Coches más eficientes

Una de las cuestiones que hay que tener en cuenta para evaluar cuánto mejora el medio ambiente cuando se compra un coche más eficiente – o cuando se incentiva su compra con programas públicos – es el total de emisiones en el ciclo de vida (es decir, incluyendo la fabricación del coche). Si la mejora de eficiencia no es muy grande, puede ser mejor no comprar coche nuevo. Bill Chameides en Green Grok hace las cuentas:

When Does a New Fuel-Efficient Car Lower Greenhouse Gas Emissions?

Years needed to pay back the CO2 emissions from scrapping an 18 mpg clunker and buying a new car as a function of the new car’s mpg rating. The arrows show the minimum mpg's required for both a new car and a new light truck under the Feinstein bill. Assumptions: embedded CO2 in the manufacture of a new car is 6.7 tons. Car is driven 13,000 miles per year.

Scrapping an old car for a new one doesn’t guarantee a lower carbon footprint even if the new car gets better fuel economy. The reason: manufacturing and delivering the new car consumes energy, and producing that energy involves greenhouse gas emissions. The amount of carbon dioxide (CO2) emitted to produce a new car has been estimated to range from about 3.5 to 12.5 tons, or an average of about 6.7 tons.

So buying a new car means an extra 6.7 tons of CO2 emissions — you wouldn’t have emitted all that pollution had you just kept your old car. Assuming your new car is more fuel efficient than your old car, you offset or work off that 6.7 tons down by driving your new car. But that doesn’t happen immediately. You have to drive and keep on driving until the amount of CO2 you save from driving the new car instead of the old car equals the 6.7 tons needed to manufacture your new car.

The time it takes to completely offset the 6.7 tons is the CO2 payback time. Clearly the more fuel efficient your car is relative to the old one, the shorter is the payback time.  How much shorter? Here’s an example.

Suppose your old vehicle gets 18 mpg (the maximum allowed in Feinstein’s cash-for-clunker bill). The graph above shows the CO2 payback time as a function of your new car's mpg.

Feinstein’s bill stipulates that the new car must be at least 25 percent more fuel-efficient than the government’s current standard, which is 27.5 mpg. So that means your new car must get at least 34.375 mpg. As you can see from the graph, the payback time will be less than three years for cars — which is a good deal for the climate.

(The fuel economy standard for light trucks is 23.1 mpg, which would mean your new car would have to get a minimum of 28.875 mpg to qualify under Feinstein’s program. And so the payback time for these cars will be a bit longer.)

Feinstein's bill also includes a cash incentive for those who scrap their old car in favor of public transit. (See details in Sen. Feinstein's press release.) Taking the old gas-guzzler off the road and not replacing it with a car at all of course would pack the biggest, most immediate climate benefit.