and land use associated with biofuel production.

New rules are being developed that will require fuel providers to account for and reduce the heat-trapping emissions associated with the production and use of transportation fuels. For example, both the U.S. Congress and Environmental Protection Agency (EPA) are considering strategies to promote low-carbon and renewable transportation fuels (including biofuels). California, the nation’s largest market for transportation fuel, is developing a Low Carbon Fuel Standard that will require fuel providers to demonstrate reductions in global warming pollution per unit of energy delivered, regardless of fuel source. More state, regional, and federal rules will undoubtedly follow, the UCS writes.

The purposes of the report are two-fold:

1. To ensure that we “count carbs” accurately, by explaining why we need a comprehensive accounting system for carbon emissions—one that measures global warming emissions over a transportation fuel’s entire life cycle. An effective accounting system will not only need to be robust enough to encompass the fuel life cycle, but also address uncertainties and allow for changes over time as better assessment tools and methods become available.
2. To “make carbs count” by describing performance-based policies that will reward low-carbon transportation fuels for their performance and help them compete against highly polluting fuels such as liquid coal (gasoline or diesel made from coal). For example, low-carbon fuel standards require a reduction in the average amount of global warming pollution per gallon of fuel.

A market for low-carbon fuels can produce a rare convergence of business, agricultural, and environmental interests that, if pursued wisely, could represent a “win-win-win” opportunity. But the promise of a lower-carbon transportation future can only be realized through federal and state policies that “count carbs and make carbs count.”

Two scenarios
The report evaluated two scenarios for alternative fuels, one carbon-intensive — meaning that it would produce significantly more global warming pollution than burning gasoline – and the other low-carbon — meaning that it would produce significantly less. The analysis assumed that alternative fuels will replace 37 billion gallons of gasoline, about 20 percent of the fuel UCS projects Americans will consume in 2030.

In both scenarios, conventional biofuels would meet 25 percent of the demand for alternative fuels. In the carbon-intensive scenario, the remaining demand would be met by liquid coal. The carbon-intensive scenario would increase emissions by 233 million metric tons — equivalent to adding about 34 million cars to the road, the number of new cars and light trucks currently sold nationally over a two-year period. By contrast, the low-carbon scenario relies on advanced biofuels to meet 75 percent of the demand. That would cut global warming pollution by 244 million metric tons, akin to taking 35 million of today’s cars off the road: