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What’s the carbon footprint of building your car, and how does that compare to tailpipe emissions?

Friday, September 24th, 2010

Mike Berners-Lee and Duncan Clark at The Guardian have a recent post in the series examining the carbon footprints of daily life activities.  Their post asks how much carbon emissions results from the direct and indirect activities of building a car.

The carbon footprint of making a car is immensely complex. Ores have to be dug out of the ground and the metals extracted. These have to be turned into parts. Other components have to be brought together: rubber tyres, plastic dashboards, paint, and so on. All of this involves transporting things around the world. The whole lot then has to be assembled, and every stage in the process requires energy. The companies that make cars have offices and other infrastructure with their own carbon footprints, which we need to somehow allocate proportionately to the cars that are made.

….The best we can do is use so-called input-output analysis to break up the known total emissions of the world or a country into different industries and sectors, in the process taking account of how each industry consumes the goods and services of all the others. If we do this, and then divide by the total emissions of the auto industry by the total amount of money spent on new cars, we reach a footprint of 720kg CO2e per £1000 spent.

This is only a guideline figure, of course, as some cars may be more efficiently produced than others of the same price. But it’s a reasonable ballpark estimate, and it suggests that cars have much bigger footprints than is traditionally believed. Producing a medium-sized new car costing £24,000 may generate more than 17 tonnes of CO2e – almost as much as three years’ worth of gas and electricity in the typical UK home.

17 (metric) tons is 17,000 kg or about 37,400 pounds.   The U.S. EPA estimates that the average passenger vehicle in the U.S. emits 5-5.5 metric tons CO2e per year, assuming 12,000 miles driven.

If you do the math, this means the embodied CO2e emissions to make a car is about 3-3.5 years worth of tailpipe emissions from driving.  Assuming that most people own their cars for longer than three years, this figure doesn’t jive with what the authors claim:

The upshot is that – despite common claims to contrary – the embodied emissions of a car typically rival the exhaust pipe emissions over its entire lifetime. Indeed, for each mile driven, the emissions from the manufacture of a top-of-the-range Land Rover Discovery that ends up being scrapped after 100,000 miles may be as much as four times higher than the tailpipe emissions of a Citroen C1.

If people held onto their cars for 10 years (assuming 120,000 miles), tailpipe emissions would equal 50 metric tons of CO2e, and embodied emissions would be about 34% of tailpipe emissions.  If people drove their cars for 20 years (assuming 240,000 miles), the exhaust emissions would rise to 100 metric tons CO2e, with embodied emissions dropping to 17% of tailpipe emissions.

While most folks generally agree with the notion of driving their vehicle into the ground (as my recently dead 16-yr-old truck illustrates), you’d have to be driving a Toyota Prius to get a lifetime tailpipe emission that equals the embodied emissions of building it (assuming that a Prius achieves three times the mpg of a typical car, which would drop CO2e tailpipe emissions from 5 to 1.7 metric tons CO2e per year, making a 10-year total tailpipe emission of 17 metric tons reasonable).

Thus, if you drive an average car for 10 years, your lifetime tailpipe emissions (50 metric tons) will be a lot larger than the embodied emissions to build the car (17 metric tons) (for a total emission of 67 metric tons).  If you drive a hyper-efficient vehicle for 10 years, tailpipe and embodied emissions may be comparable (17 metric tons each, 34 metric tons total).  This means you could buy a new Prius every three years, and the embodied emissions from all of these purchases plus tailpipe emissions would roughly equal a normal car driven for 10 years.

This raises an important question:  What matters here?  If the goal is to reduce total emissions, the best thing is to buy a car with a very high fuel efficiency and drive it for its full life, as the above examples illustrate.

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Photo credit:  atomicshark

3 Responses to “What’s the carbon footprint of building your car, and how does that compare to tailpipe emissions?”

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  1. Elizabeth Snodgrass says:

    Here’s what I, a concerned environmental consumer without a lot of time to stop to try to understand all the figures, would love to know:

    Is it more environmentally friendly to dump a normal mpg car (28-30 mpg) after 5 or so years in favor of a new hybrid or clean diesel? Or are the emissions of making all those new cars so much that it’s better to drive the more wasteful car longer, since it’s already been made?

    Thanks for any clarification!
    -Zard Snodgrass

  2. Phil Camill says:

    Hi Zard,

    Let’s see if we can figure this out. From the EPA data, we can assume that if you
    kept your current car for 5 more years, you will emit about 25 metric tons of CO2e,
    maybe a bit less since the 28-30 mph you get is better than the average used in the
    EPA study. If you buy a high-efficiency hybrid, this might emit 17 metric tons of
    CO2 to build but only emit roughly 1.7 tons of CO2e per year of driving (let’s
    round up to 2 for convenience). So if you buy and drive the new hybrid, for 5
    years, this will lead to a total of 17 + 2 + 2 + 2 + 2 + 2 = 27 metric tons of CO2e.
    This is pretty much a wash with your current car–you will be emitting more carbon
    up front in the manufacture of the new car, but you will be saving over the long
    haul because of the better gas mileage.

    If we asked what the comparison looked like if you chose to drive the cars for 7
    years instead of 5, then the balance tips even more in favor of the hybrid. Let’s
    see why:

    Normal car: 7 more years of driving = 35 metric tons of CO2 release.
    Hybrid car: New purchase now and 7 years of driving = 17 tons (to build car) +
    14 tons to drive, so this equals 31 metric tons.

    How about 10 more years?

    Normal car: 10 more years of driving = 50 metric tons of CO2e release.
    Hybrid car: New purchase now and 10 years of driving = 17 tons (to build car) +
    20 tons to drive, so this equals 37 metric tons of CO2e.

    Now you can see there is a clear benefit to driving longer with a new hybrid.

    So the answer depends on how long you intend to drive your old car. If you are
    going to drive your old car for five more years or less, it’s probably okay
    to keep it. If you plan to use your old car for 10 more years, consider buying a
    high efficiency hybrid (something that gets 50-60 mpg)–even with the embedded
    carbon emissions of making it, the new car will likely lower your overall
    emissions in the long run because the annual tailpipe emissions are lower than
    your old car.

    Hope this helps.
    Phil

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