Here's a problem I've posed to my students a few times:

I like to ride my bicycle, just for the sheer joy of it. I burn a lot of calories doing it. Where does the energy come from?

Well, um, food, no?

Very good! How does the energy stored in food get converted to my feet pushing the pedals.

Longish discussion with some hair-pulling and tongue-clicking, but we eventually slosh our way through cellular respiration and oxygen and mitochondria and all the trivia you need to master to get past your sophomore year in high school.

What happens to the food? Where does it go? (If you're lost, breathe on your hand....)

Again, some trepidation, but we get there: CO2 and water?

Excellent! Every time I ride my bike to school, I'm spewing off CO2--am I contributing to global warming?

Silence. The short ("obvious") answer is yes, but my lambs have learned this much--the short "obvious" answers are rarely so.

It's a different kind of CO2?

No, I tell them, it's exactly the same...and it is.

***

John Tierney flaunted his twisted knickers in The New York Times a couple of years ago--it's a fun column to read, despite his usual smugness--but he misses a fundamental point. While CO2 is CO2 is CO2, the source of the CO2 matters.

The source of our CO2 is, of course, food, and for most of us in the States, our food uses a lot of fossil fuel to make, especially meats.

Michael Bluejay bikes. A lot. He also thinks. A lot. He's developed a Bike vs. Walk vs. Drive calculator that allows you to enter all kinds of data to find you transportation carbon footprint.

Assuming I'm a regular American, the amount of fossil fuels used for the food that fuels my biking (190# at 12 mph) matches that of our Civic: a 3 mile trip uses about 9 ounces of petroleum for either trip.

And the news for eco-fanatics gets worse--I release 20% more CO2 than my car does over the same distance!

***

The big difference, of course, is that jamming a McDonald's milkshake into the gas tank of a car will not work--cars need fossil fuels, releasing carbon dioxide that has been trapped for millions upon millions of years back into the atmosphere.

And in early summer, I'm not a regular American--when I eat homegrown tomatoes and hand-raked clams, nibbling wild cherries and mulberries, my food bill approaches 0 gallons of fossil fuels.

I bike without a care, no matter what Mr. Tierney's smugness level is.

I am an admitted tree-huggin--squirrel-kissin' ecofanatic.
I do not, however, weigh only 190 pounds--I need to lose about 14 to get there again.

I really don't like John Tierney, or anyone else who's smart enough to get it,
but would rather sell a half-truth for his own gain than seek truth .

The old bike drawing is from Victoriana.

CO2 model from Wikimedia commons--by Jacek FH