Vehicle Exchange Programs May Not Improve Climate

The U.S. Congress designed the "Cash for Clunkers" vehicle replacement program to stimulate the economy by supporting the automobile industry. Some proponents of the program also touted it as a way to mitigate climate change: Trading in old, gas-guzzling vehicles for more fuel-efficient cars, they said, would decrease greenhouse gas emissions. Now a study of a similar Japanese program reports that these car swap programs don't change passenger vehicles' contribution to climate change (Environ. Sci. Technol., DOI: 10.1021/es1034552).

"Cash for Clunkers," more formally known as the Car Allowance Rebate System, ran for about a month in 2009. It offered consumers a voucher worth between $3,500 and $4,500 when they traded in a vehicle with a fuel efficiency of 18 mpg or less for a new one with a better fuel economy. Japan's subsidy program ran for 18 months, and gave consumers up to the equivalent of $2,500 when they traded in a car older than 13 years for a new, more environmentally friendly vehicle.

Economist Shigemi Kagawa of Kyushu University, in Japan, industrial ecologist Sangwon Suh of the University of California, Santa Barbara, and colleagues decided to examine whether such programs could actually reduce total greenhouse gas emissions. They created a model to incorporate Japanese government and industry data on vehicles from 1990 to 2000, including vehicle sales, fuel efficiency, and how often cars are replaced, as well as the emissions generated by the production of the automobiles and of the fuel they consumed. The researchers found that an idealized program that replaces the entire passenger car fleet with fuel-efficient gas-electric hybrid cars would cut greenhouse gas emissions by at most 0.2%.

That's because the replacement programs also would stimulate greenhouse gas production through manufacturing new cars, the researchers noted. In general, these emissions offset the greenhouse gas savings of the new, fuel-efficient car. In some cases, a car swap program could lead to an overall increase in emissions, if the program doesn't require high enough fuel economy standards in the new cars, Suh says.

To balance the manufacturing-related emissions with a one-year decrease in an old vehicle's lifespan, the new car's fuel efficiency would need to exceed that of the swapped older car by about 13%. In comparison, the annual rate of fuel efficiency improvement during the study period was only about 1%.

Deb Niemeier, a professor of civil and environmental engineering at the University of California, Davis, says the study is "really useful" because it reveals the complexity of how automobiles affect climate change. She adds that the study's somewhat counterintuitive conclusions should invigorate discussions of "what kinds of policies would get us a bigger bang for the buck."

The researchers suggest that more-effective policies would encourage a sharp increase in hybrid vehicle market share and an extension, rather than reduction, of vehicle lifespan.