Carlos Osorio, Associated Press
A Jeep Cherokee turned in during the Cash for Clunkers program is seen at the Riverside Auto Mall, Saturday, Sept. 12, 2009 in Marquette, Mich.

Bjorn Lomborg, an academic best known for his book "The Skeptical Environmentalist," had an interesting op-ed in the Wall Street Journal recently that got me thinking about cars and the costs of carbon.

I suppose I am not an environmentalist. I enjoy being in the outdoors, particularly hiking and backpacking. I prefer doing these in an unpolluted environment, but I’m not committed to maintaining a pristine environment regardless of the cost. I am certainly not as committed as actress Evangeline Lilly, who recently spent 32 hours traveling by airplane to attend the Forward on Climate Rally to express her opposition to the KXL pipeline, apparently without any sense of irony.

Still, like most people, I am willing to pay at least some cost in order to reduce the amount of pollution, and Lomborg’s article got me thinking about the past “Cash-for-Clunkers” program of 2009.

Lomborg’s main point is that electric cars are certainly not “zero emissions,” and he gives two reasons. First, the electricity used by the vehicles is produced by power plants that emit pollutants. Second, producing a vehicle also generates pollution. He notes that this is particularly true for electric vehicles that use lithium batteries, because lithium is mined with relatively pollution intensive methods.

Lomborg figures that about half the lifetime emissions of an electric car come from its manufacturing, while the equivalent number for a conventional gasoline-powered car is 17 percent. Of course, an electric car does generate less pollution once it begins operating. Using the numbers that Lomborg cites, I calculated that a typical electric car will use electricity that generates about 1,500 pounds of CO2 per year, while a conventional car would directly generate more than twice that, about 3,400 pounds.

Which brings me to “Cash-for-Clunkers.” If my goal were to reduce carbon emissions regardless of the cost, would it be better to a) keep my existing clunker, b) buy a more fuel efficient gas-powered car, or c) buy an electric car. The choice is not obvious. On the one hand, keeping my existing clunker means I am burning more fuel than I would if I had a more efficient late model car. However, producing that new car also generates pollution that would not exist if I kept my old car.

So I did a little number crunching. My estimates are very imprecise, as I am just trying to get a feel for the magnitude of the carbon cost or savings. However, they are not wildly off. I assume the typical car has a useful life of 20 years. One gallon of gasoline when burned produces 5.3 pounds of CO2.

The University of Michigan Transportation Research Institute reports that new cars in 2012 averaged roughly 24 miles per gallon. It also reports that in 1991 the average miles per gallon for all cars in service was 16.9. So a reasonable assumption might be that my 15-year-old clunker has an mpg of about 20.

Given these assumptions, scrapping my clunker today and buying a new gas-powered car would reduce my CO2 emissions by 5,810 pounds. If I was committed to buying an electric car when my clunker died in five years, buying one now instead would give a reduction of 26,700 pounds.

However, the total amount of carbon emitted is not the real issue; the timing of its release is just as important. It is the stock of carbon in the atmosphere at any point in time that matters for global warming, not the amount of new carbon introduced. This means a thousand pounds of carbon today is worse than a thousand pounds five years from now. By delaying the manufacture of a new car, I can reduce the effect of pollution on the environment.

A more accurate measure of the effect of emissions would be the difference in pound-years. A new gas-powered car saves 34,600 pound-years of CO2 over 20 years, while an electric car would save almost 10 times as much, 330,000 pound-years.

So if do not care about the cost, an electric car would be the way to go. However, suppose I do care about cost. Imagine I keep my clunker and produce more pollution, but I buy a carbon offset. I need to buy one that actually reduces emissions of CO2 in some other way by an equivalent number of pounds. CO2 offsets trade for between $15 and $40 per metric ton. So I could buy 16,500 pounds of CO2 reduction today for $225 at a price of $30 per metric ton. Over 20 years this would give a reduction of exactly 330,000 pound-years of CO2.

In other words, there are significantly lower-cost methods of reducing carbon emissions than scrapping an otherwise sound older car in favor of buying an electric one.

Kerk Phillips is an associate professor of economics at Brigham Young University. His views do not necessarily represent those of BYU.