Now take two cases: In the first case, everybody keeps their car until it is 20 years old; then they replace it with a new car and hold that for 20 years. Over the course of a 20 year period, 20 cars get bought, and 20 cars get junked. Each person buys one car, and each person junks one car.
In the second case, everybody trades cars (and dollars) every year, passing their car on the person with the next oldest car. So one person is always driving a new car, one person is always driving a 1 year old car, one person is always driving a 2 year old car, etc. Over the course of a 20 year period, 20 cars get bought, and 20 cars get junked. But one person buys all 20 cars, and one person junks all 20 cars.
What carbon footprint accounting method do you propose to use that would result in different accounting for the two cases? It seems to me that in both cases, all 20 people have the same automotive carbon footprint. The only difference I see is that in the second case, some people are (presumably) paying more to drive newer cars, and some people are saving money by driving older cars.
In the first scenario - 20 cars were built
In the second scenario - 40 cars were built
You also assume that there is an immediate buyer waiting to buy each car. That isn't necessarily the case. Have you noticed that dealerships in our cities keep getting larger & larger? This is happening in my city. Every dealership has expanded, and there are more of them.
From Morgan Stanley:
"- Off-lease supply: This has already doubled since 2012 and is set to rise another 25% over the next 2 years.
- Record high units of new car inventory: 2016 YE unit inventory levels were near 10% higher than 2015YE, and are continuing to trend higher in 2017"
http://www.zerohedge.com/news/2017-03-3 ... y-crash-50
Has the population in your city doubled since 2012 and/or increased in relation to the explosion of growth in the number new & used cars sitting on dealer lots? Ours hasn't. Dealers are getting larger because they need more space to put all the cars. Some apparently see that as a win for the environment. I don't.
I look at personal carbon footprint accounting in terms of the CO2 I create vs the CO2 I offset (probably like everyone else). If I buy or lease a new car every three years, by year 10, I'm responsible for the CO2 that went into the manufacturing of 4 cars. If I buy one new car, and drive it for 10 years, I'm responsible for 1 car. If I buy a used car, I'm only responsible for the CO2 caused from the gasoline, consumables, and/or grid emissions that keeps the car in operation.
I bought my EV used, adhering to the environmental principal of Reuse. The car was already built...I didn't cause it's manufacture and all of the CO2 released in the process of building a new car. The first owner did that. The manufacturing CO2 is on his or her scorecard - not mine. They also don't get to take credit for the near zero emissions that comes from charging the car from my solar arrays either...that's on MY scorecard.