Apparently, the U.S. Postal Service is considering cutting back on one day of mail delivery per week.

Personally, I suppose I’m fine with this, since I get very little time-sensitive mail. But I imagine that there are some folks who’d see this as a real hardship — yet another little blow, at a time when there are plenty of big ones to absorb.

Regardless, someone just emailed me to ask how the service cutbacks might affect global warming.

Sadly, I’ve got no time for a real answer. But Google gives me just enough information for a ballpark answer: as an upper-bound estimate, I think that a one-day-per-week cutback in mail delivery could reduce vehicle CO2 emissions nationwide by as much as 700,000 tons per year.

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Here are some numbers — and consider these very preliminary. I think the back of an envelope might be embarrassed to have all this scribbled on it:

  1. USPS says that it spent $2.35 billion on energy in 2007 — 75 percent of which was for transportation.

  2. The same source says that fuel for cars and trucks accounted for about two-thirds of all transportation fuel spending.

  3. $2.35 billion * 75% * 67% = about $1.2 billion spent on highway fuels in 2007.

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  4. Cutting mail delivery from 6 days to 5 would have saved about $200 billion in 2007.

  5. Gasoline averaged about $2.84 per gallon that year.

  6. At that price, $200 million would have bought about 70.4 million gallons of gasoline.

  7. When burned, gasoline emits about 19.56 pounds of CO2 per gallon.

  8. The CO2 savings total 70.4 million gallons * 19.56 pounds per gallon / 2,000 pounds per ton.

  9. That all adds up to 688,732 tons of CO2. Call it an even 700,000 tons.

How much is that? About as much as the annual emissions of a city of about 35,000. A drop in the bucket in a nation of 300 million, but nothing to sneeze at either.

But here’s the thing: I’m almost certain that this number is wrong! In fact, it could be too high by 60 percent or more. Y’see, even if local deliveries are trimmed, inter-city deliveries will still go on, at about the same pace — meaning that USPS trucks will still ply the nation’s highways, traveling every bit as far as they do now. Cutting delivery service by one-sixth might mean cutting total fuel consumption by much less.

So consider yourself warned — if you attribute this number to me, I’ll accuse you of not reading to the end of this post!

This post originally appeared at Sightline’s Daily Score blog.