Long ago, I used to teach literature. A couple of years into it, an unsettling pattern emerged. Some people, it turns out, cannot understand irony.

I can’t go into the reasons why because … well, because I don’t know the reasons why. I only know that after years of trying all kinds of explanations, definitions, examples and approaches to it, it was usually very difficult to clear the air. Irony simply registers vaguely, or not at all, for some people. It seems similar to being unable to distinguish a color, say, green. People talk about green objects but all you see is different shades of blue, for example. So your response is to think either 1) you’re missing something so muddy it can’t be real but you keep quiet about it because you don’t want to seem ignorant, or 2) those people are deluded fools because there obviously is no color “green” since you don’t see it.

Henry David Thoreau’s writings give some readers trouble because if you can’t see his irony, he just seems like a fatuous tree-hugging bunch of nonsense. And for some others who do detect it, his irony stings their consciences pretty hard and makes them angry. Especially when it comes to his remarks on having a civilization without wrecking the environment. Thoreau was onto this problem in the 1830s, mind you.

So I was thinking about this on a recent trip to the recycling center (see Backyard Naturalist, March 24). When I arrived, I pulled up beside a pickup truck roughly the size of a Panzer tank and got out to unload my collection. The truck driver, a neatly dressed guy around my age, was energetically unloading his cans, bottles, plastic containers, newspapers, etc., and chatting garrulously with the staff, who also helped me get my stuff out. The pickup’s diesel engine, all the time, was idling. I spent about 10 or 12 minutes unloading. When I drove off, the pickup was still rumbling in wait mode.

Direct, simple numbers about emissions from idling vehicles are surprisingly difficult to ferret out. But a study cit-ed by the Environmental Defense Fund states that in New York City, idling vehicles pour 30,000 tons of carbon dioxide emissions per year into the air. That’s New York City only, and idling vehicles only. That NYC idling burns more than 7 million gallons of gasoline and 5 million gallons of diesel — going nowhere, just making smoke. A study by Argonne National Laboratory found that if each car in the U.S. idles six minutes a day, about 3 billion gallons of fuel are turned into exhaust, to no purpose, every year.

The problem with all this useless smoke, of course, is that it aggravates air pollution. And a couple of the things we mean by “air pollution” are particulate matter like soot and smoke that cause respiratory illnesses, among other things, and greenhouse gases like carbon dioxide that are, by practically unanimous agreement among climate scientists, playing a significant role in jacking up the greenhouse effect and warming the Earth’s atmosphere to damaging levels.

So there’s New York City. Not to mention the faraway rest of the world like Bulgaria (as I have mentioned here before), where air pollution and idling vehicles are rife. In Beijing and Shanghai, where cars with smoky engines idle in halted traffic all the time (evidence: my own eyes), the air is worse than in New York. Tons and tons of poison launched into the air by idling vehicles in Boston, Hong Kong, Istanbul, London, Seoul, Bucharest, Rome, Los Angeles, Quebec. I’ve never been to New Delhi, Tokyo or Buenos Aires.

Blah, blah, blah. If you turn your engine off, you burn just as much fuel turning it back on. So goes the common wisdom.

Except, that’s true only for 10 seconds. If you idle longer than 10 seconds, you’re burning more fuel than it takes to start the engine again. The vehicles idling in the corner store parking lot for 10, 15, 20 minutes are just pouring smoke into the atmosphere.

Even if you don’t care whether you’re poisoning the air, you’re wearing down your engine and spending — depending on how much idling you do — hundreds of extra dollars a year on gas or diesel that’s doing nothing. The cost from wear of restarting is a small fraction of the cost of idling the engine beyond 10 seconds.

What if even half of us shut off idling car engines? A Canadian government fact sheet says that “if Canadian motorists avoided idling for just three minutes every day of the year, CO2 emissions could be reduced by 1.4 million tonnes annually.”

Thoreau died a couple of decades before gasoline-powered vehicles showed up, but he had their predecessor the railroad pegged: “That devilish Iron Horse, whose ear-rending neigh is heard throughout the town, has muddied the Boiling Spring with his foot, and he it is that has browsed off all the woods on Walden shore.”

Once more to the lakes: If Unity Pond and Lake Cobbosseecontee, among others, are warming up like the rest of the world (see Backyard Naturalist, March 10), there are things you can do to at least lower your participation in it. Using the recycling center is one. Shutting your engine off when you arrive, if you drive there at all, is another.

Take my word for it: It’s green.

Dana Wilde lives in Troy. You can contact him at naturalist1@dwildepress.net. Backyard Naturalist appears the second and fourth Thursdays each month.