Your car’s greenhouse-gas emissions are about 25 percent worse than you think.

Driving highway CO2

How so? Well, for each gallon of gas you burn in your engine, there’s the climate equivalent of another quarter-gallon or so embedded in your consumption. What that means is this: the gasoline you use didn’t just magically appear in your tank — it was extracted, refined, and transported to your local station. And all that activity released emissions.

It’s a curiosity of our energy system (and other systems too, such as our food system), but it’s a curiosity that bears closely on our thinking about how to reduce greenhouse-gas emissions. Stay with me for a moment.

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It’s usually assumed that each gallon of gas releases about 19.5 pounds of CO2 into the sky. (Some quibble, and argue that it’s 19.4 or 19.6. But whatever.) Basic physics dictates that a gallon of gasoline combusted will release a more-or-less fixed amount of CO2. But from a public policy perspective, physics isn’t the whole story.

When you consider the “lifecycle” emissions of a gallon of gas, it turns out that all the hidden emissions add up to about 25 pounds of CO2 (see here, here, here, and here). So basically, once you’ve factored in the activity required to get the gas out of the ground and into your tank, it’s fair to say that each gallon you burn is responsible for emitting about a quarter more carbon dioxide than it appears to.

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Now lest you think I’m beating up on gasoline — I would never — I should point out that lots of other things have hidden emissions too. Coal emissions are worse than they appear when you factor in the mining and the transportation. Just so, consumer products often have hidden greenhouse gas content. Food products, in particular, have gotten a mountain of attention for this recently as advocates tally up “food miles” and ponder carbon labeling.

But what does all this mean for climate policy? Well, it suggests that we can make bigger-than-expected cuts by reducing our use of products with high hidden emissions.

More to the point: I’m not arguing that bicycling is more climate-friendly than driving — we already knew that — but that it’s 25 percent better than we thought.