Photo: xc_kuniI was recently traveling in Europe (including in Switzerland, where 87% of trains are less than 3 minutes late). While I was in Cambridge, England, my old friend and colleague David MacKay, who shares my antipathy for bad numbers, gave me a copy of a recent article in the UK journal Physics World (“Optoelectronics: a green explosion”, May 2011, p. 5 of the optics supplement). The article touted laser-based “green technologies,” including their use in reducing carbon-dioxide emissions:
Volvo’s Johnny Larson says it is possible to shave a few kilograms off the weight of a car’s metal frame by optimizing its design for a laser process. This has knocked up to 2 kg [4.4 lbs] off the XC60, and for every one of these models that clocks up 100,000 km [60,000 miles], 24 kg [53 lbs] of carbon-dioxide emissions will be saved.
So many numbers, so little meaning! Whenever I see so many numbers, I think of what Socrates might have said: “The uncompared number is not worth knowing.” Let’s start with the 2-kg weight reduction. Because it has units, it is meaningless: It gets meaning only in comparison to another, relevant quantity with the same units—for example, the weight of the XC60 (which is an SUV). A typical SUV weight is perhaps 2000 kg [4409 lbs], so the weight reduction is 1 part in 1000 or 0.1 percent.
That number is dimensionless and meaningful, and it suggests that the weight reduction is not significant. However, it’s hard to feel the size of 0.1 percent directly. To give the comparison some bite, convert it to an area of life—salaries—where our perceptions are acute. Thus, imagine that you’re an engineer earning $60,000 per year. In the past year you’ve developed amazing new green technologies. Your supervisor comes to discuss your annual merit raise, saying, “You’ve done fantastic work! We want to thank you by giving you a raise to $60,060!” You might feel cheated by the microscopic raise—just as we should feel cheated by the minuscule weight reduction.
Now let’s apply similar reasoning to the reduction in carbon-dioxide emissions. The 24-kg reduction (over 100,000 km of driving) is, once again, a number with units; thus, it is meaningless. To make it dimensionless, I’ll divide it by the total carbon-dioxide emissions from 100,000 km of driving. Alas, I don’t know that number off the top of my head (whereas I could easily guess the vehicle’s weight when making the weight reduction meaningful).
Fortunately, having made the 2-kg weight reduction meaningful (by dividing it by the XC60’s weight) pays us a bonus dividend: we can reuse that comparison by guessing that the carbon-dioxide emissions are simply proportional to the vehicle’s weight. Thus, the emissions reduction would be 1 part in 1000 or 0.1 percent; and the total carbon-dioxide emissions would be 1000 times larger than 24 kg, or 24,000 kg.
There are at least three lessons in that result. First, and somewhat amazingly, the guess is reasonably accurate: The XC60 is rated at 154 grams of carbon dioxide per kilometer, which is roughly 15,000 kg per 100,000 km (our guess was 24,000 kg). Second, this application of lasers is green because of the color of the money that this misleading marketing will earn for Volvo. Third, a physics journal should know better!
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thus explains the problem of giving PR folk a calculator…
The more relevant metric is total cost/weight CO2 reduction. How much did the change cost Volvo, and how much CO2 will now not be emitted across the life of the fleet?
If the technology implementation was inexpensive enough, it may still be impressive on a value basis.
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While Volvo’s XC60 isn’t going to sell numbers in the Toyota Camry range, even on say, 10,000 units, you’re talking 20,000 kg less steel used (reduction of consumption there) and 24,000 kg less CO2 produced versus its non-laser-designed versions. Maybe a drop in the bucket (and meaningless marketing jargon) but on the other hand, 24,000 kg steel “saved” (continuing the 10,0oo units example) is nothing to sneeze at, either. Marketing? Yup. Efficiency gain? Yup, that too. Even if it’s 0.1 percent.
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They should have used your numbers instead of theirs. Much more impressive, since they were touting the Laser, not necessarily the car.
If they were only talking about the car, then a 0.1% reduction is pretty silly. You don’t need high-tech lasers to design a car that weighs under 2 tons.
Vehicle engineers have come up with thousands of innovations – some more revolutionary than others. A single innovation that shaves over 4lbs off the frame is, as Justin’s comment says, nothing to sneeze at. Not only because of the vehicle emissions by the way- the innovation also reduces production emissions, e.g. via reduced demand for metal. Anyway, not a small feat – unlike your article.
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p.s. freakonomics should know better.
An unexpected side-epiphany: going on a diet reduces carbon emissions when you drive. Also, it’s another reason to go potty before a long drive.
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It’s also why I don’t pick up hitchhikers.
2 kilograms? And we have people justifying this as being wonderful? Just because it is an “in car” and a PR guy calls it green?
It’s just an excuse to justify driving an SUV. Arrogance.
Seems as though you missed the point entirely. It’s not about the particular car, but about a laser welding technology. If as an example the technology can cut 4 lbs off the weight of a big SUV, it could also cut 2 lbs off the weight of a reasonable-sized car, maybe a couple of ounces off your road bike frame, and shave equivalent amounts off the weight (& material cost) of any product that’s welded. And do so more efficiently and more accurately, so the product costs less to produce…
Now you economist types can have fun adding up the savings over the whole economy.
Guess this explains why you guys are economists rather than engineers.
The emissions of the car are almost irrelevant. The average first-world baby contributes over one-million kilograms of CO2 throughout its lifetime.
Going “green”?