A key conclusion of this discussion is the fact that all sectors of the economy must achieve deep reductions, not just one or two. This certainly includes the transportation system, which in California is the largest single source of GHGs (38 percent of the total, according to the California Air Resources Board). Transportation-related emissions, in turn, are a product of three factors:
- The fuel efficiency of vehicles
- The carbon content of fuels
- Vehicle miles traveled (VMT)
What does this really mean in practice? Well, if we think of fuel efficiency and carbon content collectively as "carbon efficiency" (or carbon emission per mile), then we see that the carbon efficiency times the VMT must be reduced on the order of 88 percent per capita in forty years. If carbon efficiency were quintupled over that time period through a combination of fuel efficiency gains and decarbonization of fuel stocks, we would still need to reduce VMT by 40 percent per capita relative to today's levels. In other words, we would have to "make do" with 40 percent less personal driving, trucking, business travel and all other uses of motorized vehicles. If carbon efficiency is merely doubled, we would need to reduce the per capita VMT by 76 percent!
Are these changes feasible? Certainly. In fact, there are Americans right now who live well on 76 percent less VMT than their average compatriots. They're the Americans who live in dense urban centers such as New York and San Francisco. Bringing the transportation system into line with our 2050 climate goals means making sure (among other things) that vastly more Americans can live in those kinds of truly dense, mixed-use environments, rather than simply somewhat-more-dense suburban settings (which tend to achieve VMT reductions of only about 20-30 percent).
It can be done -- but modest changes to how we build urban regions won't get us there.