A better way to curb emissions from personal transportation, which is responsible for 19% of all man-made CO2 emissions in the United States, is by improving the vehicle itself, says Henry Jacoby, a professor of management at MIT who studies energy use and climate change. “The bigger bang will come from changing the emissions per mile of the fleet we will have in 2050,” Jacoby says. “If all new housing stock was very dense, you could cut total driving by 25 percent, but the things I’m talking about affect the other 75 percent.”
Jacoby says that a number of measures toward this end are already under way, including government subsidies for cleaner-burning biofuels and plug-in hybrids, efficiency controls on new vehicles, and higher Corporate Average Fuel Economy (CAFE) requirements. All these measures will have a greater impact on emissions reductions, and “just the recent tightening of CAFE standards has had a bigger effect than increased housing density would by 2050,” he says.
A supplemental study released by the NAS concludes that an immediate 0.1 percent reduction in the weight of all vehicles nationwide would be 10 times more effective at reducing greenhouse gas emissions than an immediate 0.1 percent increase in housing density nationwide. Kara Kockelman, an associate professor in the department of civil engineering at the University of Texas at Austin and lead author of this study, says an expansion of public transportation combined with housing density increases could actually increase CO2 emissions if the current levels of usage persist.
“If we are adding someone to a bus system that already exists, that is great, but if you are doubling your bus service, in most cases that is a bad idea in terms of carbon reduction,” she says. “If you could instead fill existing passenger vehicles or double the fuel economy of an SUV, you would get much greater CO2 reductions.”
Jacoby says that recent legislation, such as the increase of CAFE standards, will go a long way toward reducing transportation CO2 emissions, but adds that to achieve significant emission reductions through all sectors of society, much more will need to be done.