Case in point: In the past few years, the Obama administration has enacted a series of ambitious corporate average fuel economy standards that will require new cars to get around 54.5 miles per gallon by 2025. (That will translate into about 35.4 miles per gallon on the road.) That sounds impressive, but the NAS study concludes that current standards aren't enough to hit even that 2030 goal for oil use.
In fact, the report argues, it's tough to find any single technology that can cut oil use in half by 2030 on its own. Making conventional cars more efficient won't do it. A major push on electric vehicles won't do it. The only things likely to work are a massive switch over to natural-gas vehicles (which would, in turn, make it much harder to hit the greenhouse-gas goals) or a combination of efficiency, electric vehicles, and advanced biofuels:
What the NAS is doing here is estimating the impact of each technology "if it is pursued vigorously." And it found that each technology, on its own, has some promising upsides and some limitations. For instance:
Better efficiency: The report found that there's enormous potential to make conventional gasoline vehicles more efficient, from engine and drivetrain improvements to lighter vehicles. Over the past few years, automakers have started adopting many of these tactics, such as gasoline direct injection. The NAS report estimates that conventional cars could realistically average 74 miles per gallon in 2050 and hybrids 94 mpg.
That said, while improved efficiency will be necessary for big reductions in gasoline use and emissions, it's unlikely to be sufficient on its own (as the graph above shows).
Biofuels: Corn ethanol often gets a bad rap for driving up food prices and putting pressure on farmland. The NAS report, however, looks at next-generation biofuels from cellulosic materials like switchgrass or wood waste. These advanced biofuels, the report notes, could lead to big reductions in gasoline use and emissions with little new infrastructure needed. And there appears to be enough cellulosic material lying around.
The big hitch is that "achievable production levels are still uncertain." There are plenty of promising technologies out there, but cellulosic biofuel development has been sluggish so far.
Electric vehicles: The NAS report estimates that electric vehicles will catch on relatively slowly in the next few decades, even if battery costs drop by a factor of 5, because "limited range and long recharge time are likely to limit the use of all-electric vehicles mainly to local driving." What's more, it will be hard to meet long-term emissions goals through plug-in vehicles alone so long as the electric grid is still powered by fossil fuels. Still, the report notes, electric vehicles are an extremely promising way to curtail gasoline use.
Compressed natural gas vehicles: The United States is newly awash in cheap shale gas. So why not fuel our cars and trucks on homegrown natural gas instead of oil? In theory, the NAS report notes, that's doable if costs for CNG vehicles come down and the fueling infrastructure can get built (those are big ifs). The downside? Natural gas vehicles still produce plenty of greenhouse-gas emissions, and it will be hard for the United States to meet its climate targets if we're all driving natural-gas vehicles in 2050.
Hydrogen fuel-cell vehicles: A few short years ago, this was the great hope for clean vehicles — when hydrogen is used as a fuel cell, the only emission is water. Yet it's still tough to manufacture the hydrogen in a low-carbon way, and developing the fueling infrastructure for these cars could prove enormously "difficult and expensive," the report notes.
An interesting bit from the report: "The technical hurdles that must be surmounted to develop an all-purpose vehicle acceptable to consumers appears to be lower for [fuel cell vehicles] than for [battery-electric vehicles]. However, the infrastructure and policy barriers appear larger." The NAS isn't optimistic that fuel-cell vehicles will make big inroads before 2030.
Public transit and changes in urban planning: Oddly, there's not much here. The NAS report only makes one passing mention of things like telecommuting, public transportation, better traffic management, and compact land use. Which seems surprisingly brief. Surely one way to curtail gasoline use and cut greenhouse-gas emissions would be to make it easier for people to drive less.
Anyway, here's a second graph showing likely greenhouse-gas emission cuts by 2050 for each of these technologies. Notice that no one technology alone can get us to 80 percent cuts — even plug-in electric vehicles struggle to get that goal when paired with a low-carbon grid:
So, the report concludes, getting truly sharp reductions in greenhouse-gas emissions from transportation by 2050 will a variety of technologies and broad policies, from R&D to regulations to gasoline or carbon taxes.
"It is impossible to know which technologies will ultimately succeed, because all involve great uncertainty," the NAS notes. "It is thus essential that policies be broad, robust, and adaptive." That sort of sentiment has almost become a cliche in energy-policy discussions, but at least here there are some hard numbers attached to it.