He talked to two of my colleagues, then me.
An unremarkable process (I was quite direct about how little information I had about the student - basically, I wasn't aware of any negatives, but wasn't really in a position to vouch for much of anything).
As he was packing up, he was describing his day: this place, then over to that place, then back to Pennsylvania.
"You must put a lot of miles on the car."
"It's a diesel, so it's not so bad. I get 55 miles per gallon in the summer; in the winter it's in the low 40's."
And then some engineering talk that was interesting to me, about why diesels run more efficiently in warmer temperatures, which led him to describe how he tweaks some elements of the engine to increase the fuel efficiency. (I think it was the intake and something else, but I don't know enough about car engines to have retained what he said in passing.)
"And I figure, if I'm burning less fuel, I'm polluting less. If I was using a gallon to go 40 miles, and now I'm using a gallon to go 55 miles, that's got to be putting out less pollution."
In the back of my mind I'm wondering if his tweaks may result in more soot or certain other combustion products, but I don't know nearly enough of the details of how diesel engines work to know that - it's just a question that occurs to me. So I stick to what I do know:
"Well, at a minimum, you're emitting less carbon, since the CO2 is pretty much a linear function of fuel burned."
I thought I was building a bridge of common understanding. But it turned out, I had just started pulling the boards off of a bottomless chasm of mutual incomprehension.
"The plants could use the CO2," he responded.
"The plants have more than they can handle," I say - presumably, the rising levels of CO2 in the atmosphere are evidence that the plants can't keep up with what we're putting out.
His response is that, "I read that CO2 levels were higher in the Middle Ages than now."
"That's actually not true. CO2 levels are higher now than they've been in hundreds of thousands of years." (I was going off of memory, and hoped I was right. It turns out I was - to the extent that there is such a thing as "truth" anymore.)
Originally, I had unintentionally started uncovering the gap between us, but now he decided it was time to pull up the rest of the boards and make visible the size of the space between us. "I don't even think the Earth is hundreds of thousands of years old."
"There's lots of evidence that it's about four-and-a-half billion years old."
He plays his trump card: "It says in the first chapter, 'In the beginning was the Word,' and then it lists the days. We each have our beliefs."
That we do, sir. That we do.
And a prequel.