It is time to stop quivering in our boots in pointless fear of the future and just roll up our sleeves and build it.
- Ray Pierrehumbert

Monday, August 27, 2007

Does My Prius Help?

While we generally disagree on how it shakes out on this or that issue, I have always agreed with John McCarthy's principle (well, OK, one of his principles):
He who refuses to do arithmetic is doomed to talk nonsense.
Recently I have heard, both from a genuine climate denialist (the only one I have met in person, she was charming and infuriating) and from left wing bloggers that the negative manufacturing impact of a Prius outweighs its environmental positives.

Here's a peculiar video (it meanders back and forth into and out of sarcasm in a way that I find exasperating) wherein numbers are bandied about to intimidate, but no serious comparisons are made. Nevertheless, Jerry Mander, who among leftwing kvetches is one whom I admire more than most, is in there arguing against buying Priuses. He suggests that buying a used car has a smaller impact.

Before starting to think about this, that if you are more worried about peak oil than about climate change, let's stipulate that the Prius is a clear winner. If we switch to lower fuel consumption vehicles, the liquid fuel will last longer and the transition to alternatives will be smoother.

What about climate change? My initial response was that Mander's position is ridiculous. I decided to do a back of the envelope calculation to prove it, and I failed.

I did my calculation in dollars. Suppose I fill the tank of my Prius a thousand times over its lifetime (a reasonable number.) Suppose, generously, that an alternative car would have had half the mileage. So we have saved twenty or thirty thousand dollars worth of gasoline on a twenty-five thousand dollar car. Case proved, right? There is no way thirty thosand dollars worth of energy went into the manufacture of a twenty-five thousand dollar car.

But wait. Manufacturing typically uses coal, not petroleum. Petroleum has not been competitive with coal in stationary manufacturing for a long time, as liquid fuel is much more in demand for portable engines. Also, there are huge distribution costs and consumer taxes associated with gasoline that don't apply to coal. So let's backtrack to the historical pump cost of gasoline before the shortage started to bite, about a dollar a gallon, and assume only half of that is energy cost, the rest being tax and distribution. Now we have cut the actual energy savings to the equivalent of $5000 worth of fuel. Can the energy component of manufacture and delivery of a Prius exceed $5000? Hmmm, that starts looking plausible.

OK, but we aren't talking costs, we are talking impacts. Of course most stationary large scale manufacture is coal-fired, and coal has roughly double the climate impact of petroleum. This means the break-even point on climate change is on the order of $2500 worth of coal.

All very crude estimates, but a bit shocking.

So I not only failed miserably to prove that the Prius is a net gain on the climate front, I find in the end that the contrary is quite plausible. I haven't proven Mander's position, but it seems well within the range of possibility.

However, I missed a point that goes the other way. We are talking about the *saved* fuel so we should be talking about the *excess* manufacturing energy. Does it take that much more energy to manufacture a Prius than a comparable non-hybrid? That's the number we should really be thinking about.

I imagine the bulk of the extra energy cost if there is some is in the manufacture of the battery. Replacing the battery costs, what do you know, about $3000. Hmmm.

As the video points out, there is also a direct environmental cost in terms of the mining and refining that goes into the battery, but I would think this would be tiny in comparison to global climate change, no matter

So the back of the envelope analysis is inconclusive, but so far leans in favor of the Prius as a net positive in climate change, compared to a conventional vehicle, although much less obviously so than you might suspect.

The people claiming that it is a net negative really ought to come up with better numbers than these and make a quantitative case, though.

Remarkably, great minds thinking alike, Climate Progress has something to say about this peculiar meme; Joe apparently worked harder at this one than I did.

Of course, whatever we choose to drive or fly, we should drive or fly less.

Does it make environmental impact sense, as Mander argues, to preserve nasty old equipment in rather than purchasing less nasty new equipment which has manufacture costs. I guess the answer is, that depends on the new equipment and the old equipment as well as how you cost things out. You'll have to do your own arithmetic, and the information you want won't be all that easy to find.

But hey, I bought my Prius used. :-)

So I am off the hook, right?

Update 7/29/09: This silliness is still hanging around, as silliness does. There now is a more substantive response on The Energy Collective. 


Zeke said...

You seem to be making a rather heroic assumption that the majority of manufacturing costs and comprised of energy costs. I'd imagine that labor costs and returns on capital investments contribute the lions share of the cost of your new Prius. Simply monitizing the cost savings from reduced gasoline use and comparing them to production costs is largely meaningless.

Michael Tobis said...

Zeke, sure, but that is the worst case for the Prius.

To the extent you are right the Prius does better. See the linked Climate Progress article for more.

Fermi Paradox said...

Well, if manufacturing a new car has really enviromentally more impact than using a used car it is more a question of using a car as long as possible, right? So if I sell my old used (now repaired) Civic instead of scrapping it, so helping to extend its lifetime, after I replaced it with a new Prius, I am of the hook as well. Or not?

James Annan said...

Ride a bicycle you lazy buggers!

Michael Tobis said...

A young fellow who rides his bicycle everywhere stopped by my office last week.

He told me he would be taking the bus in future.

The approach to the JJ Pickle Campus is simply impassable by bicycle.

Anonymous said...

I don't think it is important to do this kind of calculation on a case-by-case basis. Most people operate on rules of thumb, and the goal is develop good rules of thumb. This gets the first order impact correct. Then, individuals tune their behavior further if they wish.

For example, I think, on average, it is better to reuse and recycle than buy new, even if the new item is more efficient. If the entire country had a strong tilt towards reuse and recycle, our impact would be lessened. Then we easily make some exceptions.

Hank Roberts said...

I heard from a guy in a carpool that "the Prius has a little engine that pollutes really badly when you're driving it on the freeway, it's only good for stop and start in town."

I don't believe it, can't find any support for it, figure the California Air Resources Board would have noticed === so this is a data point about the stories out there.

No doubt spread by General Motors (grin).

EliRabett said...

Japan gets 30% of its electricity from nuclear and considerable from LNG. Coal is also important for both energy and metallurgical purposes, which means you have to redo your calculation

Thomas Palm said...

You also have to consider that if you don't buy that used car someone else will. If you drive less than him that is a net bonus, especially if lack of used cars forces someone who drives a lot to buy a Prius instead.

Sometimes you hear people bashing environmentalists driving old, polluting cars, but if they drive little they are making a perfectly rational choice.

I suspect that Michael is wrong when he dismisses the environmental impact of mining as insignificant compared to climate change. Mining can be terribly polluting.

Marlowe Johnson said...

First, comparing the benefit of a hybrid to a used car is an apples-to-oranges comparison, and frankly not a very useful one; taken to its extreme this kind of logic leads one to conclude that we would be better of staying in our houses, never to leave at all except for foraging purposes...

On a more serious side, there are a number of models publicly available that do provide lifecycle assessments of the GHG benefits of hybrids, biofuels, and other transportation options. The two that come to mind are GREET and GHGenius (the Canadian one that I'm familiar with. It can be downloaded at

And Fermi, don't forget to include the differences in criteria air emissions (PM, NOX, VOCs) when comparing the benefits of a new hybrid to your old clunker :)

Hank Roberts said...

> Sometimes you hear people
> bashing environmentalists
> driving old, polluting cars,
> but if they drive little they
> are making a perfectly
> rational choice.

Yep. Drive it til you can junk it (in California, the state will pay $500 to get an old but legal vehicle off the road, setting a bottom on the worst case).

Luca said...

First, on the issue of used vs. new cars, someone already made the case here, but it is clear that there is no difference. You can't make the calculations based on your personal emissions, but on those of the entire system as a whole: if you buy a used car, someone else is selling it to buy a new car. If you sell your old car to buy a more efficient one, someone else is buying and driving your old inefficient car. The total number of driven miles (that is the relevant indicator) is not going to change a lot. In no way is anyone contributing the environment, except those who are destroying their old inefficient cars and stop using the car.

Second, on the Prius, I live in Europe, and while the difference in consumption measured between the Prius and a corresponding very efficient traditional gasoline-fuelled car is around 40%, the difference with diesel-fueled cars is reduced to 20%. This considering consumption, let alone emissions. If we consider emissions, the real bad emissions of diesel engines are particulate, and the new particulate-filtering exhausts reduced these emissions to virtually nothing. This means that the gap between a hybrid and a traditional diesel is reduced to 5-10%. Buying a hybrid in Europe does not make much sense, and as a matter of fact there aren't many sales of hybrid here. The real difference could be made by the introduction of the diesel-engine hybrid, announced by Citroen but not yet on the market. These engines could bring the gap with a traditional gasoline-fuelled engine to a factor of 6, i.e. with similar emissions you could drive six-times longer. That will be an achievement.

In conclusion: US need to rethink the policy they have towards diesel, and favour the introduction of the hybrid-diesel which will truly contribute to slowdown the pace at which we are saturating our environment with GHGs and the pace at which we are exhausting our hydrocarbon fuels.

Michael Tobis said...

All, I don't actually believe that a Prius has more impact than a conventional car. I was just unable to prove it without doing some research.

Follow the link from Joe Romm's linked article and you will see that the greenhouse advantage of a Prius is real but unspectacular.

The advantages to the individual owner are substantial all the same, provided you don't take rugged back roads a lot. The Prius can be a pain in rural Texas.

I agree with Luca that thinking globally it doesn't matter much who drives the used car, but rather what the efficiency of the new stock is. At what point it becomes environmentally or economically sensible to junk the used car plays a role, but on the whole it's clear to me that the number of miles driven dominates.

By the way, we will probably concede to the local mores and get a second car. We are awaiting the American release of the Smart car.

Marlowe Johnson said...


In the U.S., tier 2 emission standards are on a grams/mile basis; they don't differentiate between fuel types.

In these types of discussions it's also important to remember that driving habits can have as great an impact as hybrid vs conventional. Going 70mph vs. 50 mph, for example, can reduce your fuel economy by as much 25%; aggressive accelaration can reduce it by a further 20%; air conditioning another 20% (

These are all things that can be done RIGHT NOW to improve fuel economy. Nissan recently announced that it would be including a real-time fuel efficiency gauge in all its new models, and hopefully other manufacturers will follow suit. Getting people to change their driving habits won't happen overnight, but neither does fleet turnover (about 12 years), so it's important not to get fixated on getting the latest greatest fuel efficient vehicle just because you can.

Drive conservatively (in the true sense of the word).

Fermi Paradox said...


in your comparison gasoline/diesel, did you take into account that diesel emits about 15% more carbon per gallon than gasoline? See for example

Anonymous said...

Wheee, what fun. This is my first time visiting this blog....
Back when I was doing my undergrad I did a project with a guy who was studying the economy of Japan. His approach was to estimate the 'ecological footprint'. My contribution was to estimate how much of the earth's surface area was required to support Japan's fish diet (see Wada and Latham, uh, somewhere wrt ecological footprint). Anyway, I'm surprised that a more complete calculation isn't available out there. These footprinting studies were all the rage (it seemed) in the 90's. It's truly sad that the data aren't available to help inform choices.
Steve L

Steven said...

I can't believe I'm going to come down on the side of the Prius in this balance, but here's a thought:

Taking into acount not only existing resources, but a future cycle:
Wouldn't an investment in the Prius be an incentive towards future developments in increased efficiency, better batteries, lighter materials?

Whereas buying the used car could not fund R&D. This may sound odd, but I rarely buy brand X. They may have my mango-fusion flavored drink at less cost- but they have no R&D. Big Name Drink Brand develops new flavors that I may like.

Marion Delgado said...

I realize this is an ancient topic, but one of my science sparring partners and I went over diesel with a fine-toothed comb. The very first nit we unearthed to pick was with the idea that you could simply bring better diesel tech to the US. You can't!

The US has not only got gasoline on the West Coast that they actually inject toxic benzene into as a cheap waste disposal measure - it's no coincidence that all over Portland benzene levels are about 9 x the federally permitted amounts - it also has by orders of magnitude the dirtiest diesel in the industrial world. You drive a cute little Euro-diesel up to the pump and fill it up, and by the time you get home your catalytic converter will already be a little corroded. In, I think, a month or two it won't work very much at all.

My science sparring partner only knows about US diesel, so when I suggested diesel as part of a long term solution, including biodiesel, he looked at me aghast (I have asthma and bronchitis) and said, you of all people should know that's a terrible idea!

You can do a decent catalytic conversion but it is also true, I think, that more N02 is produced inherently by diesel combustion than by gasoline combustion.

Re all the ins and outs of used, new, prius, yarus, etc. There's a Russian saying I like very much:

Whether you whack the owl with the pole, or the pole with the owl, the owl is still dead.

Troy said...

to Fermi Paradox:
Diesel puts out 15% more carbon than gasoline, but it has 15% more energy than gasoline. This is one reason you get higher mileage off of diesel, the other is that the diesel engine is more efficient than gasoline. This is why gasoline tops out at 35 mpg for most vehicles, but diesel is 42+.