Zehner says there are also other problems that come from just looking at mpg when trying to reduce energy consumption and improve the environment.
Energy rebound effect
To understand one side effect of making more fuel-efficient vehicles in the 21st century, people need to look at a similar attempt in the 19th century. In 1865, William Stanley Jevons studied the efficiency of steam engines. As efficiency increased (getting more power from a steam engine while using less coal) he discovered more coal was being used. Increasing efficiency made the use of the energy cheaper, so more steam engines were used for more purposes and for longer periods of time.
This paradox is called the energy rebound effect. Making something more fuel efficient reduces the cost of using fuel. If it costs less to use fuel, then more fuel will be used.
"If you do increase the energy efficiency of cars," Zehner says, "then, all else equal, you could see an increase in automotive use."
And that increase could offset any societal savings in energy use, pollution, etc.
However, Zehner says, that rebound effect may not materialize if the price of energy increases for other reasons (scarcity, taxes, etc.).
"But it is difficult to predict," he says.
What concerns Zehner most is the narrow focus on better fuel efficiency, and not the other energy used to manufacture a car when it's not in use, which includes the mining required for iron and other materials used to build cars, or chemicals leaching into the ground from cars rusting in landfills.
As cars slim down to meet mpg standards, rare materials are needed such as magnesium or carbon-fiber composites.
"One of the reasons (green) vehicles are so expensive is largely a reflection of the fossil fuels that go into building those cars," Zehner says. "They are not expensive because they are green; they are expensive because they are energy intensive to build."
In other words, they are not really green.
"You can't say this much gasoline is equal to this much pollution in another country where they are manufacturing batteries," Zehner says. "There is no way to do it. It is a limitation that isn't the fault of any individual or group. It is just the way it is."
Norton sees things in a less environmental light. He says he worries that the push for better mpg will result in cars that are less safe and just as expensive.
"Now your vehicle is going to be smaller and it is going to cost you as much as a larger vehicle," Norton says. "And it is not as safe."
So far, overall fatality rates haven't been rising from smaller cars. The National Highway Traffic Safety Administration reported that 2011 saw 30,246 people die in crashes — the lowest number of total motor vehicle crash fatalities since 1949.
But safety isn't the only concern Norton has. If the new vehicles are smaller, he says that means less comfort.
"In my case we have a family of five and a dog," he says. "We are just not going to fit in a Prius — especially if we are going to go camping. What are you going to do, get a convoy of cars? Two or three cars to go off and see Yellowstone together?"
And how will people haul their RVs around?
"Or have we just decided that industry must die?" he says. "Did we have a public hearing on that first before we erected the gallows?"
Zehner also wants a broader public debate — one that looks at actions that would have greater real environmental impact. To him this means looking at zoning, how cities and transportation structures are built. "It is about how people get to work," he says. "Encouraging bicycling and walking. Rapid bus transit. That would create a huge difference."
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